High pressure builds in for the weekend.

Saturday will be mostly sunny with periods of fair weather cumulus clouds at times.  It will be comfortably non-humid and cool with highs around 77.

Ordinarily, this would be a straightforward forecast, but this high pressure center will remain to our north and a northeasterly flow of air around the high can bring in some unexpected cloudiness, especially for Sunday. The GFS keeps the immediate PHL area relatively cloud-free for Sunday, but the NAM has cloudiness developing in the afternoon here.  High temps around 75.

So there’s uncertainty with the Sunday forecast.

Additionally, both models pull in clouds and light drizzle for the immediate Jersey shore late afternoon Sunday.  Not sure about that either.


Friday 8 AM update: The front has slowed. Clouds had been expected for much of Friday, but it appears that areas of showers may linger into the afternoon in some areas. This had not been forecast.

It suggests the models don’t have a good handle on the forecast for the weekend. I’ll update this evening.

A cold front will move through tonight (Thursday) and a line of showers and thunderstorms is predicted by most models between 9PM and midnight. Some of the thunderstorms will have heavy rain Thursday night and 2-3 inches of rain are possible in the immediate PHL area.   Areas west and north of Philadelphia are showing high shear values, so some thunderstorms will likely be severe.

High pressure builds in for Friday and the weekend. Unfortunately this high pressure center will remain to our north and an easterly or northeasterly flow of air may bring some damp conditions to the Jersey shore on Sunday, according to the latest NAM. 

Most of the area will have sunny skies, but periods of some fair weather cumulus clouds are possible as well. A delightful weekend is forecast weather-wise.

High temps Saturday 77, and Sunday 76 with low humidity.

The easterly wind flow needs to be watched.  This sometimes results in more cloudiness than the models predict this time of year.  Sunday may have more cloudiness than currently forecast.


Weekend Weather Forecast

Sat 9 AM Update: The 2AM model runs (06z UT) show sun with a bit more clouds and a slightly increased chance of scattered thunderstorms in the afternoon. Highs about 88-89.

Last night’s HRDPS shows scattered showers/thundershowers about 1-3 PM Saturday.  Exact placement of the showers typically is not strictly accurate.

from last night…

A typical August pattern— a weak, poorly defined, surface pressure pattern will be with us over the weekend.  For most of both Saturday and Sunday, the wind flow will be from the SE, bringing warm and very humid air into our area.  The southeasterly flow is the result of a weak cyclonic circulation off the coast from a poorly defined stalled frontal boundary to our south.

The models are in general agreement that Saturday will be mostly sunny, hot and humid. The NAM has more morning cloudiness than the GFS, but both show clouds breaking for sunshine.  There is a large range in the temperatures predicted.  Most of the models are in the 89-91 degree range, but the GFS is showing a high of 96 in the city, probably a modeling error of this new model (which is the  FV3-GFS).  (The Canadian HRDPS shows a high of 92.)

There is a low chance of widely scattered showers/thunderstorms in the late afternoon and evening, mostly north and west on Saturday.

A repeat performance on Sunday, with the models showing temperatures  in the range of 90-92, again with the GFS model somewhat higher in the immediate PHL area.   There’s a somewhat higher chance of thunderstorms late afternoon on Sunday.

The humidity will be noticeable with dew points in the uncomfortable range of 69-72 both days.

I’ll update over the weekend to narrow down the chance and location of any showers.


So what happened to the storms and flash flooding expected for today?

Clearly, the models didn’t do a very good job here.  On Sunday, the models overwhelming predicted a severe weather event.  They continued to do this on Monday, but by Monday evening there was a lack of agreement, with some models showing the insignificant showers we ended up with.

By this morning, the severe weather parameters had become very unimpressive and I posted that change this morning.  Still, the Rapid Refresh model (RAP) available at 9:35 AM showed a line of storms about 4 PM.

While at work, the rest of this morning’s models became available between 10:15 and 12:40.   The NAM NEST, WRF-AWF, WRF-NMMB, RAP, HRRR and HRDPS are the models I look at for thunderstorms. (Hey, it’s almost a full-time job, but it’s still a hobby.)

By this morning’s model run, the models had really backed off considerably with even the rain, shunting anything developing to our south.   Too late to make an updated web announcement, while at work.

Interestingly, they were still talking about severe thunderstorms on the radio while driving home at 6:30 PM.

It’s always a tough call to cancel the call for severe weather when even the slight possibility could endanger people if it occurs.

Anyhow, the “elusive” search for the model that’s always correct is elusive for a reason.

Looking back, even the GFS, a large-scale model, did better yesterday than some of the high resolution models.  And if I had to hang my hat on a model yesterday, the Canadian HRDPS probably called it the best at the earliest time.  But it’s not always right…