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.