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Saturday Forecast Update
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.
Any comment on the “microbursts” that hit a swath of Lower Merion and Philadelphia? We were in the Lower Merion area, and I’ve not experienced winds and rain like that before (even including the tornado that struck a quarter century ago). The NWS didn’t manage to issue an alert until the storm was almost over, so I’m supposing that the prediction models don’t deal well with these kinds of events.
Hi Jeff,
I was watching it on radar as it formed. (I hadn’t done my usual biking that day because I was sure I’d be caught in something early afternoon.) As my post for the day indicated, most models showed thunderstorms occurring about 2 PM. The HRRR, usually really good with this, showed nothing for the immediate PHL area! I do recall reviewing various parameters that morning and except for high CAPE, there were no particularly impressive severity parameters in those morning model runs. By the 2 PM runs, they showed strong dynamics, but the event was already taking place. I only keep my downloaded models for 2 days because they take up so much storage, but you’ve prompted me to download that day’s model again and I’ll look it over. I would say that the models failed us with those severe storms, as neither myself nor the NWS saw the possibility of that severity in advance that morning…until it developed. I’ll let you know if I see anything with the download.