Friday, October 30, 2020 Snowfall Forecast Update

Heavy rainfall currently across Connecticut giving us some much needed rain and putting a dent in the drought. As we move through Thursday night into Friday morning, much colder air is going to filter into the state. This combined with some pretty intense upward vertical motion will result in rain transitioning over to a period of moderate-to-heavy snow across much of the state. 

By early Friday morning, computer forecast models indicate temperatures above 5,000 feet above the ground dropping to as low as -2C to -3C with temperatures down at the surface not much above freezing:

Forecast models also indicate a band of high vertical velocities traversing the state. This is indicative of rather strong upward vertical motion...favorable for the development of precipitation (in this case snow):

Forecast models also indicate we'll see some strong upward vertical motion into the dendritic snow growth zone:

Given the signals for some rather strong upward vertical motion across the state combined with cold temperatures in the lower-levels of the atmosphere and strong lift into the dendritic snow growth zone, the signals are there for what should be a 3-5 hour window of moderate-to-heavy snow across much of the state...even down to the shoreline! Given this signal, snowfall totals have been increased slightly from Thursday's forecast:

Potential Impacts:

• Rain transitions to snow across the northwest and northeast hills between between 12:00 AM and 3:00 AM Friday morning. The remainder of the state will see rain transition to snow between 4:00 AM and 7:00 AM (perhaps as late as 8:00 AM along the immediate shoreline) 
• Expect a slow morning rush hour commute due to slick roads. Travel should not be significantly impacted, however, give yourself extra time for travel. 
• Breezy during the morning with winds subsiding through the day, however, it will be cold with temperatures only into the 30's with lower 40's along the shoreline. 
• Very cold Friday night with temperatures into the teens and 20's so slick spots will develop. 

Significant Coastal Storm to Bring Rain, Gusty Winds, and...First Snows???

The potential exists for a rather significant coastal storm to impact southern New England heading into the end of the week. Much needed rainfall, gusty winds, and even some snow is possible. There are several pieces involved here and how they interact will determine the strength of the storm and the impacts we see. Let's dig into the science!

#1) Hurricane Zeta which is set to emerge into the southern Gulf of Mexico Tuesday morning. After making landfall late Wednesday in the vicinity of the Louisiana - Mississippi border, the remnants of Zeta  will rapidly progress through the Tennessee Valley, through the mid-Atlantic, and eventually pass just south of Long Island. 

#2) A closed area of low pressure across the southern Plains will eventually become an open wave as it pushes east through the Tennessee Valley. This system is a main player in the rapid speed of Zeta after it makes landfall. 

#3) A shortwave trough across southeast Canada amplifies (strengthens) as it approaches New England. 

#4) A strong area of surface high pressure slides into southeast Canada behind the shortwave trough. 

These features are all shown below using the 12z run of the  GFS from Monday looking ahead to Thursday evening:

The position of the high pressure and potential track of the low pressure (remnants of Zeta) are very critical as there will be quite a bit of cold air in Canada. In fact, 850mb temperature anomalies (temperatures about 5,000 feet off the ground) may be -10 to -20F below-average! 

The clockwise flow around the high pressure could support the transport of anomalously early season cold into portions of southern New England. Meanwhile, the counterclockwise flow around the low pressure could act to reinforce the transport of this low-level cold. 

In addition, an extremely strong jet streak will be placed north of the region with a jet streak in excess of 190 knots!!! The region will also be in a highly favorable quadrant of the jet stream which will favor vigorous upward motion and favor for low pressure strengthening off the coast. These factors also enhance the likelihood for much colder air to be drawn into the region. 

The verdict? Well, it's a bit too early for exact details and specifics, however, the signals and confidence continue to grow towards a significant coastal storm the day before Halloween. Potential impacts include; 

• Heavy rainfall (much needed!) 
• Strong and gusty winds 
• Potential for snow (especially for elevations above 1,000 feet)
• Accumulations are also possible above 1,000 feet. 
• Snow can't even be ruled out across elevations lower than 1,000 feet though the likelihood decreases quite a bit as you near the immediate coastline. 

Bottom line, this needs to be watched as many could see their first flakes of the season; including the potential for accumulating snowfall (with the highest likelihood above 1,000 feet). One final thing to mention...we are heading towards a full moon so costal flooding due to higher than usual high tides is a possibility.