Our First Alert on Thursday is two-pronged: impactful heat and impactful storms. The heat and humidity will be felt by nearly all central, southern and western New Englanders, while the storms will be strong for some, but not necessarily as widespread as the heat.
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A southwest wind gusting to 35 mph at times and even upwards of 40 mph on Cape Cod is transporting significant humidity into much of New England that will continue to increase through the day Thursday, coupled with temperatures rising near and just over 90 degrees to create a heat index value of 95 to 100 degrees in southern New England.
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Especially in a season of limited heat – Boston has hit 91 degrees twice this summer – this will impact the body and be a bit of a shock to the system, with hydration, sunscreen and limiting exertion during the hottest hours of 10 a.m. to 5 p.m. all helpful hot weather tips.
Meanwhile, an energetic jet stream level disturbance and an associated surface-level wind shift are moving east out of the eastern Great Lakes and not only deliver scattered showers to northern and central New England through midday Thursday, but then touch of new downpours and thunderstorms Thursday afternoon and evening.
While northern New England starts in the early afternoon, most communities will find the strongest storms occurring in a window between 3 and 9 p.m., generally developing from northwest to southeast. Given the recent rain of the season, localized flooding won’t be hard to achieve in downpours – plus, some of these storms will grow strong enough to produce damaging wind and hail, fueled by the abundant heat and humidity.
In fact, closest to the center of the jet stream level disturbance – namely from extreme northern Massachusetts to western Massachusetts northward through southern Vermont, central and southern New Hampshire and southern Maine – the winds will turn enough in the lowest levels of the atmosphere to raise the risk of an isolated brief tornado.
There may be an area of lesser activity between where that concerning wind profile sets up and where a line of severe storms develops from near the Massachusetts/Connecticut border to points south, but often that type of detail is determined on the scale of just a couple of hours, which is why the best idea overall is to be prepared for the potential of damaging storms in the entire First Alert area, encompassing much of New England.
The ongoing southwest wind will create substantial chop on our coastal waters and lakes, and a gale warning has been hoisted around the Cape and Islands, along with a high surf advisory. There will be a slight yet noticeable lowering of humidity behind the storms overnight Thursday night into Friday, so Friday likely won’t be as sweltering – but will be every bit as hot, with highs into the lower and middle 90s, meaning even moderate humidity will be sufficient for heat index values at or above 95 degrees. That said, the drier air will also be present aloft, limiting the chance of a Friday afternoon storm to less than 10%.
Saturday is a different story: a strong cold front approaching from the northwest will set the stage for several downpours and thunderstorms, firing up as early as late morning but passing through, off and on, through the afternoon to early evening.
The passage of the cold front will bring much cooler and less humid air to New England Sunday with highs falling shy of 80 degrees for many, and while a fair sky is expected, a few evening showers are quite possible with a follow-up jet stream level disturbance.
Comfortable air persists through next week into the start of next weekend in our exclusive First Alert 10-day forecast, with the chance of a shower Monday owing to an upper level disturbance, and then a scattered shower or storm late in the week.
