Wednesday, April 21, 2021 Low Topped Squall Line Potential

 A cold front is set to sweep across southern New England early Wednesday evening. Combination of marginal instability and rather strong dynamics and forcing from the front will promote the development of a low-topped squall line ahead of the cold front. While the greatest overall potential for damaging winds and even hail exists across New York and Pennsylvania, localized embedded damaging wind gusts and hail will be possible across southern New England (particularly western and central sections). 

Instability is not the driver here with temperatures predominately in the 60's with dewpoints into the 40's and lower 50's, however, steep mid-level lapse rates (~7 C/KM) will aid in the development of marginal instability characterized by about 500-750 J/KG of mixed-layer CAPE. 

The driver is going to be the strong forcing associated with the cold front and very strong jet stream dynamics which are characterized by 50-60+ knots at 500mb (~18,000 feet above our heads) and 35-40+ knots at 850mb (~5,000 feet above our heads). These shear values are more than enough to support the development of organized convection. 

The line initially develops across western New York and central Pennsylvania and rapidly progresses east through the afternoon and evening. Combination of dry low-level air and strong low-level winds will support the potential for damaging wind gusts on the leading edge of the squall line. Wind damage may be more scattered across New York and Pennsylvania but then become more isolated across New England. Given cold mid-level temperatures characterized by -20C at 500mb (~18,000 feet) some of the more robust convection within the line could even produce some small hail. The line is likely to move across Connecticut between 5:00 PM and 8:00 PM Wednesday evening likely causing some localized damaging wind gusts on the leading edge and even potential for some small hail:

Vigorous Upper-level Low Pressure to Bring Accumulating Snow to Parts of Connecticut Overnight 04/15/21 to 04/16/2021

 A vigorous upper-level low pressure system across New England prompts the developments and strengthening of a surface low pressure right in the vicinity of Connecticut moving through Thursday night. If the processes involved with this system occurred say another 50-75 miles farther south and east..at least the northern part of the state would be looking at a pretty significant winter storm, however, looks like the most significant snows occur across parts of western Massachusetts, southern Vermont, and perhaps around the Worcester Hills in east-central Massachusetts. 

As of Thursday afternoon moderate-to-heavy rain has overspread much of Connecticut. As we move into the overnight and the low pressure system really gets its act together, anomalous cold will transpires within the lowest levels of the atmosphere. In fact, surface temperatures look to even dip as low as the mid-30's and even colder across the hills. As this occurs rain begins to transition to snow across the hills and even parts of northern Connecticut. In fact, the majority of the state could even some some snow mix in with the rain. 

This is a highly complex setup due to the features and processes involved, however, forecast models have been zeroing in on a solution which features a band of heavy snow which becomes established across southern Vermont through the Berkshires of western Massachusetts even extending into extreme northwestern Connecticut. This would result in a several-hour period of moderate-to-heavy snow overnight. Snowfall rates could approach 2'' per hour so despite the warm ground, these rates, combined with it being overnight would result in snow accumulating, though taking a bit longer on paved surfaces. Another band of moderate-to-heavy snow looks to originate across the Worcester Hills of east-central Massachusetts with this band pivoting and clipping parts of northeast Connecticut Friday. 

Given this snow will be wet in nature and very heavy, the potential exists for downed tree limbs and power lines. In fact, scattered-to-numerous power outages could be expected in areas which receive greater than 4'' of snow...power outages could even be on the widespread side. 

This is a very dynamic storm and add in the time of year and you get a very challenging forecast which is ultimately going to come down to nowcasting...meaning we won't really know how the situation unfolds until the processes involved get going. 

What to Expect:

• Moderate-to-heavy rain persists through the overnight
• Around midnight, rain begins to mix with and change over to snow across the northwest and northeast hills with snow becoming moderate-to-heavy through the overnight. 
• The heaviest of the snow across the northwest hills winds down between 6:00 - 8:00 AM Friday morning, however, moderate-to-heavy snow may persist through much of the morning across parts of northeast Connecticut. 
• The majority of the state may see some snow mix in, however, most people will probably be counting cows when this happens. 
• Gusty and cold Friday with temperatures only into the lower 40's with a stiff northwest wind gusting 20-25 mph.

Below is what I am currently expecting in terms of snowfall accumulations: