Good afternoon from Southern Connecticut Weather! 

That was quite a storm! For the first time in a while, we had a widespread and significant all snow event for the entire state. This was a high stakes forecast, with post-Christmas travel impacted and a very aggressive last minute trend that made me grapple with whether to change our forecast numbers, which were on the aggressive side when they were issued a day before the start of the storm. We held firm and that decision paid off. 

For those new to SCW, we grade the forecasts of significant winter storms for two reasons. First, in our effort to "show our work" we want to show what the result of the forecast was to hold ourselves accountable to you. This becomes even more important in our fast paced world because we believe it helps build a record of accuracy and trust. Second, and just as important, grading our forecasts allows us to see what went right and what went wrong, so we can be better in the future. 

We weigh the key components of our final call forecast, and because snowfall accuracy is king to the public, we count snow accumulation double in our grading to ensure that it gets the proper weight in the final grade. 

Our verification map takes the overwhelming majority of National Weather Service reports and although not placed to scale we do our best to place the reported snowfall number in the right spot in a given town. It's an approximation, which doesn't mean your backyard got more or less, we're just using the official numbers to be as objective as possible. 

Now, let's get to the grade. 

Above: The storm verification map. It's like opening up a Christmas present when you place accurate numbers in your snowfall zone! 

Timing
This is where I am most disappointed. In these kind of warm air advection scenarios, the snow usually rolls in faster, and exits faster, than on the guidance. I tried to beat that by forecasting that conditions would deteriorate between 6-10pm from CT from west to east, and while that was right for most, it was too late in western CT which saw flakes start flying just after 5pm. Now, you might say it's just an hour, but in a storm where we went from light snow to near whiteout conditions in under 20 minutes, that's not great. The saving grace here is that we identified the earlier start time and put out a warning 2.5 hours before flakes started flying. 

Other than that, the forecast was great. We called the quick change from light to heavy snow, and accurately called the snow being gone before 9am, though that too was a little on the earlier side. We accurately called that the worst was essentially from onset through the early morning hours. That piece of information was vital. 

So great overall, but that missed hour really bugs me. 

Grade: A-

Snow & Ice Accumulation
Folks, I am incredibly excited that we held to our map. Not only did it seem like we were one of the first out there with a more aggressive call on Thursday, but when the models were aggressive even hours before the storm started yesterday we didn't fully buy widespread double digits that would have made us up the numbers to a 6-10 or 6-12. Not that it matters to be first, but the best is being first and right, and I feel like we were there.

Now, the result isn't perfect. It wasn't further southwest that did the best in this storm as the banding ended up further north (as we suspected it might), and in our 2-5" zone New London County did very well, with a few spots above but within an inch of our 5" ceiling and one spot with a surprise (and believable) 8", but we still did an excellent job here.

That was about as good as you can get, and I'm proud of that.  

Grade: A

Wind & Power Outages
This was always going to be an easy call. The storm wasn't windy (40+ mph winds) and we didn't have power issues. Aside from one report, we didn't hear anything contradicting that we saw a drier type of snow versus paste. 

Grade: A

Overall Impact
Once again, excellent work here. We accurately called that despite the weekend timing and timing later in the evening that this would be a moderate to high impact event. In NE CT it was more moderate, but for much of the state last night was high impact with very high snowfall rates and substantial snowfall. We stated that the banding signal suggested heavy snow that would make travel hazardous, and that was spot on. 

Grade: A

Final Grade
It is truly wonderful to know that when the stakes are highest, we do our best work, and for winter you'd be hard pressed to find a higher stakes forecast than one around Christmas. I know there was a fair amount of skepticism at taking the numbers up so aggressively Thursday, and maybe a little at not taking them up even more on Friday, but this was about as good as you can get for a forecast.

I am incredibly proud of this one. 

Final Grade: A

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 and trusting SCW.

​-DB

Good evening and merry Christmas from Southern Connecticut Weather! 

Hopefully, everyone has had a wonderful day. If you are a winter weather lover, it looks like Santa has brought more digital snow to the state for the coming storm! 

We thought there would be adjustments given the highly sensitive environment ahead of this one, and the adjustment has been in the direction of a more impactful event. Now, we're taking the numbers up statewide, and a significant to potentially major snowfall is on tap.

For reasons stated below, this forecast is a little more on the aggressive side than what we usually do, and while most will likely end up in the lower to middle part of this snowfall range, I wanted to use a higher ceiling for the potential that some last minute shifts or a slightly more robust/longer lasting period of banding leaves a stretch of an area with higher totals.

Above: Our new snowfall map. We took the snow forecast up across the board, and expanded the highest zone further east into NW CT and more of western CT. We also adjusted the eastern zone to cover more of southern CT because of questions over banding. 

Overall Setup
Not much has changed. We knew it was a sensitive forecast, and the trend yesterday and today has been toward a more significant early burst of snow and snowfall expanding further northeast. It's already breezy out there, and as another Arctic front passes we will get cold air put in place overnight tonight. Tomorrow will be colder than normal, which is obviously critical in advance of our storm.

While more adjustments are possible, there is high enough confidence now to put up these snowfall zones and make a final call forecast. 

Above: GFS depiction of the coming storm. The general evolution is now agreed upon by the guidance with a burst of snow initially followed by a period of lighter snow across the state. With cold in place and a track far enough south, it looks like we will remain virtually all snow. There may be a chance of some brief sleet in extreme SW CT, but this is a snow storm. 

Timing
In our fast flow regime quick moving systems have been the rule and this is no different. Most of the day tomorrow looks absolutely fine for travel in Connecticut. Conditions will deteriorate tomorrow between 6-10pm from west to east as the storm moves into the region. It may start out as virga briefly as the column moistens enough for snow to fall, but once that happens snow will quickly go from light to moderate and possibly heavy, especially in western and central CT. 

The 12z Euro below shows this nicely. We quickly move the banding of heavier snow into western and central CT early in the storm and it will be a thump. From there as the storm deamplifies we will see the intensity of the snow diminish before the storm exits fully Saturday morning, likely before 9am. 

The worst of the storm is essentially from onset through the early morning hours. If you are traveling for a flight, there may be delays and cancellations Friday evening into Saturday morning, especially if you are flying in or out of a New York City area airport. 

Snow & Ice Accumulation
Above is our final call map again. Much like I said yesterday, the further southwest you are the better odds you have of a major snowfall. The band of heavy snow is now projected to cross the state, especially the western and central portions of the state. One thing to acknowledge here is that when we have strong frontogenesis we often end up with banding further north than guidance signals and warm air advection like this can push precipitation further northeast than we'd otherwise see.

Above: The 18z GFS showing 700mb frontogenesis. Note how there is strong frontogenesis over western CT. This is a good signal for moderate to heavy snow further northeast in the state.

This band of heavy snow is still pretty narrow geographically. Lighter snow will be present further northeast and folks on the edge of the snow may struggle as banding usually creates subsidence (read: dry slots). It is still the case that even a 25-50 mile shift makes this more or less significant. Given the overall trend toward a more amplified system, I think a more aggressive track is reasonable here. Things hinge on how strong the initial burst is, and while most likely fall on the lower to middle part of the range there's going to be spots that over perform. The speed of the system and its rate of deamplification caps the ceiling for this, but some folks could see a major snowfall. 

Wind & Power Outages
Much like last night's expectation, I don't think this is a windy storm. Snow should be on the fluffy side and the storm is weakening as it rolls into the region, There may still be some sort of pressure gradient so it may be breezy, but I don't anticipate significant power outages. 

Overall Impact
With the snow totals coming up the impact comes up. Even with the timing and it being a weekend this is a moderate to high impact event. The banding signal suggests heavy snow tomorrow night that will make travel hazardous, and during the storm we may have widespread flight delays/cancellations and hazardous travel conditions. Eastern CT is most likely to have a lower impact event, but even there travel is likely to be rough for a bit late Friday into early Saturday. 
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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.

​-DB

Good morning and merry Christmas Eve from Southern Connecticut Weather. 

I'm working overtime this holiday. 

A few days ago I talked about how despite the pattern change across much of the U.S. that will bring record warmth to much of the central portion of the country, we are sitting on the edge of the battle zone between warmth and cold.

This "Razor's Edge" pattern has indeed played itself out as I expected, with New England for the most part being on the colder side of that zone. While yesterday's event was a little too warm for meaningful snow for most, it did snow, and despite forecasts for mild temperatures for Christmas Eve and Christmas, here we are with seasonable temperatures for the holiday. For the most part, the trend has been toward cold, and that continues a theme that goes all the way back to August. 

This coming storm has been the embodiment of that theme. A few days ago, it looked like a rain event. Then it looked like a mix event, trending colder. Now, we expect a well timed Arctic blast (another theme of the season so far) to produce an all snow event. The trick for this one however is the angle of approach and amount of blocking in place for the track. 

We're on the edge of a significant winter storm. Let's talk about what we see right now. 

Above: The first call snow map. At this time I expect 3-6" in much of Fairfield County, with 2-4" in NW and central CT and 1-2" in eastern and NE CT. This is a highly volatile forecast where just a little trend west or east will make the difference between a significant snowstorm or minor snow event. 

Overall Setup
Basically, whether or not we see a significant winter storm is all made possible by an atmospheric traffic jam. A few days ago, when some believed that we were in for a mild Christmas and especially post Christmas period, this was because guidance was effectively showing a more zonal flow on the other side of the big ridge that's developing over the central United States.  

Just three days ago, this was the GFS for December 26. It had 50s in the state! 

The signal for some type of post-Christmas storm had long existed, but with this depiction, the forecast was for rain. 

So what happened? The trend in the 500mb pattern became substantially more blocked, which causes two critical things to happen. First, it allows for less of a zonal flow, meaning that cold from our source region in Canada can get shunted down into New England. Second, it forced our departed system and a follow up clipper to slow down to our northeast, jamming up the flow and forcing the next system in this fast flow regime to dive southeast. Note the "blue ball" to our northeast become stronger, and the ridge to our north into Greenland trend west and stronger. 

These are massive shifts, and as a result, the shift at the surface have been massive as well. It should be noted that the Euro has generally had the right idea for longer, I am weighing that more than the GFS in this forecast. 

That was a long overview of how we got here, but it's important in understanding what's happening with this storm and what's likely to happen after. While we will see storms that are too warm for snow, it is becoming increasingly likely that this blocked pattern will continue the delivery of Arctic cold and more storms into January. 

Back to the storm. With the block in place, first we will get a shot of Arctic air to reach the region Christmas night into Friday. That's step one. Our fast moving storm will be running into that cold, and with the strength of the block will not be able to take a more eastward angle or intensify too much. In fact, this will be another situation where the system is probably weakening aloft or middling along as it moves into the region.

This means that the strength of the block is crucial to how far northeast the precipitation shield can get and how much space it has to gracefully deamplify. Basically, the stronger the press of the block, the further west/south the storm goes. 

Above: the 500mb evolution of the storm. Note the Christmas Day clipper to our north and how it slows down and tries to back up at the far right of the image. That's our block. 

Basically, very small changes make a big difference here in CT. Let's get to the forecast. 

​Timing
Once again, we are dealing with a quick mover. If you plan to travel earlier in the day on Friday, you should be fine. By early evening, probably around 7pm, we're likely to have snow on our doorstep and overspreading the state afterward. The good thing for travelers is that this looks like a Friday evening to Saturday morning event, meaning that daytime travel shouldn't be impacted in CT too much. Depending on the track though, expect flight delays and possible cancellations, especially if the storm is stronger. Some guidance keeps the snow around into Saturday afternoon, and while that's plausible it is just something worth watching right now. 

Above: Storm evolution on the 06z Euro. Snow is in and out in a hurry, which also lowers confidence in snowfall totals. 

Snow & Ice Accumulation
Here's the first call map again. The further southwest you are the better odds you have of a significant snowfall. There is likely to be a narrow band of heavy snow at the front end of the system, with lighter snows the further east and northeast you are. As it stands now, SW CT is in the potential jackpot zone, with a lighter 2-4 inches possible for most of NW and central CT, and a zone of 1-2" in eastern CT. Just a 25-50 mile shift in either direction makes this a more significant winter storm or another nuisance event. 

With cold air in place, despite the similar track to yesterday this is a very different outcome. This should be all snow from start to finish, unless there's a dramatic change in the track, which seems unlikely at this point. 

Wind & Power Outages
Because this is likely weakening as it rolls into the region, I don't think this is a windy storm. That said, there may still be some sort of pressure gradient so it may be breezy. I don't anticipate significant power outages. 

Overall Impact
This may be the most difficult question to answer. If you are in SW CT and it's snowing into Saturday morning, it may be high impact. If you are in eastern CT and this is a glancing blow it may be minor. For now, given the timing around the holiday, potential for heavy snow, and likelihood of a gradient with regard to snowfall, let's call this a moderate impact event for now. This will be refined tomorrow. 

Stay tuned. 

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! 

This will be a short discussion because I don't have a ton of time and this isn't worth much proverbial ink. We have a weak system that will traverse the region tomorrow and it will give us our chance at a white Christmas. The timing makes it a little more tricky than it would otherwise be, and last minute trends haven't been great if you like winter weather. 

Above: The SCW final call snow map. While most of the state can still expect 1-3", there's just too much risk at the coast for warmth the warrant a higher floor and ceiling. I almost put the whole state in the 1-3" zone, but I just couldn't do it. 

Overall Setup
The setup is pretty straightforward and has been for days. A weak clipper in the very fast flow regime we've had much of the month will dive down from Canada tomorrow. With cold air in place we will see overrunning over that cold, which will produce snow. For coastal and southern CT however, the gradual southerly flow will erode the cold that is in place, and should force the precipitation to change to rain eventually. This is a fast moving system, so by the evening we should be cleared out. 

Above: A 12z Euro model depiction of tomorrow's clipper. The other guidance generally have the same evolution, with differences in how much precipitation falls. Overall however, a general 1-3" of snow inland with up to an inch at the coast seems most likely. 

Timing
This is a quick moving storm, like our other winter weather events this season. There is a bit of a split on the guidance on start time, but I think we see snow develop between 4-8am from west to east. The snow may very well quickly go to moderate in spots, and it'll be later in the morning/afternoon when precipitation becomes lighter that we see warmth really erode the snow in southern CT, maybe up into the CT River Valley. The storm is out of here by 8pm. 

Snow & Ice Accumulation
For inland areas, a general 1-3" has seemed like a high confidence forecast for a few days now. For the coastal region however, it gets more tricky. You could make a plausible argument that with the initial burst of snow that some coastal areas actually do better than the current ceiling of one inch in our forecast. However, the combination of a little more warmth showing up on the guidance, and some guidance cutting back on the amount of precipitation makes that argument more tenuous. If I were at the coast, I would expect an inch at most and be pleasantly surprised or disgusted if your town surpassed an inch. Inland, the CT River Valley is at the greatest risk of ending up on the lower end, but I think most finish with 2-3 inches of new snow. Enough for a white Christmas. This looks like more of the pasty wet snow, especially in lower elevation places. 

Wind & Power Outages
Not a factor here. Weak system. 

Overall Impact
Everyone's asking if this is enough to shut down schools. Normally, I would say no, but the timing of the onset of snow, and candidly--the fact that it's right before the holiday--probably increases the odds that there are widespread cancellations. Overall however this is a minor impact system in the whole scheme of things, with more moderate impact on the roads, especially in interior CT.

Looking Ahead
In the wake of this clipper, Christmas Eve and Christmas look quiet and seasonable with highs in the mid to upper 30s, maybe 40 in a few of our warmest spots--this is a far cry from much of the country which may be in record warm territory for Christmas.

We have trended colder as we stay on the colder side of the boundary between warmth and cold. This is critical for our next storm in the fast flow, which may very well be significant. If you are planning to travel on Friday, be prepared for a possible winter storm. 

Stay tuned. 

​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! 

The first half of December has been frigid, but as expected, a move toward milder weather is afoot. The supply of deep could from our source region in southeast Canada is being reduced as a massive ridge develops over the central U.S. in the coming week.

Before we likely warm up however, we will be on the razor's edge of the boundary between warmth and cold. In a continued fast flow regime with several weak systems rocketing through, it looks like we have one candidate for a weak winter storm right before the Christmas holiday, and perhaps another right after Christmas. The signal for the earlier event is far stronger, but let's take a quick look at both. 

Above: the 18z GFS model depiction of the Tuesday system, which even has an inverted trough bringing snow on Christmas Eve verbatim. 

Tuesday's Potential
This is our shot at a white Christmas, if you're into that kind of thing. 

If you like winter, be glad you're not living in Missouri this coming week. For most of the month, we've had a favorable WPO for cold air intrusions overwhelming other more muted teleconnections. This has allowed repeated legitimate Arctic blasts, as the coldest temperatures in the hemisphere have been pushed over North America. 

As the WPO's power wanes, a massive ridge will develop over the central U.S. and absolutely torch much of the continental U.S., perhaps through the rest of the month. Note on the EPS below how the colder blasts over our region get cut off as the ridge over the central U.S. develops and flexes. Note also how the biggest anomalies are to our west. That puts us in the battle zone between cold and warmth. 

Overall, for Tuesday we are mostly on the colder side of the boundary. Another cold front will pass through the region tomorrow, bringing in a fresh cold air mass for Monday and Tuesday. 

As this happens, a weak ripple in the flow will try to amplify as it dives down from Canada. This depiction, also from the 18z Euro, shows how it tries to amplify at 500mb...

And the result at the surface:

Now if you're comparing and contrasting between the GFS and Euro runs I posted, there's a split in the overall evolution. Yes, it looks like there will be enough cold for a period of snow on Tuesday, but there are still significant questions on track and amplitude. 

First, on one hand, a system diving down from Canada (basically a clipper) can be good for ensuring a colder flow--depending on the track. A further north track would keep a surface front to our north, likely causing a change from snow to rain, especially at the coast. A further south track keeps the column colder for longer. 

Second, how strong will this get? There are some models that try to turn this into a redeveloping coastal storm, which introduces a different type of evolution and heightened potential for eastern CT depending on the speed of development and the storm itself. For now, that's discounted but something to watch. 

Third, is there really inverted trough potential? Now, longtime readers know that inverted troughs are always highly suspect and are very difficult to predict. If there is some sort of inverted trough feature as some of the guidance tries to develop, that could lead to higher accumulation potential. 

The bottom line is that although this looks like a weak event, there's a lot to be sorted out. Stay tuned. 

December 26 Potential
This won't get much discussion yet, but in the wake of our Tuesday event we get a reinforcing shot of cold for Wednesday (Christmas Eve). That will be short lived, as it looks like we will warm up some on Christmas Day. This doesn't look like a torch though, like much of the rest of the country. 

Above: An 18z Euro surface temperature anomaly depiction. It's warm just about everywhere, but it's relative. While it's a blowtorch in the Midwest, here are our Christmas afternoon temperatures--mid to upper 30s for most. 

Above: 18z Euro temperatures Christmas afternoon. Image courtesy of Weathermodels.com. 

The temperature profile in the wake of the Tuesday event is extremely important for the post-Christmas period, as models are signaling a potentially stronger system moving though the region the day after Christmas.

There may be enough cold in place at the surface for a mixed bag event, or a rain event. While the GFS keeps us on the warm side of the boundary here to bring a rain event. 

The Euro has a colder temperature profile that could bring an impactful winter weather event on December 26. 

That above image might not look like much, but that's trouble for travel if we trend colder. 

Stay tuned. 

​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 and trusting SCW.