Monthly Archives: July 2023

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#weather #paweather #wx #pawx #philadelphia #phillywx

Friday Forecast Update

Updated Fri 08/04 @ 2:11 PM — I’ve delayed this update to allow time for clarification regarding the line of storms expected with the cold front passage this evening.

There’s still a wide range of forecasts, with some models having the line of showers/storms move through between 6 and 9 PM while other forecast guidance has the storms closer to 9 PM to midnight.

Most, but not all, models have the storms losing upper level support causing them to weaken considerably as they approach Philadelphia.

The latest HRRR continues with that scenario, but does have some potentially severe storms in Lehigh Berks counties and upper Bucks/Montgomery counties. Those storms weaken considerably as they move southeastward towards Philadelphia.

Here’s the 15z HRRR showing elevated CAPE and highly elevated helicity in Berks county.

The latest HRRR has a later arrival of any showers/storms here, between 9 PM and midnight—

Today’s 15z HRRR shows simulated radar and elevated surface CAPE (orange-yellow). Highly elevated helicity values (contours) in white box. (Click on image for a larger view.)

We’ll see some sunshine break through this afternoon. The more sunshine, the more energy for storms.

As for Saturday, the forecast promise of a perfect weekend is not all that clear. Several models are showing considerable cloudiness lingering on Saturday as low pressure develops on the stalled front near Virginia. More details later this evening.

Friday Forecast

Update Thu 8/03 10:01 PM — Tonight’s models show any shower or thunderstorm activity tomorrow to be unimpressive, especially as they approach Philadelphia tomorrow evening. It appears they will lose their upper air support and weaken as they approach the city.

Updated Thu 08/03 @ 5:47 PM — There’s some uncertainty around the forecast for tonight and Friday. The HRRR and HRDPS both show clouds early Friday morning breaking for sunshine by noon. Then a fast moving cold front moves through between 7 and 10 PM Friday evening with showers and thunderstorms, some strong.

On the far other end of the spectrum is the latest NAM-NEST which shows considerable shower activity this evening and tonight into Friday morning. The NAM-NEST continues with the showers into the afternoon and has little in the way of sunshine. I’m going to downplay this NAM-NEST forecast for now and go with the drier HRRR/HRDPS forecast for the daytime Friday.

Some strong storms possible Friday evening with the cold front. Stay tuned.

Thursday Update
Thu 5:44 PM Forecast Review — The skies looked threatening at times early this afternoon, but only a few isolated showers broke out about 5 PM in Bucks county. Overall, we had more clouds than had been forecast and much less in the way of showers.

Updated Thu 08/03 @ 9:31 AM — Most of our NCEP models continue to keep us dry during the daytime today (Thursday). but notably, the ECMWF, RGEM and HRDPS forecast some light scattered shower activity in the afternoon. To try to determine which is correct, let’s see if there’s any satellite imagery that supports showers—

This morning’s IR Satellite shows several weak disturbances in the jet stream flow. I’m now leaning towards some widely scattered shower activity in the early afternoon. Most areas won’t have any showers. (Click on image for a larger view.)

I’m now leaning towards some widely scattered shower activity in the early afternoon. Most areas won’t have any showers.

Thursday Outlook

Update Wed 8/02 10:59 PM — After reviewing tonight’s early models, my earlier forecast still holds.

So mostly sunny, a few periods of cloudiness. During the afternoon, a few isolated showers west of the city, but most areas dry. More scattered showers possible in the evening, again mainly west of the city.

Updated Wed 08/02 @ 5:23 PM — The previously forecast warm front isn’t looking that impressive for Thursday, yet some models are cranking out periods of sun and clouds and very widely scattered showers on Thursday afternoon and evening. Most of the showers are forecast to be light and west of Philadelphia, but we’ve seen that forecast accuracy of location of precip is less than stellar.

So mostly sunny, a few periods of cloudiness. During the afternoon, a few isolated showers mostly west of the city, but most areas dry. More scattered showers possible in the evening, again mainly west of the city.

A weak warm front will cause some of these showers, but it will be enhanced by a rather strong jet streak.

Read more: THIS WEEK’S WEATHER
12z ECMWF jet stream wind forecast for Thursday at 11 AM The jet streak position (white arrow) (red/magenta =high winds) puts our area into the left exit region (white box) of the jet streak . Rising motion occurs in the left exit region; with moisture, showers develop. (Click on image for a larger view.)

The forecast for tomorrow will be interesting. The latest NAM-NEST and HIRESW and RGEM models have scattered shower activity in the afternoon and evening. The latest HRRR and HRDPS show virtually no shower activity in the afternoon.

I’m leaning towards the sunnier and drier forecast for tomorrow, but it’s a bit lower confidence than average.

So mostly sunny, a few periods of cloudiness. During the afternoon, a few isolated showers mostly west of the city, but most areas dry. More scattered showers possible in the evening, again mainly west of the city.So mostly sunny, a few periods of cloudiness. During the afternoon, a few isolated showers mostly west of the city, but most areas dry. More scattered showers possible in the evening, again mainly west of the city.

Wednesday -Friday Update

Updated Tue 08/01 @ 7:49 PM — Not much happening weather-wise. Another beautiful day on Wednesday.

On Thursday, a warm front approaches with an increase in cloudiness, but the showers and clouds appear to be unimpressive.

Friday looks to be the most unsettled with periods of light showers, but the models aren’t showing any severe weather at this time. A cold front moves through Friday night with some showers and thunderstorms.

The weekend looks good!

Previously Posted Mon 10:47 AM —

Overview

There isn’t much to talk about weather-wise for the next three or so days. We’re in an upper level trough, with associated cooler weather, while the Midwestern US will be very hot, although not to levels seen for much of earlier July.

Expect clear and sunny weather through Wednesday, with low humidity.

On Mondays, I usually begin This Week’s Weather with a jet stream level map (250 mb level in summer). I’ll mix it up this week with a 500mb map (mid level atmosphere ~ 18,000 feet) this week, (which looks similar jet stream level map)—

Today’s 06z NAEFS 500 mb forecast map for Tuesday morning. Mid- level low and trough (1) bringing cooler weather here while mid and upper level high (2) keeps hot weather over the central US, all the way up to the Canadian Border.

Notice the orange contour that snakes across Florida and extends up the Canada in the central US. South of this 5476 “500-1000 mb thickness” contour, it’s extremely hot and humid. (Notice it’s just south of the jet flow winds.)

In this setup, impulses move along and near this thickness level with rain and unsettled weather in most cases. By Thursday, the mid level flow flattens out and the orange contour will be closer to our area, bringing increased chances of showers and thunderstorms.

Enjoy the good weather!

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Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Update Sat 7/29 5:06 PM — A quick mobile update. Based 18z HRRR and current radar/satellite imagery, heaviest activity into NJ and South Jersey. Philadelphia and immediate western suburbs may just get clipped by current track of storms, mostly going eastward.

#Philadelphia #weather #PAwx

Update Sat 7/29 12:40 PM — A quick mobile update. Latest HRRR and HRDPS has the storms between 4:30pm and 6pm. The heaviest activity will move just north and just south of the immediate Philadelphia area. We’ve seen that storm placement often is inexact.

#Philadelphia #weather #PAwx

Updated Sat 07/29 @ 9:17 AM — Yesterday’s HRRR and HRDPS combined yielded the best timing forecast for those storms last night. As always, predicting the precise location of the storm’s path is beyond the capability of today’s numerical weather models. (I provide it only as a guide.)

Kudos to Channel 6’s meteorologist Brittany Boyer for her candid acknowledgement that their previous day’s thunderstorm forecast wasn’t what they had predicted and for directly acknowledging that the models of late haven’t been too accurate with thunderstorm predictions. Intellectual honesty and humility isn’t something you see often with many of the of the other TV weather people. Refreshing!

As for today, the 06z HRRR and 06z HRDPS show thunderstorms moving into the immediate Philadelphia areas and suburbs from the west as early as 3:30 PM to 5 PM. Heaviest activity is currently forecast to our south, in Delaware.

What’ most impressive with today’s severe parameters is elevated helicity and shear. Jet stream winds also higher than previous days.

Those severe parameter mean that storms should stay together and there’s a very slight risk of tornadic activity. Straight line wind gusts of 40+ mph possible.

This morning’s HRRR shows that this system will be faster moving than previously thought.

Here’s this morning’s 12z HRRR just available showing the model’s guess of where the heaviest rainfall will be—

12z HRRR total predicted rainfall for Saturday late afternoon into evening. (Click on image for a larger view.)

Updated Fri 07/28 @ 7:36 PM — Approaching storms at 7:28 PM—

Previously Posted Fri 5:28 PM —

Our first focus will be this evening (Friday), where both the 12z HRDPS and the latest 18z HRRR show an upper disturbance setting off thunderstorms between 9 PM and 1 AM tonight, mostly just north and west of Philadelphia.

With PWATs in the 2.1″ range, the HRRR is cranking out some locally heavy rain this evening in some areas—

This afternoon’s 18z HRRR shows some heavy rainfall in some areas just north and west of Philadelphia. (Click on image for larger view.)

Saturday

The disturbance affecting us Friday evening may linger into early Saturday morning with additional widely scattered showers. Sunshine expected by mid to late morning. Saturday will be hot and very humid. While temperatures will be a bit lower than Friday, humidity levels are expected to be much higher.

An approaching cold front will bring an increasing chance of thunderstorms later Saturday afternoon. The models show this front coming through slowly and the initial batch of showers may go to our north and our south. Additional showers and thunderstorms as late as midnight and 1 AM Sunday morning.

High temperature 92.1º sd 2.0º ( NBM model— location Blue Bell, PA)

Sunday

There’s some uncertainty with Sunday’s forecast. Some models have some widely scattered showers around as the front will linger somewhat near us. The general headline is sunny and cooler, but it may not be that simple.

We’ll go with some lingering clouds possible in the early morning, then sunshine. But don’t be surprised if there’s a slight forecast change. It will be less hot and less humid.

High temperature 83.8º sd 3.0º ( NBM model— location Blue Bell, PA Note that the high sd of 3.0º reflects some of the uncertainty. )