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.