It’s not January, but the current agony of a will it/won’t it line feels awfully similar to a winter storm. Of course, a cozy snow weekend day has lower implications than the potential for flash flooding on a busy summer Saturday. And that’s the type of setup we have making an uncomfortably close pass to southern New England to start the weekend. No one likes an ‘uncertain’ forecast or wants to hear about ‘probabilities.’ We all want a deterministic yes or no answer when it comes to the weather. In some patterns, that’s easy to do. In others, it just can’t be done. Here we weigh some options and talk about the most likely scenario for how this one will pan out.
Let’s start with the things that are locked in. This is an unusual setup. A dramatically deep (and possibly record setting) upper level trough will dig in across the Mid-Atlantic, spawning a storm system that’s reminiscent of cold season more than midsummer. The difference between this happening in July instead of November is that there is a TON of atmospheric moisture to work with. We look at PWAT values (precipitable water in the column) to decipher where these tropical atmospheres lurk. And when it comes to Friday and Saturday, there are some obscene levels of water vapor to work with. What does that mean? Torrential, pounding rainfall. The storm track isn’t a difference between showers and steady rain. It’s the difference between nothing and flooding. Simple as that.
The most likely location for high-impact flash flooding and some tremendous rainfall totals looks to target the Mid-Atlantic. Here we have all the ingredients. Upper level divergence, a high PWAT air mass, a deep anomalous trough, and a potent shortwave swinging in to an existing active boundary. That’s a lot of meteorological mumbo-jumbo for ‘it’s gonna rain.’ Areas of Virginia, New Jersey, Maryland, and Pennsylvania seem at highest risk for these high-end totals.
Then we get to the New England side of the equation. We have high pressure nosing down at the surface from the northwest and quite a bit of dry air aloft as an upper-level ridge tries to collapse down over the top of the region. At the same time, this potent axis of heavy rain is trying to make a move north. And we’re on the line.
Model guidance has been all over the place, as if often is in these scenarios. Computer models are notoriously bad when there is a lot of convection in a storm system. These elements can wreak havoc on the expected rain amounts, the location of rainfall, etc. So we have to rely on pattern recognition and observations to figure out how things will go down.
At this time, odds for any steady rainfall favor the immediate South Coast. I don’t think the shield of heavy rainfall will be able to get up into the Boston area, and certainly not north of Boston. The heaviest rain of all should be right near/south of Nantucket on Saturday…and it may be a game-time nowcast to see whether it can make it up across the Islands/Cape or will dip south on us at the last minute. The range is literally showers to 5″ on Nantucket…so it would be a wise to keep a close eye on this one! I think we could get a half inch to possibly 1.5″ on the Cape, with dramatically lower rain totals once you head northward from there. Anyone north of the Pike is looking mainly dry or maybe a couple light showers. New Hampshire, Vermont, and Maine will enjoy a great weekend of dry weather and comfortable temperatures!
Will I be shocked if the entire rain shield misses south? No. I know you’re thinking ‘oh the dumb weather guy can’t come up with a forecast’ here but frankly that’s just how it is with these types of systems and we have to eat some humble pie every now and then. On Friday, we should hopefully be able to lock down the final outcome. But I wouldn’t bail on Saturday plans yet. If the low cuts off and drifts just a little farther south we’ll just be left with a cool and gusty day. It’s a matter of just a few miles.
Regardless of rainfall, northeast winds will keep temperatures below average this weekend. Highs won’t make it past the 60s to low 70s on Saturday, with highs ranging from the low to upper 70s from southeast to northwest on Sunday. Wind gusts will peak 20-45mph on Saturday, strongest across Cape Cod and the Islands…and drop about 10mph on Sunday from there.
This gyre of low pressure should drift far enough south to get out of our hair on Sunday, providing another cool-ish but dry day for the second half of the weekend. Again, the theme will be brighter skies and warmer temperatures the farther north/west you go. For those looking for a beach day, it should be doable. Just don’t expect it to be a summer sizzler. Maybe a benefit of gusty winds will be fewer greenheads on the beach?
It looks like this northeast flow will continue as a trailing wave of low pressure wobbles around near the coast Monday and Tuesday. If it can float back close enough our way, we could get another round of clouds and showers late Monday into Tuesday. That’s still a little TBD in this uncertain pattern. At the very least, temperatures stay cooler than average for Monday and Tuesday and the seas will stay quite rough, perhaps even leading to some beach erosion issues with multiple days of NE winds.