#Philadelphia #weather #PAwx #PhillyWx

Originally Posted Fri 5:53 PM —For Friday evening/night, there are thunderstorms to our west, but a strong area of downward vertical motion (omega) over the city will cause these storms do dissipate by 8 PM. It doesn’t appear that any storms will make it into the city.

High pressure pushing down from Canada will push the edge of the hot air to our south on Saturday with a backdoor cold front. Many pop-up storms will develop in our area as early as noon on Saturday. The humidity and storms will be far to our south on Sunday.

Saturday

The backdoor cold front will bring showers and storms as early as noon to 2 PM. Precipitable water will be high near the city and southward; any storms could bring heavy rains.

Highs (Model Blend): Blue Bell- 85 º Philadelphia 86º ±2.3º Average sd and uncertainty.

Sunday

Mostly sunny in the morning; Some cloudiness increases from the west in the afternoon. Highs (Model Blend): Blue Bell- 84 º Philadelphia 86º ±2.4º Average sd and uncertainty.

#Philadelphia #weather #PAwx #PhillyWx

Friday Forecast Update

Posted Friday 07/10/26 @ 9:06 AM — First, let’s look at the total rainfall for Thursday—

A few thunderstorm triggers are visible in the pipeline this morning—

For today, Friday, high temperatures will range from 88º (Blue Bell) to 90º (KPHL). It will very humid with dew points in the low 70s.

Sunshine through high cirrus clouds this morning. Increasingly cloudy (sunshine through thicker clouds) after 1 PM from northwest to southeast. A few isolated -widely scattered storms after 2 PM. Organized activity moves in after 4- 5 PM from the west; the heaviest activity will be sliding just south of Philadelphia, into Chester and Delaware counties, but other areas may see scattered activity. The total rainfall (RRFS) shows this pattern—

The NBM (model blend) thunderstorm coverage at 5 PM gives a similar view—

Muggy Friday

Posted Thursday 07/09/26 @ 7:50 PM — The rain will be exiting our area later this evening. Some clearing is expected and by Friday morning we should see sunshine through thin high clouds. What will be apparent will be the heat and humidity. While temperatures will ‘only’ be in the mid to upper 80s, dew points in the 70s will make it feel much more uncomfortable.

Clouds thicken between 2 and 3 PM Friday afternoon as another area of showers and storms moves in between 3 and 5 PM. Initially, these showers will be very widely scattered and isolated. Areal coverage of these showers should increase in the early evening.

There is a lack of model agreement regarding these showers. The GFS shows them not making it into Philadelphia; the NAM-NEST doesn’t show much shower activity at all. The HRRR as well as the RGEM shows showers many places but not near Philadelphia or the immediate suburbs. The RRFS and HRDPS are the most emphatic with these pop-up showers/storms and it’s what I’m leaning towards.

Thursday Update

Posted Thursday 07/09/26 @ 8:27 AM — A few light sprinkles moved through some areas earlier. Additional storms and triggers for storm formation are in the pipeline.

“Tracking” these storms like a freight train is really meaningless; additional storms will dynamically form near and over our area, particularly areas north and areas south of us.

Zooming in, the vertical motions directly over us have turned negative (downward) inhibiting rainfall, but just 2 hours ago, it was strongly positive. Very dynamic.

The morning models have some showers/thundershower developing and moving through as early as 11-12 noon. Additional showers during the afternoon and evening hours. It won’t be raining in any one area continuously but most areas will receive some rain today. Last night’s models suggested heaviest rains to our north and to our south. That is less obvious with today’s (06z) model runs. More importantly, look at what I call a “parameter match” with the RRFS, REFS, HRRR, HRDPS and NAM-NEST shows poor matches with actual current 3 hour conditions in the RTMA. This means that the models have a poor handle on the specifics.

Thursday Forecast

Posted Wednesday 07/08/26 @ 7:59 PM — I wanted to review a few more models. The HRRR also has storms developing before noon, somewhat earlier than the RRFS—

The HRRR also shows (not illustrated here) a second widespread batch of storms about 3-5 PM over the immediate Philadelphia area and southward.

The heaviest activity is forecast from the city southward, but again, the model placement of the heaviest activity is almost never literal.

With the exception of lightning and heavy rain potential, I don’t see parameters indicating severe weather like tornadoes. But the NWS has public safety as its mission, so expect severe thunderstorm watches and warnings tomorrow.

Posted Wednesday 07/08/26 @ 6:11 PM — Over the past couple of days, I’ve used water vapor (WV) images (with RAP model overlay) to show the advance of very moist air and higher temperatures back into our area. Here’s the latest WV image showing the edge of the heat dome and high humidity advancing towards us. The 576 thickness contour shows the advance compared to yesterday—

The models are showing the possibility of scattered light showers to our west in the morning, but all are showing much more significant rain/thunderstorms moving in around 2 PM Thursday. The amount of water in the air that can fall as rain (referred to as “precipitable water” or PWAT) is in the 2.2″ to 2.4″ range Thursday afternoon. That is very high. That means that any showers and storms has the ability to produce heavy downpours and possible flooding.

I’d like to look at additional models and I hope to post again this evening. Stay tuned.

Wednesday through Friday Outlook

Posted Tuesday 07/07/26 @ 8:04 PM — The low stratocumulus cloud layer was difficult to scour out today. We should see sun Wednesday.

Wednesday will be the nicest day of this week. Highs 82º-84º

From Thursday through Saturday, things become unsettled with not much model agreement regarding the advance of very hot air now in the Southeastern US. Some models have the boundary of this warm air moving back and forth over us. Other models keep it south of us.

Either way, it looks like we’ll be in the position for numerous showers and thunderstorms Thursday, Friday and Saturday. Exact timing is unclear at this time, but the setup favors afternoon showers and storms but possible other times as well. Stay tuned.

Tuesday – Wednesday Outlook

Posted Tuesday 07/07/26 @ 12:08 PM — The latest RRFS, just available, keeps the low clouds in our area through at least 3 PM with some brightening in some areas.

Posted Tuesday 07/07/26 @ 8:43 AM —

Tue 8:43 AM —Forecast Review — Much of the city and westward didn’t see the flareup in the rains in the afternoon as predicted by the RRFS. Some impressive rainfall was received by some areas. Here’s the two day total rainfall for our area—

For today, Tuesday, a few widely scattered showers in the morning, then clouds should thin enough by noon or 1 PM to allow some sunshine through clouds, especially from the city and westward.

For Wednesday, sunshine returns, but the edge of the tropical heat dome is pushing on our doorstep by Wednesday evening. The return of heat and humidity is not expected to be as extreme as this past heat wave.

Mon 9:37 AM —Forecast Review — The 11 PM forecast time of the RRFS was impressive; with the exception of the very heavy rainfall in some areas, there wasn’t much severe weather reported. (Berks county, Kent county DE and Huntingdon County, NJ had flooding and strong wind gusts reported.)

Overall the SWEAT index wasn’t all that useful. The relatively new CWASP from the NBM (not used) wasn’t all that impressively severe and may be a better indicator of severe weather in the future.

No tornadic activity (helicity was only moderate) and timing of the main system wasn’t at time of peak CAPE.)

Here’s the rainfall totals through 8 AM today—

Monday

Posted Monday 07/06/26 @ 9:48 AM —Rain continues to develop this morning due to a convergence of thermal level winds at mid levels—

The RRFS shows clouds thinning for some bright areas during the early afternoon. Heating of the day will cause more thunderstorms about 4-6 PM

Severe Weather Likely Sunday Evening

Originally Posted Sun 5:25 PM — The periphery of the “heat dome” (as captured by the 5760 thickness line) will be slipping slowly south of our area tonight. Strong waves and impulses are moving along this periphery and will spawn thunderstorms, some likely to be severe Sunday evening into the night.

This morning’s GFS captures this with the energy of the first wave and the edge of the heat dome—

Storms are already developing in Western and Central PA. From the model timing, it appears that the storms in Western PA will more directly affect Philadelphia between 8 PM and midnight, with the storms ahead moving somewhat north of the Delaware Valley.

Of great interest and concern is the “Severe Weather Threat Index” or SWEAT. I put this calculation together with the help of AI last year and it’s part of my routine RRFS model download post-processing. I have never seen it this high over the past year.

Also of interest and concern is the “echo height”, a standard parameter of several models. The RRFS shows 52,000 feet at 11 PM at a different location. Again, a number I have rarely seen before.

Finally, the simulated radar and rain at 11 PM—

Below is my “severity index”. I’ve revised it from past years to now use the new RRFS —

It appears that the boundary/front will be near us to spark showers thunderstorms Monday afternoon and evening.

#Philadelphia #weather #PAwx #PhillyWx

Sunday Forecast Update

Sun 9:00 AM —Forecast Review — — Yesterday’s high temperature (airport) was 101º. The rain moved in about 9 PM and tapered about 11:15PM. While Philadelphia was in line with generally the lowest rainfall amounts, areas to our north were in the band of significantly higher totals. The rains didn’t dissipate as much as I had expected in the immediate PHL area. Here’s the rainfall totals, according to the MRMS.

Posted Sunday 07/05/26 @ 8:38 AM —For today, the big question is the amount of cloud cover and how early any thunderstorms will develop. (A slowly moving front is moving south over us tonight with the edge of the heat dome (760 thickness line) moving to somewhat south of us.

Based on the latest RRFS, I’m going with clouds thin for sunshine by late morning or early afternoon. While a few isolated storms will form as early 2 PM, the RRFS shows much of the area dry through 5-6 PM with the exception of southern NJ Cape May county, which may have more numerous scattered storms late afternoon.

Widespread heavy rains/thunderstorms develop and move down from the north and west between 7 PM and 9 PM and continue through much of the night. Expect some severe thunderstorm warnings this evening.

With the edge of the heat dome right over us, expect plenty of clouds Monday with scattered showers and storms especially with daytime heating.

Lightning or Fireworks?

Updated Saturday 07/04/26 @ 5:47 PM — A quick update. The latest radar shows a series of storms on a path towards Philadelphia.

The latest RRFS and HRRR show this area of storms reaching us by 9 PM (RRFS) or 10 PM (HRRR). The storms dissipate as they move through and may end in the city by 11 PM.

This is another “out on a limb” forecast, but I guess that’s why you read this blog- to get a different perspective on the weather.

Posted Saturday 07/04/26 @ 10:01 AM — Today looks to be very hot again with the model blend high temperatures returning to 102º ± 2.4º for Philadelphia (KPHL). Just to the west of the city, as shown in last night’s temperature map, it will likely be ‘less hot’. (I can’t use the word ‘cooler’ here and dew points will be near 70º at times.)

The big question is the rain and thunderstorm threat for the afternoon and evening. Several models suggest that the immediate PHL area will be in a relative doughnut hole of precipitation/storms this evening.

The NAM-NEST shows strong triggered ‘forcing’ of storms to our west but the 700 mb wind flow may take it just north of the city and south of the city—

Trying to predict thunderstorms is like trying to predict where boiling water will create its first bubble at the bottom of a large sauce pan. This week’s thunderstorm forecasts timing and forecast tracks haven’t been very accurate.

Supporting the relative calm for Philadelphia this evening with the city being in-between main forecast tracks is the current water vapor image.

So, in summary, (and I’m going out on a weak limb here), fireworks in Philadelphia beginning around 9-10 PM may escape the onset of natural fireworks (lightning). I’ll try to update later this afternoon.

When it comes to lightning, always listen to the latest official NWS forecasts and warnings.

Originally Posted Fri 8:21 PM —

A Friday Update was posted

Another hot day is expected on Saturday while a high pressure system tries to move southward, slowly pushing the extremely hot air mass (with the heat boundary defined here as the 760 thickness line) to our south—

Numerous showers and thunderstorms will break out during the day to our west and by 5-6 PM, they will be moving in to the Philadelphia area. These storms are not the result of a fast moving cold front and as such, they will develop , re-develop and linger for much of Saturday evening. Nature will be producing its own fireworks.

By Sunday evening, the 760 thickness line is just south of Philadelphia causing many showers and thunderstorms. Exact details to follow in my Weekend Updates.

Saturday

Sunny with some high thin cirrus clouds for much of the day. Storms develop and move in between 5 and 8 PM. Model Blend Highs: Blue Bell- 99º Philadelphia 101º ±1.9º (average sd, average uncertainty)

Sunday

Considerable cloudiness. Plenty of showers and thundershowers around beginning about 2 PM. Highs (Model Blend): Blue Bell- 91 º Philadelphia 93º ±4.7º Humid! Very high sd and high uncertainty in this forecast.