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.
Sat 03:45 PM Forecast Review — The shower coverage is not what was forecast. Why? The position of the current mid level low is not where it was forecast to be and the strength of the jet streak is less than forecast.
GFS forecasts comparing yesterday and today differ significantly in the in the position of the mid-level low and the strength of the jet streak (Click on image for a larger view.)
Water Vapor Image showing actual position of mid-level low. Much closer to this morning’s GFS model prediction, not very close to yesterday’s GFS model prediction. (Click on image for a larger view.)
Here’s current radar with water vapor image
Current radar with water vapor. The showers are near Philadelphia, but much of our area is not seeing showers right now.
The weather this weekend will be interesting from a basic meteorological viewpoint; several basic weather phenomena will be in motion, including “horizontal wind convergence” and “jet streak exit region” vertical motion.
The bottom line will be unstable, shower-like precipitation with considerable clouds on Saturday (and maybe some breaks of sun). More rain Sunday afternoon with an old fashioned low pressure system.
To start, on Saturday, an area of horizontal moisture convergence will result in rain showers very early Saturday morning. (Winds from the east will run directly into winds from the west. Where they meet, the air has nowhere to go but up, resulting in precipitation.)
Horizontal Moisture convergence zone (red shading) where easterly winds run into westerly winds. This phenomenon is not always this easy to see. (Click on image for a larger view.)
This area of low level convergence will lift off to our northeast after daybreak and Saturday mid morning will become drier.
Friday night’s HRRR has sunshine Saturday morning before the showers develop.
Additional convergence develops in the afternoon along with a negatively tilted upper air trough (1), a very strongjet streak (2) just to our southwest resulting in our area being in the what’s called the “right exit region (3) with vertical lift”.
Negatively tilted upper trough (1) (blue lines), strong jet streak at 145+ mph (2), right exit region (3) The right exit region of a jet streak has considerable vertical motion upward. This jet streak is expected to lift towards us during Saturday afternoon. (Click on image for a larger view.)
From comet.ucar.edu Shows the right exit region of jet streak with vertical motion. Corresponds to area #3 in the map above.
By 3 PM, we’ll be sitting under a strong closed mid-level closed low. Showers will develop under this low. With weak winds, any showers may be slow movers.
500 mb closed mid level low superimposed on jet streak winds. Saturday 3 PM Note that most of the action doesn’t occur right under the jet streak….it occurs in the exit and entrance regions.
Ok, enough! What about the forecast for Saturday and Sunday?
Showers tapering off early, about 8 AM or so. Mostly cloudy mid to late morning with this break in the showers. Periods of sun and clouds possible by late morning and early afternoon.
Any sun will be “self-destructive sunshine” and lead to showers.
Plenty of dark clouds will develop. Impossible to time, but showers develop about noon and become widespread by 2 -5 PM. The showers will develop in-place and become numerous. High 58.2º sd 2.0º Blue Bell (NBM)
Tonight’s models show the showers to have somewhat less coverage than the map below.
HIRESW-ARW-MEM2 Model captures the situation for Saturday afternoon. Simulated Radar with low/middle level clouds at 4 PM (Click on image for a larger view.)
Showers taper off early Saturday evening.
Weak high pressure builds in for Sunday morning. Some mid-level clouds will lower and thicken by around noontime. An advancing low pressure system in the Midwest will quickly move in with clouds late morning and even some showers by 2 -4 PM according to the latest HRRR. High 61.5 º sd 1.6º NBM Blue Bell.
Friday night’s HREF shows most of the shower activity on Sunday afternoon moving off to our north. A nicer day is possible than previously forecast.
Winds
NBM Wind meteogram Blue Bell PA (Click on image for a larger view.)
