Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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Sunday Forecast Update

Posted Sunday 06/23/24 @ 4:18 PM —If you were expecting today’s rainfall to fully water your garden or lawn, you may be disappointed. Numerous models have the the showers diminishing in intensity in the immediate Philadelphia area. We could use the rain, but I’m not convinced we’ll see all that much. A few areas may get a good soaking but it seems that many won’t.

MRMS radar with superimposed HREF CAPE probability (purple shading) and vertical motion yellow contours (Max Omega) (Click on image for a larger view.)

Posted Sunday 06/23/24 @ 10:27 AM — The approaching cold front is in Ohio this morning with a line of showers and storms in western PA—

Water Vapor and MRMS radar summary with 500 mb windstreams at 10 AM I’ve drawn the approximate location of the cold front. In the oval is another disturbance ahead of the front that may triggers some storms this afternoon. Click on image for a larger view.)

This front will be a slow mover, not likely passing through here until 4-5 AM Monday.

Clouds well ahead of the front don’t appear to have changed the forecast high of 96-98º for the immediate Philadelphia area.

An increasing chance of thunderstorms after 1 PM with widely scattered storms possible west of the city as early 1-3 PM. Most likely period here in the immediate PHL area between 5 and 9 PM, but some scattered activity possible into the pre-dawn on Monday. It still appears that the heaviest activity will be in NJ this evening between 7 PM and midnight.

Total rainfall doesn’t look that impressive according to the NBM—

NBM total rainfall for Sunday through 5 AM Monday. (Click on image for a larger view.)

The Canadian HRDPS shows greater rainfall totals—

HRDPS total rainfall through 5 AM (Click on image for a larger view.)
Sunday Forecast Update

Posted Saturday 06/22/24 @ 7:52 PM — We had some thundershowers in the early to mid afternoon Saturday, not predicted by many models. (The Canadian HRDPS forecast some early storms, mostly west). These showers kept the high temperatures ‘only’ in the 96º-98ª range.

NexRad radar Saturday at 2:27 PM. Fast moving small storms. (Click on image for a larger view.)

For Sunday, a weak front is expected to move through late in the day, preceded by a line of showers and thunderstorms. Most likely time for the storms is about 4 PM far western areas, 5-6 PM in Philadelphia. As the storms enter NJ, they’re forecast to intensify.

Following our last round of poorly forecast storms a week ago, I mentioned I would be changing my approach. So I’m changing my methodology for forecasting thunderstorms.

It’s no secret that thunderstorms are best forecast with a probability-based approach. As such, I’m going to rely on the probability parameters of the HREF, NBM and the HRDPS, instead of my past approach of favoring one model’s timing and storm placement over another.

For Sunday, high temperatures look to be less than previously forecast. Highs will ‘only’ be in the 96º-98º range, similar to today. However, dew points will be near 70º, making it feel as hot if not hotter.

While there may be a main line of storms that move through 4-7 PM in Philadelphia, additional showers and storms, before and after the main line.

While the highest chance of storms in the immediate Philadelphia area will be between 4 PM and 7 PM, there are indications that some storms may move in from the northwest and blossom earlier in western suburbs, 1-3 PM —

HREF probability of CAPE >2000 joules/kg at 2 PM. Storms will likely develop or enhance in this region between 1-3 PM. Some storms will move in from the far northwest at this time. (Click on image for a larger view.)

Most models show any storms that move through Philadelphia may become more severe in NJ. By 6- 7 PM, the highest probability of storms has entered into NJ and are expected to intensify—

NBM Probability of thunderstorms. (Click on image for a larger view.)

While all thunderstorms may be locally severe, the overall severity parameters on Sunday are unimpressive, except closer to NYC.

Saturday Forecast Update

Posted Saturday 06/22/24 @ 10:16 AM — The model forecast guidance continues with a forecast of high temperatures in the 99º range in some areas today. Dew points are forecast to be near 70º, making the “apparent temperature” (heat index) over 100º around 3 PM—

This morning’s 12z NBM heat index forecast for Saturday at 3 PM. (Click on image for a larger view.)

As for showers/thunderstorms, while possible, the hourly probability in the immediate Philadelphia -Delaware Valley area is less than 13%. That’s not zero, so a widely scattered thunderstorm is possible later this afternoon, but when it comes to hourly probabilities, I usually look for 18% and higher. Those POPs great than 18% are far north of our area today.

The Canadian HRDPS has some storms as early as 2 PM around Doylestown and areas far west of the city. The experimental RRFS also has a few storms, mostly north. The HRRR has little in the way of storms around here, but a few in central and northern NJ.

Previously Posted Fri 5:33 PM —I’m pretty sure everyone reading this blog knows it’s going to be hot weekend. What value add can I provide to that forecast?

With high temperatures approaching 99º Saturday and possibly exceeding 100º on Sunday, there’s a slight chance of widely scattered thundershowers Saturday evening and a better chance of organized thunderstorms later Sunday evening or night with the approach of a cold front. Most areas will not see any showers Saturday.

A big factor this weekend will be the concomitant increase in humidity with the high temperatures. Some areas will have dew points in the mid 60s while others will have dew points near 70º

Current satellite water vapor image

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), potential vorticity (fine violet contours) with superimposed MRMS RADAR. Notice the 576 thickness line just north of the Pennsylvania border. The disturbances along the front in the northern US are expected to drop through our area late Sunday into Monday. Right now the imagery doesn’t show this movement southward. (Click on image for a larger view.)

Low pressure is expected to form along the boundary to our north and its circulation will move a weak front through Sunday night. To mix it up a bit, here’s the latest ICON model forecast for 8 PM Sunday—

Today’s 12z German ICON model showing low pressure to our north. A weak front moves through late Sunday into Monday. Note the 576 thickness line (orange line) still to our north (Click on image for a larger view.)

Saturday

Sunny and very hot and humid. Slight chance of widely scattered thermally driven thundershowers late afternoon or evening.

High temperatures (latest NBM model)

NBM high temps Saturday (Click on image for a larger view.)

Sunday

Sunny in the morning, increasing clouds after 2 PM from west to east. Showers and thunderstorms early evening into night. Difficult at this time to access the the actual thunderstorm threat. I’ll update over the weekend.

NBM high temperatures—