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Breaking Down Friday's Thunderstorm Threat for New England

After a beautiful and dry summer day on Thursday, unsettled weather will return for one day on Friday in the form of thunderstorms. These storms will break out along a cold front that will be dragged across the region throughout the day. The front will strengthen during the day as it pushes southeastward. The front may also slow down as we head toward the late afternoon as the system that doused Florida moves up the southeast United States coast.

Showers and storms will likely break out ahead of the front's arrival in the morning across northern Vermont, New Hampshire and Maine. This morning arrival reduces the severe threat significantly for these areas as instability will be lacking in the morning.

HRRR showing potential weather around mid-morning Friday:

Heading toward midday, more isolated storms will likely begin to fire up across central and southern New England, mainly in areas farther west. The storms will likely begin to fill in with a more widespread line taking shape later in the afternoon and potentially into the evening.

With the front slowing down as it pushes southeastward, the storms may begin to lose strength as they push closer to the coast, possibly transitioning to more in the way of downpours. Storms may also try to reignite farther north in New England, but this will be dependent on cloud cover, which may be stubborn after the morning round.

HRRR showing potential weather around midday Friday (1st image) and Friday 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 possibly southern New Hampshire. These areas have the best chance to see storms in the early to mid-afternoon, when the ingredients for severe weather (and the front itself) will be at their strongest.

As stated before, the storms may begin to slow down heading into the evening. Storms need to maintain a brisk forward motion to maintain strength. When that forward motion is lost, the storms can "rain themselves out" and weaken. Just how far east stronger storms can push will be something to watch during the day.

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. Shear will be strongest north and west. The shear present is supportive of a more organized line of storms developing with widely scattered storms ahead of the line.

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.

HRDPS showing potential CAPE values Friday afternoon:

The primary threats from the severe aspect will be damaging straight line wind gusts amid a more linear storm motion. The large hail threat is rather low at this time with meager lapse rates (the rate of temperature change with height, a steep lapse rate can result in a rapid decrease of temperature with height, promoting hail). With that said, small hail (pea to nickel sized) is always a possibility in storms.

With a moisture-rich atmosphere, there will be the potential for torrential downpours to accompany storms. While storms generally look to be progressive, especially earlier in the afternoon when the storms will likely be their strongest, rainfall rates may become heavy enough to produce localized ponding in poor drainage areas. The Weather Prediction Center does have southern Vermont, New Hampshire, Maine, much of Massachusetts and northern Connecticut in the "marginal" zone (level 1 of 4) for excessive rainfall.

With the front slowing down, showers may linger for Friday night and into Saturday morning, especially across the Massachusetts South Shore. Overall, the weekend still looks to be beautiful, with comfortable temperatures and humidity on the downward trend. The weekend weather will be temporary as significant heat still looks to build for next week.

There’s still a question about whether or not unsettled weather may ride over the edge of the ridge for New England, which would help damper heat somewhat, but a portion of New England is looking increasingly likely to see their first heat wave of the season. The heat currently looks to peak in the Wednesday-Thursday time frame.



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