WEEKEND WEATHER FORECAST

Friday, June 28, 2024 5:47 pm GFS

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Weekend Rainfall Summary

Here’s the 48 hour weekend rainfall summary. Includes Saturday and Sunday. Some areas received plenty of rain; others, not so much.

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) Red circle is the immediate Philadelphia area. (Click on image for a larger view.)

Sunday Forecast Update

Often associated with severe thunderstorms and even tornadoes, a sky of mammatocumulus clouds (also referred to as mammatus clouds) was visible before sunset tonight—

mammatocumulous clouds at sunset. (Click on image for a larger view.)

Posted Sunday 06/30/24 @ 11:12 AM —

This morning’s models continue with the forecast of heavy rain between 5 PM and 8 PM, but there’s the possibility of of showers and thunderstorms throughout the afternoon.

Posted Sunday 06/30/24 @ 8:59 AM — The latest HRDPS shows extremely heavy rain rates develop over Philadelphia between 6 and 8 PM tonight—

06z Canadian HRDPS shows extremely heavy rain rates develop over Philadelphia between 6 and 8 PM tonight. (As we’ve often seen, the actual exact placement and timing can often differ from the models. but it suggests a fairly good bet that we’ll get some heavy rain tonight.) (Click on image for a larger view.)

Sun 6:55 AM —Forecast Review — Last night’s rains were not as strong as originally forecast and came in two waves; the first diminished in intensity as it approached Philadelphia and headed northeastward.

The main group arrived after midnight, as forecast, and dropped light to moderate rainfall with a few localized heavier areas. Here’s the MRMS rainfall summary —

For today, Sunday, a very moist and unstable airmass will produce thunderstorms as a cold front approaches from the west. Some widely scattered storms are possible anytime after noon time, mostly north of the city, but the heaviest activity will be from 5 PM to 9 PM in the immediate Philadelphia area. Higher instability values (CAPE, helicity and vertical shear) may result in some storms being severe. Localized heavy rain is possible.

06z HREF shows *hourly* probability of strong radar echoes at 7 PM. Much of the area has high probability. (Click on image for a larger view.)

Weekend Forecast Updates

Posted Saturday 06/29/24 @ 5:37 PM —

Another shift in the forecast with tonight’s storms significantly diminishing in intensity as they approach the immediate Philadelphia area.

Posted Saturday 06/29/24 @ 1:28 PM —

This morning’s models are trending towards the showers and thunderstorms approaching the Delaware Valley area earlier than previously forecast— as early as 9-10 PM Saturday evening and the main batch moving in between 10 PM and 1 AM. Scattered storms may persist until 4 AM.

Moisture levels (precipitable water) is approaching 2.4″ which is almost tropical— Heavy rain and gusty winds are possible with any storms and based on elevated shear and helicity values, storms could be severe.

Posted Saturday 06/29/24 @ 10:07 AM —

Some sprinkles have broken out in some areas, but they may not be reaching the ground. These should dissipate and a mix of sun and clouds for most of the late morning and early afternoon.

Radar and MRSMS combined at 9:42 AM. These sprinkles should dissipate this morning. (Click on image for a larger view.)

Showers and thunderstorms expected between midnight and 3 AM Sunday morning. Additional showers and thunderstorms expected Sunday mid to late afternoon into the evening. Heaviest activity expected in NJ.

Posted Friday 06/28/24 @ 10:34 PM —

Tonight’s NAM-Nest is suggesting low clouds and possibly some drizzle early Saturday morning. Skies should brighten by afternoon.

Previously Posted Fri 5:47 PM —A warm front with a strong flow of moisture ahead of it will move through Saturday night and a cold front will move through later Sunday afternoon or evening. Both features are visible on this afternoon’s satellite water vapor imagery—

Friday afternoon’s water vapor imagery shows the two main systems that will affect our weather this weekend. A warm front with significant moisture followed by a cold front. I’ve drawn the current positions of the cold front and warm front. (Click on image for a larger view.)

Saturday

Saturday presents a difficult forecast challenge. When it comes to warm fronts, the degree of cloud cover ahead of the front can be tricky to predict. A best guess is that it will be cloudy in the morning Saturday, giving way to some bright spots through high clouds late morning into very early afternoon. Then increasingly cloudy mid to late afternoon into evening.

Regarding showers, the latest ECMWF the NAM-NEST and the model blend (NBM) show very light, widely scattered showers possible in some places during the mid to late afternoon. Most areas will be dry during the daytime.

Heavy rainfall with thunderstorms are expected late Saturday evening into the predawn hours of Sunday with the actual warm front. Heavy thunderstorms expected 1 AM- 4 AM. Incredibly, the latest HRRR has the heavy activity skipping past the immediate Philadelphia area. So there’s uncertainty with the rainfall as well.

Highs 85º (Blue Bell) 87º (Philadelphia). Average uncertainty. Very humid with dew points into the low 70s.

Sunday

Following the warm front passage, clouds in the morning break for hazy sunshine by the afternoon. Heavy thunderstorms late afternoon or evening.

Highs 90º (Blue Bell) 93º (Philadelphia). Higher than average uncertainty ± 2-3º Very humid and uncomfortable with dew points into the mid 70s.

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

Monday, June 24, 2024 12:51 pm GFS

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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—

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—

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.

Philadelphia Weekend Weather & Storm Forecasts