Monthly Archives: August 2021

Philadelphia Weather Outlook

COMPLEX WEATHER THIS WEEK

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Thu 7:00AM— Some areas got very heavy rain. The activity did not make it east of Philadelphia. The tornadic potential was luckily over-forecast.

NEXRAD based rain accumulation (“storm totals’) estimates from last night’s storms. Some areas received 3-5 inches of rain, but the storms never made it east of Philadelphia.

Henri looks to remain far enough off the Jersey coast, affecting Long Island and Cape Cod later Sunday-Monday.

Wed 10:05 PM—Latest models are looking less impressive with activity getting east of Philadephia, as current storms rotate up northward instead of eastward.

Current radar 10:05 PM

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Wed 7:30 PM— Latest HRRR has slowed down the passage of storms in the immediate PHL area. Current timing is between 11 PM and 2 AM. The HRRR also shows some areas of high helicity and gusty winds with the passage of these storms in our area.

HRRR forecast midnight Wednesday. Elevated Helicity shown. Possible tornadic activity between 11 PM and 2 AM Thursday. (Click on image for a larger view.)

Still watching Henri which may indirectly affect NJ on Sunday.

Wed 1:20 PM— Latest GFS has the thunderstorms moving through our area somewhat earlier, starting about 5:30 PM. Strongest dynamics remain to our far northwest, closer to Allentown.

Wed 7:14 AM- Last night’s models have moved the chance of tornadic activity a bit more westward into Lancaster, Berks and Lehigh Counties. Most likely time for thunderstorms in our area 8-11 PM tonight.

As for Henri, last night’s models have it encountering blocking high pressure and an upper low with it moving more westward, now impacting Long Island with indirect effects possible at the Jersey Shore on Sunday. This complex situation continues to result in a changing forecast.

Tues 10:15 PM— Tonight’s models just becoming available show very strong dynamics ( specifically “helicity” and upward vertical motion ) between 7 and 10 PM Wednesday evening, especially west of the Delaware and specifically in Chester and Lehigh counties. Tornadic activity is possible.


Tues 8:53 PM— An interesting and highly complex weather scenario is unfolding this week as the remnants of tropical storm Fred move up the Appalachians Wednesday into Thursday and a new tropical system, “Henri” moves up near the coastline over the weekend.

First, Fred. The moisture remnants of Fred will stay mostly west and northwest of our area with most of the heavy rain also staying west of Philadelphia and its immediate surrounding counties. The model forecast has been very consistent with this path.

Rain should move in during the evening hours Wednesday and depart by Thursday morning.

More interesting is a dramatic shift in the forecast path of Henri, developing in the Atlantic. It appears that an upper air closed low may draw Henri closer to the coastline.

While the major impact is currently forecast to affect Cape Cod and New England, the combination of moisture and upper dynamics may bring some significant rain thunderstorm activity to our area Saturday and Sunday. The ICON model is forecasting some very heavy rain on Saturday. (Although the ICON has been over-forecasting rain lately.)

It should be noted that if Henri stays a certain distance from us, Saturday may turn out to be exceptionally nice, as sinking air (subsidence) occurs a certain distance west of a tropical system.

GFS Forecast for Henri early Sunday morning. (Click on image for a larger view.)

This is a complex scenario and it’s likely that the current forecast will change significantly over the coming days. I’ll keep an eye on it.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Forecast Updated Sun 10:45 AM in green box below

For the past two weeks, the weather models have been not been at their best. I’m not sure why. Most of the models have been updated since the fall (HRRR, RAP, NBM, HIRESW, HREF and GFS) and this is their first summer hot season. Clearly, there’s something wrong, as they are not handling very hot weather and convective activity accurately.

I haven’t been able to find any public discussions or presentations from the scientists at NOAA in the “Model Diagnostics and Verification Group”. One thing to notice— new model development is conspicuously missing from their website.

Here is the current GFS bias error (comparison of forecast to observed measurement) for the past two months. Notice that over the past 10 days, the negative bias error has been greater than average. (no surprise here!)

GFS Bias error temperature at about 5000 feet. Notice how the trend is below the average bias error during the very hot weather. July 20th was also a hot period. (Click on image for a larger view.)

I’m sure the people responsible for evaluating model performance are working on it.

WEEKEND FORECAST:

A cold front will slowly pass through tonight while a secondary cold front moves through Saturday afternoon. The above discussion about model error was ‘inspired’ by the fact that there are huge differences in the forecast for Saturday.

Friday Evening

The approaching cold front will trigger showers and thunderstorms, but most models have the storms diminishing as they approach the immediate PHL area. Some storms may remain intact to our north (Trenton) and just to our south (Wilmington DE). A few isolated storms may sneak through. (Latest GFS hot off the wire shows some rain in Philadelphia 8-9 PM ) Similar to yesterday, they encounter high CIN and a dome of stable hot air. Here’s the latest HREF with its 1 hour rain forecast—

Latest HREF with rain accumulation at 8 PM showing eastward most extent. (Click on image for a larger view.)

Saturday

Highest chance of thunderstorms today. (Below 18% is low) Widely scattered and relatively low probability.


The HREF has showers/thunderstorms, scattered,starting at 4 PM and extending into 9 PM. Most of the activity west, east and south of Philadelphia.

The NBM still has a high of 89.4º ± 1.1

Here’s where things get less certain. A secondary cold front moves through between 1-3 PM. Many models have Saturday mostly sunny. The ICON model and the SREF have some showers and thunderstorms with the frontal passage during the afternoon. Additional scattered storms possible in the evening Some models (GFS) are less impressive with the showers/thunderstorms.

The HREF has showers/thunderstorms, scattered,starting at 4 PM and extending into 9 PM. Most of the activity west, east and south of Philadelphia. Some of the rain will be locally heavy. (With current track record, I wouldn’t put too much stock in the exact forecast placement.) Here’s the HREF forecast for rain (hourly accumulation) Saturday at 7 PM—

HREF hourly rain accumulation 7 PM. Some of the rain may be locally heavy (Click on image for a larger view.)

Sometimes, the best forecast is to indicate that forecast is uncertain and lower than usual confidence.

Sunday

Forecast Update Sun 10:45 AM The front that moved through Friday has stalled to our south.

Moisture from this stalled front is moving northward. The three versions of this morning’s HIRESW model all maintain some degree of high and mid-level cloudiness through noon and then considerable high level cloudiness in the afternoon with some sun filtering through. Next week looks very unsettled as tropical storm remnant moisture will move in to our area.

Sunday updated forecast. Little change. Clouds give way to partly sunny to mostly sunny. High 85º. Next week looks very unsettled as tropical storm remnant moisture will move in to our area.

Sunday looks to have some cloudiness in the morning, especially south of the city, becoming sunny and cooler. NBM high temp 84.1º ± 2.0º.