Major Winter Storm Likely for Northwest Hills of Connecticut late Thursday night and Friday (12.16.2022)

We are very close to a significant winter storm for the majority of Connecticut outside of the coast, however, it appears the pieces just won't come together for that to happen with the exception of northwest Connecticut. 

The forecast weather pattern for Thursday evening features a very large closed upper-level low pressure system over the upper-Midwest region embedded within a longwave trough. Along the southern periphery of this upper-level low, embedded within the jet stream, will be several vorticity maxima's. Our area of focus (highlighted in purple box) is vorticity which is curling poleward towards our region:

As we now look at the 500mb jet stream and stick to the purple highlighted box, we see a very favorable region of surface convergence and upper-level divergence which will net the development and strengthening of a secondary low pressure right along the mid-Atlantic coast. As the system strengthens low pressures will also form aloft:

So why isn't the whole state getting snow? While we have cold air in place ahead of the storm and a supply of cold air to the north, unfortunately where the storm is strengthening and how the strengthening occurs will result in a surge of warmer air moving into much of the state. Due to the storm track colder air may remain locked in across the northwestern part of the state, keeping all precipitation in the form of snow.

As cyclogenesis occurs close the coast (and just inland) a strong easterly flow will develop within the lower-levels of the troposphere and at the surface. Unfortunately, since we are on the eastern side of this development, we'll see an easterly flow become established across the state which will yield an influx of warmer marine air:

The biggest question we still currently face is, where does this low pressure and resultant mid-level low pressure track? If the track was more northeast or even a bit more east-northeast, we would turn the wind direction across the state to more northeast to north-northeast and would lock colder air in place and that would give a predominately statewide snow event. However, guidance continues to suggest the low pressure will track more north-northeast. This will keep the majority of the state in a warmer east-northeast wind direction, with the exception being the northwest hills. 

Forecast models indicate that intense lift will be traversing the state Thursday night and the first half of Friday with the first wave of intense lift moving into the state during the early overnight. It is very possible that under the most intense lift, the northern half of Connecticut could see a period of heavy, wet snow with large flakes. Where the temperature profile is favorable in northwest Connecticut, this will yield very heavy and wet snow:

18z/14 NAM bufkit sounding from northwest Connecticut shows this intense lift described above with over 40 units of omega within the dendritic snowgrowth zone for a period overnight Thursday. This would result in a period of extremely heavy snowfall with rates upwards of 2-3'' per hour:

So what can we expect?

• Precipitation begins to move into the state Thursday evening. Across the northern part of the state, precipitation may be a rain/snow mix, however, late evening and early overnight when stronger lift traverses the state, this may yield a few hour period of heavy went snow. 
• Any areas with mixing will changeover to rain as warmer air filters in and the degree of lift weakens. 
• The northwest hills should remain all snow for this event, however, there could be some mixing during any lulls of precipitation when lift is weaker.
• Precipitation begins to wind down Friday evening and as colder air filters in on the backside the majority of the state (with the exception of the shoreline) could end as some snow with maybe an inch or two of accumulation.
• It will also be on the gusty side with sustained winds 10-20 mph with gusts 25-30 across the state.
• Isolated power outages possible across northwest Connecticut due to the wet nature of the snow.

Below is my current forecast:

Accumulating Snowfall Event Likely Sunday, December 11, 2022

 A potent piece of shortwave energy traversing the jet stream passes southeast across New England Sunday afternoon and undergoing minor amplification:

A weak wave of low pressure develops during the amplification process. The system will acquire moisture as the shortwave tracks across the Great Lakes and will draw in moisture from the Atlantic Ocean. With a cold enough airmass in place across the state precipitation should be all snow. The only exception may along the immediate shoreline where surface temperatures may be a bit too warm for snow, however, the airmass will be quite cold aloft and the warm layer will be relatively thin. 

Forecast models are in strong agreement that the low pressure will track over Connecticut Sunday afternoon into early Sunday overnight with forecast models indicating a several hour period of moderate lift traversing at least the western part of the state. The 18z/09 GFS bufkit for Windsor Locks, CT (BDL) indicates moderate lift within the dendritic snow growth zone early Sunday evening then briefly again overnight Monday. With sufficient moisture within the snow growth zone a period of moderate snow is likely:

Based on the forecast model agreement and likelihood for a period of moderate lift to traverse the state, a widespread 2-4'' is likely across the state with totals held down a bit along the immediate shoreline where surface temperatures may be a bit on the warm side. 

What we're looking at:

• Light snow arrives during the afternoon with snow coming down moderate at times during the evening and early overnight with snow tapering off during the overnight. 
• Roads will become slick as the afternoon progresses and expect a slick evening commute. 

Below is my initial forecast:

December 2022 Second Half Preview

 If you're a snow lover or an active weather pattern lover in the Northeast then there is a lot to be excited about as we move into the second half of December and for the final stretch of 2022. There has been strong consistency and agreement within long-range computer forecast models, global teleconnections, and evolution of the stratosphere in that the ensuing northern hemispheric pattern will be one that has historically favored an active storm pattern. Now this does NOT guarantee it will be snowy, but the potential will be there. I want to stress...while the potential is there and the look of the pattern is favorable there are some flags and we need to keep these flags in mind. So let's dive right into it. 

North Atlantic Oscillation (NAO):

One of the signals we will assess is the North Atlantic Oscillation (NAO). Computer forecast model ensembles are in strong agreement that the NAO will becoming extremely negative moving through the first week of December. The NAO measures the difference in sea-level pressure between the Azores high pressure and Icelandic low pressure. We typically get a visualization of the NAO by assessing 500mb height anomalies with a focus on the domain centered around Greenland. A negative NAO is typically characterized by above-average height anomalies around Greenland (higher pressure) with a positive NAO characterized by below-average height anomalies around Greenland (stronger low pressure). 

As we move towards mid-month, the NAO remains negative, but becomes less negative. Historically, during periods where the NAO is transitioning from extremely negative to less negative (or even positive) that transition period has been tied into East Coast storms. Why is this? Without getting too much into detail (otherwise it would take me until after this period to complete the post) the negative phase of the NAO correlates to a suppressed jet stream which can mean a storm track south of the Northeast. The negative NAO (also coined the term "block" can keep a storm track close to the coast. So if we visualize this, if a storm is coming up the East Coast (so associated with a trough- dip in the jet stream) the response upstream has to be a ridge. As the storm is evolving you'll start getting this dip in the jet stream to poke into the domain where the NAO is measured - this results in a weakening negative NAO - hence the rising index. Again, it's way more complex then that and this a very basic explanation. 

The NAO can also help drive Arctic cold into the eastern United States, however, this is not always the case as structure and placement of the core anomalies are critical and you must have a build-up of cold air in Canada. 

Arctic Oscillation (AO):

The Arctic Oscillation (AO) is a very close relative of the NAO, however, this is a measure of the polar vortex. you may have heard about the polar vortex as it has made it's way into the news the past several years. The polar vortex exists 27/7/365 and is a fixated low pressure over the Arctic. It's typically extremely weak during the summer and strengthens moving into winter. However, the strength of the polar vortex can fluctuate significantly during the winter season and there are some seasons where it can be very strong or very weak. The AO measures this. When the AO is negative, the polar vortex is in a weakened states and that can correlate to a suppressed jet stream with an increased likelihood for Arctic air to invade the United States. When combined with a negative NAO that can lock-in this cold. The caveat here, however, is you have to have a build-up of cold air in Canada to advect south and you need a cross-polar flow to direct this south.  

Like with the forecast of the NAO, computer forecast models are in strong agreement that the AO will follow suite and tank as we move through the first week of December then start to rise and become less negative towards mid-moth. This would help reinforce a blocking pattern which essentially would decrease the chance for any weather system to track off to our west (which typically results in warm-air advection meaning more ran and less snow) and can decrease the potential for a storm to track too far off the coast to miss:

East Pacific Oscillation (EPO)

The East Pacific Oscillation (EPO) is very similar to that of the NAO in the Atlantic. The EPO is a dipole of anomalies with one pressure anomaly in the Gulf of Alaska vicinity and one pressure anomaly of opposite sign south of this. When the EPO is negative this corresponds to above-average height anomalies within the Gulf of Alaska region (indicating a ridge of high pressure) with a trough south of this across the southern Pacific. It can said that the EPO (negative phase) can be a huge driver in delivering Arctic cold into the United States as the ridging into Alaska can efficiently displace colder air over the Arctic southwards. When in tandem with a negative NAO/AO cold Arctic outbreaks are nearly a given in the East. 

The forecast for the EPO is to become quite negative trough this first week of the month before becoming less negative moving towards the second week of the month. What really stands out here, however, is the extreme uncertainty moving towards mid-month as evident in the very large spread. This is important and will be discussed a bit later on:

Pacific-North American (PNA):

The Pacific-North American (PNA) is to the EPO as the AO is to the NAO (for the most part). The PNA measures the height anomalies in the vicinity of the eastern Pacific and western United States. When this index is positive it corresponds to above-average height anomalies within this region (indicating a ridge of high pressure). This enhances the likelihood for a trough to dig into the East. So, when in tandem with a negative EPO the potential for an Arctic outbreak of cold weather becomes enhanced across the East (especially when you have a negative NAO and AO. When the index is negative you have a reversal of this pattern (trough in west and ridge in east). 

Like with the forecast for the EPO, there is an extremely large spread in the state of the PNA moving towards mid-month after the negative PNA in place during the first week of the month. This will be important too and discussed below:

Assessing the Day 5-day averaged 500mb height anomalies for the day 12-16 period (December 12-17) we see a classic, textbook blocking scenario with the negative NAO/AO signal with heights well above-average across Greenland and the Arctic. As we look at the Pacific, we don't see a terrible look, however, the signal is much more mixed. How the Pacific evolves as we move into mid-month will be extremely critical:

So, what can we take from all this? There is a strong signal for storminess around mid-month. This storminess may also be a precursor to what could be a very active pattern for the second half of the month. for snow lovers, the mid-month period may be a bit difficult with the exception of interior Northeast/New England. Unfortunately, despite the blocking, we may be lacking a good solid Arctic airmass and may be dealing with an airmass that is more temperate and Atlantic nature (this is evident by the lack of cross polar flow and despite the fact we have negative height anomalies in the East during that period, they are retrograding from the Atlantic so the source region is that region as opposed to the Arctic.

HOWEVER, as we move into the second half of the month, there are signals that we could get enhanced support from the stratosphere and a Pacific which may become better configured. A better configured Pacific could be the key to dump Arctic air into the East, however, we first need to get Arctic air into Canada which the signals would indicate is a good chance.

All in all, if you're a snow lover in the East there is a lot to be excited about, HOWEVER, there are still some flags and details to iron out but at this stage, all you can ask for is potential and a favorable pattern and that's what we have. We'll see how this goes.