We have the threat of severe thunderstorms firing across Northern New England this afternoon and evening that could produce heavy rain, hurricane-force winds, and medium-sized hail, with an embedded supercell or two potentially spinning up an isolated tornado. A rare enhanced severe storm threat has been issued for the northern tier of Vermont, New Hampshire, and Maine.
The thunderstorms will peter out before reaching Massachusetts and Greater Boston, so there’s no concern for the Bay State into the night.
There will be plenty of instability in the atmosphere to produce multiple severe storms up north, thanks to a potent cold front sliding in from Canada and forcing the hot, humid air up to form these storms.
Meteorologists use all sorts of different variables to assess the potential for such severe weather, and one of them is the “convective available potential energy,” or CAPE for short. Think of this like the energy that’s being built up when you put the lid on a boiling pot of pasta. Eventually, the energy builds up enough that it spills over. In the case of thunderstorms, eventually the energy builds up enough that instead of spilling over, it rapidly rises, creating big thunderstorms.
Today is very interesting because there are other factors that may greatly diminish or negate the formation of any big storms.
This morning, we’ve seen a mesoscale convective system tracking along the top of a ridge of high pressure down through Northern New England from Quebec. This system is an organized cluster of storms that feed off of each other and can last several hours. Sometimes these MCS’s can act like opening that lid on the boiling pot of water, just briefly, and releasing some of the energy. Another way to look at it is that the morning thunderstorms and showers steal some of that energy from the afternoon. When this happens, it can occasionally — but not always — prevent the most intense storms from developing in the afternoon or evening.
To complicate matters further, the cloud cover from the morning showers could linger long enough to prevent much heating from occurring. So that’s like turning down your stove a little bit so the water doesn’t boil over.
Other factors: Wildfire smoke, maximum heating
The other interesting part of this is the wildfire smoke today. You might actually notice the hazy sky this morning. The smoke can act like a bigger lid on the pot and also prevent the energy from being released, and basically prevent the sun’s heating from reaching the surface. So you have two factors: one, perhaps releasing the energy early; the other, perhaps not allowing the energy to release into the evening.
There’s also the fact that the greatest instability in the atmosphere is likely to occur after maximum heating this afternoon — one of those quirks where the hottest hour of the day isn’t necessarily the most volatile one, since instability depends as much on what’s happening in the atmosphere aloft as it does on the sun. And while it is humid, it’s not the intense humidity or heat that we saw at the beginning of July and the end of June.
The bottom line here is that while we have upper-level support for thunderstorms, some of which could become severe today, we also have other factors working against severe weather. All this is to say: if you’re wondering why sometimes you see a chance of showers and thunderstorms, and you think, “Why can’t they be more definitive?” It’s because the atmosphere is a very complicated neighborhood, and knowing which neighbor is going to be loudest certainly isn’t always predictable.
Stay tuned!
