The storm that initially looked as though it would slow, delivering rain to New England, has developed too far south to bring any precipitation to the region. The clouds, on the other hand, made steady northward progress into the southern half of New England pre-dawn Monday and has held steady since then. In the boundary layer, the atmosphere moistened with the persistent onlshore flow, which brought lowering cloud bases for a time, though that trend will turn around later Monday afternoon through Monday evening as dry advection from the high pressure center over Quebec is pumped south into New England on the strong northeast flow. Of course, this northeast flow is so strong thanks to the pressure gradient between the Eastern Canada high and the low pressure center over the Mid-Atlantic. The result is a strong east-northeast wind through a marine layer, maximized overnight Monday night when 940 mb winds of 45 kts should mix down periodically as some 50 mph gusts on Cape Cod and perhaps the far South Shore. Widespread gusts have already been 30+ and should go 40+ mph in Eastern coastal MA before we're done. The dry advection in the lower levels is credited for holding off an otherwise moist flow. In fact, that dry air will really come spilling south once the storm center starts to move east and a north-northeast wind takes hold later Monday night, bringing clearing skies southward. This sets most areas up for sunshine early Tuesday, though some clouds will probably start the day on Cape Cod thanks to a continued flow across ocean water, though it's cool and dry advection by that point, so I wouldn't expect the deck to last long.
From that point on, Tuesday improves, with winds gradually diminishing as the storm center pulls away from New England. Mixing is remarkably deep on Tuesday...perhaps as high as 800 mb and this will mean a super-dry adiabatic lapse rate. Some cirrus cover is still probable behind the departing low with a lobe of strengthening vorticity that still needs to swing through with elevated dynamic lift. Nonetheless, a gorgeous day is expected and temps will rebound handily as the air that produced daytime highs in the 60s across the Canadian border on Monday comes south and couples with tremendous mixing to boost back up into the mid 60s yet again. Was tempted to go warmer given the mixing, but a northerly wind and the fact that the airmass itself is cool and dry in origin suggested that wouldn't be wise.
The storm departing over the Atlantic becomes a broad, expansive upper level low by midweek, with cyclonic flow extending from Nova Scotia and Newfoundland south to Bermuda and beyond over the Western Atlantic. This upper level trough is followed by an amplified ridge that muscles into New England on Wednesday, bringing subsidence (sinking air) and some weak return of warm advection. Of course, the best warm advection comes behind the ridge axis, so it's tough to get any deep warmth into New England on Wednesday, and a bit easier to do that over New York State. Nonetheless, gradual warming and continued sunshine seems like a good bet, with a decrease in the cirrus deck that was present on Tuesday.
By this point, it will be important to take a step back and look at the big picture. The longwave pattern from Monday to Wednesday transitions from Eastern Pacific ridge/Western US trough/Central US ridge/Eastern US trough, to amplified EPac/Western US ridge/Central US trough/Eastern US northern stream trough & southern stream flat ridge/Western Atlantic trough. The ridge that previously had been amplifying over the Northeast is being beaten down by polar energy rotating around the base of the polar vortex, and this sets up an unstable wave pattern. To make this simpler...we don't have an equal and opposite reaction for the amplified, broad, expansive ridge in the Western US and Eastern Pacific, and we need one. There needs to be some sort of wave change here, and that transition will occur from Thursday into the upcoming weekend, as the necessary deep and broad Eastern US trough develops to bring equilibrium to the pattern. This will come in stages, with multiple shortwaves exerting influence on New England - both the southern fringe of the polar vortex energy spinning through Quebec, and northern stream but farther southward displaced energy ejecting out of the base of a positively tilted Midwest trough, racing over New England.
This type of a pattern sets up timing dilemmas, without fail, because the key is now to time TWO fast moving streams of energy. Good luck! It's a fairly good bet, though, that the northernmost of these vorticity maximums will throw a cold front south out of Canada on Thursday, and though boundary layer warm advection will maximize ahead of this front, the question for Thursday temperatures at the surface will revolve around how much insolation we see...which may be limited by cirrus and altocumulus riding ahead of the front...so actually throttled back just slightly as compared to the dry warmth of Wednesday...understanding, also, that moistening a dry airmass will favor cooling it evaporatively, even if warm advection is underway.
Again, this only begins the process of dropping the heights over the Northeast and Eastern United States, and the next vorticity maximum moving out of the Midwest rides through on Friday. In my on-air broadcast this morning, I had Friday dry and decent with precipitation missing us to the south and a bubble of high pressure moving in. While that's still a possibility, as I look at this setup a bit closer, I wouldn't bet the farm on it! The problem here is that, while there is excellent agreement in the guidance of suppressing the precipitation south of New England on Friday, there's also pretty good agreement on carrying the upper level energy directly overhead. Those of you who read regularly know already which one I prefer to follow...the upper level energy. And with a west-southwest flow developing in the mid-levels, and coming from the Midwest trough that will have at least some tap to the Gulf of Mexico, I'm not comfortable with a dry forecast.
The potential for wet weather really cranks up this weekend, when there's going to have to be something that gives somewhere between Lake Huron and New England! That's a broad and silly sounding statement to make, but the fast and expansive lowering of heights with numerous strong shortwaves dumping into a carving Eastern US trough...along with a steady feed of Gulf Moisture...will have to result in a significant precip event, and I wouldn't be surprised to find a Great Lakes primary and coastal secondary as the trough carves east on Saturday, with plenty of leftover energy and cold pool aloft on Sunday. This will become clearer with time, but that seems like a reasonable evolution.
Matt
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