Overview: A semi- tropical type storm in the Atlantic brought an easterly damp flow to us on Friday. That low pressure system will depart Friday night.

High pressure will build in with a southwesterly flow that will bring warm temps on Saturday, but a weak front will move through during the afternoon.

GFS 5 PM upper air wind/contours and areas of mid level vorticity (1) (concentric circles) with weak front move through on Saturday, setting off isolated light showers mostly west of the city. Most areas dry. (Click on image for a larger view.)


Mostly sunny, occasional brief periods of low clouds both in the morning and afternoon. A slight chance of isolated showers, especially north and west of Philadelphia in the mid to late afternoon. Most areas dry. Warm and somewhat humid. High temp 84.2º ± 1.3º NBM model, Blue Bell.


Sunny and and pleasant. Much less humid. High temp 80.5º ± 1.8º NBM model, Blue Bell.

The Week Ahead

Autumn arrives next week. Close to schedule, in the Wednesday to Thursday time frame, will be the arrival of a strong cold front. Some interesting weather possible with this front.

In the same time frame, the ICON model, for several model runs, has been forecasting a coastal tropical-type low pressure system moving up the coast with a sharp dip in the jet-stream. The Canadian Global GEM and to some extent, the GEFS, show a somewhat similar development, but they differ considerably in the extent of development and sharpness of the developing jet trough.

Interestingly, a derived parameter called the 1000-500 mb thickness line (specifically, the 540 dm line) (the winter-time rough determination of the rain-snow line) makes its first appearance in several months on our weather maps next week.

ICON model forecast for 8 AM Wednesday. Red line is the “1000-500mb : 540 Thickness line” (Click on image for a larger view.)