Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx #PhillyWx #wx #cycling classic

Question of Timing

Posted Saturday 08/29/26 @ 8:24 PM — You’ll soon understand why this post is in small font size. This section will not everyone’s cup of tea.


There’s a large range of forecasts regarding the timing of the showers Sunday. All models agree that the heavy action occurs after 6-7 PM, but they disagree with onset of the light scattered rain during the afternoon. Tomorrow’s forecast is an excellent time to try out a new tool that I asked Anthropic Claude to put together for me . It’s called “Q-vector analysis” and and the Q is for convergence vectors based on “quasigeostrophic equations” (Wikipedia Link).
In very simple terms it locates areas of convergence and divergence causing upward and downward motion in the atmosphere based on winds and thermal gradients. Upward motion with high humidity causes rain.

Here’s the Q-vector analysis of the latest RRFS data at 2 PM Sunday—

18z RRFS calculated Q-Vectors at 700 and 500 mb heights and relative humidity at 700 mb at 2 PM Sunday. Maroon is high relative humidity (needed for rain) . The orange area as well as the black oval contours are upward motion caused by Q-vector convergence. The green is downward motion. (no rain here!) (The white arrows are the actual Q vectors.) The white oval is the area of likely showers, especially in the orange shading and is where the upper (black contours) and the lower level 700 mb (orange) area overlap; this should be enough upward motion for showers. (Click on image for a larger view.)

So based on the Q-vector analysis, I would say that scattered showers are possible as early as 1-3 PM in western suburbs and even into northwestern parts of the city as early as 2-3 PM. This forecast is supported by this afternoon’s Canadian RDPS.

Last week, showers broke out in the morning, but no rain was forecast by the models. However, examination of the Q-vectors would have shown the showers developing. This is an experiment. It’s why I do this stuff.

We’ll see what happens.

What happened?
Posted Sunday 08/30/26 @ 7:54 PM — The entire forecast changed from last night. The 00z and 06z were very different The change in the location of the Q-Vectors was the least of the changes. The heavy showers and thunderstorms expected at this time haven’t materialized. Not a good test for the Q-Vector technique. That said, the rainfall we received between 11 AM and 3 PM strangely resembles the white oval area above. Frankly, I think it’s a coincidence.

MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED today between 11 AM and 3 PM. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)


2026 Cycling Classic Forecast Update

Posted Saturday 08/29/26 @ 5:18 PM — This afternoon’s models’ forecasts have moved the showers in Philadelphia to the mid afternoon or early evening hours; a positive development! Here’s the latest RRFS simulated radar at 6 PM showing the rain just begins to encroach on Philadelphia—

18z RRFS simulated radar forecast for 7 PM Sunday— (Click on image for a larger view.)

The latest HRRR is somewhat more aggressive with the rain at 6 PM, but the race will be over by that time.

The latest Model Blend (NBM) still has light rain about 2-3 PM, still an improvement. (The Model Blend tends to lag somewhat with changes in rain onset.)

19z NBM 1 hour accumulated rain at 3 PM. Means that rain started and accumulated sometime between 2 and 3 PM. (Click on image for a larger view.)

Windy and gusty from about 11 AM to 2 PM.

2026 Cycling Classic Forecast Update

Posted Saturday 08/29/26 @ 10:02 AM — The models have mostly come together with a forecast of light showers developing as early as late morning in the western suburbs and moving into Philadelphia around 1-2 PM. These showers are the result of an upper air disturbance moving over the upper air heat dome ridge into our area. The showers will be widely scattered initially.

Water Vapor image at 10 AM with superimposed RAP model Mean Seal Level pressure (black contours), 500-1000mb Thickness (yellow contours), RAP potential vorticity contours (violet fine contours) with MRMS Radar with surface wind barbs (Click on image for a larger view.)

By 3 PM, the REFS (which combines the new RRFS and HRRR models) shows the 1 hour precipitation to be scattered—

06z REFS 3 PM snapshot of 1 hour previous precipitation (Click on image for a larger view.)

Once the rain starts, it will increase in areal coverage by 5-6 PM and continue into the evening.

Winds will be from the SSW-SW, according to the latest HRRR—

Today’s 12z HRRR Winds and Wind Gusts (Click on image for a larger view.)

Originally Posted Fri 8:16 PM — High pressure is building in behind a front that brought showers to large areas of NJ today.

Water Vapor image at 6 PM with superimposed RAP model Mean Seal Level pressure (black contours), 500-1000mb Thickness (yellow contours), RAP potential vorticity contours (violet fine contours) with MRMS Radar with surface wind barbs, The white box is the upper air disturbance that affects Sunday’s weather. (Click on image for a larger view.)

An upper level disturbance moving around a heat dome in the center of the U.S. looks to bring some showers on Sunday afternoon or evening. There appears to be some uncertainty with the timing of these showers and their placement.

Saturday

Sunny and pleasant. Light winds. Highs (Model Blend): Blue Bell- 82 º Philadelphia 83º ±1.8º Average sd and uncertainty. Comfortable dew points near 60º

Sunday

A mix of sun and clouds early with an increase in cloudiness during the afternoon. Showers may develop in the western suburbs and areas northwest during the early afternoon. They’re not expected to move into the immediate Philadelphia area until 5 PM. There’s uncertainty with this.

Highs (Model Blend): Blue Bell- 87 º Philadelphia 85º ±1.8º Lower than average sd and low uncertainty.

Higher humidity- dew points near 70º. The Canadian Model is showing it somewhat windy/gusty in the afternoon from the SSW.

There is considerable model spread regarding the rain onset for Sunday. I’ll update over the weekend.