An area of low pressure will pass to New England’s north on Wednesday. This will drag a warm front through the region, putting New England into the warm sector. Within the warm sector, scattered showers and thunderstorms will develop across the region. It won’t rain all day anywhere, but storms will be on and off through the day, especially in the afternoon and evening.
Showers and storms will slowly begin to develop throughout the morning and lift in a northeast direction. As the morning goes and transitions to the early afternoon, storms will become more widespread across New England. A secondary round of thunderstorms will try to develop later in the afternoon; these storms will most likely be focused across western New England (the Berkshires, Vermont and northern New Hampshire).
HRRR showing potential weather around midday (1st image) and late this afternoon (2nd image):
Storms are unlikely to become very organized. They will be scattered around, seemingly randomly, throughout the day. This is due to the fact that the environment is weakly forced, so there isn't a strong force for storms to organize along, such as a strong frontal boundary.
While severe weather isn't favored in this setup, it is ripe for torrential rainfall within this heavily moisture-rich environment. PWAT values (a measure of water vapor in the atmosphere that could turn into precipitation) will be in the 1.5-2+" range. This indicates the potential for heavy rainfall to develop within storms.
Unfortunately, Vermont and northern New Hampshire are the most likely areas to see storm training (when multiple storms move over the same area) develop with the secondary round later in the afternoon. This, along with the already heavily saturated soils from Tuesday morning, has prompted a flood watch for most of Vermont and northern New Hampshire.
With an environment highly favored for torrential downpours at times, any storm will be capable of producing a quick 1-2 inches of rain. With the scattered nature of the thunderstorms, rainfall amounts will be HIGHLY varied across New England today. There will be locally much higher and lower amounts throughout the region. The big question is where the storms end up setting up and what areas end up with multiple storms.
The threshold for flash flooding to occur is around 1.5 to 2 inches in an hour for Vermont, northern New Hampshire and much of Maine, however, the Northeast Kingdom is as low as 1 to 1.5 inches. Three hour flash flood thresholds are currently in the 2 to 3 inch range for most of New England, but, again, the Northeast Kingdom is lower, around 1.5 to 2 inches. Overall, any flash flooding today is more likely to be isolated given the scattered nature of the storms.
The threat of severe weather is low today. Looking at the ingredients for severe weather, there's more than enough moisture in the air to get storms off the ground, lift and shear are both weak. With clouds dominating the skies today, instability won't be off the charts, with CAPE values (a measure of instability) being around 1,000 to 1,500. Overall, the setup is much more favorable for heavy rainfall rather than severe weather (strong winds & hail).
The system's weak cold front will cross New England this evening into Thursday. This will help wind down the storms heading into the overnight hours. With the weakening nature of the cold front, it won't lead to much of an air mass change, with both temperatures and humidity remaining elevated through the rest of the week.
Thursday and Friday will begin to dry out before another frontal system arrives for the weekend, bringing more widespread thunderstorm activity back into the picture.
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