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Saturday Forecast Update

Sat 4:59 PM —Forecast Review — My experiment with the HRRR, despite an excellent surface parameter match with the RTMA, showed the HRRR to be wrong about the thunderstorms today. It’s reassuring that our newest model, the RRFS, while over-forecasting rainfall did much better than the HRRR, as did the older NAM-NEST and the Canadian HRDPS. I’ll have to rethink my parameter match approach. Here’s today’s MRMS rainfall estimate through 4 PM —

Posted Saturday 07/11/26 @ 5:05 PM — Another batch of storms are to our northwest, but the steering winds will be moving towards a northerly direction, keeping them away from Philadelphia.

High pressure builds in for Sunday with fair skies.

Heat returns next week as the edge of the heat dome pushes in from the west. The 90s return on Tuesday and mid 90s on Wednesday—

Posted Saturday 07/11/26 @ 11:30 AM — There appeared to be no question about the showers and storms expected to develop here during the mid afternoon. However, this morning’s HRRR casts serious doubt on this forecast. The question is, which model is correct??

This morning’s 12z HRRR forecast precipitation shows no rain for much of the immediate Philadelphia area; the area of rain to our west occurs during the evening—

Here’s the same 12z model run of the new RRFS—

Finally, here’s the 12z NAM-NEST forecast for Saturday—

So, this is an interesting forecast.

It was several years ago that the HRRR predicted no rain and I was on a bike ride where I got positively drenched and there was severe flooding in some areas.

So I know first-hand that the HRRR can be wrong.

In an effort to determine which model is best, I do something that I call a 3 hour parameter match with the actual conditions (RTMA), Today’s HRRR came up with the best match today.

So this is an experiment. We may get very little rain if the HRRR is correct. But three out of four models have substantial showers and rain developing mid afternoon lasting into the evening. I guess we’ll find out.

Originally Posted Fri 5:53 PM —For Friday evening/night, there are thunderstorms to our west, but a strong area of downward vertical motion (omega) over the city will cause these storms do dissipate by 8 PM. It doesn’t appear that any storms will make it into the city.

High pressure pushing down from Canada will push the edge of the hot air to our south on Saturday with a backdoor cold front. Many pop-up storms will develop in our area as early as noon on Saturday. The humidity and storms will be far to our south on Sunday.

Saturday

The backdoor cold front will bring showers and storms as early as noon to 2 PM. Precipitable water will be high near the city and southward; any storms could bring heavy rains.

Highs (Model Blend): Blue Bell- 85 º Philadelphia 86º ±2.3º Average sd and uncertainty.

Sunday

Mostly sunny in the morning; Some cloudiness increases from the west in the afternoon. Highs (Model Blend): Blue Bell- 84 º Philadelphia 86º ±2.4º Average sd and uncertainty.