Monthly Archives: July 2021

Philadelphia Weather Outlook

ON THE COOLER SIDE OF THINGS

While extreme heat will continue to bake the middle of the country, the Philadelphia area will trend towards cooler temperatures by the end of the week. (The pattern that will set up is similar to the less humid, pleasant weather that we experienced last Thursday through Saturday.)

The dip in the position of the jet stream can readily be seen on the GEFS model position of winds at about 31,000 feet (250 mb). The red arrows overlay the ‘jet streaks’, areas of higher winds within the overall flow—

GEFS 250 mb forecast for Saturday at 5 AM. Notice the position of the jet streaks (red arrows), areas of enhanced winds.

The position of the jet stream is important, but equally important is the location of the jet streaks.

Areas on either end of the jet streaks are referred to as the “entrance and exit regions”.

Jet streak showing entrance and exit regions with different upward/downward motions depending on the specific side of the jet.

On the side of these regions you will find rising or falling vertical motions, causing rain, clouds, or nice weather. While the winds in the jet stream move in excess of 100 mph, the jet streak areas move along at a much slower speed.

Next Saturday looks to be very nice. We’ll be in the right exit region of the jet streak, with downward vertical motion.

Next Sunday, we’ll be in the right entrance region of the jet streak. A surface trough will develop and will move through next Sunday evening with possible showers/thunderstorms.

Weather Updates

SUNDAY FORECAST UPDATE

Updated Sun 7:25 AM highlighted below:

This update is really a cut and paste from my original forecast from Friday.

A weak warm front will move in before daybreak on Sunday. Some scattered showers before daybreak. Clouds early will break for sunshine by afternoon. 

As a result of this warm front, Sunday will have noticeably higher humidity (dew points) than Saturday. Dew points will move back into the uncomfortable 70º range.

Today’s models have the weak front moving through earlier on Sunday afternoon than previously forecast. Latest models have scattered showers and thunderstorms developing in western suburbs as early as 2 PM. High temp tomorrow 89.4º 88.3º± 1.2º Dew points uncomfortable, near 70º

Most models have the activity diminishing as it moves into NJ.

More rain later Sunday night, after midnight.

Meteogram timing still on-track—

NBM precip forecast for Sunday, Blue Bell, PA (The peak probability is 2-6 PM and is not that impressive. ) Click on image for a larger view.
Canadian GEM regional model forecast 2 PM Sunday (Click on image for a larger view.)

Water Vapor Imagery and Advancing Warm front

I love looking at the clouds and seeing what they tell about the model weather forecasts. Today’s approaching warm front is visible on water vapor imagery. Latest water vapor imagery is can be seen at this link.

Unlike visible satellite imagery, the colors of water vapor imagery indicates the water vapor temperature and height. Here we see upper air disturbances spilling out ahead of the warm front just west of central PA about 2 PM Saturday.
The advancing upper air disturbances visible on water vapor to the north and west caused the development of some mixed clouds and cumulus clouds with vertical growth on the Pennsylvania side of the Delaware River at about 3 PM Saturday.