Category Archives: Philadelphia Weather Outlook

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx

Friday Forecast

Posted Thursday 07/04/24 @ 7:56 PM — Friday will be similar to Thursday, but somewhat hotter. Highs in the mid 90s and humid. Unlike today, the greatest chance of showers and thunderstorms will be far northwest, north and northeast of us, possibly affecting the north Jersey shore.

Here’s the 7:45 PM MRMS radar, which seems pretty close to this morning’s HRDPS forecast posted below.

MRMS radar with RAP “potential precipitation placement” contours (yellow) (Click on image for a larger view.)
4th of July Forecast

Posted Thursday 07/04/24 @ 12:42 PM —

No significant change in the forecast posted just below, based on this morning’s models.

Posted Thursday 07/04/24 @ 9:28 AM —

A mix of sun and occasional cloudiness for most of today. Cloudiness should be mostly thin, high cirrus cloud type.

Uncertainty remains regarding any thunderstorms – timing and location. The model trend over the past 12 hours has been a slight increase in the chance of widely scattered thunderstorms as early as 2 PM. Most likely location is far west of the city.

The latest ECMWF just available has some heavier showers/thundershower just south of the city during the evening hours. The HRDPS sort of captures the ECMWF and NOAA models with its precip forecast—

HRDPS precip forecast for 7 PM Thursday evening (Click on image for a larger view.)

I’ll do a quick update later this morning

July 4th Weekend – Thursday – Sunday

Posted Wednesday 07/03/24 @ 7:41 PM —

The latest GFS model is similar to the Canadian model and has a few showers and thunderstorms moving into the Philadelphia area Thursday evening. Any showers and storms will be scattered.

Unfortunately, an unsettled pattern develops for Friday and Saturday with a chance of of somewhat more widespread showers and thunderstorms, especially late afternoon and evening, but there’s some possibility throughout period. Otherwise sunshine with periods of cloudiness.

The greatest chance of thunderstorms in the immediate Philadelphia region will be late afternoon Saturday afternoon and evening.

All days appear to be hot with highs in the low 90s!

Dew points in the uncomfortable 70s through Saturday. Dew points drop on Sunday, so it will feel somewhat cooler. Sunday will be dry.

Posted Wednesday 07/03/24 @ 5:46 PM —

The Canadian models just became available after my recent post. They lean more toward some thunderstorm activity later in the evening, even into NJ. I’ve been impressed with the Canadian model’s thunderstorm predictions.

Posted Wednesday 07/03/24 @ 5:21 PM —

The trend for July 4th is hot and steamy with thunderstorms possible western areas. Temperatures will be in the 91-92º range (± 1.4º) for both Philadelphia and Blue Bell locations.

Dew points are forecast to be in the uncomfortable 70s, creating a heat index in the upper 90s—

NBM heat index for July 4th.

It’s still a tough call about potential thunderstorms Thursday late afternoon into evening. They can’t be ruled out, especially west of the city—

Today’s 18z NBM thunderstorm probability at 6 PM. The probability does NOT increase in Philadelphia over the evening hours. While 26% doesn’t sound like much, that is an hourly probability, which is a decent chance. The 15% in Philadelphia is low, but still a possibility. (Click on image for a larger view.)

The HREF gives an indirect probability of thunderstorms through its probability of radar echoes greater than 40 db at 6 PM —

HREF probability of radar echoes greater than 40 db. at 6 PM (Click on image for a larger view.)

So the best we can say for tomorrow right now is that the further east of the city you are, the less chance of showers/thunderstorms in the early evening.

Look for an update tomorrow morning.

Posted Wednesday 07/03/24 @ 8:24 AM —

As updated last night, the latest models are showing an increased chance of thunderstorms moving further into the Philadelphia area than previously thought. Most of the activity should remain west of our area, but there is now a significant probability of some scattered showers and thunderstorms moving in as early as the afternoon and more likely in the evening hours.

06z HREF 1 hour probability of radar echos >40 db. at 9 PM Thursday (Click on image for a larger view.)

Posted Tuesday 07/02/24 @ 9:57 PM —

Tonight’s HRRR shows some showers moving in Thursday afternoon, somewhat negating my earlier outlook. This may change again.

Posted Tuesday 07/02/24 @ 8:46 PM —

Thursday is looking hot again with temperatures at or just above 90º in the city, upper 80s surrounding areas.

There’s been a consistent forecast of showers/thunderstorms that barely make it into Philadelphia during the evening hours. Indeed, most models keep any showers and storms just to our west. This forecast continues to hold with the latest models, so fireworks displays in city and immediate surrounding areas. Western suburbs may see some showers/storms. It’s going to be close, so I’ll continue to monitor it.

Here’s the latest NAM-NEST, showing the eastward extent of the showers/storms at 11 PM Thursday evening—

18z NAM-NEST shows the eastward extent of the showers reached at 11 PM. They seem to hit a wall as they go further east and dissipate. (Click on image for a larger view.)
Monday through July 4th

Originally Posted Mon 8:20 AM —High pressure builds in for Monday and Tuesday with beautiful weather. By Wednesday, the high is offshore and a southerly flow of much warmer (hotter) air moves in late Wednesday—

NAEFS model statistical “mode” version with overlayed GEFS model “bias-corrected” precipitation forecast for Wednesday at 2 PM Notice the orange 576 line, the approximate edge of the heat dome will be pushing back into our area. The next weather maker is the low pressure system entering the Dakotas. The hurricane in the Caribbean ocean isn’t visible on this map. (Click on image for a larger view.)

By Thursday, July 4th, the heat dome has moved up past us and moisture associated with the low to our west is on our doorstep—

By Thursday, July 4th at 8 PM, the GFS has the heat dome edge north of our area and some moisture and showers at our doorstep with a warm front. (Click on image for a larger view.)
Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx

Thu 8:14 AM —Forecast Review —Yesterday’s storms came through between 10 PM-2 AM, later than originally forecast in the morning, but the afternoon update seemed to be on target.

The axis of heavier rainfall was nowhere near where it had been forecast. Some areas received a good soaking, other areas had little rainfall.

Here’s the MRSMS summary—

MRMS rain-gauge measured rainfall combined with rain-guage 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.)

Update

Posted Wednesday 06/26/24 @ 5:52 PM — The latest NBM hasn’t changed significantly, but several of this afternoon’s models have moved the timing of the storms here in the Delaware Valley later, from 10 PM through 2 AM.

At 5:40 PM, things aren’t quite popping.

MRMS radar with RAP model parameters. Nothing happening in the ? areas. (Click on image for a larger view.)

Posted Wednesday 06/26/24 @ 1:32 PM — The 15z NBM includes more of this morning’s models. It’s a little more optimistic about the storms making it into Philadelphia, but then dissipating afterwards. This is consistent with the newest HRDPS.

NBM thunderstorm coverage (scale 1-4) at 9 PM —

15z NBM thunderstorm coverage map. This is more optimistic about the heavier rain at least getting into Philadelphia this evening. (Click on image for larger view.)

Posted Wednesday 06/26/24 @ 11:12 AM — While the new HRDPS is not yet available at this hour, this morning’s NOAA models are increasingly trending towards rainfall dissipating as the storms approach and move past Philadelphia. Much needed rain may only reach western suburbs.

NAM-NEST forecast 7 PM and 9 PM showing significant storm dissipation as it moves into unfavorable conditions for convection (thunderstorms) (Click on image for a larger view.)

As we’ve seen, the exact placement is rarely forecast exactly in advance. But a much needed rainfall may not materialize for some areas.

Some of the factors responsible for this thunderstorm dissipating trend are reviewed in my post on dry weather from 2022. Many of the features are the same.

Thunderstorm Update- Wednesday Evening

Posted Wednesday 06/26/24 @ 9:44 AM — A quick update. This morning’s HRRR just became available. As expressed below, I have concern that large “convective inhibition values” (CIN) and low soil moisture feedback mechanisms may adversely affect rainfall totals in the immediate Philadelphia area. The latest HRRR seems to suggest that.

I guess we’ll see tonight.

Posted Wednesday 06/26/24 @ 8:29 AM — In keeping with with my revised probability approach to thunderstorm prediction, I’m going to lean on the probability elements of the HREF, HRDPS and NBM for tonight’s thunderstorm forecast. We can use the rain!

The cold front tonight will trigger showers and thunderstorms. While strong storms are likely in some areas, I would put the severity (hail, high winds) probability as a 1 out of 3 in the Philadelphia area and immediate surrounding counties. The latest HREF shows the areas of greatest rainfall and severity at 9 PM—

HREF hourly probability of strong radar echos at 9 PM Highest probaility of storms is northwest and north of the city. (Click on image for a larger view.)

Showers and thunderstorms will be spotty in areas west of Philadelphia as early as 5-6 PM. The main area of showers and storms moves in between 7 and 10 PM and showers linger into the predawn hours of Thursday.

The NBM shows the highest density of thunderstorms (“thunderstorm coverage”: on a scale of 1-4 ) at 9 PM—

06z NBM thunderstorm coverage at 9 PM Purple is high coverage, yellow is scattered moderate coverage. (Click on image for a larger view.)

The HRDPS has the following rainfall totals (which is on the high side for the immediate Philadelphia area compared to other models) —

06z HRDPS Total Rainfall. The HRDPS did very well with the last thunderstorm rainfall totals, but many models show less rain than this. (Click on image for a larger view.)

I’m cautiously optimistic that we’ll get some much needed rain tonight, but with many models showing a “tongue” of the heat dome pushing into our area at the beginning of the event, I’m concerned that a wall of convective inhibition (CIN) and low soil moisture in our area may cause the storms to lose intensity as they move from west to east.

I’ll update later if things change.

Thunderstorm Probability Wednesday Evening

Posted Tuesday 06/25/24 @ 8:13 PM —The models have been inconsistent today regarding thunderstorm coverage and probability for Wednesday night ahead of a cold front.

Earlier today, it appeared that any thunderstorms would “hit a wall’ and decrease in intensity as they entered Philadelphia and attempted to move east into NJ. Rainfall totals this morning were forecast to be somewhat on the low side.

The latest models (18z – run 2 PM EDT) have increased the chance of thunderstorms with locally heavy rainfall in the Philadelphia area, with lesser amounts in eastern NJ. Most likely timing in Philadelphia and immediate suburbs is 8 PM to 11 PM Wednesday evening.

Here’s the current 18z NBM probability of thunderstorms at 10 PM—

NBM 18z forecast thunderstorm probability (Click on image for a larger view.)

The latest Canandian HRDPS shows a maximum at 9 PM—

18z HRDPS thunderstorm Probability (Click on image for a larger view.)

The HRDPS did well with our last frontal passage. Here’s its current total rainfall forecast —

HRDPS total rainfall. Blue-violet is > 1inch. Some areas will again have unimpressive rainfall. (Click on image for a larger view.)

Right now, I’m skeptical about heavy rain here and things will likely change tomorrow. Check back for an update tomorrow morning.

Strong storms are possible in some areas.

Previously Posted Mon 12:51 PM —

Mon —Forecast Review — As forecast, Sunday night’s storms blossomed in NJ but most of the Philadelphia- Delaware Valley area west of I-95 received little or no rainfall.

Particularly impressive was the HRDPS forecast which closely captured the band of heavy rain in NJ.

Here’s the MRMS rainfall summary for Sunday night—

MRMS rain-gauge measured rainfall combined with rain-guage 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.)

For today, Monday, the heat dome is suppressed to our south and we’ll be in a cooler cyclonic jet flow north of the heat dome.

NAEFS shows jet stream dip over the Northeastern US on Wednesday. Of interest is the cold pool of air still visible in Canada demarcated by the red 540 thickness line. It’s strange to still see this feature this late in June. Note the heat dome, demarcated by the orange 576 thickness line is suppressed to our south for now.

By Tuesday and Wednesday, the heat dome starts poking up northward and temperatures return to upper 80s Tuesday and near and above 90 Wednesday.

Here’s the surface map for late Wednesday showing a cold front moving through late in the afternoon. It looks like we’ll have some rain/thunderstorms late Wednesday into Wednesday night.

NAEFS late Wednesday forecast with approaching cold front. (Click on image for a larger view.)

After a brief cool-down Thursday, temperatures look to rise again into the 90s by Saturday and Sunday. Another cold front moves through next Sunday night.