I didn't have much of a meteorological offering here today, as the discussion linked from the weather summary link at the top of this page (the discussion that appears on the NECN blog) was a 10 minute video discussion, which pretty much covered all of my thoughts on the situation for this weekend.
One theme was banding of precipitation through the weekend - rainfall that comes and goes, but falls heavily when it comes down. Of course, the intensity is a result of the tropically loading airmass with a persistent southeast wind and the deep tropical tap. The banding comes as the composition of the atmosphere changes on a horizontal plane - that is, we began the evolution with cool and dry air in place. Advecting (moving) a warm and moist airmass in from the tropics and off of the Atlantic resulted in a clash of airmasses over New England, and this clash resulted in a shield of rain that was solid and persistent. With time, a series of frontal boundaries became laid out across New England at varying altitudes. The surface front has been draped across Southern New England, while the front at 925 mb and 850 mb has lifted farther north, and this results in multiple areas of temperature differences from south to north, or more accurately, from southeast to northwest.
Though a northeast wind persists at the surface, north of the surface front, a southeast and southerly flow is taking hold a few thousand feet off of the ground. Of course, with a tap to tropical moisture, these southerly winds are carrying increasingly warm and humid air into New England. Not only is this warm and humid air naturally buoyant (remember, warm and moist air rises!), but as it encounters the frontal boundaries at various locations and altitudes across New England, that warm and moist air is forced upward, causing enhanced clouds and therefore enhanced bands of rainfall. This phenomenon, coupled with repetitive surges of increased warmth and moisture, results in the banded nature of precipitation.
This can be a frustrating proposition for forecasters and for everyone else. For forecasters, the frustration comes because our bread and butter is accurately telling folks where and when rain will fall, and for how long. We simply can't do that accurately in a complex, varied and constantly evolving atmosphere like the one described here. We occasionally see such a setup in a winter storm, resulting in heavier snow bands, but even in those situations, though it's tricky, it's a bit more maneuverable because cold air is heavy and dense, and therefore, everything moves slower. With warm season airmasses, there's a great deal more liquidity to the airmass, which means the target to hit is moving much faster.
The result? "Periods of rain, heavy at times." As we get closer to the event, we can start to nail down the time and duration of breaks in the action, but even such "breaks" are more accurately lulls, as so much tropical moisture can easily breed convective showers and thunder even in between better defined bands. Of course, this is equally frustrating for folks who rely on the forecasts! In such circumstances, I'm often very honest with my viewers that trust in the forecast should be minimal outside of 24 hours, increased within that 24 hour window, further increased inside of 12 hours, and reliable within about 6-8 hours. Though that's not often the case - that such short-term solutions are necessary - when a tropically loaded atmosphere with a number of frontal boundaries exists, the scenario becomes far more volatile.
Matt
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