Sun 08:01 PM Forecast Review — The rain stayed along the coastline of Southern NJ today. Instead of moving back as a warm front there are indications that the front will be joined by a cold front in western PA.  This setup will allow the remnants of IDA to approach our area on Wednesday.

Updated Sat 10:30 PM— Tonight’s models show more clouds than previously forecast on Sunday and some sprinkles in the morning. A chance of some heavier showers in the early afternoon, especially from PHL and eastward.

The slowly moving frontal boundary just to our south has spawned a somewhat stationary weak low pressure system off the Delaware/Maryland coastlines.

Sunday will start cloudy, but enough dry air will filter in on Sunday to give us a mix of broken clouds and sunshine by noontime. Some widely scattered showers are possible later Sunday afternoon (mostly in NJ) due to instability but most areas will be dry.

It will be very humid. High 83.0º ± 1.8º NBM, Blue Bell PA

Hurricane Ida’s track looks like it will begin to affect our area late Tuesday through early Thursday.

If and when Hurricane IDA becomes a threat to our area, I’ll have a link to follow the storm’s effects.


Update Sat 8:10 AM— Some areas had heavy rain after midnight. For Saturday, the HRRR continues with areas just to the west of Philadelphia being relatively rain-free with most of the rain from PHL into NJ. The NAM-NEST have a more westward extent.

The HREF which blends these models shows the rain today as just that, a blend—

HREF Dark Blue is low clouds. Green/yellow is rain/radar. Green contour lines area standard deviations in rain. (Click for larger image)

No change in Sunday’s forecast below.

Update Fri 10:40 PM — The axis of predicted heavy rain tonight has fallen about >50 miles to the south. Tonight’s HRRR optimistically keeps the rain on Saturday out of the western suburbs.

The NAM-NEST and HIRESW-ARW are not as optimistic—

NAM-NEST Saturday precip

The weather on Saturday may be better than previously forecast.

A frontal boundary is attempting to move to our south Friday evening. The boundary is resulting in showers and thunderstorms as had been expected. These chance of these storms will linger through the evening and night as the front barely makes it south of our area.

Unfortunately, the high pressure system behind the frontal passage will remain to our north as it encounters a firmly entrenched upper air high over the middle and southeastern section of the country.

NAM surface forecast for Saturday at 2 PM Showing the upper High to the northeast and the Bermuda high to our south. The Philadelphia area will be in the convergence zone of moisture and winds. (Click on image for a larger view.)

The NAM-NEST shows maximum moisture convergence and upward vertical velocity about 9 AM Saturday. The NAM-NEST also shows a weak coastal low pressure system forming off the Delaware coast on Saturday, lingering into Sunday.

NAM-NEST moisture convergence and upward vertical velocity 9 AM Saturday. (Click on image for a larger view.)

What does this all mean?


Clouds, rain and thunderstorms from Friday night into at least the early afternoon of Saturday with a lower chance later in the afternoon. High 77.9º ± 2.8º NBM Blue Bell PA


The low pressure system that forms off the Delaware coast will remain nearly stationary Saturday night into most of Sunday. Considerable cloudiness in the morning. Some bright skies and sun in the afternoon with a chance of showers/thunderstorms on Sunday afternoon. High 82.4º ± 1.8º NBM Blue Bell PA

Update Fri 7:30 PM : I’ve gone with the NBM model for cloud cover on Sunday. Some models have much more cloudiness Sunday afternoon.

If and when Hurricane IDA becomes a threat to our area, I’ll have a link to follow the storm’s effects.


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.