This will be a relatively quick update with lots going on in soon-to-develop severe weather setup. Bottom line is that New England has successfully transitioned into the warm sector, though the wind shift/warm front is, as of this writing, just north of Boston and located near Essex County. I expect this boundary to lift north into Southeastern NH or extreme Southern ME, as per NMM forecast. The result of dry air in the midlevels has been for plenty of sunshine and temps have spiked as a result. This insolation, combined with dewpoints running in the middle to upper 60s, will push CAPE values to in excess of 2000 J/kg and perhaps even as high as 2500-3000 J/kg in some spots of Northeast MA/Southern NH where the most sun has been found, and surface based lifted indices may drop as low as a highly unstable -8. This is, of course, a dangerous combination of a nearby warm front in a CAPE loaded location, and the result is a high EHI (energy helicity index) value. The EHI represents the combination of available energy for convective (thunderstorm) development, and the helicity (tendency for air to form a helix as it rises - to spin) of the atmosphere. Of course, a high number represents a combination of energy and spin that can result in sustained vigorous updrafts resulting in possible rotating supercells. The low level helicity enhancement comes with strongly veering winds in the lowest 9000 feet of atmosphere, and this was a major part of my reasoning for mentioning possible isolated tornadoes in my morning forecasts both on NECN and on the Forecast Page here on my website. The greatest threat for supercells and an isolated tornado will be in Northeast MA, Southeast NH and far Southern ME, closest to but just south of the warm front. A wave of low pressure crossing New England provides additional backing of the surface flow. The cold front marching east across New York State will slow as it moves through New England this evening and tonight, carrying thunderstorms with it from west to east and overturning a warm and muggy airmass to keep scattered thunder of diminishing intensity through the overnight.
Drier air for Friday but upper low nearby will spawn renewed convection in the North Country and the stalling cold frontal boundary over extreme southeast MA may return northwest just a bit before becoming stationary, and may spark an afternoon storm over the South Shore of MA.
That's all for now.
Matt
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