Weekend Weather Forecast

Last Updated

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…


Last Updated

Tues 10 AM Update: This morning’s RAP (Rapid Refresh) model has thunderstorms breaking out 3-4 PM around Philadelphia and continuing on and off until 8-9 PM. Over past weeks, the morning RAP model has had a fairly good track record.

from earlier Tues morning:

Tues 8 AM Update: Last night’s models have backed away from the severe weather in the immediate PHL area. The severe parameters (Helicity, Vertical Shear) have reduced from yesterday’s model runs; the expected lack of sunshine will reduce the chance of severe weather in our immediate neck of the woods.

One can still expect showers and thunderstorms to develop as early as noon and continue into the afternoon. Some of the rainfall may still be locally heavy in spots. It may be very windy in some thunderstorms.

As mentioned yesterday, the heavier dynamics now appear to develop to our south and east.

from yesterday…

Tuesday has been in the crosshairs for severe weather for several days.  The bullseye for the severe weather had been the immediate Philadelphia area, but the latest models show that the most extreme aspects looks to occur to the south of us, near the Washington DC area.

That said, there are plenty of ingredients that are poised to come together in our area.

A warm front will pass through Tuesday morning, possibly causing showers and thunderstorms before daybreak with another possibility around noon.  A cold front associated with a low pressure system moves through during the late afternoon and early evening hours.

Thunderstorms with heavy rain and high winds are possible both during the afternoon and early evening.  Some of the severe weather parameters (helicity, vertical shear) are highly elevated giving us a  risk of high winds and slight risk of tornados.  Luckily, we may not have that much sunshine, limiting the available energy and reducing the potential for very extreme weather.

The models are still evolving and have changed their forecast considerably over the past 24 hours.  The NWS does a great job with these events and it’s suggested to stay tuned to their forecasts.


Last Updated

Fri 10 PM Update: Tonight’s RAP (Rapid Refresh) model shows an upper air disturbance moving through to our south on Saturday. Significant high, thin cloudiness is predicted, especially south of the city, late Saturday morning into the afternoon.

This weekend’s forecast is as straightforward as it gets.

High pressure builds in from the west.  Sunny skies both Saturday and Sunday, near zero chance of rain.  Some fair weather clouds at times in the afternoon.

Highs Saturday 83, Sunday, 81.


Storm Forecasts & Other Things "up in the air".