Good evening from Southern Connecticut Weather! 

Thank goodness for that final burst inland. 

At this point yesterday, I was quietly beginning to resign myself to a bust. That term gets thrown around, but I'll explain what it really should mean later. The storm was a touch too slow coming together, and mixing held on for what felt like an eternity in SE CT. However, as I wrote on Facebook, you cannot declare victory or failure until all the data is in. 

This is a review of our final forecast. 

Above: the verification map of our snowfall forecast. As a reminder, we take the reports published by the National Weather Service offices from Albany, Boston, and New York to fill in the map, and approximate locations because obviously the map isn't to scale. To the greatest extent possible, we try to include all reports, and throw out obviously erroneous ones--such as people reporting snow before the storm is over. 

Defining "Bust"
Alright, so let's just start at the top. What is a bust? That gets thrown around a lot during these events. We define a bust as a result that drastically departs from a forecast. Getting 2.5" of snow when your floor is 4" is a miss, not a bust. Getting 1" when your floor is 4" is a bust. I know that's subjective, but if every time a backyard is marginally off in snowfall it's a bust there's no reason to write a forecast. I note that because if you look closely, you see that some towns had quite a bit of difference over just a few miles.

Snowfall is highly localized. Your location, elevation, and even measuring style make the difference in your final total. There's a huge difference between measuring on your driveway and in your grass (don't do either). There's a difference between the top of the hill in your town and measuring in the town center. That's why there are snowfall ranges, to account for that incredibly local variation. 

What we see above overall is mostly a miss, and that'll be reflected in the grading below. It's not a disaster though, with the exception of SE CT, which...my goodness that was awful. One day our friends down there will get real snow again. Unfortunately yesterday was not that day. 

The Forecast
Let's take a quick look at what happened. First, the original forecast of 4-8" in hindsight would've been best for the majority of the state. There was never any contemplation of taking the forecast totals down in SE CT, but there should have been. There are other spots in southern and coastal CT, like Shelton and Lyme that have consistently underperformed over time. Just something interesting that may be reflected in future forecasts. 

Simply put, the storm came together slightly slower than anticipated. The banding appeared, did not get organized until it was east of the state, in large part in my opinion because of the speed at which the storm was moving. 

​Let's look at the radar at 8pm--prime time for heavier snow in our forecast. 

Above I circled the banding. There's moderate to heavy snow happening in these bands, but the bands are narrow and not consolidated. What does that mean? Look at Stamford--lighter precipitation. West Hartford--lighter precipitation. Far SE CT? Something is falling from the sky but it's so light you can't pull down the cold air as efficiently that's just over your head. With a more consolidated precipitation shield, you have more uniform snowfall. 

But there's a second piece to this.

We talked about having higher ratio snow at some point during the storm, and that was a great call. This was honestly a bit of a surprise however, and it made the snow forecast mostly respectable. Once the storm started to pull away, it did organize nicely enough for us to take advantage of better dynamics at the end. Between about 11pm and 2am, we saw moderate to heavy snow develop and slowly move across the state. With the entire state well below freezing by then, we saw excellent snow growth and that allowed some places, particularly in central and eastern CT pile up accumulation. 

Many were saved from an outright bust at the end. 

Timing
Timing worked out very well. Here's what we said:

While the individual models have bounced back an forth a little, there's not much change in my mind. I still think we see the onset of light snow and/or mixed precipitation between approximately 2pm-6pm on Sunday from SW to NE. The heaviest is likely to occur after sunset in a period between about 7pm and 1am, followed by tapering off between 3-5am. 

Precipitation started right on time, the heaviest occurred between 7pm and 1am, and the snow was gone between 3-5am. Spot on. 

Grade: A

Wind/Power Outages
This was an easy part of the exam. It wasn't a terribly wind storm. We did not have power outage issues. We were right about the inland snow being more powdery. Gusts didn't exceed expectations. 

Grade: A

Snow Accumulation
Alright, here's where people really care. I wish that we kept the forecast to 4-8", but even then, the shoreline was going to be in trouble. Fortunately, interior CT carried us here. The overwhelming majority of locations below 5" were within an inch of the floor. A miss, not a bust. Central and part of eastern CT did well. 

The shoreline zone however, that was bad for most. In SE CT it was horrific. Objectively. Exceedingly few got close to the floor of 4", and in SE CT folks have a right to complain--very good snowfall reporters reported less than an inch near the shoreline. That's really terrible for a storm that ended up being cold in the end, especially considering that the Gulf Coast is likely to see many times that amount over the next few days. I digress however. 

This wasn't our best work. Grading the zones interior CT receives a B- while the coastal zone is an easy F with SE CT busting and the rest of the coast being a pretty bad miss. The average leads to a D+, which is double weighted because again, here is where people really care. I'm arbitrarily dropping it to a D because I didn't respect the decade of coastal futility enough.  

Grade: D

Impact
Impact is a little tricky given the shoreline miss/bust. I do think this was still a moderate impact event, as roads did deteriorate, but it's hard to say we nailed it given what happened in SE CT. 

Grade: B

Final Grade
Overall, we weigh snowfall 2x because we don't want to game the grading. Every other part of the forecast was really good, but we know what people care about and what's hardest to predict. 

Final Grade: C+

​A​s always, please like, share, and interact with us on our social media--on Facebook to share our discussions, and follow us on Twitter @southernctwx to retweet our posts. Hit the buttons below to join.
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Thank you for reading SCW.

​-DB

Good evening from Southern Connecticut Weather! 

A mix of winter weather headlines are up for the state in advance of our first significant snow event of the winter season. Most of the details seem sorted out, but the most important one, snow accumulation, is not settled yet. That said, it's time for our final call. Let's dive in. 

Above: our updated snowfall forecast, which slightly increased expected snow totals in most of CT from 4-8" to 5-10", and slightly raised the SE CT zone to 4-8", in line with the rest of coastal CT. 

Below: the current winter weather headlines for the state. 

The Forecast
As a reminder, we get graded on the elements of this forecast. The overall setup has not changed. It was a warmer than normal and rainy Saturday as pre-frontal precipitation reached the area. A boundary will cross the region and the first press of Arctic air will arrive tomorrow, right as our storm system gets cranking to our south. 

The expected track has shifted toward the state, increasing the chance we see more precipitation thrown back into the state. However, as we noted over the last few days, a closer track also introduces the risk of mixing at the start of the storm, especially along the shoreline. 

Timing
While the individual models have bounced back an forth a little, there's not much change in my mind. I still think we see the onset of light snow and/or mixed precipitation between approximately 2pm-6pm on Sunday from SW to NE. The heaviest is likely to occur after sunset in a period between about 7pm and 1am, followed by tapering off between 3-5am. 

Wind/Power Outages
No changes here. This doesn't look like a very windy storm. There's no big pressure gradient and the low itself isn't looking particularly powerful. Part of the reason for slightly increasing inland numbers is the likelihood of more powdery rather than wet snow. That reduces power outage risk. I think that'll be the case along the shoreline too, but it may take a little more time. I don't expect anything more than isolated power outages with gusts below 35mph. 

Snow Accumulation
Last night I wrote that if I were to make an adjustment it would be up or down slightly. That ended up being the case. This gets weighed twice as much as the other factors, and it's a pretty close call here. There was never a real thought of taking numbers down, but there was a lot of consideration--which ended up winning out--of taking the numbers up slightly for interior CT. Let's briefly talk about why.

Above are the last five runs of the European model (not including the latest at 18z). There has been a clear trend toward more precipitation thrown back toward New England. This is in large part because of a more GFS like track closer to the coast. That increased confidence that even a more middling Euro depiction could bring widespread 5" snow amounts to most of the state, especially considering that ratios are likely to be somewhat greater than 10:1 as the storm intensifies.

Most will end up with 5-8" I think, but the ceiling is higher if strong banding can cross over CT tomorrow night. I want to account for that. I also want to account for the possibility that we trend more amped in the last 24 hours, which often happens. 

On the other side of that however is coastal CT. It'll likely be a significant storm there too, but the closer track means that there could be some mixing issues at the start. This is something that really needs to be respected. Any mixing cuts down on potential snow totals, and the possibility of banding adds another wrinkle as it means there will likely be areas of subsidence (read: dry slotting). That's extremely hard to predict this far out, but using frontogenesis forecasts we can get a sense of where the banding could be.  

Above is the GFS looking at 700mb. It's not perfect actually as you generally want the best forcing SE of you (as it puts the banding a bit NW), but as it stands that's probably a good signal for heavy snow especially in inland NW CT tomorrow night.

At the same time, the soundings tell the story of where things could be at the time. Let's quickly look at Hartford first. You have good lift through the DGZ--or dendritic growth zone (note on the left) with temperatures solidly below freezing. There's no mix risk here at this timeframe. If you get good enough snow growth inland, you can put up good numbers even in a quick hitter. 

Now look at Groton below. Temperatures at the surface are marginal, as in just above freezing. However, it's likely snowing given the column above. The ratios are not great here as well because of the relatively poor lift that's mostly happening below the DGZ. 

Finally, let's take a look at Bridgeport at the same time. It's very different from SE CT. Solidly below freezing through the entire column, but the lift is also not ideal here either. Plenty good for snow and maybe a lot of it if you can get banding over you for a time, but it's far from perfect--which is to be expected in a storm like this. 

Boom Scenario: Widespread 10-14" totals (20%)
Bust Scenario: Widespread 1-2" totals (10%)

Above are the boom/bust scenario probabilities. No change from yesterday. Let's be clear here though. If you're in the 5-10" zone and you end up with 4", that's not ideal but it's not really a bust. The same goes if you end up with an isolated 11 or 12" report on the map. At the coast, 3" in a 4" floor isn't great but it's not terrible. Boom or bust scenarios require widespread errors.

​The boom scenario happens if the models amp up at the last minute--which has happened fairly frequently in recent years once we've been able to bring a storm our way. The bust scenario happens if the storm cuts well to the west, which is unlikely, or it comes in even weaker at the last minute. That's something to watch but I think that's unlikely as well. 

Impact
Overall, this still appears to be a moderate impact event. Travel conditions will deteriorate by late afternoon tomorrow with the worst happening Sunday night. Snow is likely to be moderate to heavy at times. Otherwise, the timing is actually great considering that Monday is a holiday and we don't expect powerful winds. 
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​A​s always, please like, share, and interact with us on our social media--on Facebook to share our discussions, and follow us on Twitter @southernctwx to retweet our posts. Hit the buttons below to join.
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Thank you for reading SCW.
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​-DB

Good evening from Southern Connecticut Weather!

The week ahead is looking the most wintry we've seen in years, and it starts with this weekend. A wave developing along a boundary that will bring our initial press of Arctic air is poised to enter our third window for significant snowfall. However, today's trends have left a significant gap between two camps--one that sees the wave becoming more amplified and tracking closer to the coast, and the other, seeing the wave as weaker with a further southeast track. This is particularly glaring given how close we are to the start of the event, but as I said last night, this is where forecasters matter. 

Given everything I've seen thus far, I think a compromise between the two camps makes the most sense for the first forecast. For this forecast and moving forward, we're going to add probabilities for "boom" and "bust" outcomes. This isn't much different from what that Capital Weather Gang have done for a while in DC, but I think it'll help enhance how we communicate what risks we see with the forecast.

With that in mind, let's dive in.  

Above: our first call snow map for the weekend storm. We are utilizing a compromise here, weighing the Euro and its ensembles about 60% here, while weighing the GFS and its ensembles 30% and other guidance 10%. What this means is that while we currently expect a more southeast track, it won't be as flat and weak as the Euro/EPS. As a result, we anticipate a conservative 4-8" of snow for most of the state, with 3-6" for southeast CT. This map accounts for some mixing at the start, with a flip to snow for everyone relatively quickly. 

Overall Setup
The overall evolution of this one has been straightforward for days now. There is unanimous agreement that pre-frontal rains arrive tomorrow as a boundary approaches the region. It will cross the region as we see energy diving down from the Midwest along the trough. That will begin to spawn a storm system Sunday.

Above: the 500mb depiction of the 12z Euro from Saturday afternoon into Monday morning. Note the kink in the deep trough that's moving into the U.S., which will bring the cold. That energy on the right side is the vort that will become our storm. On the bottom we have the result at the surface. The Euro as it stands is the least robust, and this is still the surface solution, lending us confidence in the floor for the system. 

Zoomed in. Note how weak the precipitation shield is. There's a solid track for mostly to all snow, but the system is weak, flat, and doesn't produce too much as a result. 

This is one camp and the Euro/EPS are pretty much alone. We give enormous respect to the Euro, hence the weighing, but it seems unlikely at this stage that it's got this perfectly right this far out. 

The other camp is the GFS/GEFS and most of the other guidance. You can see clear differences early at 500mb, and it leads to a stronger system with a closer to the coast track. This introduces mixing risk especially early in the storm. 

The surface result, zoomed in. 

Here the stronger low and closer track brings heavy snow over the state, with the rain/snow line very close to SE CT until the temperatures crash on the backside. Longtime followers of our nor'easters know that you often need to be close to the razor's edge to get big snows, and that happens here, with the GFS toying with double digit snowfall totals for a significant part of CT. 

This seems imperfect at this stage too, but I do think that the track will be less flat, which is significant. 

So let's talk about the forecast. 

First Call Forecast

Timing
This part should be straightforward. We're expecting the storm to take shape starting Saturday well to our southwest. The timing on the guidance had shifted a touch as I noted this morning, but has since stabilized. I think we see the onset of light snow and/or mixed precipitation between approximately 2pm-6pm on Sunday from SW to NE. The heaviest is likely to occur after sunset in a period between about 7pm and 1am, followed by tapering off between 3-5am. 

Snow Accumulation
As you know by now, we weigh snowfall accumulation twice as much in grading because that's what everyone cares about. The first call here is a bit tricky but here is what we have after blending the guidance. 

Most of CT: 4-8"
I think the least is right along the shoreline because of early mixing, but I can't rule out a higher result (in this range) because I think there is going to be banding in this storm that will create winners and losers. The more dynamic the storm is, the more track matters both for mixing and the greater banding. In addition, most of the state away from the shore is going to flip to higher ratio (more powdery) snow faster. 

SE CT: 3-6"
Here, I'm not taking much off the overall accumulation. I just have less confidence that there will be widespread 4" as a floor in this zone. Again, we're watching out for banding, but that won't come into better focus until tomorrow.  

Boom Scenario: Widespread 10-14" totals (20%)
Bust Scenario: Widespread 1-2" totals (10%)

Above are the boom/bust scenario probabilities. The boom scenario happens if the GFS turns out to be right and continues to amp up (though that would increase the mixing risk in SE CT). The bust scenario happens if either 1) the storm cuts well to the west, which is highly unlikely, or, 2) the storm whiffs to the southeast. There is less chance of a bust here than the opposite, but both are unlikely. 

If I were to adjust a final forecast it would be up or down slightly. 

Wind/Power Outages
This doesn't look like a very windy storm, even though cold will be advecting in. As a result, I do not expect anything more than isolated to scattered outages at this time. For those wondering about snow type, the snow should start out on the wetter side, but we will see temperatures drop through the afternoon Sunday as the Arctic cold behind the boundary begins to arrive and the storm itself reinforces the cold assuming a more offshore track.

Our wet snow or mix as a result will flip to cold powder statewide as temperatures drop into the low 20s and teens Sunday night and possibly single digits by sunrise Monday. As an aside, the latest Euro has the coldest stretch in likely a decade in its latest run. Fresh snowpack will help raise the odds that temperatures decouple at night, bringing below zero temperatures. We'll see if that holds as we get closer to next week.

Below are 18z Euro temperatures Monday morning. Frigid even before adding the wind chill. 

Impact
Overall, this looks like a moderate impact event. I'm going with moderate rather than low impact because I do think that travel conditions Sunday evening will be pretty bad. Otherwise, the timing is actually great considering that Monday is a holiday and we don't expect powerful winds. That said, going from wet snow to frigid temperatures means that whatever falls could become a glacier. That could lead to issues if the snow is not cleaned up quickly. 

​A​s always, please like, share, and interact with us on our social media--on Facebook to share our discussions, and follow us on Twitter @southernctwx to retweet our posts. Hit the buttons below to join.
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Thank you for reading SCW.
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​-DB

Good evening from Southern Connecticut Weather. 

Last week I spent time discussing the pattern change that has now arrived and settled in across much of the country. In that discussion, I talked about colder than normal temperatures being all but guaranteed, but stated that as we think about snow possibilities, we should look at them in terms of windows of opportunity, and that those would be far more difficult to forecast than the cold itself. 

The week ahead starts with the first chance of snow essentially failing for the state. It'll be cold throughout the week, but nothing totally out of the ordinary, or at least what a normal January should be. By far the biggest element of the forecast for the week ahead is our second snow window that opens this coming weekend. On social media you are probably already seeing people throw around model images and start hyping about blizzards and such, but the truth is much more complicated.

The truth is that there is enormous potential for some type of winter storm to develop, but whether that potential is realized is highly uncertain at this time. Let's dive in. 

Above: the 18z GEFS (GFS Ensemble) with the 500mb depiction over the next week. How pieces of energy from the north and south interact this week will determine the intensity, timing, and track of a possible storm Saturday or Sunday. 

Monday
First things first. Our first window for a snow event is staying to our south. You can tell this pattern change is filled with potential generally because much of the Midwest is seeing a historic blizzard taking place right now, with the DC area poised to receive its biggest snow event in years when the storm arrives early Monday. As I wrote last week, the risk of "too much of a good thing" ended up being the case for New England.

A combination of blocking and confluence to our northeast locked in the cold, but overpowered the storm track--allowing the system to pass harmlessly to our south as it rocks the Mid-Atlantic. If the blocking weren't present, the storm likely would have taken on an entirely different evolution, cutting well to our west and bringing rain. Just a little less confluence however would have allowed the storm to move more northward, bringing a meaningful snow event here. 

That said, we still need to be on the lookout for some light snow tomorrow, especially in southern CT. Nothing shows a meaningful snow event, but I would not be surprised to see far southern CT, especially SW CT, end up with a coating. This snow window closes without doing anything of note for the state.

It'll be plenty cold however tomorrow with most likely staying below freezing. In fact, for most, we're probably going to be below freezing for most of the week ahead. 

Above: the 18z GFS depiction of our "Southern Slider" for tomorrow. There's a chance of flurries or light snow in the air, especially in far SW CT, but nothing significant is expected. 

Tuesday-Friday
Mostly sunny. Breezy. Cold. The middle to end of the work week looks like a carbon copy of one another. We should get plenty of sun each of those days, but with plenty of cold around and breezy conditions we will have wind chills that are lower than daytime temperatures. Much like last week, with cold advecting in we will have the chance of random snow showers that don't amount to much. Thursday in particular could produce more scattered snow showers, and we will update if it looks more widespread. 

But the story here is the cold. Again, this is more standard cold, nothing exotic. The way January should be. 

Above: the EPS (European Ensemble) depicting surface cold anomalies during this period. It's cold all across the country, which is unusual. 

Saturday-Sunday
Now let's let to the real period of interest--the coming weekend. As I mentioned at the start, this potential has always been signaled, but going from potential to reality is a huge leap. This looks like an all or nothing kind of setup as well, where we either see an incredibly intense storm develop and bring a major storm to the east coast, or nothing develops close enough to bring us anything meaningful.

These are highly sensitive setups, but hold enormous potential. You've probably heard of the "bomb cyclone" term in recent years from media, which describes an actual meteorological event (a low pressure system deepening 24 millibars within 24 hours). This kind of potential is thermonuclear. 

Above: the 18z GFS depicting the 500mb level in heights and vorticity (read: energy). Your attention should be focused on the area that starts on the west coast and then traverses the country. In this depiction, the northern and southern energy phases (combines) to form a mammoth storm off the east coast. If the phase is missed however, nothing really happens, especially in our neck of the woods. 

The specific model depictions should not be taken to the bank, no matter what model you use, because the outcome our way is highly sensitive to what happens with the potential phase. Let's use the European model as an example of this. 

We're going to use the same type of animation above, looking at what's happening "under the hood" along with what the result is on the surface. You'll see why you cannot lock anything in at this time. 

Yesterday Afternoon
​I start and end the animations at the same forecasted point for each for consistency. Yesterday afternoon the Euro animation starts with our system trying to take shape in the southwest part of the country. 

Note the energy in the northern Plains diving down and combining with the southern energy. From that moment forward, it's game on. The trough starts winding up and although its messy (you want more of a bowling ball look than a line of energy) you see the result with intensifying vorticity and then the surface depiction. 

In this roll of the atmospheric dice, the phase results in a storm taking off once it hits the coast. We get hit hard. 

Last Night
Here is what happened last night in the Euro run. Here, the phase kind of happens twice, with multiple pieces of northern energy, and the result is nuclear.

The timing of the phase determines details like where the heaviest banding sets up, but that's useless at this range because it's entirely dependent on whether there is even a phase. With this kind of phase, the region gets buried in a blizzard. 

This Afternoon
But again, nothing is locked. This is a highly sensitive setup, and a phase is far from a guarantee. 

Above you have this afternoon's run, where we get a highly inefficient consolidation of energy in the southwest, with the northern stream secondary energy staying to the north in later frames and failing to phase. The result is virtually nothing happening over our region, other than northern stream driven snow showers. 

The runs tonight continue to show the idea of multiple pieces of the northern stream trying to phase into the southern energy, and should phases occur, the timing of the fusion will matter a lot. 

I spent a lot of time discussing potential because it's worth understanding why the surface model depictions of a blizzard...or whiff...don't matter at all right now. As is always the case, what happens at the surface is driven by what happens aloft.

Nothing is settled at this time, but the potential is high. 

The Dailies
Monday: Mostly cloudy with a chance of snow showers. Highs in the upper 20s to low 30s. Chance of snow 20%. 

Tuesday: Mostly sunny, breezy, and seasonably cold. Highs in the upper 20s to low 30s. 

Wednesday: Mostly sunny, breezy, and seasonably cold. Highs in the upper 20s to low 30s. 

Thursday: Mostly sunny, breezy, and seasonably cold. Snow showers possible. Highs in the upper 20s to low 30s. Chance of snow 10%. 

Friday: Mostly sunny and seasonably cold. Highs in the upper 20s to low 30s.

Saturday: Increasing clouds. Highs in the low 30s. 

Sunday: Mostly cloudy with a chance of snow. Highs in the upper 20s to low 30s. Chance of snow 30%. 

​A​s always, please like, share, and interact with us on our social media--on Facebook to share our discussions, and follow us on Twitter @southernctwx to retweet our posts. Hit the buttons below to join.
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Thank you for reading SCW.
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​-DB