A large low pressure circulation located in Canada will rotate areas of vorticity through our region over the Memorial Day Weekend. The overall cyclonic flow and cold pool of air aloft will result in considerable instability cloudiness mixed with sunshine throughout the weekend. (Friday’s sky cover will be a good example.)
Exact timing of these disturbances can be difficult to pin down for Sunday and Monday.
Saturday will start with sunshine, but instability cloudiness will develop. A small area of vorticity moves through late Saturday afternoon, with thicker clouds and some light showers. High 73
Sunday currently looks to be the better day, with more sunshine and no precipitation. High 74
After morning sunshine, another weak disturbance moves through Monday afternoon, with increased cloudiness and the possibility of light scattered showers. High 76
Updated 8pm Friday in italics
A weak cool front has moved through on Friday and winds have shifted to the west. High pressure will nose down from the north and will be over our area on Saturday. Circulation around the high will be from the east, giving us significantly cooler conditions.
Saturday will start with some clouds, but skies will quickly clear as an easterly flow develops.
The GFS clears us out on Saturday, but the latest NAM suggests the chance of some brief showers about noontime on Saturday as the winds shift to the east. It’s beginning to look like there may be considerable cloudiness on Saturday.
The high drifts off on Sunday and low pressure attempts to move in from the west. Sunday will start sunny, but an increase in clouds is expected during the late afternoon with a chance of showers early evening. Highs near 68.
This weekend’s forecast is
above average confidence, although easterly flows sometimes results in more clouds than forecast by the models.
(Some heavy rain expected for Monday.)
The well-advertised rain storm for Saturday is upon on. Rain moves in around midnight and becomes heaviest during the morning hours on Saturday, then tapers during the afternoon hours. Depending upon the model, rainfalls of 1.25- 2.00 inches are likely, much of it falling before noon on Saturday. It will be very cool, with high temperatures about 52.
Rain ends Saturday evening.
For Sunday, a mix of sun and clouds is expected. The latest GFS has a fair amount of sunshine; the NAM has considerable cloudiness with a slight chance of brief shower late in the afternoon. Confidence in the sun-cloud mix is a bit lower than average for Sunday. High near 70.
The low pressure system that brought the heavy rains to Philadelphia this morning will gradually move northeastward, as a sharp upper air trough digs down over our area. The sharp upper trough will “pinch off” into a closed upper low pressure system by Tuesday and will be centered over the northeast. The developing closed upper low pressure system will result in very cool weather Monday into Tuesday.
Upper air lows are notoriously difficult to model, especially regarding amounts of sun vs. cloudiness and precipitation, so this forecast is a bit below average confidence.
For Saturday, the low pressure system and cyclonic flow in the upper atmosphere will result in considerable low level cloudiness, after some early morning bright spots. It will be breezy and mild; temperatures will reach a high of about 69.
A weak cold front approaches late Saturday afternoon with the possibility of light scattered showers. The cold front moves through early Sunday morning.
Following the frontal passage, Sunday will be have a mix of sun and clouds. With the cold, low pressure in the upper atmosphere, any sunshine will be “self-destructive”; heating from sunshine will result in more cloudiness due to the unstable thermal profile. Temperatures will be cool on Sunday; Highs about 60.