Technical Discussion
Synopsis
The storm system that brought us periods of heavy rain and thunderstorms is currently racing towards the northeast away from the Commonwealth. Expect gradually decreasing clouds Monday night. Tuesday will feature abundant sunshine for majority of the state. The temperatures will be near the seasonal values. Expect more sunshine on Wednesday, though a few clouds may mix in as a weak cold front passes late Wednesday afternoon. We could see some warmer temperatures creeping back into the region on Friday. Expect increasing clouds throughout the first half of the weekend and rain showers for the later half.
Short-term (Tuesday – Wednesday evening)
A ridge of high pressure building in behind the departing storm will bring calmer weather conditions on Tuesday. Ahead of it, the winds will be coming from the north, which will bring cooler air down south from Canada. The good new is that this fresh shot of cooler air does not look terribly cold. The high temperature for Tuesday will only fall slightly to near seasonal values. Once the high pressure slides off to our east later Tuesday evening, the winds should shift from northerly to westerly; which will allow some warmer air to enter the region. Thus, do not expect a significant drop in temperature Tuesday night. A cold front will sweep through the region Wednesday afternoon. However, this cold front will be moisture starved and will only bring a few passing clouds across the state, especially for the northern portions.
Mid-term (Thursday – Friday night)
Another high pressure will build into the region behind the cold front, bringing another shot of cooler air. The models do not suggest a large temperature gradient to follow behind the cold front. Thus, the temperatures will only fall slightly to near seasonal or just below seasonal on Thursday. Any lingering clouds associated with the cold front will clear out early in the day; give way to mostly sunny skies. However, the clouds will once be on the increase Thursday night as the high pressure loses its footing and slides offshore. Warm air advection occurring on the western side of the high pressure will help to build some high clouds in the upper level of the troposphere. A short wave passing through the Great Lakes will drag a stationary front stretching across the Midwest. We may see a brief period of shower on Friday if enough moisture is present along the stationary front. However, the main story will be the low pressure that is forecast to develop along this stationary front as we enter the weekend.
Long-term (Saturday – Sunday)
Once again, the long-term forecast looks to be highly uncertain. A region of positive vorticity advection coming off the Rockies will generate a low pressure system along the stationary front. The 500-mb trough will gradually become more amplified as cold air advection on the backside of the storm really cranks up the system. The question remains on how much this 500-mb trough will amplify. The GFS has really backed off from its original idea showing a much less amplified 500-mb trough. The latest GFS run depicts a shallower 500-mb shortwave swing around the base of a long wave over Central Canada and the surface low remains intact throughout the period. As result, a long-duration rain event that may possibly end with some snow now looks unlikely according to the GFS. If the GFS is true, then expect a mainly rain event for Saturday as the majority of precipitation will fall while the warm air is still in place. However, some of the foreign models including the Euro and Canadian are still showing the development of a rather amplified 500-mb trough. The Canadian model is on the extreme side of this spread as it amplifies the trough so much that it eventually cuts it off over the Gulf states. As result, a series of slow progressing surface lows will ride along the aforementioned stationary front, feeding in copious amount of moisture from the GOM and the Atlantic as it does so. If this solution is correct, we could see a long-duration rain event, possibly heavy at times on Saturday. As that stationary front slowly progresses eastward, the cold air on the east side of the front has the potential to change the rain to periods of snow Saturday night, especially for northwestern PA. The Euro model is less extreme on this, however, with a similar idea.
David Wang
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