FIRST ALERT: Extreme Heat, Severe Thunderstorm Potential Wednesday

A First Alert has been declared for Wednesday, Friday, Saturday and Sunday of this week

What to Know

  • A First Alert is in effect for Wednesday for mid-90s heat, potentially severe, damaging thunderstorms and maybe even a possible tornado
  • A First Alert has also been declared for Friday, Saturday and Sunday for dangerously high temperatures that could approach 100 degrees
  • It's likely most of New England experiences a heat wave (three straight days of 90-plus temperatures) sometime between Friday and Monday

When you see a First Alert stamp on our exclusive NBC10 Boston First Alert 10-day forecast, that’s an indication our First Alert Weather Team thinks we should all have our antennae up for an especially impactful weather day.

In a somewhat rare setup, we have a total of four First Alert stamps on Tuesday’s 10-day forecast. After a delightful Tuesday of summer warmth and continued relatively low humidity, the now diffuse remnant of once-Hurricane Barry is drawing slowly closer to New England, and this will bring vast changes to our weather.


Humidity continues increasing slowly but steadily at our South Coast Tuesday afternoon into evening, and as the evening wears on, that tropical air will continue migrating north across New England. When dew point temperatures, used to measure the amount of moisture in the air, reach 70 degrees, the air feels tropical, and we expect that milestone to be eclipsed overnight Tuesday night, making for an uncomfortable sleeping night and leading to a sultry Wednesday.

Morning showers will already be ongoing Wednesday in northern New England, where off and on showers, downpours and thunder will persist through the day. In central and southern New England, an isolated shower is possible morning through midday, but for the most part we’ll find sun through clouds, raising high temperatures to around 90 with heat index values – the combination of heat and humidity for a "feels like" temperature – reaching the middle 90s.

This, in and of itself, would be sufficient for a First Alert declaration, as these heat index values take a toll on the body, but the weather situation turns more impactful Wednesday afternoon ahead of a slow moving cold front interacting with the tropical moisture associated with Barry’s remnant, resulting in erupting thunderstorms Wednesday afternoon and evening.

In central and especially southern New England – including Boston, Manchester, Worcester, Springfield, Hartford and Providence all the way to the South Coast and Cape – thunderstorms may grow severe and damaging in spots, not only bringing torrential rain fueled by the abundance of humid air, but also producing damaging winds. Although not a high probability, a tornado can’t be ruled out in southern New England.

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While damaging thunderstorms will weaken Wednesday night, showers and downpours will continue, off and on, into Thursday morning, particularly south of the Massachusetts Turnpike, where Thursday may dawn with some lingering downpours before less humid, cooler air pushes in, actually bringing delightful air and sun to the Northern half of New England Thursday.

Shifting winds are expected to return heat to New England starting Friday and lasting through the weekend. Though the abundance of hot and lack of hot/cool clash means the chance of storms is fairly low this weekend outside of perhaps an isolated Green Mountain thunderstorm, heat will take center-stage as temperatures climb to around 90 Friday, provided early clouds clear as currently anticipated, with temperatures certain to climb well into the 90s Saturday, even making a run at the century mark. Combined with humidity, the heat index seems quite certain to surpass 100 degrees Saturday afternoon, making hydration, relief from the heat and avoiding overexertion all critical parts of weekend plans.

Although the heat may not be quite as searing on Sunday, we still expect temperatures to surpass 90 degrees, with heat index values into the middle 90s. Monday is likely to bring a slow cold front with developing thunderstorms, so it’s tough to say exactly how hot we get before the new air arrives, but another 90 degree day isn’t out of the question.

Of course, three consecutive days of 90 degrees or hotter qualifies as a heat wave, so it’s likely most of New England experiences a heat wave somewhere between Friday and Monday.

Cooler but still summery air is expected my midweek in our exclusive First Alert 10-day Forecast.

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