A respite as the atmosphere reloads…ready to dump more heavy rain midweek

Posted by Matt Noyes July 28, 2009 at 11:26 am

The deep tropical moisture that fueled afternoon and evening thunderstorms on Monday has pushed east, allowing for a dry Tuesday in most communities. Though there are signs moisture – and a chance of precipitation – will return soon, for now we’ll enjoy a humid respite.

The thunderstorms that brought damaging wind and large hailstones late Monday to some communities died while shifting offshore Monday night, and left in their wake has been appreciably drier air aloft. At the surface and through the lowest few thousand feet of the atmosphere, a frontal boundary has stalled over North-Central New England, from Southern Vermont to Central Maine. The new, drier air several thousand feet in elevation will prohibit thunderstorm growth for most of us today, but where somewhat more substantial moisture returns to Western New England, the southern tip of the stationary front may instigate a terrain-enhanced shower or thunderstorm Tuesday afternoon in Southern Vermont or Western Massachusetts, from the Green Mountains to the Berkshires.

Though there may be drier air aloft, there’s no question that most of us have another sticky and very warm day here at ground level. The abundant moisture in the lower atmosphere resulted in morning fog for our valleys, and along south-facing coastlines, and as temperatures cool from afternoon highs in the middle and upper 80s, down to the 60s overnight, new fog is likely to develop in similar areas. This fog and any similar low-altitude fog that develops in the tropical air overnight is likely to be stubborn for South Coastal communities on Wednesday morning, and along the coast of Maine. For many other areas, fog will burn off fairly quickly, but clouds will be on the increase again as the day wears on, a product of increasing moisture in advance of a juicy disturbance ejecting northeast from the Mississippi River Valley. That disturbance has been evident on satellite and radar imagery throughout Monday night and Tuesday, producing areas of heavy rain across Louisiana, Arkansas and Mississippi, expanding northeast throughout Tuesday and Tuesday night, and heading for New England for midweek. The increasing moisture ahead of this disturbance – returning on the deep southwest wind through a thick layer of the atmosphere – will not only bring thickening Wednesday clouds, but also increasing coverage of thunderstorms through the day, really cranking up Wednesday afternoon and then depositing areas of heavy rain and thunder overnight Wednesday night as the heart of the disturbance moves through New England, infused with tropical moisture. Total rainfall amounts between Wednesday afternoon and night, into Thursday morning when the rail will end from southwest to northeast, should total 2″-3″ in a west-southwest to east-northeast oriented band, with some locally higher amounts. Not coincidentally, this heavy axis of precipitation very closely reflects the location of today’s stalled front across Central New England. Exact placement of this front on Wednesday will determine exact location of the heavy rain band, but localized flooding certainly is a possibility where heaviest rainfall amounts are observed.

Thursday’s early rain will subside, giving way to some emerging sunshine, though the lingering frontal boundary and abundant tropical moisture will breed a few new afternoon storms. This front is likely to be stubborn on Friday, with a few upper level disturbances still moving over New England, caught in the jet stream winds aloft that will dip into a “trough” over the Great Lakes, and that leaves a moist and active southwest wind aloft over the Northeastern United States, bringing a combination of energy and moisture for scattered storms again on Friday. There are signs, however, that some less humid air will not only bring more comfort on Saturday, but also the likelihood of a dry day for the start of the Pan-Mass Challenge.