Category Archives: Philadelphia Weather Outlook

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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#Philadelphia #weather #PAwx

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 Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx

Posted Thu, 08/29/24 @ 7:44 PM— Clouds moved back in Thursday afternoon and low level cloudiness is expected for much of Friday. Showers and storms in central PA this evening are again having trouble moving into the Philadelphia area intact.

The models (again with the exception of the ECMWF) have some light showers making into the immediate PHL area in the predawn hours Friday. Any rainfall here will be light.

Low clouds and even some drizzle is expected on Friday with an easterly wind providing the moisture. Some additional showers may pop up Friday afternoon due to moisture convergence.

Saturday is looking fairly cloudy, except at the shore. A cold front moves through with showers and storms Saturday night and some showers will linger into Sunday, especially in NJ.

Sunshine for Monday, but some high clouds may linger at the shore due to a low pressure system developing on the front to our south.

The model forecasts haven’t been too stellar this week. My guess is that changes in the Labor Day Weekend Forecast are likely.

Thu 8:10 AM —Forecast Review — The areas of showers and thundershowers last night went to our north and to our south, as shown by this morning’s MRMS 24 hour precip summary—
MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)

The expected complex of storms expected to move just near Philadelphia at 3 AM didn’t materialize. Numerous models got this wrong, with the notable exception of the ECMWF.

Many parts of our area haven’t had very much rainfall in weeks. The forecast for tonight (Thursday night) into Friday is for significant rainfall in the Delaware Valley and the ECMWF is currently on board with this forecast.

Thunderstorm Possibility

Posted Wednesday 08/28/24 @ 7:43 PM — So far, our area hasn’t seen any scattered storms. The current water vapor and radar shows storms developing to our north and southwest at 7:30PM—

Water Vapor and Radar with RAP model wind streams and vertical motion / moisture convergence. I’ve put a box around an area showing strong vertical motion. (Click on image for a larger view.)

The afternoon models have storms to our north and just to our south about 9-10 PM. I can see why. Nonetheless, there’s an area of strong vertical motion to our immediate northwest and it might lend a hand to some storm activity in our immediate area.

The models continue to have another batch of storms move through between 2AM and 6 AM, although the intensity expected has diminished from previous model runs.

Here’s the experimental RRFS forecast for 3 AM—

08-28-24 18z RRFS forecast for 3 AM. Precipitation, Precipitation Rate and clouds (black) (Click on image for a larger view.)

While some rain is possible in most areas, only a few areas will have much needed heavier rainfall.

Wednesday Forecast Update

Posted Wednesday 08/28/24 @ 9:45 AM — The thunderstorm and rain forecast for today remains below average confidence. The ECMWF continues to forecast minimal rainfall for the Philadelphia area.

Our NOAA models and the Canadian models had been forecasting a main group of storms moving through about 2-4 AM Thursday, with earlier scattered activity during the evening.

However the latest 12z HRRR just became available and it shows minimal rainfall and highly scattered activity. The experimental 00z and 06z RRFS continues to show a heavy group of storms moving through 2-4AM, but the 12z RRFS is not yet available.

So the trend is for minimal rainfall in the immediate Philadelphia area, a big shift from what was forecast yesterday and closer to the ECMWF. As mentioned in several previous updates this week, the models have been unusually inconsistent.

Here’s the latest HRRR total rainfall through 8 AM Thursday. The HRRR has all of this falling after midnight tonight —

Wednesday Forecast Update

Posted Tuesday 08/27/24 @ 5:29 PM — There continues to be large model differences in the forecast of showers and thunderstorms Wednesday late afternoon and Wednesday night. As a result, this is a low confidence forecast.

Basically, temperatures are still expected to reach 96º-97º or so, according to the latest NBM.

The big question is the timing and coverage of any showers and storms. The models have lacked continuity between themselves and their own previous runs.

Some models have storms developing and moving through late afternoon 4- 6 PM, mostly scattered. The HRRR and RRFS both have a large complex of storms moving through between 2 AM and 6 AM Thursday morning. The ECMWF showed this earlier, but the latest run has much of the rain skipping over the city.

The latest HREF and Canadian RGEM just available perhaps has the best compromise. Scattered storms starting as early as 4 PM, and scattered storms through the evening. Then a large complex of strong storms moves through 2-6 AM Thursday morning.

Here’s the current HRRR forecast for 5 AM Thursday —

18z HRRR Precipitation rate for 4 AM Thursday morning. (Click on image for a larger view.)
Wednesday Heat Update

Posted Tuesday 08/27/24 @ 8:43 AM — The forecast continues to evolve for this week and the weekend. Wednesday continues to be a very hot day with high temperatures 96º (Blue Bell) -97º (Philadelphia). Dew points will be in the humid 70s and the heat indices will be over a 100º—

Here’s the NBM heat indices in the late afternoon Wednesday—

06z NBM heat index (“apparent temperatures”) on Wednesday afternoon. (Click on image for a larger view.)

Regarding potential rainfall, the forecast seems to change from day to day.

Last night’s models now have some heavy thunderstorms moving through the immediate Philadelphia area late Wednesday evening and into the after-midnight hours of Thursday. A significant short wave is expected to provide the energy for the showers—

06z Experimental RRFS simulated radar forecast for 4 AM Thursday morning. Heavy rain and thunderstorms move through. (Click on image for a larger view.)

The location and timing of the above is currently supported by the ECMWF model, but again, there’s been changes in this forecast almost daily.

The ECMWF also shows more thunderstorms moving through Saturday evening—

00z ECMWF 3 hour rainfall forecast for 8 PM Saturday (Click on image for a larger view.)

There’s uncertainty with the rest of the Labor Day Weekend forecast. The front was originally expected to move way to our south, but may get hung up closer to us. A cool trend that looked almost certain for next week is not a certainty now.

Forecast Update

Posted Mon, 08/26/24 @ 7:22 PM— There’s been a few forecast changes since yesterday. High temperatures Wednesday will be somewhat lower than forecast yesterday, with highs 95º-96º. Dew points will be in the low 70s and the heat index will be over 100º.

Today’s 18z NBM high temperatures for Wednesday (Click on image for larger view.)

For those waiting for some decent rainfall, it now looks like it won’t happen late Wednesday through Thursday. It looks like the bulk of the rainfall stays west of the city. Some showers are possible here, but total rainfall looks to be less than 0.25″.

The Labor Day Weekend looks to be unsettled for Saturday and first half of Sunday. Plenty of clouds with some showers possible. I expect this forecast to change.

This Week and Labor Day Weekend Weather

Originally Posted Sun @ 5:28 PM — —The cool weather we’ve been having for the past week will be replaced by possible record high temperatures Wednesday. The edge of the ‘heat dome’, which has been far south of our area, will move north of us by Tuesday into Wednesday.

Regular visitors to this site know I use the 576 thickness line (500-1000mb thickness) to demarcate the edge of the heat dome. It’s visible north of us by Wednesday—

NAEFS forecast for Wednesday at 5 PM (Click on image for a larger view.)

The NBM forecast high temperatures are in the mid to upper 90s on Wednesday!

NBM high temperatures Wednesday (Click on image for a larger view.)

By Saturday a series of cold fronts will have moved through, the heat ( and the 576 thickness line) will be well to our south, and we’ll be returning to closer to ‘normal’ temperatures here —

Saturday (Labor Day Weekend) shows the heat dome well to south again. (Click on image for a larger view.)

Next Saturday—

NBM high temperatures for next Saturday (Click on image for a larger view.)

A series of fronts may provide some light but much needed rain, possibly Monday night for southern parts of our area and again Wednesday night into Thursday morning. The rainfall late Wednesday night could be more substantial.

There is some uncertainty regarding the southward push of the cold front for next weekend. The front may stall to our south, with showers and thunderstorms for our area. The NAEFS forecast for Saturday above assumes a better weekend. This morning’s GFS has the front stalled to our south. Too soon to know for sure.