There's not a lot of change to the forecast to pass along today, as the synoptic setup remains the same as described yesterday - low level cool and dry air moving in at the same time middle and high altitude warmth and moisture moves in.
This differential advection assures a tricky sky cover forecast, but there's good agreement among the guidance. This means dry and cool advection from the east overnight Friday night into Saturday, with relative humidity below 850 mb decreasing, while 850 mb and up remains saturated for most of New England - especially around 700 mb where mid-level front is established. This puts Maine in sunshine but the rest of New England will be mostly cloudy with perhaps thin enough cloud cover in Northern NH for some dim sunshine. The other slight possibility for sunshine is in SW CT, under the mid level ridge axis and behind the mid-level front, but this seems unlikely given the nearby boundary layer front. Lack of shortwave means no precipitation likely for all areas.
Saturday afternoon thru Saturday night, strong vorticity maximum will spawn strong and severe thunder from Ern OH Valley into NY/PA. Tho this stays west of NewEng Sat night, seems likely it will propagate east on Sunday AM. While the lift is impressive, the convective band will probably outrun the dynamic support, and will be coming into dry and stable airmass, so weakening and loss of organization is possible. Even so, strengthened cyclonic vorticity advection ahead of the vorticity maximum is likely to renew rainfall Sunday with an increasing likelihood for a north-south oriented band of moderate to heavy rain, as the typically favorable setup of differential advection causing precipitation cranks up, especially Sunday afternoon/evening.
Originally it seemed likely this upper low would move slowly enough to keep a meandering coastal surface low reflection just off the coast of Cape Cod with periodic rain Monday and perhaps lingering into Tuesday, but the trend in 12Z runs Friday is for quicker progression with lingering rain and a cool northeast flow Monday, then improvement on Tuesday. At this point, the implied northeast flow into Monday suggests we shouldn't be planning on any significant temperature rises.
The overall pattern of Eastern US troffing is likely to hold the first half of May, as the ridge upstream over the Western US shows no signs of diminishing, and the wavelength is unlikely to change across the Lower 48.
Matt
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