Category Archives: Weather Outlook


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.


With a complicated forecast, let’s try a different approach. I’m going to use two different model’s meteograms showing precipitation for Blue Bell, PA. (remember that a meteogram only shows the weather for one specific location; the Jersey shore’s meteogram may look much different for the same time frame.)

Both show heavy rain (1 inch) starting about 2 PM Thursday afternoon.

The ICON shows significant rain again on Saturday. The model blend does not. (Not shown here, the GFS has showers in the late afternoon Saturday.) Both show Sunday and Monday essentially rain free.

ICON model—

ICON model rain meteogram for Blue Bell, PA Is it over-doing the rain on Saturday? (The GFS only has rain in the afternoon on Saturday.) Why the ICON model? It did well with last night’s rain, while the GFS model totally over-predicted last night’s rain. (Click on image for a larger view.)

Important clarification: the meteogram times indicate prior 1 hour accumulations. If you were to take those times as literal (and you shouldn’t), it would mean that the actual rain start times could have been as much as 1 hour earlier. Accumulation also does not show light rain that might not have accumulated enough to show in the scale of the meteogram.


NBM meteogram for Blue Bell PA (Click on image for a larger view.)

So after some heavy rain Thursday and early Friday, much of the Fourth of July weekend may be relatively rain free. (Considerable clouds on Saturday with possibly some showers in the afternoon, according to the GFS.)

Sunday and Monday look much better weather-wise.

It will be cool on Saturday (high in the 70s) and still below average highs on Sunday (about 80º)


Tue 06:19 PM Update — The models are coming together to forecast a severe weather day Thursday.  It looks like we’ll have very heavy thunderstorms in the afternoon. The GFS is showing rain accumulations in the range of 2 inches.  Stay tuned.

It’s been well-advertised that Tuesday and Wednesday will be very hot and humid days, with high temperatures in the mid to upper 90s and apparent temperatures (heat indices) approaching 100.

Update: NBM has highs on Wednesday 97º ± 2º (Blue Bell) The ICON has a high of 98º. It appears it will be in the low 90s on Thursday.

The latest NBM high temperatures for Wednesday—

NBM High Temps Wednesday (Click on image for a larger view.). (This has been updated above)

Starting Wednesday night, a front will affect our area and a significant pattern change, at least for the short run, will occur.

From Wednesday night through at least Saturday, the models are in surprisingly good agreement that a cold front will slowly move through and stall just south of the Philadelphia area. Rain, possibly heavy, appears to be the current forecast for Thursday through Saturday, as waves of low pressure move up along this front and an upper cyclonic flow and an upper closed low support rainy conditions. We may clear out by Sunday. Hopefully before.

Here’s the current GEFS forecast for Friday 8 PM showing the upper low near Chicago—

GEFS forecast Friday 8 PM, showing an upper closed low in the Great Lakes and an oblong area of low pressure centers along a stalled front. (Click on image for a larger view.)

Some of the models are cranking out a total 1-2 inches of rain by Saturday.

When fronts are forecast to stall, it’s not always a sure thing. So there’s always hope that the forecast will improve. Stay tuned.

I’ve added a weather model glossary to this site.  Hover/click on the underlined model acronym for pop-up information.