Hello and welcome to the 2016-2017 Southern Connecticut Weather Winter Forecast!
Introduction
Each fall, we produce a winter forecast, based off of some of the factors that influence the larger-scale pattern and hence the sensible winter weather in our area. We’ll identify some of the key variables that go into the outcome of a winter, devise a weighting scheme to build a final forecast, analyze each of the variables and what it suggests for the upcoming winter, and finally, tie it all together into a seasonal forecast. We focus primarily on meteorological winter, that is, December through February, but we’ll touch on March were appropriate. Fair warning; this is a long discussion. If you’re just looking for the forecast, scroll down to the end of the piece, but I highly encourage you to read through the discussion and analysis, as it will give you a much better understanding of why the forecast is what it is.
Before we get into the forecast, a word of caution. To quote last year's winter forecast:
“Long-range outlooks are an extremely imperfect science, and, as with any forecast, are not the rule of thumb. To mitigate this somewhat, we try to work in probabilities and ranges rather than absolutes, to give a more reasonable boundary of what may happen, but things can and do change in meteorology on a dime, and, as a result, any long-range outlook, including this one, should be taken with a grain of salt, and seen as more informational rather than predictive. Nonetheless, we do strive to be as accurate as possible, and the science behind these methods is real, if a bit shaky at times. We'll keep an eye on how we're doing throughout the winter, and will likely have periodic updates to confirm or refine our thinking from this initial outlook.”
Essentially, the variables that make up a winter forecast are numerous, and if even one or two of them don’t turn out as expected, it can result in a significant shift in the overall paradigm of ground truth. So take it with a grain of salt.
Methodology
With that out of the way, let’s take a look at the variables that make up the winter forecast. There are five main data spaces that can contribute to a seasonal forecast. Here they are, along with their approximate weights towards the final forecast.
Our end goal is to produce nine forecasts; temperature and precipitation departures from normal for the winter as a whole as well as monthly for December, January and February as well as a snowfall departure for the winter as a whole. Let’s get to it!
Seasonal Models
Let’s start by taking a look at the seasonal models. There are three of them; the Euro seasonal, the JAMSTEC, and the CFS. Unfortunately, due to copyright issues, the Euro seasonal can’t be reproduced, and I have not seen the actual images myself. So, we will focus on the latter two.
A note on how the seasonal models work. Essentially, they attempt to model the upcoming months in much the same way that the midrange global models attempt to model the upcoming forecast period, that is, start with a set of initial observations and apply some physics package to those observations to make future forecasts for the next period on some interval, then, that forecast is fed back into the model as the initial observations for the next period to be created off of, and so on. As you can imagine, small inaccuracies in initial data or calculations are exponentially magnified as the recursive cycle continues, and as a result, global models, especially the seasonal ones, are notoriously inaccurate, hence our relatively low weight of them compared to other factors. They are still worth considering and looking at, as what they show (and don’t show) can be valuable when taken in the right context.
First, let’s take a look at the CFS. Here are temperature and precipitation departures from normal on the CFS for December, January, and February.
Introduction
Each fall, we produce a winter forecast, based off of some of the factors that influence the larger-scale pattern and hence the sensible winter weather in our area. We’ll identify some of the key variables that go into the outcome of a winter, devise a weighting scheme to build a final forecast, analyze each of the variables and what it suggests for the upcoming winter, and finally, tie it all together into a seasonal forecast. We focus primarily on meteorological winter, that is, December through February, but we’ll touch on March were appropriate. Fair warning; this is a long discussion. If you’re just looking for the forecast, scroll down to the end of the piece, but I highly encourage you to read through the discussion and analysis, as it will give you a much better understanding of why the forecast is what it is.
Before we get into the forecast, a word of caution. To quote last year's winter forecast:
“Long-range outlooks are an extremely imperfect science, and, as with any forecast, are not the rule of thumb. To mitigate this somewhat, we try to work in probabilities and ranges rather than absolutes, to give a more reasonable boundary of what may happen, but things can and do change in meteorology on a dime, and, as a result, any long-range outlook, including this one, should be taken with a grain of salt, and seen as more informational rather than predictive. Nonetheless, we do strive to be as accurate as possible, and the science behind these methods is real, if a bit shaky at times. We'll keep an eye on how we're doing throughout the winter, and will likely have periodic updates to confirm or refine our thinking from this initial outlook.”
Essentially, the variables that make up a winter forecast are numerous, and if even one or two of them don’t turn out as expected, it can result in a significant shift in the overall paradigm of ground truth. So take it with a grain of salt.
Methodology
With that out of the way, let’s take a look at the variables that make up the winter forecast. There are five main data spaces that can contribute to a seasonal forecast. Here they are, along with their approximate weights towards the final forecast.
- Seasonal models(10%): These models are longer range versions of the global models that we use for short-range forecasting. While they do provide day by day forecasts, they are almost completely useless. Instead, we look at their composite averages. They have some skill, but have several distinct biases, and as such we weight them relatively little.
- Analogs(25%): We look at past years that have similarities to this year, whether it be conditions observed during the fall, ENSO state, teleconnectors, or previous winters.
- ENSO(25%): El Nino Southern Oscillation; it represents water temperatures in the central and eastern equatorial pacific. El Nino represents warmer than normal temperatures, La Nina cooler than normal. These phases have distinct impacts upon the larger-scale pattern, and as a result on our sensible weather.
- Teleconnectors(20%): There are many indices measuring current conditions in different parts of the world, from upper level features to regional ocean temperatures. We will discuss them, analyze them, and explain what their current and forecast states mean for our weather.
- Current conditions(15%): Current surface conditions such as siberian snow cover and surface temperatures, along with the current pattern, can influence our expected winter patterns.
Our end goal is to produce nine forecasts; temperature and precipitation departures from normal for the winter as a whole as well as monthly for December, January and February as well as a snowfall departure for the winter as a whole. Let’s get to it!
Seasonal Models
Let’s start by taking a look at the seasonal models. There are three of them; the Euro seasonal, the JAMSTEC, and the CFS. Unfortunately, due to copyright issues, the Euro seasonal can’t be reproduced, and I have not seen the actual images myself. So, we will focus on the latter two.
A note on how the seasonal models work. Essentially, they attempt to model the upcoming months in much the same way that the midrange global models attempt to model the upcoming forecast period, that is, start with a set of initial observations and apply some physics package to those observations to make future forecasts for the next period on some interval, then, that forecast is fed back into the model as the initial observations for the next period to be created off of, and so on. As you can imagine, small inaccuracies in initial data or calculations are exponentially magnified as the recursive cycle continues, and as a result, global models, especially the seasonal ones, are notoriously inaccurate, hence our relatively low weight of them compared to other factors. They are still worth considering and looking at, as what they show (and don’t show) can be valuable when taken in the right context.
First, let’s take a look at the CFS. Here are temperature and precipitation departures from normal on the CFS for December, January, and February.
Next, here’s the temp and precipitation departures on the JAMSTEC for the winter. Unfortunately, the model does not provide monthly breakdowns.
Looking at the two models, the general theme appears to be fairly similar. Both models are wet, with normal or above normal precipitation in all three months on the CFS and the JAMSTEC being above normal for the winter. As far as temperatures go, both have the same idea in placing the warmest temperature anomalies across the southern US, with cooler air across the northern tier. This corroborates with what we have seen since late this summer due to the persistent presence of a strong southeast ridge, and both of the models keep the ridge in the picture through the winter, as is typical for La Nina winters. Both models show our area as slightly above normal. However, both have the area as a relatively cooler spot relative to average when compared to areas to our south and west. Seasonal modeling (especially the CFS, which is notoriously warm biased), often has difficulty identifying mesoscale features that lead to localized cold in certain areas. We’ve seen that this fall, where New England, while still above normal, has been much closer to normal (thanks to some well-timed Quebec highs) than areas to our south and west which have seen record breaking temperatures this fall. As a result, I’m inclined to say that the models are overdoing the warmth a bit.
It’s worth noting that both the CFS and the JAMSTEC, even taken at face value, are not all that warm; the seasonal average for both is an average of around half a degree to a degree above normal throughout the winter. In the grand scheme of things, that’s a relatively small signal, especially compared to what we saw last fall on the CFS where it was calling for three to five degrees above normal.
Overall summary of the seasonal models: Slightly above average temperatures and slightly above average precipitation.
Analogs
Each day, the CPC produces a list of analogs that match the upper air patterns over the US. Greg has been carefully watching and compiling them; looking for frequent matches and seasons that correlate with to what we’ve seen looking up to this winter. We’ve identified six that are strong matches: 1983-1984(The upper air pattern this fall is almost an exact match), 1995-1996(Sea surface temperatures this fall are closest to fall of 1995 by far), 2003-2004(Another strong upper air analog), 2013-2014, 1959-1960, 1980-1981(Both 1959 and 1980 were strong matches with the PDO and QBO, respectively).
When you combine all of those analogs, you get seasonal composites that look like this.
It’s worth noting that both the CFS and the JAMSTEC, even taken at face value, are not all that warm; the seasonal average for both is an average of around half a degree to a degree above normal throughout the winter. In the grand scheme of things, that’s a relatively small signal, especially compared to what we saw last fall on the CFS where it was calling for three to five degrees above normal.
Overall summary of the seasonal models: Slightly above average temperatures and slightly above average precipitation.
Analogs
Each day, the CPC produces a list of analogs that match the upper air patterns over the US. Greg has been carefully watching and compiling them; looking for frequent matches and seasons that correlate with to what we’ve seen looking up to this winter. We’ve identified six that are strong matches: 1983-1984(The upper air pattern this fall is almost an exact match), 1995-1996(Sea surface temperatures this fall are closest to fall of 1995 by far), 2003-2004(Another strong upper air analog), 2013-2014, 1959-1960, 1980-1981(Both 1959 and 1980 were strong matches with the PDO and QBO, respectively).
When you combine all of those analogs, you get seasonal composites that look like this.
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Quite the difference from the seasonal models! You see an impressive cold pool across most of the eastern portion of the country, with a warmer area in the west. This looks like a classic wintry pattern, with a -AO and -NAO leading to blocking in the east and a +PNA leading to a ridge in the west. This creates a path of cold air down from western Canada into the northern Plains and then spreading eastward, and you can see the edge of that path clearly through the borders of the cool vs. warm anomalies on the map. Like the seasonal models, precipitation is above normal; suggesting an active storm track.
When looking at the monthly breakdowns, things start to differ a bit. Here they are for temperature and precipitation.
When looking at the monthly breakdowns, things start to differ a bit. Here they are for temperature and precipitation.
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In December and January, we see temperatures similar to the composite. However, in February, we are much warmer, with well above normal temperatures across the area. La Nina events are often characterized by cold starts and early springs(For example, 2010-2011, our last Nina winter), so seeing this in the analog package is no surprise. As far as precipitation goes, December and February are above normal, with January a bit below, but all are fairly close to the mean.
Overall Summary of the Analogs: Below normal temperatures and slightly above normal precipitation.
ENSO
Last year, ENSO was clear and overwhelming; we experienced either the strongest or second-strongest El Nino event on record(which it is depends on which source and area you use to measure), and it had a dominating effect on the pattern. The “super Nino” never allowed any real blocking to develop and generally kept the flow fast and progressive. This year, the signal for ENSO is much murkier.
Here’s the progression of sea surface temperatures in the ENSO region for the past several weeks, as well as a longer-term view which is broken out by region.
Overall Summary of the Analogs: Below normal temperatures and slightly above normal precipitation.
ENSO
Last year, ENSO was clear and overwhelming; we experienced either the strongest or second-strongest El Nino event on record(which it is depends on which source and area you use to measure), and it had a dominating effect on the pattern. The “super Nino” never allowed any real blocking to develop and generally kept the flow fast and progressive. This year, the signal for ENSO is much murkier.
Here’s the progression of sea surface temperatures in the ENSO region for the past several weeks, as well as a longer-term view which is broken out by region.
As you can see, we are currently in a weak La Nina, representing cooler than normal temperatures in the equatorial pacific. This generally correlates with a less-active tropical jet and a generally calmer storm track overall, as weaker northern stream systems tend to dominate the pattern. As such, the blockbuster storms that have been a hallmark of our past few winters are less likely. La Nina’s, when they are not very strong, do correlate with cooler than normal temperatures in our area, but the correlation is weak, and the cold is often more of the transient variety. In general though, the correlation between La Nina and Northeast weather is lower than the correlation between El Nino and Northeast weather, especially when the Nina is weak, as this one is.
Here’s the forecast for the ENSO regions over the next several months.
Here’s the forecast for the ENSO regions over the next several months.
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As you can see, the forecast is for generally rising temperatures in the ENSO area over the course of the winter, and a better than average chance of reaching neutral status by the end of the winter.
It’s worth noting, however, that while we are in a weak La Nina, thus far, the atmosphere has shown signs of behaving like an El Nino with a strong Pacific jet and a progressive flow. While there is no concrete scientific evidence behind an ENSO “Hangover”, one would logically think that such an anomalous event as the one we saw last year would leave some sort of lingering impact. This has also been anecdotally observed as well following strong ENSO events. I don’t think it’s a major portion of the upcoming winter, but it’s worth noting and keeping in the back of our mind when making a forecast.
In general, the weak ENSO state, combined with the lack of correlation between weak Nina and Northeast weather in general, makes me think that ENSO will take a back seat this year to some of the other factors in the outlook. In general, a weak or neutral ENSO allows us a blank canvas for some of the other elements that can be overshadowed in a strong ENSO to play a larger role in the shaping of the winter. We’ll look at many of those elements in the next two sections. For now though, we have to characterize ENSO somehow, and we’ll go with the averages for a weak Nina: slightly below average temperatures and average precipitation.
Overall Summary of ENSO: Slightly below average temperatures and average precipitation
Teleconnectors
If you’re a regular reader of our discussions, you’ll notice that we mention the teleconnectors a lot. Teleconnectors are indices measuring current conditions at certain placements in the atmosphere,most commonly, the presence of a ridge or a trough, relating to pressure. Measuring these conditions can provide significant insight into the overall pattern and its impacts on our sensible weather in the short to medium range. Although there are many that we can discuss, generally, forecasters in our region focus on four major teleconnectors--the AO, NAO,PNA, and EPO. As I mentioned, there are a whole host of secondary indices that can have an impact as well.
These are important, so I want to take a little time to discuss each.
Arctic Oscillation (AO)
First, the Arctic Oscillation, or AO. This is an index measuring pressure anomalies in the Arctic, and is correlated with temps for our area for the winter months. A positive phase of the AO consists of below average geopotential heights. A negative phase consists of the opposite. In a negative phase of the AO, the polar vortex over the Arctic is weaker, resulting in weaker upper level winds that make it easier for Arctic air to penetrate further south. Another result is a further south storm track. With a positive AO, the opposite is true. Stronger upper level winds keep the polar vortex in check, making it less likely for deep cold air intrusions and ideal storm tracks. The AO is an important teleconnection for understanding temperature potential.
Here's a look at what a positive AO looks like graphically. This map is from 1993 and is not representative of our current conditions!
It’s worth noting, however, that while we are in a weak La Nina, thus far, the atmosphere has shown signs of behaving like an El Nino with a strong Pacific jet and a progressive flow. While there is no concrete scientific evidence behind an ENSO “Hangover”, one would logically think that such an anomalous event as the one we saw last year would leave some sort of lingering impact. This has also been anecdotally observed as well following strong ENSO events. I don’t think it’s a major portion of the upcoming winter, but it’s worth noting and keeping in the back of our mind when making a forecast.
In general, the weak ENSO state, combined with the lack of correlation between weak Nina and Northeast weather in general, makes me think that ENSO will take a back seat this year to some of the other factors in the outlook. In general, a weak or neutral ENSO allows us a blank canvas for some of the other elements that can be overshadowed in a strong ENSO to play a larger role in the shaping of the winter. We’ll look at many of those elements in the next two sections. For now though, we have to characterize ENSO somehow, and we’ll go with the averages for a weak Nina: slightly below average temperatures and average precipitation.
Overall Summary of ENSO: Slightly below average temperatures and average precipitation
Teleconnectors
If you’re a regular reader of our discussions, you’ll notice that we mention the teleconnectors a lot. Teleconnectors are indices measuring current conditions at certain placements in the atmosphere,most commonly, the presence of a ridge or a trough, relating to pressure. Measuring these conditions can provide significant insight into the overall pattern and its impacts on our sensible weather in the short to medium range. Although there are many that we can discuss, generally, forecasters in our region focus on four major teleconnectors--the AO, NAO,PNA, and EPO. As I mentioned, there are a whole host of secondary indices that can have an impact as well.
These are important, so I want to take a little time to discuss each.
Arctic Oscillation (AO)
First, the Arctic Oscillation, or AO. This is an index measuring pressure anomalies in the Arctic, and is correlated with temps for our area for the winter months. A positive phase of the AO consists of below average geopotential heights. A negative phase consists of the opposite. In a negative phase of the AO, the polar vortex over the Arctic is weaker, resulting in weaker upper level winds that make it easier for Arctic air to penetrate further south. Another result is a further south storm track. With a positive AO, the opposite is true. Stronger upper level winds keep the polar vortex in check, making it less likely for deep cold air intrusions and ideal storm tracks. The AO is an important teleconnection for understanding temperature potential.
Here's a look at what a positive AO looks like graphically. This map is from 1993 and is not representative of our current conditions!
North Atlantic Oscillation (NAO)
The NAO is the North Atlantic Oscillation, and is a close relative of the AO. Here, forecasters look at two pressure centers in the Atlantic--an area of low pressure near Iceland, and a high pressure system near the Azores. Generally, during the winter we’re paying close attention to heights around Iceland. The strength and exact placement of these features tend to have a significant impact on the placement of the jet stream--absolutely critical for both cold air placement and storm track. During a negative phase, we see higher heights over the Iceland area, and below average heights over the western Atlantic and (usually) the eastern US. The opposite generally occurs with a +NAO. As we’ve seen the last few years, a persistently negative NAO is not necessary for cold and snowy winter, but as research has suggested, having a -NAO, even if transient, can be a major factor in producing some of our biggest winter weather events, as a -NAO usually implies more blocking is available, while a +NAO represents a faster pattern with less room for storms to amplify. Further complicating matters is the position of the heights. A west based -NAO is more likely to produce a cold and stormy period than an east based -NAO.
Again, here's a historical look at a negative NAO from 1966.
The NAO is the North Atlantic Oscillation, and is a close relative of the AO. Here, forecasters look at two pressure centers in the Atlantic--an area of low pressure near Iceland, and a high pressure system near the Azores. Generally, during the winter we’re paying close attention to heights around Iceland. The strength and exact placement of these features tend to have a significant impact on the placement of the jet stream--absolutely critical for both cold air placement and storm track. During a negative phase, we see higher heights over the Iceland area, and below average heights over the western Atlantic and (usually) the eastern US. The opposite generally occurs with a +NAO. As we’ve seen the last few years, a persistently negative NAO is not necessary for cold and snowy winter, but as research has suggested, having a -NAO, even if transient, can be a major factor in producing some of our biggest winter weather events, as a -NAO usually implies more blocking is available, while a +NAO represents a faster pattern with less room for storms to amplify. Further complicating matters is the position of the heights. A west based -NAO is more likely to produce a cold and stormy period than an east based -NAO.
Again, here's a historical look at a negative NAO from 1966.
Pacific/North American Pattern (PNA)
Although it is nice to have the AO and NAO on your side during the winter if you like cold and snow, I am a big believer that without a good Pacific pattern, nothing else matters. No PAC, no winter! The PNA is one of the most important teleconnections, with a sprawling influence over North America. A positive PNA phase occurs when there are higher heights over the Pacific and western US. This correlates to ridging over the western US and a trough (transporting colder air) over the eastern US. In a negative phase, the opposite occurs. It is important to note that the research has shown that ENSO has been shown to influence the PNA quite a bit.
Although it is nice to have the AO and NAO on your side during the winter if you like cold and snow, I am a big believer that without a good Pacific pattern, nothing else matters. No PAC, no winter! The PNA is one of the most important teleconnections, with a sprawling influence over North America. A positive PNA phase occurs when there are higher heights over the Pacific and western US. This correlates to ridging over the western US and a trough (transporting colder air) over the eastern US. In a negative phase, the opposite occurs. It is important to note that the research has shown that ENSO has been shown to influence the PNA quite a bit.
East Pacific Oscillation
Last but certainly not least, we have the East Pacific Oscillation, or EPO. This has become one of my favorite teleconnections over the last few years, and with good reason. The EPO is a major player in delivering cold air or torching the lower 48. Here, we’re looking at heights over the northeast Pacific and Alaska. Higher heights over this region brings warmer than normal weather over that region, but it serves up an almost ideal mechanism for cold air delivery in the continental US, as a resulting Arctic high in northwest Canada and trough over the US work in tandem to drop the Arctic hammer. If we get enough of a negative EPO, we can get what is called “cross-polar flow”. This is the extreme cold that comes straight from the pole without much moderation. I’m getting excited just visualizing it.
On the other hand, with a positive EPO phase, we (potentially) see the worst of what winter can offer if you like cold and snow. In a +EPO, we get a persistent low (vortex) in the northeast Pacific and Alaska, which creates favorable conditions for the dreaded PAC jet to inundate Canada and much of the continental US with warm Pacific air. If you have this kind of pattern persist--turn out the lights, it is going to be a rough (or beautiful) winter.
Here's a look at the positive and negative phases of the EPO and their effects on our sensible weather.
Last but certainly not least, we have the East Pacific Oscillation, or EPO. This has become one of my favorite teleconnections over the last few years, and with good reason. The EPO is a major player in delivering cold air or torching the lower 48. Here, we’re looking at heights over the northeast Pacific and Alaska. Higher heights over this region brings warmer than normal weather over that region, but it serves up an almost ideal mechanism for cold air delivery in the continental US, as a resulting Arctic high in northwest Canada and trough over the US work in tandem to drop the Arctic hammer. If we get enough of a negative EPO, we can get what is called “cross-polar flow”. This is the extreme cold that comes straight from the pole without much moderation. I’m getting excited just visualizing it.
On the other hand, with a positive EPO phase, we (potentially) see the worst of what winter can offer if you like cold and snow. In a +EPO, we get a persistent low (vortex) in the northeast Pacific and Alaska, which creates favorable conditions for the dreaded PAC jet to inundate Canada and much of the continental US with warm Pacific air. If you have this kind of pattern persist--turn out the lights, it is going to be a rough (or beautiful) winter.
Here's a look at the positive and negative phases of the EPO and their effects on our sensible weather.
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Teleconnector Analysis
Let’s start with the Pacific. Over the past few weeks, we have had a fairly hostile Pacific pattern for cold and snow lovers. With a strong vortex in the gulf of Alaska, there is no funnel for Arctic cold to move into the US, and we are left with warm Pacific air overspreading the country. Interestingly enough, we have generally seen a +PNA pattern during this time, which is generally a signal for troughiness in the east, but we have also seen a +EPO, which is negating the benefits that a +PNA provides by keeping a trough in the Pacific northwest and not allowing any of that cold air to move into the country. I don’t see a huge motivating factor to push it persistently negative. That’s a red flag in my book. As such, I’m inclined to think that we aren’t going to get a ton of help from the Pacific this year, and we’re going to have to rely on Atlantic blocking (the NAO and AO) to get the help that we need to bring cold and snow into our area. We’ll hit more on the NAO and AO in current conditions, but for now, here are some long range forecasts for them.
Let’s start with the Pacific. Over the past few weeks, we have had a fairly hostile Pacific pattern for cold and snow lovers. With a strong vortex in the gulf of Alaska, there is no funnel for Arctic cold to move into the US, and we are left with warm Pacific air overspreading the country. Interestingly enough, we have generally seen a +PNA pattern during this time, which is generally a signal for troughiness in the east, but we have also seen a +EPO, which is negating the benefits that a +PNA provides by keeping a trough in the Pacific northwest and not allowing any of that cold air to move into the country. I don’t see a huge motivating factor to push it persistently negative. That’s a red flag in my book. As such, I’m inclined to think that we aren’t going to get a ton of help from the Pacific this year, and we’re going to have to rely on Atlantic blocking (the NAO and AO) to get the help that we need to bring cold and snow into our area. We’ll hit more on the NAO and AO in current conditions, but for now, here are some long range forecasts for them.
Generally, the NAO and AO have been volatile over the past few months. These forecasts expect the indices to move towards the negative states, indicating the development of high-latitude blocking. Given that these forecasts are based off the CFS, I’m not especially confident in them, but the general trend of taking the indices negative fits with what we’ve been seeing in the stratosphere (more on this in a moment).
The PDO, or Pacific Decadal Oscillation, after trending towards a negative state throughout the past several months, has jumped sharply positive over the past couple of weeks. A positive PDO generally is correlated with the attributes that a -EPO brings (farther west GOA low, high latitude ridging, etc.), which results in the development of cooler conditions across our area. In a year like this one, where ENSO is relatively weak, the PDO likely will play a larger role than usual in influencing the conditions in the Pacific, and it may serve to counteract the negative signals from the PNA and EPO regions.
The PDO, or Pacific Decadal Oscillation, after trending towards a negative state throughout the past several months, has jumped sharply positive over the past couple of weeks. A positive PDO generally is correlated with the attributes that a -EPO brings (farther west GOA low, high latitude ridging, etc.), which results in the development of cooler conditions across our area. In a year like this one, where ENSO is relatively weak, the PDO likely will play a larger role than usual in influencing the conditions in the Pacific, and it may serve to counteract the negative signals from the PNA and EPO regions.
The QBO, or Quasi-Biennial Oscillation, is an index that has been puzzling forecasters recently. It generally has a fairly standard 12-15 month cycle between the negative and positive phases, and this spring, was on track to move into the negative phase as expected. However, as you can see on the chart above, the negative movement abruptly stopped and has reversed to move back into the positive phase. It’s rather unclear why this has happened, with some attributing it to the residual effects of last winter's El Nino. A positive QBO can generally correlate to a stronger Polar Vortex, making it more difficult for sudden stratospheric warming and vortex displacement to occur. However, the correlation between QBO and our sensible weather is fairly weak, and given that we have already seen significant stratospheric activity, I am inclined to discount the index somewhat.
In general, it appears that there are no strong signs either way from the teleconnectors other than the signal for a warmer than normal Pacific and even that is cancelled out somewhat by the PDO. However, as the Pacific is crucial to getting sustained cold into the area, I do think that the lack of a cooperative EPO will reduce the risk of a sustained Arctic blast like we saw in the exceptionally cold February of 2015, and we are more likely to see shorter shots of cold air from Quebec courtesy of the NAO and AO. I do think that the lack of a strong Pacific will allow for intrusions of warm air across the CONUS (much like we are seeing currently), and so think that temperatures can be swung to average somewhat above normal as a result. Precipitation is a wash with the teleconnectors; really none of them have an excellent correlation for our area, and they generally have more influence into how a system tracks rather than whether it is able to develop.
Overall summary of the teleconnectors: Slightly above normal temperatures and normal precipitation.
Current Conditions
Here’s a look at the current state of snow cover across Siberia.
In general, it appears that there are no strong signs either way from the teleconnectors other than the signal for a warmer than normal Pacific and even that is cancelled out somewhat by the PDO. However, as the Pacific is crucial to getting sustained cold into the area, I do think that the lack of a cooperative EPO will reduce the risk of a sustained Arctic blast like we saw in the exceptionally cold February of 2015, and we are more likely to see shorter shots of cold air from Quebec courtesy of the NAO and AO. I do think that the lack of a strong Pacific will allow for intrusions of warm air across the CONUS (much like we are seeing currently), and so think that temperatures can be swung to average somewhat above normal as a result. Precipitation is a wash with the teleconnectors; really none of them have an excellent correlation for our area, and they generally have more influence into how a system tracks rather than whether it is able to develop.
Overall summary of the teleconnectors: Slightly above normal temperatures and normal precipitation.
Current Conditions
Here’s a look at the current state of snow cover across Siberia.
Snow cover in Siberia is well above normal, while snow cover in North America is slightly below normal. Each matters for different reasons. Snow cover in North America(specifically, in Canada) generally signifies(and promotes) a colder northern tier. As Canada and the Arctic are the usual source for our cold shots, having a baseline of cold there makes it more likely that some of the cold can be brought down into our area, while a warm Arctic forces us to rely on well-timed areas of high pressure to advect cold air down into our area. The correlation between a cold Canada and cold New England is very strong, but keep in mind that while it may be in a certain state now, it can change rather quickly as patterns change.
Snow cover in Siberia has a much more questionable correlation by way of the SAI. The SAI is an index developed by Dr. Judah Cohen that claims that the extent of snow cover in October in Siberia is correlated to the development of a Sudden Stratospheric Warming event and hence the development of a -NAO and -AO. Until 2014, the index had performed very well, and there were high hopes for it’s future. However, the past two years have not been kind to the index, as it had fairly spectacular failures in both 2014-2015 and 2015-2016. Last years failure can probably be partially explained by the highly anomalous ENSO state, and it is possible that this year, with a relatively neutral ENSO state, the index will be able to play a larger role. We’ll weigh it for now, but this is definitely a “prove it” year for the index as far as to its long-term effectiveness.
Here’s a diagram that explains the logic behind the index.
Snow cover in Siberia has a much more questionable correlation by way of the SAI. The SAI is an index developed by Dr. Judah Cohen that claims that the extent of snow cover in October in Siberia is correlated to the development of a Sudden Stratospheric Warming event and hence the development of a -NAO and -AO. Until 2014, the index had performed very well, and there were high hopes for it’s future. However, the past two years have not been kind to the index, as it had fairly spectacular failures in both 2014-2015 and 2015-2016. Last years failure can probably be partially explained by the highly anomalous ENSO state, and it is possible that this year, with a relatively neutral ENSO state, the index will be able to play a larger role. We’ll weigh it for now, but this is definitely a “prove it” year for the index as far as to its long-term effectiveness.
Here’s a diagram that explains the logic behind the index.
While the index is predicting a SSW to occur later in the winter, it’s interesting to point out that the first steps of one already appear to be in progress. We have already seen the PV split earlier in October, which will likely go down as the earliest PV split on record. The vortex is clearly weak and vulnerable(as illustrated by the split), and so I believe that the likelihood of a SSW, potentially a very early one, is substantially elevated from the baseline average. I don’t see solar activity being an issue this winter; given that it will be low and moving towards a solar minimum, strong solar activity will likely not be the reason that a SSW fails to develop if our thoughts prove incorrect.
Here’s a map of current sea surface temperature anomalies.
Here’s a map of current sea surface temperature anomalies.
s you can see, water temps are quite warm in our immediate area, with some portions of the northeast seeing near record warm waters offshore. Further to the south, they are cooler, with below normal temperatures just offshore of the southeast coastline. The warmer sea surface temperatures in our area could promote a stronger storm track; as systems move over the warm water, they intensify and generate additional moisture. Whether that means a bunch of rain or a heavy snowstorm depends on the airmass that the storm has in place to work with; a marine influence can only do so much when dynamics are involved, especially away from the coastline. I do think that the warm SST’s could hurt the coastline compared to the interior with regard to dealing with surface temperatures during winter weather events, although anything can be overcome with a cold enough airmass.
Overall summary of current conditions: Elevated likelihood of SSW development and resulting cooler temps due to development of blocking. Potential for stronger storms due to record-warm sea surface temperatures, but unclear as to their impact on snowfall totals.
Gut Feeling
I always say that meteorology is more than science alone; sometimes, you have to trust your gut. As such, I like to factor in my gut feeling into the forecast. Sometimes, when you’re pouring over the data, you develop a vision for what you expect but can’t quite explain it using the data you’ve got. More often than not, I find that that vision is just as accurate as whatever the data spits out.
I’ve gone back and forth this fall between a near-normal winter and a winter that wasn’t. The talk this November has reminded me of 2011-2012, where the talk was always that the pattern change was 10 days away, but it never got any closer than 10 days away and the pacific ruled the roost. While it does look like the pattern change is legitimate and coming at the end of this weekend, one wonders how long it will last before it goes back to the baseline state of the past several months of a poor Pacific overwhelming the pattern. I am not quite as bullish on the winter as the overall data suggests, and strongly feel that when you do not have a favorable Pacific(especially the EPO), you are fighting an uphill battle. I think the same will be true this winter. With that in mind though, I don’t see a total bust; there are too many factors that are positive for me to see that happening. I do think that we are going to see an earlier period of sustained winter; December and January are much more likely to be wintry than February and March in my opinion.
Overall summary of gut feeling: Slightly above normal temperatures, normal precipitation, and slightly below normal snowfall.
Overall summary of current conditions: Elevated likelihood of SSW development and resulting cooler temps due to development of blocking. Potential for stronger storms due to record-warm sea surface temperatures, but unclear as to their impact on snowfall totals.
Gut Feeling
I always say that meteorology is more than science alone; sometimes, you have to trust your gut. As such, I like to factor in my gut feeling into the forecast. Sometimes, when you’re pouring over the data, you develop a vision for what you expect but can’t quite explain it using the data you’ve got. More often than not, I find that that vision is just as accurate as whatever the data spits out.
I’ve gone back and forth this fall between a near-normal winter and a winter that wasn’t. The talk this November has reminded me of 2011-2012, where the talk was always that the pattern change was 10 days away, but it never got any closer than 10 days away and the pacific ruled the roost. While it does look like the pattern change is legitimate and coming at the end of this weekend, one wonders how long it will last before it goes back to the baseline state of the past several months of a poor Pacific overwhelming the pattern. I am not quite as bullish on the winter as the overall data suggests, and strongly feel that when you do not have a favorable Pacific(especially the EPO), you are fighting an uphill battle. I think the same will be true this winter. With that in mind though, I don’t see a total bust; there are too many factors that are positive for me to see that happening. I do think that we are going to see an earlier period of sustained winter; December and January are much more likely to be wintry than February and March in my opinion.
Overall summary of gut feeling: Slightly above normal temperatures, normal precipitation, and slightly below normal snowfall.
The Forecast
To recap, here are the summaries of our six variables that make up the forecast.
Overall summary of the seasonal models: Slightly above average temperatures and slightly above average precipitation.
Overall Summary of the Analogs: Below normal temperatures and slightly above normal precipitation.
Overall Summary of ENSO: Slightly below average temperatures and average precipitation
Overall summary of the teleconnectors: Slightly above normal temperatures and normal precipitation.
Overall summary of current conditions: Elevated likelihood of SSW development and resulting cooler temps due to development of blocking. Potential for stronger storms due to record-warm sea surface temperatures, but unclear as to their impact on snowfall totals.
Overall summary of gut feeling: Slightly above normal temperatures, normal precipitation, and slightly below normal snowfall.
Temperatures:
Looking at temperatures, we have three factors calling for slightly above normal temperatures, two calling for slightly below normal temperatures, and one calling for below normal temperatures. The two factors with the highest weight are both calling for below normal temperatures, but I think that the analogs in this case are likely overdoing it compared to what we will likely see due to the poor state of the Pacific. As such, I’m thinking we’re looking at a fairly close to normal winter temperature wise, and will go with an overall temperature composite of up to half a degree from normal in either direction.
When looking at it monthly, I am reasonably confident that February will be the warmest month of the three. The analogs are convincing in arguing for a warm February, and a front-loaded winter is common in a La Nina state. Furthermore, the early development of a SSW, which I think is possible, may mean that it’s effects have already played out by the end of the winter, negating that potential influence. Likewise, I am reasonably confident in a below-normal December; shorter range weekly guidance is already suggesting that, and a cold start to the winter is a strong fit with the analogs and ENSO along with any potential stratospheric help we may get.
To me, the wild card is January. Will we see a cold dump into the east as a result of stratospheric action or the development of high latitude blocking, or will we see the country flooded with warm Pacific air as the La Nina flexes it’s muscles? At this point, I could see January going either way, but will lean towards a continuation of the December pattern, as patterns often take longer to change than expected and any stratospheric activity would likely influence the pattern most strongly during this time. I will trend January slightly warmer than the above suggests to account for the potential of an earlier flip or a warmup during the end of the month, and so will go with near-normal temps for the month.
Summarizing, temperatures look like this:
December: 1 to 2 degrees below normal
January: Normal to 1 degree below normal
February: 2 to 3 degrees above normal
Winter Composite: .5 degrees below normal to .5 degrees above normal
Precipitation:
Everything we’ve looked at suggests normal or slightly above normal precipitation, which is fairly standard in a La Nina. We’ve finally moved into a wetter pattern the past couple of weeks, putting a dent in our drought, and it looks like the pattern will stay active for the foreseeable future. Breaking precipitation down monthly is more difficult than temperature, as one storm can make or break your forecast, but in general, I don’t see anything to suggest an imbalance towards wetter or drier than normal in any given month. If a month were to be the wettest, I would lean towards December only because it appears we have an active pattern developing, and the opposite for February as an early end to winter could bring a warm and dry pattern(See March 2012, our last significant La Nina winter).
Here’s the forecast for precipitation:
December: 90% to 130% of normal
January: 80% to 120% of normal
February: 75% to 115% of normal
Winter Composite: 80% to 120% of normal
Snowfall:
Snow is always the trickiest part of a forecast, only because it’s a variable that relies on factors that are hard to predict at the macro level. Take last January for instance; in one of the warmest winters on record and in a pattern that was extremely hostile, portions of the east coast saw their largest snowfalls on record in what was unquestionably a historic blizzard. At the same time though, it was a tiny shortwave that served as a kicker to keep the storm from moving north and dumping feet of snow on all of New England, which would have completely changed the seasonal totals across the area and made the winter appear far more prolific than it really was. We can take our best estimate on what is likely considering the forecast pattern, but as every storm setup is unique, it’s difficult to put out a number with great confidence, especially with conflicting signals this year.
We’re looking at near normal temperatures for the winter along with near normal precipitation. One would think that would mean near-normal snowfall. My concern with forecasting that, however, is that I’m not sure we’ll see the timing align if we don’t get the development of high-latitude blocking. I think the cold will be transient in nature as opposed to a sustained block like we saw in February and March 2015, and as such, I think that we will see a lot of storms where the front end starts as snow, but, as there is no mechanism to hold the cold in place, it is pushed out and the storm brings warm air north with it. If we do see the development of a -NAO/-AO, that would go a long way towards higher snowfall, as the block will force systems out under us, resulting in our area remaining in the cold sector and an all snow type of event. For now, I’m going to go with very slightly below normal snowfall, but I think there’s a lot of bust potential here in both directions. Unlike last season, with an active southern jet stream, I think that clippers and Miller B type systems are going to be our most common storm track. That favors light snowfalls and messy systems that may sneak up under us or cut to the west.
Winter Snowfall: 75%-115% of normal
To sum it up, the 2016-2017 SCW Winter Forecast
The Winter Ahead
The issuance of the winter forecast represents the start of the winter season, and along with it, our gradual ramp-up in coverage to daily updates, Don’s medium range outlooks starting December 1st, and our verification maps after winter weather events. We’ll be aiming for four forecasters discussions per week(posted on our website and linked on our social media), increasing to daily in the mid and short range leading up to a wintry event, and will be filling in the gaps with shorter updates on our social media platforms. You can also follow us on Twitter @SouthernCTWX for continued high quality analysis, including model interpretations and quick thoughts. We’re looking forward to our fifth winter of forecasting and we hope it’ll be the best one yet!
With that, I’d like to thank you for reading this year’s Winter Forecast. I hope you enjoyed it and learned something from it, and ask that if you did, you consider passing it along to family and friends in your network who you think may enjoy it as well. As is always the case here at SCW, we love your questions and comments, and are looking forward to hearing from you! Drop us a line in the comments section here or on Facebook, tweet to us @SouthernCTWX, or shoot me an email at [email protected].
On behalf of Tim Wrightington Jr., Greg Petredis and Don Bell, I’d like to welcome you to SCW Winter 2016-2017!
-Spencer Aronstein(SA), Partner and Lead Forecaster, Southern Connecticut Weather
A reminder that this and all content on SCW is Copyrighted by Southern Connecticut Weather in 2016, and all rights are reserved. Reproduction is prohibited without express written consent, but consent will likely be granted for non-commercial uses upon request, so please reach out.
To recap, here are the summaries of our six variables that make up the forecast.
Overall summary of the seasonal models: Slightly above average temperatures and slightly above average precipitation.
Overall Summary of the Analogs: Below normal temperatures and slightly above normal precipitation.
Overall Summary of ENSO: Slightly below average temperatures and average precipitation
Overall summary of the teleconnectors: Slightly above normal temperatures and normal precipitation.
Overall summary of current conditions: Elevated likelihood of SSW development and resulting cooler temps due to development of blocking. Potential for stronger storms due to record-warm sea surface temperatures, but unclear as to their impact on snowfall totals.
Overall summary of gut feeling: Slightly above normal temperatures, normal precipitation, and slightly below normal snowfall.
Temperatures:
Looking at temperatures, we have three factors calling for slightly above normal temperatures, two calling for slightly below normal temperatures, and one calling for below normal temperatures. The two factors with the highest weight are both calling for below normal temperatures, but I think that the analogs in this case are likely overdoing it compared to what we will likely see due to the poor state of the Pacific. As such, I’m thinking we’re looking at a fairly close to normal winter temperature wise, and will go with an overall temperature composite of up to half a degree from normal in either direction.
When looking at it monthly, I am reasonably confident that February will be the warmest month of the three. The analogs are convincing in arguing for a warm February, and a front-loaded winter is common in a La Nina state. Furthermore, the early development of a SSW, which I think is possible, may mean that it’s effects have already played out by the end of the winter, negating that potential influence. Likewise, I am reasonably confident in a below-normal December; shorter range weekly guidance is already suggesting that, and a cold start to the winter is a strong fit with the analogs and ENSO along with any potential stratospheric help we may get.
To me, the wild card is January. Will we see a cold dump into the east as a result of stratospheric action or the development of high latitude blocking, or will we see the country flooded with warm Pacific air as the La Nina flexes it’s muscles? At this point, I could see January going either way, but will lean towards a continuation of the December pattern, as patterns often take longer to change than expected and any stratospheric activity would likely influence the pattern most strongly during this time. I will trend January slightly warmer than the above suggests to account for the potential of an earlier flip or a warmup during the end of the month, and so will go with near-normal temps for the month.
Summarizing, temperatures look like this:
December: 1 to 2 degrees below normal
January: Normal to 1 degree below normal
February: 2 to 3 degrees above normal
Winter Composite: .5 degrees below normal to .5 degrees above normal
Precipitation:
Everything we’ve looked at suggests normal or slightly above normal precipitation, which is fairly standard in a La Nina. We’ve finally moved into a wetter pattern the past couple of weeks, putting a dent in our drought, and it looks like the pattern will stay active for the foreseeable future. Breaking precipitation down monthly is more difficult than temperature, as one storm can make or break your forecast, but in general, I don’t see anything to suggest an imbalance towards wetter or drier than normal in any given month. If a month were to be the wettest, I would lean towards December only because it appears we have an active pattern developing, and the opposite for February as an early end to winter could bring a warm and dry pattern(See March 2012, our last significant La Nina winter).
Here’s the forecast for precipitation:
December: 90% to 130% of normal
January: 80% to 120% of normal
February: 75% to 115% of normal
Winter Composite: 80% to 120% of normal
Snowfall:
Snow is always the trickiest part of a forecast, only because it’s a variable that relies on factors that are hard to predict at the macro level. Take last January for instance; in one of the warmest winters on record and in a pattern that was extremely hostile, portions of the east coast saw their largest snowfalls on record in what was unquestionably a historic blizzard. At the same time though, it was a tiny shortwave that served as a kicker to keep the storm from moving north and dumping feet of snow on all of New England, which would have completely changed the seasonal totals across the area and made the winter appear far more prolific than it really was. We can take our best estimate on what is likely considering the forecast pattern, but as every storm setup is unique, it’s difficult to put out a number with great confidence, especially with conflicting signals this year.
We’re looking at near normal temperatures for the winter along with near normal precipitation. One would think that would mean near-normal snowfall. My concern with forecasting that, however, is that I’m not sure we’ll see the timing align if we don’t get the development of high-latitude blocking. I think the cold will be transient in nature as opposed to a sustained block like we saw in February and March 2015, and as such, I think that we will see a lot of storms where the front end starts as snow, but, as there is no mechanism to hold the cold in place, it is pushed out and the storm brings warm air north with it. If we do see the development of a -NAO/-AO, that would go a long way towards higher snowfall, as the block will force systems out under us, resulting in our area remaining in the cold sector and an all snow type of event. For now, I’m going to go with very slightly below normal snowfall, but I think there’s a lot of bust potential here in both directions. Unlike last season, with an active southern jet stream, I think that clippers and Miller B type systems are going to be our most common storm track. That favors light snowfalls and messy systems that may sneak up under us or cut to the west.
Winter Snowfall: 75%-115% of normal
To sum it up, the 2016-2017 SCW Winter Forecast
- Temperatures ranging within a half a degree of normal in either direction, broken down into -1 to -2 in December, 0 to -1 in January, and +2 to +3 in February
- 80%-120% of normal precipitation, broken down into 90%-130% of normal in December, 80%-120% of normal in January, and 70%-110% of normal in February
- 75%-115% of normal snowfall
The Winter Ahead
The issuance of the winter forecast represents the start of the winter season, and along with it, our gradual ramp-up in coverage to daily updates, Don’s medium range outlooks starting December 1st, and our verification maps after winter weather events. We’ll be aiming for four forecasters discussions per week(posted on our website and linked on our social media), increasing to daily in the mid and short range leading up to a wintry event, and will be filling in the gaps with shorter updates on our social media platforms. You can also follow us on Twitter @SouthernCTWX for continued high quality analysis, including model interpretations and quick thoughts. We’re looking forward to our fifth winter of forecasting and we hope it’ll be the best one yet!
With that, I’d like to thank you for reading this year’s Winter Forecast. I hope you enjoyed it and learned something from it, and ask that if you did, you consider passing it along to family and friends in your network who you think may enjoy it as well. As is always the case here at SCW, we love your questions and comments, and are looking forward to hearing from you! Drop us a line in the comments section here or on Facebook, tweet to us @SouthernCTWX, or shoot me an email at [email protected].
On behalf of Tim Wrightington Jr., Greg Petredis and Don Bell, I’d like to welcome you to SCW Winter 2016-2017!
-Spencer Aronstein(SA), Partner and Lead Forecaster, Southern Connecticut Weather
A reminder that this and all content on SCW is Copyrighted by Southern Connecticut Weather in 2016, and all rights are reserved. Reproduction is prohibited without express written consent, but consent will likely be granted for non-commercial uses upon request, so please reach out.
