Thursday will be the third and final day of the heat wave for most in New England as a cold front approaches. This front will push into northern New England before very slowly moving southward. This will allow another hot day across New England today with highs well into the 80s for the northern tier and well into the 90s once again for central and southern New England. Heat index values will again range from 97-107° today.
As for the thunderstorm threat today, it will likely be similar to Wednesday with a similar environment in place. The big difference is that today there is a cold front and shortwave energy in the vicinity of New England, which will provide additional forcing. This will allow scattered to widespread thunderstorms to break out once again across northern New England with generally less activity in southern New England. With that said, everywhere in New England will have a chance for a storm with the exception of northern Maine.
GFS showing potential weather late this afternoon/early this evening:
Also like yesterday, there will be a chance for strong to severe thunderstorms. Any storm that forms will have a chance to become severe with damaging winds being the greatest threat. Large hail will also likely occur in storms, but given the freezing level is around 15,000 feet, it won't be widespread. With that said, should a severe storm develop with strong updrafts, hail would become a larger concern. Severe storms will be most likely across central New England (southern New Hampshire, Vermont, Maine, northern Massachusetts).
As for the ingredients for severe thunderstorms, lift will be increased today with the arrival of the cold front. There will be more than enough moisture with the excess heat and humidity developing once again. Shear will be modest and on the weaker side, but it was also on the weaker side yesterday and several severe storms were able to pop up along with more widespread non-severe storms. With the increased forcing, this weaker shear likely won't hinder development too much, but it may make it harder for strong storms to last.
Instability will be present with most unstable CAPE values likely climbing to the 1,500 to 2,500 range. With the abundant moisture present, storms will also be able to produce locally torrential rainfall. A quick inch or two of rain is possible under a storm, which may lead to isolated instances of flash flooding.
Friday will officially mark the end of the heat wave for a majority of New England as the cold front slips southward. The front likely won’t clear New England on Friday as it gets held up and stalls out. Along with the front, an onshore flow will develop in the afternoon, sending cooler marine air over eastern New England.
This will likely result in a cooling trend for the afternoon across the coastal plain, with temperatures dropping into the 70s. The one exception to the cooler weather may be Connecticut and western Massachusetts, where a fourth 90+° day is possible.
Heading into the weekend, Saturday will likely be similar to Friday with plenty of clouds and scattered showers and storms. Sunday will see warmth and humidity begin to rebuild, but not nearly to the extent of this past week. A system will pass to the north of New England, bringing its fronts across New England Sunday into Monday, leading to more rounds of showers and storms.
The Weather Prediction Center has much of northern New England highlighted in the “slight” category (level 2 of 4) for excessive rainfall Sunday into Monday morning as a moisture rich atmosphere will allow for heavy downpours to develop. If the system trends farther north, this threat will be decreased.
Looking into the extended forecast, signals continue to point toward generally above average temperatures returning. It’s important to remember that above average temperatures doesn't mean a return to 100°, but warmth does look to dominate in the long run over the cooler weather we’ll see Friday afternoon and Saturday.
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