Damn the NAM!
That’s the phrase that was likely uttered by operational meteorologists across the northeast today, as what was a fairly well defined consensus on the midday runs was completely shot to pieces by the 18z NAM, which brings an all out blizzard to the state this weekend. However, despite it’s impressive solution and some minor north nudges from the GFS, it remains on it’s own. Considering that the NAM is a) an outlier; b) known to over amplify systems and favor extreme solutions; and c) is notoriously poor in accuracy, we are not inclined to believe it’s solution, and tend to favor closer to a consensus blend of the global guidance that results in a relatively small or nonexistent impact to the state this weekend. However, having the NAM in the fold, coupled with a slow but noticeable north trend on the GFS, complicates things a bit in the forecast, and thus, gives us something to talk about this evening.
Models and Trends
Most of what we’re going to do in this discussion is break down the various pieces of guidance and the possible solutions they represent to show just how complicated a forecast this is. We talked about the synoptic setup of the storm extensively here last night; if you haven’t read that discussion yet and are interested in the science behind the models and the movements we are seeing, I highly suggest you check out that discussion as well as this one. Tonight, we will talk on a ground level, and look at the possible impacts to the state as well as the uncertainties.
12z GFS
Let’s start with the lowest impact. The GFS has been wobbling around between an impact and a miss for our area, but generally, had been further north than the Euro, which has been consistently showing a relatively minor solution for several model cycles. Todays 12z run, however, caved to the Euro almost completely, and took any significant snow out of the forecast for Connecticut. While the model verbatim shows an inch of snow or so along the south coast and flurries throughout most of the state, I don’t think that we would see any precipitation in the state if the GFS verified.
The reason for this, which should be carried across to the interpretation of all guidance, lies in the very dry airmass to the north of the system, which will allow light precipitation to evaporate and result in virga, which is when precipitation is falling aloft, and being detected on radar, but not hitting the ground due to evaporation. Further compounding this area of subsidence and dry air is the fact that modeling is consistent with a strong mid level band of energy on the north flank of the system, which will pull the lighter moisture in front of the feature into it. This will result in a sharp gradient of snowfall across the state, with perhaps only a few miles separating warning criteria snowfall (6”+) from near nothing. We have seen this combination of dry air and strong midlevel fronto banding several times in recent history; with February 6th, 2010 being the most notable example in the weather world, but last March and the three back to back to back storms in January/February of 2014 are also good examples of this phenomenon. As such, I am firmly convinced that the cutoff with this system will be sharp and severe with a matter of potentially a few miles separating flurries from significant snows. Where that sets up, however, is up in the air, and that makes this system a very difficult one to forecast. Folks along our gradient lines should be prepared.
Enough about cutoffs and back to the models. Here’s the previously mentioned snow map for the GFS. Remember that 1mm =.04” = ~.4” of snow with 10:1 ratios.
That’s the phrase that was likely uttered by operational meteorologists across the northeast today, as what was a fairly well defined consensus on the midday runs was completely shot to pieces by the 18z NAM, which brings an all out blizzard to the state this weekend. However, despite it’s impressive solution and some minor north nudges from the GFS, it remains on it’s own. Considering that the NAM is a) an outlier; b) known to over amplify systems and favor extreme solutions; and c) is notoriously poor in accuracy, we are not inclined to believe it’s solution, and tend to favor closer to a consensus blend of the global guidance that results in a relatively small or nonexistent impact to the state this weekend. However, having the NAM in the fold, coupled with a slow but noticeable north trend on the GFS, complicates things a bit in the forecast, and thus, gives us something to talk about this evening.
Models and Trends
Most of what we’re going to do in this discussion is break down the various pieces of guidance and the possible solutions they represent to show just how complicated a forecast this is. We talked about the synoptic setup of the storm extensively here last night; if you haven’t read that discussion yet and are interested in the science behind the models and the movements we are seeing, I highly suggest you check out that discussion as well as this one. Tonight, we will talk on a ground level, and look at the possible impacts to the state as well as the uncertainties.
12z GFS
Let’s start with the lowest impact. The GFS has been wobbling around between an impact and a miss for our area, but generally, had been further north than the Euro, which has been consistently showing a relatively minor solution for several model cycles. Todays 12z run, however, caved to the Euro almost completely, and took any significant snow out of the forecast for Connecticut. While the model verbatim shows an inch of snow or so along the south coast and flurries throughout most of the state, I don’t think that we would see any precipitation in the state if the GFS verified.
The reason for this, which should be carried across to the interpretation of all guidance, lies in the very dry airmass to the north of the system, which will allow light precipitation to evaporate and result in virga, which is when precipitation is falling aloft, and being detected on radar, but not hitting the ground due to evaporation. Further compounding this area of subsidence and dry air is the fact that modeling is consistent with a strong mid level band of energy on the north flank of the system, which will pull the lighter moisture in front of the feature into it. This will result in a sharp gradient of snowfall across the state, with perhaps only a few miles separating warning criteria snowfall (6”+) from near nothing. We have seen this combination of dry air and strong midlevel fronto banding several times in recent history; with February 6th, 2010 being the most notable example in the weather world, but last March and the three back to back to back storms in January/February of 2014 are also good examples of this phenomenon. As such, I am firmly convinced that the cutoff with this system will be sharp and severe with a matter of potentially a few miles separating flurries from significant snows. Where that sets up, however, is up in the air, and that makes this system a very difficult one to forecast. Folks along our gradient lines should be prepared.
Enough about cutoffs and back to the models. Here’s the previously mentioned snow map for the GFS. Remember that 1mm =.04” = ~.4” of snow with 10:1 ratios.
Not very exciting. This is the miss scenario and we weight this scenario fifteen percent in the snow map.
12z GGEM, 12z ECMWF
The next tier of models consists of the 12z GGEM and ECMWF. The Euro has consistently been advertising a scrape to the state, with the GGEM teaming up with it today after being a bit further to the north last night. While this solution isn’t a complete shutout like the GFS, it’s still rather pedestrian. The GGEM and Euro allow the system to come just far enough north as it scoots east to clip the coastline with the edge of the significant snow band, and the result would probably be a short period of heavy snow overnight Saturday on the coastline that would result in an inch or three on the coastline and not much else anywhere else. While I think this solution is less likely than it was before the 18z models (which we’ll get to in a moment), I think it’s a very real possibility. Any further movement south on the consensus of guidance right now and I think this becomes the maximum that we can hope for in terms of snowfall totals, with a complete whiff a-la the 12z GFS then being favored in that case. However, we’re not there yet on the models, and given the slow slide north on the 18z suite, I’m still not convinced that this option is all we see.
Here’s the previously mentioned map for the GGEM, the Euro is similar. Same scale as for the GFS map, and the same caveats about light precipitation amounts being virga apply. We weight this scenario thirty percent in our eventual snow map.
12z GGEM, 12z ECMWF
The next tier of models consists of the 12z GGEM and ECMWF. The Euro has consistently been advertising a scrape to the state, with the GGEM teaming up with it today after being a bit further to the north last night. While this solution isn’t a complete shutout like the GFS, it’s still rather pedestrian. The GGEM and Euro allow the system to come just far enough north as it scoots east to clip the coastline with the edge of the significant snow band, and the result would probably be a short period of heavy snow overnight Saturday on the coastline that would result in an inch or three on the coastline and not much else anywhere else. While I think this solution is less likely than it was before the 18z models (which we’ll get to in a moment), I think it’s a very real possibility. Any further movement south on the consensus of guidance right now and I think this becomes the maximum that we can hope for in terms of snowfall totals, with a complete whiff a-la the 12z GFS then being favored in that case. However, we’re not there yet on the models, and given the slow slide north on the 18z suite, I’m still not convinced that this option is all we see.
Here’s the previously mentioned map for the GGEM, the Euro is similar. Same scale as for the GFS map, and the same caveats about light precipitation amounts being virga apply. We weight this scenario thirty percent in our eventual snow map.
18Z GFS, 12Z UKMET, 18z GEFS
Now it starts to get a little more interesting. The 18z GFS, after being a complete whiff on it’s 12z run, followed the 18z NAM(which we’ll get to shortly), and came well north of it’s 12z run. It is in good agreement with the 12z UKMET, which along with the NAM was the northwest outlier on the 12z suite. These models weaken the convection to the east of the low and the confluence to the north of it, and thus allow it to track more NNE as opposed to ENE. As a result, the low makes it to a latitude that is northerly enough for southern areas of the state to get in on the band of heavy precipitation, and in the case of the UKMET, central portions of the state as well. As of now, I believe that this is the most likely of the four modeled scenarios to verify for a couple of reasons. First off, I think that there will be a strong midlevel area of frontogenesis on the northern portion of the low, and as such, it will allow for a heavy deformation band to form to the north of the main area of precipitation, likely over at least a part of Connecticut. Second off, the shift on the 18z guidance was definitive. While some of the other bumps that we have seen have been small wobbles either way, both the NAM and GFS, as well as their ensembles, shifted definitively north from their earlier runs. Finally, and I will admit this is an unscientific reason, this storm doesn’t feel right to me. The features that the models show keeping this system to our south seem, to me, rather insignificant, and I find it somewhat hard to believe that such a strong system in a strong El Nino with a potent Subtropical Jet will just hit a wall and escape. I don’t think it will be the blockbuster that the NAM shows, as that seems like an extreme solution that is typical NAM for the reasons described in the opening paragraph, but I do think this is a realistic upper end, and as a result, I have organized our snowfall map to closely mirror this scenario, and give it fifty percent weight in the forecast.
Here’s the 12z UKMET and 18z GFS total QPF. Same scale as earlier.
Now it starts to get a little more interesting. The 18z GFS, after being a complete whiff on it’s 12z run, followed the 18z NAM(which we’ll get to shortly), and came well north of it’s 12z run. It is in good agreement with the 12z UKMET, which along with the NAM was the northwest outlier on the 12z suite. These models weaken the convection to the east of the low and the confluence to the north of it, and thus allow it to track more NNE as opposed to ENE. As a result, the low makes it to a latitude that is northerly enough for southern areas of the state to get in on the band of heavy precipitation, and in the case of the UKMET, central portions of the state as well. As of now, I believe that this is the most likely of the four modeled scenarios to verify for a couple of reasons. First off, I think that there will be a strong midlevel area of frontogenesis on the northern portion of the low, and as such, it will allow for a heavy deformation band to form to the north of the main area of precipitation, likely over at least a part of Connecticut. Second off, the shift on the 18z guidance was definitive. While some of the other bumps that we have seen have been small wobbles either way, both the NAM and GFS, as well as their ensembles, shifted definitively north from their earlier runs. Finally, and I will admit this is an unscientific reason, this storm doesn’t feel right to me. The features that the models show keeping this system to our south seem, to me, rather insignificant, and I find it somewhat hard to believe that such a strong system in a strong El Nino with a potent Subtropical Jet will just hit a wall and escape. I don’t think it will be the blockbuster that the NAM shows, as that seems like an extreme solution that is typical NAM for the reasons described in the opening paragraph, but I do think this is a realistic upper end, and as a result, I have organized our snowfall map to closely mirror this scenario, and give it fifty percent weight in the forecast.
Here’s the 12z UKMET and 18z GFS total QPF. Same scale as earlier.
We already gave you a hint of what the NAM shows in our opener, but now we’ll talk about it for real. The NAM, essentially, says “to heck with the confluence!”, and drives the system well north of any other model. As a result, it brings a prolonged period of heavy snow to the entire state, and the net result is blizzard conditions pretty much statewide, with 10”+ of snow across the entire state and upwards of 20” in southwest portions of the state. It would come with a dose of heavy wind and coastal flooding, and all together, create a big old classic blizzard. As much as the snowlover in me wishes that this would verify, I peg it’s chances of verification extremely low at around five percent for the reasons described earlier in the opener. It’s a nice model fantasy to look at, and something we can dream about happening, but is it likely? Not really. We mention it because it’s out there and it’s not totally inconceivable, but I don’t think that it’s a solution that is realistic at this timeframe. However, should we see the other guidance jump north again at 0z while the NAM holds serve, you may be waking up to a much heavier snow map all around. For now though, we keep a cautious eye on it and leave it at that. As such, it is weighted at five percent.
Forecast and Timing
Here’s our revised snow map for this event.
Here’s our revised snow map for this event.
In this map, we have tightened up the zones to account for the near certainty of a tight gradient of snowfall totals across the state, and have also modified the high end and the low end of snow totals a bit to try to better capture the inherent uncertainty that remains in this forecast.
In the northern third or so of the state, I don’t expect this storm to have much if any impact. It’ll be a cloudy and cold day on Saturday and Sunday, with some gusty winds and maybe some flurries or light snow expected, but I think that the heavier band stays south of this area, and as a result, I have cut forecast snow accumulations from 1-3” to up to 2”, with 0” being a realistic possibility. We would need to see a NAM like solution from other guidance to feel more confident that we would see more than a couple of inches in this zone. While not impossible, it is certainly a large long shot.
The next small slice of the state represents the transition zone. In this zone, I expect a somewhat bipolar representation, with snow starting up as the storm wiggles a mile or two north, only for it to shut down again as dry air overpowers it, then rinse and repeat. I think that this zone will have off and on snow showers throughout Saturday afternoon and overnight, with at least a short period of steady accumulating snow likely, and that will be enough to add up to one to three inches of snow. I think that the possibility of totals increasing in this zone, while not high, is there, as it would not take a shift to a total NAM solution to see heavier snow totals here. However, the consensus blend that we have used still keeps this area, for the most part, on the outside looking in, which is why I am not confident in higher totals here. Keep a watchful eye on the forecast though.
The next zone is where I expect steady snow to make it to during the course of Saturday afternoon and overnight. I think that for at least a short time, this area will make it into the deformation band which I have described, and as a result, a quick thumping of a few inches of snow is likely, with lighter snow on either side. I think winds will also begin to play a factor in this zone, as it will be close enough to the stronger area of the storm for some stronger gusts to work their way in. As a result, I think that some quick periods of low visibility and difficult driving conditions, especially Saturday evening, are likely here, and as a result the impact is a bit higher.
This zone should keep a very close eye on future forecasts, however, as it would only take a slight shift north on the models to bring the edge of consistent heavy snow further north and into this zone. In fact, looking now at the ensembles of the GFS, which were just released a moment ago, they are consistently northwest of the operational model and as a result would mean this zone would be factored into the 4-8” area as well. While I don’t currently have enough confidence in that to move the zone further north, this area, out of all the zones, I think is the most likely to see a potential increases in totals.
Finally, on the immediate shoreline, I have painted a small stripe of 4-8” of snow. I think that the system will come just far enough north to get the immediate coastline into the meat of the banding for a decent period of time Saturday late afternoon and evening, and as a result, I expect a period of moderate to heavy snow along the south coast that will result in four to eight inches of accumulation. However, out of all the zones, it is this one that I have the least confidence with. The models place the knifes edge of the gradient right along the shoreline, and a small wiggle of perhaps ten miles could make the difference between this zone verifying and busting. To be completely frank, I included this zone instead of just leaving the snow map at 2-5” to the shoreline because I wanted to show the possibility of higher amounts making an appearance in the state, and I figured that if we had to adjust the map higher in future updates, it would be easier to explain and convey the reasoning behind it to the readership if we had previously mentioned the possibility by way of the 4-8” zone. I do think that as of right now, most areas in the zone will fall closer to 4” than 8”, but it would not surprise me to see a few measurements in the 8” range, especially in the “panhandle” in the far SW corner of the state. We’ll see what ends up happening with this one, but I think it needed to be included in this map.
Impacts
Our next update will be mid-day tomorrow after we have been able to look at the overnight and some of the midday guidance, and a final detailed update will be issued tomorrow evening. Until then, please like, share to help spread awareness, follow us Facebook and on Twitter @SouthernCTWX if you haven’t already, and as always, ask any questions you may have either here, on Facebook, Twitter, or by emailing me at spencer@southernconnecticutweather.com. Thanks for tracking this storm with us tonight!
-SA
In the northern third or so of the state, I don’t expect this storm to have much if any impact. It’ll be a cloudy and cold day on Saturday and Sunday, with some gusty winds and maybe some flurries or light snow expected, but I think that the heavier band stays south of this area, and as a result, I have cut forecast snow accumulations from 1-3” to up to 2”, with 0” being a realistic possibility. We would need to see a NAM like solution from other guidance to feel more confident that we would see more than a couple of inches in this zone. While not impossible, it is certainly a large long shot.
The next small slice of the state represents the transition zone. In this zone, I expect a somewhat bipolar representation, with snow starting up as the storm wiggles a mile or two north, only for it to shut down again as dry air overpowers it, then rinse and repeat. I think that this zone will have off and on snow showers throughout Saturday afternoon and overnight, with at least a short period of steady accumulating snow likely, and that will be enough to add up to one to three inches of snow. I think that the possibility of totals increasing in this zone, while not high, is there, as it would not take a shift to a total NAM solution to see heavier snow totals here. However, the consensus blend that we have used still keeps this area, for the most part, on the outside looking in, which is why I am not confident in higher totals here. Keep a watchful eye on the forecast though.
The next zone is where I expect steady snow to make it to during the course of Saturday afternoon and overnight. I think that for at least a short time, this area will make it into the deformation band which I have described, and as a result, a quick thumping of a few inches of snow is likely, with lighter snow on either side. I think winds will also begin to play a factor in this zone, as it will be close enough to the stronger area of the storm for some stronger gusts to work their way in. As a result, I think that some quick periods of low visibility and difficult driving conditions, especially Saturday evening, are likely here, and as a result the impact is a bit higher.
This zone should keep a very close eye on future forecasts, however, as it would only take a slight shift north on the models to bring the edge of consistent heavy snow further north and into this zone. In fact, looking now at the ensembles of the GFS, which were just released a moment ago, they are consistently northwest of the operational model and as a result would mean this zone would be factored into the 4-8” area as well. While I don’t currently have enough confidence in that to move the zone further north, this area, out of all the zones, I think is the most likely to see a potential increases in totals.
Finally, on the immediate shoreline, I have painted a small stripe of 4-8” of snow. I think that the system will come just far enough north to get the immediate coastline into the meat of the banding for a decent period of time Saturday late afternoon and evening, and as a result, I expect a period of moderate to heavy snow along the south coast that will result in four to eight inches of accumulation. However, out of all the zones, it is this one that I have the least confidence with. The models place the knifes edge of the gradient right along the shoreline, and a small wiggle of perhaps ten miles could make the difference between this zone verifying and busting. To be completely frank, I included this zone instead of just leaving the snow map at 2-5” to the shoreline because I wanted to show the possibility of higher amounts making an appearance in the state, and I figured that if we had to adjust the map higher in future updates, it would be easier to explain and convey the reasoning behind it to the readership if we had previously mentioned the possibility by way of the 4-8” zone. I do think that as of right now, most areas in the zone will fall closer to 4” than 8”, but it would not surprise me to see a few measurements in the 8” range, especially in the “panhandle” in the far SW corner of the state. We’ll see what ends up happening with this one, but I think it needed to be included in this map.
Impacts
- Accumulating snow for the majority of the state from Saturday afternoon into very early Sunday morning. Warning level amounts are possible along the immediate coastline.
- High winds, especially along the coastline and in southern areas. Blowing snow and scattered power outages are possible.
- Coastal flooding, especially in the western portion of the coastline. In fact, Coastal Flood Watches are up for coastal Middlesex, New Haven and Fairfield Counties for Saturday night into Sunday for minor to moderate water level rise above astronomical high tides.
- Possibly difficult travel Saturday afternoon and evening, especially along the shoreline. Should you be planning to head south on Saturday, I would reconsider those plans, as snow will increase in quantity and intensity very quickly as you head south of CT, especially south of New York City.
Our next update will be mid-day tomorrow after we have been able to look at the overnight and some of the midday guidance, and a final detailed update will be issued tomorrow evening. Until then, please like, share to help spread awareness, follow us Facebook and on Twitter @SouthernCTWX if you haven’t already, and as always, ask any questions you may have either here, on Facebook, Twitter, or by emailing me at spencer@southernconnecticutweather.com. Thanks for tracking this storm with us tonight!
-SA
