Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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Not a Drought Buster

Posted Thursday 09/05/24 @ 5:53 PM — Little change in the forecast for Friday and Saturday.

Friday will have clouds with some light drizzle or sprinkle early in the day as a warm front approaches.

Saturday will be cloudy with light sprinkles possible in the morning, showers move through with a cold front early Saturday afternoon, clearing early evening.

We need some rain and the total rainfall looks disappointing, with many areas coming with 0.20 inches or less of rain. The trend has been towards lower rainfall totals.

Here’s the model blend (NBM)—

18z NBM rainfall totals forecast by 8 PM Saturday. (Click on image for a larger view.)

Posted Wednesday 09/04/24 @ 8:57 PM — Latest models showing minimal rainfall for Saturday and a faster frontal passage

Posted Wednesday 09/04/24 @ 6:00 PM — One more day of incredible weather on Thursday.

Friday‘s weather is looking much less outstanding. Earlier this week, it looked like some clouds would move in with an easterly flow. Several model runs now show some light showers and possibly some drizzle by early afternoon on Friday with the easterly flow. This is especially true at the NJ Shore.

The rainfall expected on Saturday will not be a drought buster and may be a disappointment. A front moves through during the daytime Saturday with showers and even some thundershowers. Showers may start as early as mid to late morning and continue through the early evening.

Depending upon the closeness of a low pressure system in the Atlantic, an unknown amount of moisture may be available for showers here on Saturday.

The ECMWF and ICON models have the low further away and the total precipitation here may only be about 0.2-0.3 inches. The GFS and Canadian GDPS have the Atlantic low closer to us, with more moisture available here. Amounts may range from 0.4″ to 0.6″.

The model blend (NBM) is showing the average, about 0.3″ rainfall. Not a drought buster.

12z GFS forecast for 1 PM Saturday. Front moving through our area. Low pressure off the North Carolina coast. Exact position of this low will determine how much moisture becomes available for rain in our area. (Click on image for a larger view.)
Forecast Update

Posted Tuesday 09/03/24 @ 6:07 PM — Wednesday and Thursday continues to promise beautiful weather here in Philadelphia with gradually increasing temperatures approaching or exceeding 80º —

18z NBM meteogram forecast for KLOM Blue Bell, PA Temperatures approach or exceed 80º. (Click on image for a larger view.)

Clouds increase on Friday, but rain is currently expected to wait until Saturday, as an approaching cold front and moisture from a system in the expected to develop near the North Carolina coast that may take on some tropical characteristics.

Here’s the latest ECMWF- AI (machine learning) model forecast for 2 PM Saturday—

12z ECMWF AI machine learning model for 2 PM Saturday. This is my first posting of an AI-model forecast. (Click on image for a larger view.)

At this time, total rainfall in the Philadelphia area on Saturday will range from 0.40 to 0.50 inches. which is already trending lower from yesterday. The higher resolution models will provide better clarity with the areal coverage of the rainfall, but Saturday is currently beyond the forecast range of these models. Some thunderstorms are also expected on Saturday ahead of a cold front passage.

More Diminished Rainfall for Philadelphia

Posted Tuesday 09/03/24 @ 8:10 AM — I’m continuing with the thread about the current trend of rainfall diminishing as it approaches Philadelphia.

Our next chance of rain will be Saturday, with clouds on the increase Friday, possibly with some light showers east on Friday. Here’s the latest ECMWF and GFS forecast rainfall for Saturday, ending Saturday night.—

GFS total rainfall by Saturday midnight (Click on image for a larger view.)
ECMWF total rainfall by 2 AM Sunday morning. Frankly, I would be surprised if so much rain falls into Philadelphia. (Click on image for a larger view.)

These numbers and areal coverage will certainly change by Saturday. The observation that the rainfall drops off towards Philadelphia isn’t by chance; there is a true mesoscale physical process at work, captured in the models, in the current weather pattern. This process is likely related to the points made in the boxed comment below.

Regarding our dry conditions, prompted by a reader’s comments —

I’ve been doing much reading behind the scenes to try to understand why the rain is dissipating in the immediate PHL area. I’ve seen this behavior over the years, during what I call “drought-like” times. Last time was the 2022.

One thing seems to be a anticyclonic curvature of the upper air winds and the development sort of a high pressure wall. Certainly true with the 576 thickness makes a curvature just east of Philadelphia.

The other thing is that Philadelphia is located just east of what they call the ‘fall line’ – the terrain altitude drops 200-300 feet from parts of upper Montgomery and Chester counties to the river. That drop in altitude seems to counteract any upward vertical motion needed for the rain to occur. By the time disturbance gets to the Delaware river, the terrain altitude is flat, so there is no other loss of vertical motion and whatever weak ‘forcing’ upward of air for rain can resume without being negated. So it skips over Philadelphia.

There is something called a local heat dome, with the “urban heat island” so warm that normal convection is inhibited right over the city.

Finally, there is a moisture feedback loop. Once soil moisture is depleted, you get into a dry begets dry situation that results in a dry feedback loop.

Any and all of these theories could explain what we’re seeing.

Originally Posted Mon @ 10:10 AM — —High pressure builds in and moves due eastward, giving us fair weather and comfortable temperatures/humidity Monday through Wednesday and possibly Thursday—

NAEFS forecast for Wednesday at 11 AM High pressure (1) will be giving us sunny skies, but with an easterly cool flow of air. Low pressure (area 2 and and 3) will develop and bring rain for possibly as early as Friday and likely Saturday. There are differences in the GFS and ECMWF regarding the rain and the area of eventual low pressure development. (Click on image for a larger view.)

There is also potential tropical development in the Gulf of Mexico later this week, but it’s unclear if it will actually occur.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Labor Day Weekend Forecast

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Forecast Update

Posted Sunday 09/01/24 @ 9:35 AM — The forecast continues to have cloudy skies (perhaps with some bright spots at times) and not much rainfall as a front is stalled over our area and small disturbance will move up towards us—

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Stationary front over our area. (Click on image for a larger view.)

A few weak disturbances will move along the front with some light showers possible Sunday afternoon and again this evening as a secondary cold front is expected to move through late.

Increasing sunshine from northwest to southeast Monday. Cooler with lower humidity.

Unseasonably cool weather this week. Sunny with little chance of rainfall.

Sat 4:30 PM —Forecast Review — We had some sun and clouds, but the amount of sunshine was considerably less than forecast. As a result, high temperatures were lower than forecast, closer to 82º and dew points were only in the upper 60s instead of the low 70s. Current radar shows a line of storms and showers near Harrisburg, not forecast by the models.
Radar at 4:35 PM. The complex of storms near Hazelton is dissipateding as forecast. (Click on image for a larger view.)

So this weekend’s forecast is continuing with the lack of accuracy and low confidence that had been the situation of this past week.

Rain is forecast by the models to reach us about 11 PM and after midnight. It’s also expected to weaken considerably as it approaches, with heavier rain in northern Bucks county. There’s low confidence in this forecast, since the current situation isn’t being accurately modeled.

While the timing is uncertain, I do think there will be weakening of the storms as they approach our area.

Saturday Forecast Update

Posted Saturday 08/31/24 @ 8:24 AM — Last night’s 06z models show more sunshine for today than previously forecast in many, but not all, areas.

Here’s the latest RRFS cloud cover forecast for noon—

Experimental RRFS cloud- radar forecast for 12 PM Saturday. Blue=low clouds, maroon=mid-level clouds, tan = high cirrus clouds (Click on image for a larger view.)

Regarding showers, the showers associated with the approaching cold front move in late from the northwest, about midnight tonight (Saturday).

Heaviest showers appear to skip over the Philadelphia area, keeping many areas in a rainfall deficit.

RRFS cloud-simulated radar forecast for midnight. Blue=low clouds, maroon=mid-level clouds, tan = high cirrus clouds (Click on image for a larger view.)

For Sunday, low clouds break for sunshine through high cloudiness by early afternoon. Cloudy at the shore. Some scattered instability showers develop after 3 PM throughout the area.

Originally Posted Fri 5:19 PM — A warm front will pass north or our area Friday night into Saturday. An approaching cold front will bring showers and storms Saturday night into Sunday morning. The front will slow on Sunday.

Here’s the current water vapor image showing these features—

Current (4 PM Friday) satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. (Click on image for a larger view.)

The front will slowly sag south as weak low pressure develops along the front, causing clouds and showers to linger through Sunday.

Saturday Forecast

Some areas will see sunshine in the morning, some areas west will have more clouds and some spotty sprinkles. Overall, very changeable skies in many areas. The Jersey shore looks to have a fair amount of sunshiine.

Here’s the RRFS cloud/radar forecast for 2 PM—

RRFS experimental Clouds with simulated radar at 1 PM Saturday. Darker is cloudier. (Click on image for a larger view.)

Humid with dew points in the low 70s.

NBM high temperatures: Blue Bell, PA 85º Philadelphia, PA 86º
Above Average Uncertainty (based on standard deviation): ± 2.3º

Sunday Forecast

Cloudy with lingering showers in the morning. Brighter skies at the west of the city in the afternoon with clouds and possible showers lingering in NJ and at the shore.

Here’s the latest experimental REFS depicting showers at 2 PM Sunday—

REFS forecast for 2 PM Sunday. (Click on image for a larger view.)

Increasingly breezy. Humid with dew points in the low 70s.

NBM high temperatures: Blue Bell, PA 85º Philadelphia, PA 86º
Average Uncertainty (based on standard deviation): ± 1.8º

Monday Forecast

Sunny and breezy. Low humidity

NBM high temperatures: Blue Bell, PA 77º Philadelphia, PA 79º
Low Uncertainty (based on standard deviation): ± 1.3º

Weather… and Other Things 'Up in the Air'