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.