Synopsis:
A broad trough centered over eastern Canada will keep the heights low for much of the forecasting period. A disturbance moving across the middle of the nation will tap in some Gulf moisture and give us a chance of snow showers starting Friday evening. The light snow will continue into Saturday, gradually tapering off later in the night. Clouds will linger into Sunday, with northwestern parts of the state receiving some lake effect snows. A high pressure building in from the south will end any remaining snow showers in the region on Monday. Clouds and snow showers will move in once again as another weak disturbance propagates northeastward from the lower Midwest.
Short-term (Friday to Saturday):
As mentioned in the last Friday's forecast discussion, a polar vortex has set up over eastern Canada this week. However, the scenario where a southern stream phases with one of shortwaves rotating down from central Canada does not look likely, at least for now. The polar vortex has set up too far north and east due to the lack of blocking over Greenland. The southern cutoff upper-level low has stalled in Baja California. As result, a full phasing of subtropical and polar jetstreams look unlikely.... However, the last few runs of GFS and SREF models have recognized some additional energy crossing the Rockies might slightly amplify the upper-level trough dipping down from central Canada. Leeward trough located just east of Sierra Madre Oriental also provides some extra moisture flowing up from the Gulf of Mexico. As result, troughs may phase sooner than expected and its associated surface low may form a little closer to the east coast once the upper level trough reaches the east coast. As for now, only expect a period of light snow starting Friday evening as the surface low begins to take shape over the Tennessee Valley. The snow will continue into Saturday as the surface low moves offshore and intensifies. The general accumulation across Pennsylvania should range from 1-3 inches, with the heaviest amount in southeastern portions of the state. Closer to the coast, snow amounts could range from 4-8 inches in Massachusetts. It would be interesting to see if this westward trend continues on Friday. If the northern upper-level trough becomes a little sharper than expected, then a S.E.C.S cannot be ruled out over the weekend.
Mid-term (Sunday to Monday):
As the storm departs to the northeast, the strong northwesterly flows behind the system will provide us a breezy and cloudy weekend. With cold winds advecting over the abnormally warm Lake Erie waters, snow squalls will set up throughout Sunday. Areas west of the Appalachians may see some decent snow amounts, especially areas in the downwind trajectory off Lake Erie. Elsewhere around the state, some snow showers may make their way into the valleys of central Pennsylvania, while the eastern parts of the state should see a few passing clouds on Sunday. A high pressure ridge building from the lower Midwest will gradually shut the lake-effect snow machine off on Monday as the winds shifts to southwesterly. Heights will slowly build on Monday to break up the lingering clouds.
Long-term (Tuesday and beyond):
A shortwave crashing into the Pacific Northwest over the weekend will make its way across the Rockies. Interactions between a broad 500-mb long wave and this piece of energy will spun a weak surface low pressure over the lower Midwest. This disturbance is forecast to slowly propagate eastward, spreading a wide area of precipitation northward. The precipitation should stay on the light side. However, there is a large model spread now because the event is still too far into the future. The system might get more amplified then expected, but if it does so, warmer air will flow into the region to make the precipitation type mixed. The trough may also not amplify at all to keep the region free of any precipitation.
David
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