Convection (vertical development of clouds) has been taking off this afternoon beneath the cold pool associated with the upper level storm that's parked along the US/Canadian border. This upper level low will be the main feature through the forecast period as it meanders across Eastern Canada and flexes its muscle in the skies of New England from time to time. One flex came with a shortwave pinwheeling around the center of the low Sunday afternoon and evening with an area of rain and an associated surface cold front, while the remaining shortwaves are mostly driving surface troughs rather than fronts. One such trough is crossing New England and helping to focus convection this afternoon that already was destined to develop in favorable steep low and mid-level lapse rates and with the aid of dynamic forcing owing to the midday shortwave, not to mention orographic assistance in the favored terrain with the fast westerly flow. The combination of a dry adiabatic lapse rate in the lower levels and a downsloping wind continues to propel wind speeds higher until the pressure gradient slackens as the trough pulls away, then as the diurnal mixing dies down, that will also help to quiet the wind somewhat.
Nonetheless, Monday night brings a rather active breeze, which will help to keep temperatures above the dewpoints that sit in the 30s and 20s, though some valleys in Northern and Western New England that get a lighter breeze will drop into the 30s, as will somewhat elevated terrain, thanks to the steep lapse rate. Tuesday actually brings a break without any discernible shortwave here in New England, so even though it's still cold aloft, the dry slot evident on the north and northwest side of the cyclone today should also rotate overhead, and I'm thinking most spots escape with a dry day. There will be a dampened, flattened, progressive shortwave moving east-northeast out of the Ohio Valley later Tuesday, though it looks as though the cyclonic vorticity advection would stay south of New England, for the most part, meaning only the South Coast may get a graze with some showers Tuesday evening, but even that seems to be a rather limited possibility.
Both Wednesday and Thursday feature a morphing of the upper low from an elongated cyclone, stretched west to east over Eastern Canada, to a more round and symmetric cyclone that settles over Northern New England both days. The combination of the cold pool overhead and the proximity of shortwaves rotating like spokes on a wheel around this low will mean scattered convection comes back into the forecast both days, and probably one more shot of reinforcing cool air.
By Friday, the upper low is finally ready to start lifting out. Strong anticyclonic vorticity advection in the morning should preclude any precipitation early, and though a weak vorticity maximum rotates around the back of the departing low in the afternoon, and does come through during a time of warm advection (implying at least cirrus clouds are likely, if not a more substantial mid-level deck), right now it looks like enough dry air and a downsloping flow would be in place to keep any showers quite limited, and for now I'll keep the forecast dry. As ridging slowly but surely replaces the upper level low, expect fair weather to move in just in time for the Memorial Day weekend! Temperatures will rise rather quickly aloft, with the deep southwestern US heat (as of this writing, SW US temps are heading for 110 daytime highs) filtering north and northeast with the ridge, but at the surface, the center of high pressure will move to our west, then our southwest, meaning a northerly flow prevails. This may drive fire danger up as it implies a dry and increasingly mild (though not hot) flow as warmth wraps out of the Western US, into Central and Eastern Canada, then southward into New England.
Matt
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