Daily Archives: Tuesday, August 24, 2021 9:16 pm

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

Thu 10:30 PM — Tonight’s models just becoming available shows heavy thunderstorms developing Friday between 4 PM and 8 PM and continuing into late evening as the cold front mentioned in my Wednesday update tries to make further progress southward.
(The HIRESW-ARW has storms as early as 2 PM)

High CAPE values and high PWAT will make for strong storms and heavy rain.
Saturday looks rainy according to the latest NBM. My regular weekend weather forecast will cover the details.

Thu 6:50 AM — The GFS, RAP and HRRR have backed away from numerous showers/thunderstorms this evening. Still some scattered around. The HI Resolution models and NAM still have showers as shown in last night’s graphic..

Wed 8:02 PM— Thursday’s forecast has come into somewhat better focus as a weak front will try to get its toe in the door late Thursday with little success as it’s pushing against strong upper level high pressure entrenched over much of central and southeastern US. Today’s models have some showers and possible thunderstorms in the Philadelphia area late Thursday afternoon as the front tries to move through. Most, but not all, of the activity is is currently forecast to be north and northeast of Philadelphia.

HREF 1 hour rain forecast for Thursday at 7 PM. Just a single time snapshot. Any storms break out as early as 4 PM in some areas. (Click on image for a larger view.)

A slight change in wind direction on Friday; the front will make another attempt to move through Friday evening with more showers and thunderstorms, likely more prevalent.

The weekend weather outlook still looks uncertain, but a mix of clouds, some sun and showers/thunderstorms is the most likely scenario for Saturday as the front hangs up somewhere near us. Sunday looks to be the better day.

Also on the horizon is another tropical storm that the GFS is forecasting to become a hurricane in the Gulf and approach the US coastline, possibly Louisiana, as early as Monday.

Current GFS Forecast 2 AM Monday (Click on image for a larger view.)

Tue 9:16 PM— After the ‘exciting’ weather from Henri, this week’s weather will be relatively unexciting, as a large high pressure ridge in the upper atmosphere continues with the high heat and the humidity—

GEFS jet stream (250 mb wind/height) forecast for Saturday (Click on image for a larger view.)

There really isn’t any sign that crisp cold air from Canada will descend upon us any time soon.

Wednesday through Friday appear to be hot and humid (although without the mid 70s degree dew points we have recently had). It will still feel very hot.

A meteogram is worth a thousand words. Here’s the NBM Temp/Dew point/Heat Index (“Apparent Temperature) for Blue Bell this week—

NBM model meteogram for Blue Bell. Wed-Fri. Note that the near 100º is the Heat Index (in the model world, known as the “apparent temperature”) Also included is the standard deviations. (Click on image for a larger view.)

While the general pattern is for continuation of hot/warm weather, small ripples in the upper air flow will result in possible thunderstorms Thursday late afternoon (especially far north) and Friday evening.

Following a cold front Friday evening, high pressure will try to move in over the weekend. Unfortunately, it appears that the main high pressure system will remain in Canada and we’ll get some easterly winds as the front stalls somewhere just to our south. That means somewhat cooler weather, but it also can mean some clouds and some showers.

The weekend forecast is too uncertain at this time to even try.