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Stormy Afternoon Incoming for Parts of New England

A classic summer cold front will slice through New England today. This front will produce scattered thunderstorms this afternoon and into the evening. The main window for storms will be from 12pm-8pm, with isolated activity centralized in western areas earlier, with storms filling in and sliding eastward during the course of the afternoon. Storms will likely begin to lose strength as they push eastward, especially if storms hold off until mid to late afternoon.

HRRR showing potential weather early this afternoon (1st image) and heading toward late afternoon (2nd image):

As far as the severe threat goes, there will likely be a more narrow area with the greatest potential for severe weather. This will likely be western and central Massachusetts, northern Connecticut and southern New Hampshire/Maine. These areas have the best chance to see storms when the ingredients for severe weather (and the front itself) will be at their strongest.

Heading toward eastern Massachusetts, Rhode Island and eastern Maine, the atmosphere will likely be more stable with weaker shear, which will reduce the severe threat and allow storms to weaken.

Overall, the greatest change in the trends for severe weather was to see Maine's threat increase. This comes as Maine may see the most destabilization and shear for storms along with favorable timing of the front.

The environment is favorable for clusters of storms to develop along a line through New England. The primary threats from the severe aspect from this will be damaging straight line wind gusts as storms are expected to take a more linear motion. The tornado threat is extremely low as storm cells favor clusters rather than individual supercells.

The hail threat has somewhat increased since yesterday due to an increase in how steep lapse rates will be (a more rapid decrease in temperatures with elevation is now expected, which would help create hail). Still, the freezing altitude is around 11,000 to 12,000 feet above sea level, so large hail will have a tough time making it all the way to the ground, though smaller hail remains a possibility.

Another aspect to watch with the storms will be the potential for torrential downpours to be embedded with the storms. Overall, the threat of flooding downpours has dropped since yesterday. This comes as storms are expected to be more progressive in nature with a low training threat. As instability profiles increase slightly for hail, it has decreased slightly for torrential rainfall. With that said, there could be localized issues in poor drainage areas in places that see a stronger storm.

As far as the four main ingredients for severe weather go, there will be plenty of moisture from a deep southwest flow transporting humid southern air into New England ahead of the front. Lift will be ample in the form of the cold front. Sufficient shear will also be in place, with the strongest levels of shear likely to set up across northern and western New England. 

The question, as usual in New England, will be the level of instability. Guidance generally shows CAPE values at 1,200 or less, which doesn't exactly jump off the page when it comes to severe weather. When CAPE values hit 1,000, that is when the atmosphere is considered to be in an unstable state for severe storms. With that said, there is no exact threshold on how high (or low) CAPE values need to be for severe storms to develop as other factors come into play.

HRRR showing potential CAPE values early this evening:

The corridor of greatest instability will likely be more narrow in nature (as seen above). Increased cloud cover in the afternoon ahead of storm arrival will limit the amount of destabilization that can occur. The extent of clouds and morning/early afternoon showers will be key in how widespread and long lasting stronger storms will be. Farther east in New England, the later the storms move through, the more stable the atmosphere will be when they arrive. Any early afternoon showers would also zap some energy out of the atmosphere.

Heading into Friday night, the cold front will slow down as it gets blocked by an area of low pressure well offshore of New England. This will promote showers lingering into the night and Saturday morning. By the morning, light, scattered showers may still be lingering near the coastal plain, but these should clear out rather quickly. High pressure then builds in for the weekend with comfortable temperatures, lowering humidity and fair skies. 

As we’ve been saying for several days now, significant heat will be coming to New England next week as a heat dome looks to set up favorably to bake New England. The peak of the heat looks to occur from Tuesday through Thursday. 

The National Weather Service's New Heat-Risk map, showing significant heat possible. This map is for next Thursday (June 20):

Temperatures well into the 90s will be possible with heat index values pushing into the low 100s in favorable places. Thunderstorm chances have dropped during this time frame, but not completely out of the picture quite yet, especially for Vermont. Early indications show a cold front providing sweet relief late in the week, but timing is (naturally) very uncertain at the one week mark in the forecast.



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