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Friday & The Weekend Outlook

Posted Thursday 07/18/24 @ 7:26 PM — I’m updating the forecast for Sunday. The latest ECMWF and Canadian RGEM show scattered showers on Sunday while the latest GFS keeps us dry with just high clouds. As I said below, a stalled front presents forecasting challenges. There’s too much uncertainty to hang one’s hat on either forecast Sometimes the best forecast is to indicate that we just don’t know for sure right now.

Posted Thursday 07/18/24 @ 5:32 PM — The front that went through Thursday night is slowly moving southward and drier air will filter in for Friday, eliminating the cloud cover. Sunny and pleasant should capture it for Friday.

The front boundary is expected to become stationary and persist for several days. Here’s the NAEFS depiction for Sunday afternoon—

Stalled frontal boundaries often present a forecasting challenge. Waves can develop along the front and a battle sets up between warm air to the south trying to move north while cooler, drier air tries to move south.

With potential wave development along the front, moisture can move over the boundary, causing clouds and the chance of showers.

This movement of moisture north of the front will cause cloudiness for Saturday. Light showers are predicted far northwestern areas of the forecast area, in Berks and Lehigh counties. Dry conditions expected in the immediate Delaware Valley area. Highs 86º -87º

For Sunday, the high pressure system to our north is expected to push a bit further south, helped by the wave that exits eastward. Sunday should be another very nice day, although somewhat warmer. Highs 89º-90º

As I said stalled fronts can be a challenge. Any unexpected change in the position of the front, or an unexpected development of a wave can ruin a forecast. We’ll see what happens.

BTW, this stalled front is currently expected to drift northward by Monday and next week may be very wet and unsettled. Stay tuned.

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Wednesday Thunderstorms

Posted Wednesday 07/17/24 @ 3:49 PM — A few strong scattered storms have developed and moved northeastward between 3 and 4 PM. The main activity will reach northwestern Montgomery and Bucks counties between 6:30 PM and 7:30 PM and move through the immediate Philadelphia area between 7:30 PM and 10 PM.

The models are suggesting that weak low pressure will develop over us as the front moves through, enhancing rainfall intensity.

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Posted Wednesday 07/17/24 @ 9:39 AM — A strong cold front will move through tonight with showers and some strong thunderstorms developing and moving through ahead of the front.

It appears that this will be a broad line of showers and storms and areas may receive several rounds of storms and rainfall.

Things are a little less clear about the timing. There’s the possibility of scattered storms breaking out well ahead of the main batch, as early as 3-4 PM. The main group of storms moves through between 5:30 and 10 PM.

Gusty winds and periods of heavy rain are likely.

Tuesday Evening

Posted Tuesday 07/16/24 @ 7:23 PM — A new line of storms, with a better wind trajectory to reach Philadelphia has formed. We’ll see if they hold together—

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Posted Tuesday 07/16/24 @ 5:50 PM — The storms are diminishing in intensity and likely will move to our north, if they stay intact.

Posted Tuesday 07/16/24 @ 5:18 PM — Storms have developed pretty much as had been forecast by NAM-NEST and HRDPS, but not with the HRRR, GFS or ECMWF.

While the NAM-NEST does have them moving through Philadelphia around 6:30 PM- 8 PM, the upper air winds may take them just to our north, unless the line extends and builds further southward.

More interesting weather for Wednesday. Check back.

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Tuesday Forecast Update

Posted Tuesday 07/16/24 @ 12:56 PM — A quick update. Some of this morning’s models have backed off on the chance of thunderstorms for this evening. Isolated to scattered thunderstorms still possible, north of the city. (The Canadian models are the most aggressive with a continued chance of storms, the ECMWF and GFS the least.)

Posted Tuesday 07/16/24 @ 8:39 AM — Latest Model Blend (NBM) forecast high temperatures for Tuesday—

As for thunderstorms, the latest HREF has the greatest chance of thunderstorms from the city north and west—

Thunderstorms possible (isolated) as early as 4:30 PM. Scattered storms from the city west from 5-9 PM.

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Posted Monday 07/15/24 @ 4:43 PM — A small upper level wave will move through late afternoon Tuesday. With high temperatures and humidity and with instability also high, thunderstorms will be triggered. Currently the time frame is from 7 PM through 11 PM Tuesday evening.

Some of these storms may be severe, especially northwest of the city.

The storms may weaken as they move through Philadelphia, as the wave will already be to our east and upper air “heights” will begin to increase again. Here’s the NAM-NEST model showing the wave, visible in the jet-stream level wind streams —

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Posted Monday 07/15/24 @ 10:50 AM —Monday will be hot, with humidity and temperatures creating an “apparent temperature”, also referred to as a “heat index” or “feels like” temperatures of over 100º in many areas—

There’s a chance of isolated thunderstorms northwest of our area.

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Originally Posted Sun 8:41 PM —Our current heat wave will continue through Wednesday. High temperatures will be in the mid 90s on Monday and approach or exceed 100º on Tuesday. With some increased cloudiness from an approaching cold front, Wednesday will ‘only’ be in the mid 90s.

A cold front is expected to move through late Wednesday. Showers and thunderstorms will accompany the front—