Significant Winter Storm Update

 I have been very impressed with the potential for heavy banding to materialize across Connecticut all along and we we get closer to the event I've become way more impressed. There are signals for a hellacious band of extremely heavy snow to impact southwestern Connecticut and northeastern Connecticut. It is a bit more challenging in the Connecticut River Valley (Windsor Locks, Hartford, this corridor) as subsidence could lead to totals on the lower end of the range. 

Forecast models are indicating extreme frontogenesis traversing the state by Monday afternoon into the early evening. Now, usually when forecasting higher end totals (getting above 15-18'') you would like to see the signal for banding stall and pivot. While this isn't necessarily happening here, very high snowfall ratios (perhaps 15:1 - 18:1) combined with hefty QPF totals tell me that it will dump 2-3'' per hour for several hours:

Below is looking at White Plains, NY. While not Connecticut, this is just over the border and not far from Greenwich. The two isotherms (-12C and -18C) highlight the dendritic snow growth layer (DGZ). This is the layer in which snow growth is maximized. At this location, this is showing the most extreme lift just below the DGZ, however, in southwest Connecticut, we may maximize this lift into the layer. This is a very strong signal for extremely heavy snow:

Let's break all this down:

• Snow begins likely during the pre-dawn hours Monday morning (especially across southern Connecticut) with snow spreading north through the early morning. 
• Snow rates quickly increase by late morning and early afternoon. 
• The absolute heaviest of the snow falls between 12:00 PM - 8:00 PM EST. This is when snowfall rates could approach 2-3 inches per hour (potentially even 4).
• Some thunder is possible
• Winds increase out of the northeast to 15-25 mph (25-35 along the shoreline) with gusts 40-50 mph (upwards of 55 along the shoreline). This will bring about blizzard conditions with extremely poor visibility, drifting snow, and isolated power outages.
• Afternoon and evening commute may be near impossible and extremely dangerous due to rapidly accumulating snow, very poor visibility, and high winds. 
• Light snow continues well into Tuesday morning.
• There will likely be some sleet and freezing rain involved along the shoreline and even extending well inland, especially during the overnight as precipitation intensities wind down and the storm takes a wonky track. 

Below, is my update:

Significant Snowstorm to Impact Connecticut to Start February

 There continue to be signals for a significant and long-duration winter storm to impact Connecticut as we begin the month of February. While all the pieces are there, a lot has to play out correctly for this to happen. I am on the aggressive side with this forecast, however, I feel quite confident these pieces will evolve in such a way that a band of very heavy snow traverses the state. Should this system materialize just a bit quicker, more of the state could get into even heavier snows. Why am I so aggressive? I'll explain below. Note: Just because I'm using graphics from a particular forecast model does not mean I am basing my forecast off that model. I am just using the graphics for illustrative purposes. 

Baroclinic Zone:

I am very impressed with the temperature gradient in the lower-levels of the atmosphere which will be in place across New England. Choosing the 850mb level (~5,000 feet above the ground) too illustrate, temperatures in the warm sector may as as warm as +7C. Meanwhile, temperatures at the same level across interior New England may be as cold as -6C to -7C. Also note the very strong easterly winds in the warm sector (the flags indicate 50+ knots of wind feeding into the storm). This will provide sufficient moisture into the storm. This is all even more impressive if we were to look down around 2,500 feet (925mb). This favors very strong lift (upward vertical motion) across the region.

Forecast models indicate the 700mb low closing off south of Connecticut with the low tracking to our south and east. There are uncertainties regarding the exact evolution of this feature which poses some challenges to the overall forecast. As modeled, this might be a bit farther south than I would like for heavy snow into Connecticut, however, given this low does not become very tightly compacted (the more "rings" or circles" the more compacted and tightly closed it is) and is more elongated indicates to me that the band of heaviest snow would actually be displaced farther northwest than usual. That would place this band over Connecticut:

In terms of how much liquid will fall (this can be a huge assistance when forecasting snow totals), forecast models generally are not very impressive here in Connecticut as they keep the heaviest south and east of the state. I feel very strongly that this is underdone based on analysis of the setup. I feel based on the degree of warm inflow into the very cold air mass that will be in place combined with the strong baroclinic zone and favorable region-wide large-scale lift, we will certainly see a band of heavy snow traverse the state.

Here is a breakdown of my thoughts:

• Snow begins during the overnight Sunday into Monday morning. 
• Snow intensities begin to increase rapidly thorough Monday morning (especially the southern part of the state) with conditions deteriorating rapidly. 
• Winds begin to increase out of the northeast or north-northeast and may gusts between 40-50 mph during the height of the storm. This may lead to blizzard-like conditions at times along with potential for drifting snow and isolated power outages. 
• The heaviest of the snow falls from late Monday afternoon through early overnight Monday. Light snow may even stick around through the first half of Tuesday. 
• Snowfall rates during the peak range between 1 to 2 inches per hour. 
• Monday late afternoon and evening commute will be extremely difficult due to heavy snow and snow covered roads. Given how cold it is the snow will stick rapidly and easily. 
• There are some wildcards to consider, including the potential for dry air to filter in the lower-levels from the north. This could put a major wrench in totals.
• Below is my initial forecast:

Significant Storm Potential Around February 1, 2021

 There are signals for a rather significant winter storm to begin the month of February. In a winter which has been well...non-existent in terms of the snow department and chances few and far between, it makes monitoring this that much more fun. For us here in Connecticut, there are a wide range of possibilities from a small hit, very large hit, or no hit at all as the storm could miss out out to sea or well off to the south. How the overall pattern progresses through the weekend will determine what type of solution we have.

Since our massive snowstorm back around mid-December two things have been constant;

1) A negative North Atlantic Oscillation (NAO)

2) A positive Pacific-North American (PNA) (however, the PNA has become more negative recently)

Now...typically when you see this sort of combination there are two things you can expect;

1) Colder temperatures

2) Snow...and lots of it

Well neither of these have happened...but we won't get into that here. 

Moving towards the first of February we start to see some changes occur. The NAO trends more positive while the PNA, which has become more negative recently, is expected to become more positive:

When both the NAO and PNA are in this transitory process, this can be a signal for a major East Coast storm. 

The 12z GFS ensemble mean for 500 hPa height anomalies come Monday morning features a very exciting look for snow fans;

1) A ridge across the Inter-mountain West and Great Plains region

2) Trough amplifying across the East with a trough axis into the Southeast

3) A large area of anomalous above-average height anomalies over much of Canada. This will create a "block" and when combined with the look of #1 and #2 sharply increases the likelihood for a storm to develop and either sort of ride up the coast or if it comes from the Ohio Valley, the block should keep the storm farther south and also supply us with cold air. 

One thing we've been lacking all winter is a baroclonic zone off the East Coast. Baroclinicity is a huge driver in cyclogenesis and that has been lacking. 12z GFS forecast temperatures at 850mb indicate there could be a pretty decent barloclinic zone across our area with temperatures about 5,000 feet above the ground to our south as high as +6C with across northern New England they could be as cold as -10 to -12C. This type of steep temperature gradient favors strong upward motion and favorable conditions for low pressure development and strengthening (cyclogenesis):

Forecast models are indicating the potential for energy riding around the crest of the ridge out west digging and amplifying as it moves into the eastern trough. This will be a focal point for the development of low pressure which could rapidly intensify. 

Being nearly a week out, it's way too early to get into details or specifics, however, the signals are there for a pretty significant winter storm which could impact Connecticut around February 1. Details should become clearer moving through the week.

Tuesday, January 19 - Wednesday, January 20, 2021 Clipper Potential

 After the mid-December snow event, winter has been a zzzzzzzzzz fest in the snowfall department and in the cold department for that matter. While it may not seem like it, the overall pattern hasn't been all that terrible, however, a few nuisances; particularly with the state of the Pacific have prevent us from cashing in on the pattern. We have also seen a more southern stream dominant pattern which well...sometimes works and sometimes doesn't. However, moving into the third week of January, we will be dealing with northern stream energy and these can offer up clipper systems and sometimes these clipper systems produce...so will this one? Let's take a look!

A textbook rex block pattern will evolve across the west coast at the beginning of the week with a cutoff low pressure across the Southwest (or just off the Southwest coast) with high pressure building into the Pacific-Northwest region. This results in the jet stream splitting into two across the western United States with the two pieces finding love again across the eastern United States:

Within this pattern, there are two key features to focus on for the clipper potential. Shortwave energy digging in from southeast Canada within the fast-moving northern stream and shortwave energy across the southwestern United States:

If that shortwave energy across the Southwest remains more off the west coast (or even weakens more rapidly than forecast) this will provide further room for the northern stream energy to amplify as it digs into New England. Should this amplify enough with the main piece of energy passing south of Long Island, this would promote a high likelihood for a rapidly strengthening system with a rapidly blossoming precipitation field. Given there will be sufficient cold air available, this precipitation would be in the form of snow!

Should this northern stream energy amplify enough to strengthen, we could see a light-to-moderate snowfall event unfold Tuesday night into Wednesday morning. Accumulations would certainly be possible. What kinds of accumulations would be looking at? Too early to tell, however, something certainly just getting into plowable territory would be in the cards. 

This is still several days out, however, that potential looms and for those starving for snow, we hold onto any hope we can get in these dire times.

Happy Snow Watching!

Major Blizzard To impact Connecticut: 12/16/2020 - 12/17/2020

 Confidence continues to grow in the likelihood for a significant winter storm with blizzard conditions to impact Connecticut Wednesday, December 16, 2020 into Thursday, December 17, 2020. Significant snowfall accumulations, strong winds, scattered power outages, and severe travel disruptions are all anticipated.

As of Monday morning, the energy responsible for the developing storm which digging into the western United States as part of a long-wave trough. This energy will slide through the southern Plains Tuesday and then begin lifting northward through the Ohio Valley and into the Northeast through Wednesday:

Very impressive upper-level jet dynamics are expected as this system strengthens with the Northeast in a very favorable position for extensive large-scale lift. This will result in the blossoming of moderate-to-heavy precipitation across much of the region:

There are two features which will help to produce a very significant snowstorm across a large area;

1) An area of high pressure just to the north of Maine - This will reinforce colder air into the state and also work to keep a storm track a bit farther south. 

2) A weak negative North Atlantic Oscillation (NAO) with the NAO actually becoming more negative over the next few days. This will setup a 50/50 low or a "block". The block helps to keep a storm track offshore which favors a colder airmass and greater likelihood for snow:

The biggest uncertainties and challenges being faced right now are;

1) The overall track of the surface low pressure.

2) How the low pressure at 850mb (~5,000 feet above the ground) evolves and where that tracks.

3) How the low pressure at 700mb (~10,000 feet above the ground) evolves and where that tracks. 

All 3 of these are keys as these will heavily influence exactly where the band of heaviest snowfall traverses. The heaviest snowfall occurs generally 50-75 miles northwest of where these low pressures track. Forecast models are in general agreement for the surface low to track just south of Long Island. There are disagreements, however, regarding how the 850 an 700 low pressure evolve and where those track. There are some indications the 700mb low could even track over Connecticut, this would then introduce the potential for a dry slot to punch into the state and significantly reduce snowfall totals.

With high pressure to the north and a strengthening low pressure passing to our south and east, this will result in a very tight pressure gradient across the state which means strong winds. Winds will increase out of the Northeast through the day Wednesday to between 15-25 mph by the overnight with gusts 35-45 mph. This will bring about blizzard conditions with extremely low visibility at times. In addition, scattered power outages are possible. 

Despite some of these uncertainties above, forecast models are showing very impressive upward vertical motion into the dendritic snow growth zone with fairly high snow ratios too, however, the NAM forecast model is more impressive than the GFS forecast model. The NAM would indicate extremely heavy snowfall Wednesday night into Thursday morning with snowfall rates upwards of 2'' per hour:

However, what goes up must go down. This will pose another significant challenge regarding snowfall totals. Where there is a band of extremely heavy snow (which indicates intense upward vertical motion) there must be a region of intense downward vertical motion (the atmosphere is always trying to get to a balance). This will result in what is called subsidence and anyone under this subsidence will see snow rates must reduced and this would result in much lower totals for these areas. This is exceptionally difficult to forecast more than a few hours out. 

In the event of this storm, I am leaning more on the aggressive side with snowfall totals and impact. This system will feature very impressive dynamics with large-scale lift. Snow growth should be maximized with higher-than-average ratios given the extent of the lift and how cold the airmass is. Below is what to expect;

• Snow starts to breakout between 6:00 PM and 9:00 PM EST Wednesday evening and quickly becoming moderate-to-heavy moving into the overnight.
• Heaviest snow falls in the window between 11:00 PM EST Wednesday and 5:00 AM EST Thursday. During this window, snowfall rates may approach 2'' per hour.
• Winds increase out of the Northeast through the day Wednesday and peak Wednesday night between 15-25 mph with gusts 35-45. This will bring about blizzard conditions with drifting of snow. 
• High winds and heavy snow may result in scattered power outages.
• Periods of light snow persist into early Thursday afternoon, however, it remains cold and windy with temperatures barely getting above 30.

Below, is my initial snowfall map. As confidence increases regarding the banding potential this map will be updated:

Significant Coastal Storm to Bring Strong Winds, Rain, and Snow to Connecticut Saturday, December 5, 2020

 Looks like Connecticut will JUST miss out on a state wide significant snow event, however, parts of the state could see several-plus inches of snow. An area of low pressure passes just to the south and east of Connecticut Saturday. As it does so, the low pressure will rapidly strengthen. While the entire state will start out as rain, as the low pressure passes to our south and east and strengthens, a process known as dynamic cooling will transpire. Combination of dynamic cooling and heavy precipitation rates will rapidly cool the lower-levels of the atmosphere. This will result in rain changing over to heavy, wet snow across much of the state moving into Saturday afternoon. The northwest and northeast hills will see a changeover occur around mid-morning. 

The forecast track of this low pressure places much of Connecticut on the northwest side of the track. This is a very favorable place for moderate-to-heavy snow. If this whole process occurs say even 2-3 hours faster, then much of Connecticut could be looking at a significant snow storm. It will be very close!

The NAM forecast model shows a potent area of frontogenesis developed and progressing across eastern CT by early afternoon. This will be associated with a band of very heavy snowfall. Snowfall rates could approach 2-3'' per hour under this band. There is even the potential for thunder and lightning as the upward vertical motion will be very intense:

Below is an atmospheric profile for Willimantic, CT. The two red lines running across the image labeled -12 and -18 represent the dendritic snow growth layer. Snow growth is maximized between this temperature range. The color shadings represent atmospheric rising or sinking motion with rising motion denoted by negative values and sinking motion denoted by positive values. Notice the very low negative values penetrating the snow growth zone? This is indicative of very intense lift, however, one thing to note is the dendritic snow growth zone is relatively high (up around 600-550mb). Lift will have to be exceptionally vigorous to transport enough moisture into the snow growth zone to really maximize snowfall production and rates. The red box shows exceptional lift but well below the dendritic snow growth zone. 

While surface temperatures may be warmer than 32°F (perhaps around 34 or so), the depth of this warm layer should be very shallow and given the degree of lift and rapidly cooling lower-levels of the atmosphere, rain should have no problem changing to snow. This will result in snow being wet and very heavy. With the for several-plus inches in spots, scattered-to-numerous power outages would become likely and could persist for a few days. Winds will also become rather strong out of the north-northeast at 15-25 mph with gusts 30-35. This would result in very poor visibility. 

So let's sum this up into what we can expect:

• Rain for everyone Friday night into Saturday morning. 
• The northwest and northeast hills begin to see rain rapidly transition to snow mid-to-late morning with the remainder of the state (especially northern Connecticut) transitioning by late morning and early afternoon. 
• A band of very heavy snow quickly materializes across northern and northeastern Connecticut with snowfall rates 2-3'' per hour possible.
• Thunder and lightning possible. 
• Winds increase out of the north-northeast 15-25 mph with gusts 30-35 by afternoon. 
• Scattered-to-numerous power outages likely.
• Snow winds down by late afternoon. 

Below, is my updated snowfall map. There were some significant adjustments from my previous forecast as the band of heaviest snow will traverse eastern Connecticut where I had western Connecticut earlier.

Significant Weather System to Close Out the Month?...But It Will Be Rain NOT Snow

Signals for a potential storm system to impact the Northeast to close out the month of November have been in place the past several days and as we move closer to the end of the month these signals remain in place with confidence growing. The signals have been the following;

1) A sharp rise in the Pacific-North American (PNA) teleconnection pattern (PNA becoming more positive)

2) A rapid drop in both the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) (AO and NAO becoming less positive)

Without going in great scientific depth regarding these teleconnection's and oscillation's when the PNA becomes increasingly positive, this typically corresponds with a ridge across the western United States with a trough either across the eastern United States. When the AO/NAO become increasingly negative, this typically correlates to a trough across the eastern United States. However, it needs to be noted and clearly stated that there is more to whether or not these are positive or negative, the strength, placement of the height anomalies associated with each, and structure of the anomalies play a significant role in connection to the pattern across the United States.

Moving into the final week of November and into the first of December, forecast models indicate a sharp rise in the PNA as shown below:

At the same time, forecast models indicate a sharp drop in the AO and NAO as shown below:

How can we visualize what this means on the weather pattern? I am going to use the GFS forecast model and the 500mb height anomalies to provide a visualization. Here we see a ridge (orange/red shadings) across the western United States with a trough (darker blue shadings) across the eastern United States: 

So the question becomes, for New England (since my blog typically focuses on New England) why aren't we looking at the chance for snow? Well snow can't be ruled out, especially in the higher elevations of northern New England, but unfortunately snow lovers, this just is not a look for snow. 

Perhaps the biggest reason overall is there is just lack of sufficient cold out ahead of the developing weather system. Looking at around November 27 (a few days before we can expect a storm to materialize) and looking at 850mb temperature anomalies across Canada there just isn't much in the way of cold. Now...if this were say January or February, 850mb temperature anomalies might not be sufficient to look at here. Given those months are getting into the climatological coldest part of the season, even "anomalous warmth" at this level could still be composed of an air mass more than sufficient for snow, however, we are just entering December so above-average temperatures just won't cut it:

Another big factor is where does cyclogenesis take place? Note: It is never a good idea to judge this based on one forecast model, however, for illustrative purposes I will use the same graphic that was posted above the 850 temperature anomaly graphic. While using that for illustrative purposes, analysis of other long-range forecast models plus model ensembles was conducted to generate a forecast:

Based on the analysis, the best likelihood for cyclogenesis would be right around the base of center of the black line indicating trough. This would promote low pressure development well West of New England which favors warmth ahead of the system. This is also shown by the positive height anomalies (red shadings) into New England. 

At this juncture, the period between November 30 - December 2 is likely to see a significant weather system develop. This would include heavy rainfall, gusty winds, and perhaps potential for thunderstorms. Again, some snow can't be ruled out across the higher elevations of northern New England. 

So what happens behind this...well I want to again post the PNA and AO/NAO charts from above to highlight the look just to the right of what I have circled:

Notice how there is "chaos" going on. Does the AO/NO head well negative, does it trend more positive? Does the PNA quickly become less positive? This chaos indicates uncertainty with how the weather pattern will evolve post-storm. While we may face a decent blast of cold air behind this storm, unfortunately, the pattern does not look very supportive to maintain this cold and we may quickly go back into a pattern which favors average-to-above-average temperatures. Those hoping for snow moving into December...good luck.