Aug 10, 2016 12:01 AM

NH1 First Alert Weather: Humidity returns to NH, heat to follow

NH1 Chief Meteorologist

It's back! The Humidity is returning to New Hampshire, and this time it's going to stick around awhile. In fact, this humidity will likely be the worst we have seen so far this summer. This upcoming humidity and heat will become oppressive and quite uncomfortable for those without air conditioning.

This Wednesday morning will start out with some hazy sunshine, but will fade to increasing cloud cover. A a batch of showers and storms will push from New York State into Vermont this morning but synoptic and dynamic forcing weaken quickly as the system moves eastward. Scattered showers and a few thunderstorms will be crossing the state during the late morning through early afternoon with a few downpours, and the potential for a few storms to be accompanied with some gusty wind.

Behind these showers, dewpoints will rise rapidly into the mid to upper 60s, especially cross the southern half of New Hampshire. Depending on the amount of clearing late in the day, we could see enough instability to generate some pop up showers or T'storms that could produce brief heavy downpours in the late afternoon and early evening, along with the potential for some gusty winds. Highs will be limited somewhat, to the upper 70s to low 80s given clouds and early precip in the south. Wed night should see most precip come to an end. It will be a warm and humid night with lows from the low 60s north to upper 60s south, along with the potential of patchy dense fog.

The good news is some beneficial rain now seems likely for drought-stricken areas of New England, but we must first endure another round of extreme heat and humidity. The jetstream is providing a steady westerly flow which will pump warm air from the center of the continent eastward into New England. At the surface, high pressure shifting into the Atlantic promotes a southwesterly flow transporting moisture all the way from the Gulf of Mexico up into New England. Thursday will be the hottest day with plenty of sunshine and temperatures reaching the mid 90s. Dewpoints in the mid to upper 60s and Lwr 70's will produce heat index values near or over 100 degrees in southern New Hampshire. Very important to prepare for this heat and have a plan to stay cool on Thursday.

A cold front will stay draped across the Canadian border Thursday, but sag south on Friday. This front may serve as a focus for shower and thunderstorm activity Friday afternoon as it moves toward the hot, humid, and increasingly unstable environment being set up over New England. Cannot rule out the possibility of an an isolated severe t'storm, but the bigger threat may well be heavy downpours as such abundant moisture is available. With more cloud cover over the area, daytime high temperatures should not be quite as hot as Thursday, but humidity will be higher. Some models are quite aggressive with dewpoints on Friday as well, with dewpoints climbing well into the 70's. Even with afternoon highs only in the low 90s, dewpoint values in the low to mid 70s would bring the heat index back up to 100 or more in southern New Hampshire.

Over the weekend the cold front slips slowly southward through the region, with multiple distinct waves of low pressure moving along it, each one pushing the front a little further southward. The first wave moves through central New Hampshire and Maine Friday night into Saturday, with a round of showers and isolated storms as it moves through. Temperatures north of the front will only be in the 70s or even the 60s, while south of the front, the heat and humidity will continue with highs in the low 90s. This wave sends the front south toward southern New England with the next wave tracking eastward along the front through southern New England Sunday into Sunday night. This will be the best chance of rain for the hardest-hit areas of drought in southern New Hampshire. With all the moisture available ahead of this front, the potential exists for some beneficial heavy rainfall, possibly over 1 inch.

With high pressure dropping south behind the front, through eastern Maine early next week, winds will be shifting onshore directing in low clouds and temperatures below seasonal normal for both high and low temperatures as a cool/dry air mass moves in. Storm track shifts primarily to our south by midweek, so any subsequent waves bringing more rain should mostly stay south of our area, though cannot rule out rain chances into Tuesday. Temps will be in the 70's, with even a few 60's Sunday, Monday & Tuesday. Some relief from the heat thanks to a backdoor cold front.

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