THIS WEEK’S WEATHER & MEMORIAL DAY WEEKEND OUTLOOK | Philadelphia Weekend Weather & Storm Forecasts

PERIODS OF SHOWERS AND STORMS FRIDAY

Update Fri @ 5:04 PM — Not much in the way of strong storms or severe weather in our immediate area. (Although severe weather was picked up on radar in NJ.) The amount of rain so far has been less than forecast. More showers expected later this evening and around/after midnight.

Update Fri @ 9:44 AM — Latest HRRR and RAP have converged on a peak time for storms to start: about 3 PM, earlier in western areas.

Update Fri @ 9:10 AM — Reviewing the latest hourly models, the previous forecast time frame for showers and thunderstorms has remained unchanged in our immediate area: a large range of time, from 2 PM to well after midnight.

Based on helicity values and storm motion/shear vectors, the strongest storms appear to be most likely this evening, after 5:30 PM, although CAPE values are already high at 2 PM.

Different severe parameters and factors peak at different times. There’s a strong jet stream influence later in the evening after midnight.

The severe parameter values are similar to those shown in my severe parameter table from last night.

Last September‘s Tornadoes (9/1/21)	Thursday’s RAP 21z Model	Today’s RAP 09z Model	!!
CAPE 3500-4200 Joules/kg	1200	1200
Helicity 800-1100 m^2/s^2	350	250	⇓
Vertical Shear 35-45	16-20	25	⇑
Precipitable Water 2.0″	1.8″	1.9″
Lifted Index minus 7-9	minus 5	minus 5
Peak Wind Gusts 40-50	35	32
Aligned Storm Motion and Shear Vectors- YES	YES (early)	YES 6 -9 PM	⇑
Jet stream- Highly Cyclonic	Flat to anticyclonic	More Cyclonic
Jet Stream Speed – High	Moderate	Mod-High (midnight)	!!
500mb – Highly Cyclonic	AntiCyclonic	Cyclonic midnight	!!

Updated Update Fri @ 8:44 AM

As was my forecast last night, these storms can be strong. I don’t expect severe weather, but I’m sure the NWS will issue severe storm watches and warnings. Base your plans on the NWS.

As for this weekend’s weather, this has been an especially challenging forecast, especially for Saturday, with models having inconsistent and shifting forecasts from day to day. The upper low has been hard to pin down. The ICON, whose forecast had been totally dry for Saturday has added showers and even some thundershowers for mid-day Saturday. Indeed, several models have a mix of sun, clouds and convective showers (thundershowers) during the morning and early afternoon.

I’ll post the weekend forecast this evening.

Updated Thu 11:30 PM — Tonight’s models continue with forecast from the earlier RAP— showers and thunderstorms develop late morning/early afternoon Friday and continue through at least midnight.

Several batches of storms during this period— Some late morning activity possible to start things off —then early afternoon (2PM) and some potentially stronger storms 6-9 PM. Another batch around midnight. Some models have some showers even Saturday morning before the major clearing trend starts for the balance of the Memorial Day weekend.

Updated Thu 7:38 PM — Prior to what’s shaping up to be a rather nice weekend, we’ll have to get through Friday.

An approaching upper air trough will trigger the development of showers and thunderstorms on Friday. My table of severity parameters below compares tomorrow model forecast (RAP) to what has become my standard of extremes- last year’s (9/1/21) tornadic storms.

Last September‘s Tornadoes (9/1/21)	Fridays Storms RAP 21z Model
CAPE 3500-4200 Joules/kg	1200
Helicity 800-1100 m^2/s^2	350
Vertical Shear 35-45	16-20
Precipitable Water 2.0″	1.8″
Lifted Index (minus) 7-9	minus 5
Peak Wind Gusts 40-50	35
Aligned Storm Motion and Shear Vectors- YES	YES (early)
Jet stream- Highly Cyclonic	Flat to anticyclonic
Jet Stream Speed – High	Moderate
500mb – Highly Cyclonic	AntiCyclonic

As you can see, the current severe parameters are in the middle range. I expect some strong storms tomorrow. I’ll use the disclaimer that severe weather is always possible with thunderstorms, but I don’t expect much in the way of severe.

What appears to be the most important aspect of tomorrow’s storms will be the extended time frame that scattered storms develop and move through. Some early activity late morning, the bulk of the activity from about 2 PM to midnight.

**This afternoon’s 21 RAP model radar/accumulated rain forecast at 2 PM Friday. Large area of rain (1) for the afternoon. A second batch of rain (2) catches up and moves in around midnight.** (Click on image for a larger view.)

I’ll keep an eye on things. Stay tuned.

Update Thu @ 8:47 AM —This week’s ever-changing weekend forecast has moved in a very positive direction. The slow moving “closed low” that had been forecast by the GFS and then the ECMWF is now forecast to “open up” on Saturday and move off to the northeast. ( A very different forecast from the past two days, but not too different than the ICON’s forecast which has consistently predicted for us to have a good weekend.)

The forecast transition from a “closed” upper low to an “open” upper low will occur Friday night and this transition improves our weather for the entire weekend—

"Closed" upper air low
"Open" upper air low

As a result, Saturday is looking to have more sunshine, with significantly less chance of showers, especially by the afternoon. Sunday and Monday look very good.

Updated Wed 7:32 PM — Reviewing some of the other models, it appears that Saturday will have a mix of sun, clouds and pop-up showers throughout the day. More sunshine, less clouds and significantly reduced chance of showers Sunday. Mostly sunny Monday.

Update Wed @ 5:13 PM — Today’s GFS and ECMWF models are in reasonable agreement with forecasts for rain Friday and portions of Saturday. Both models have partly cloudy/partly sunny skies on Sunday and sunny on Monday. The Canadian GEM Global model and the NAM has a similar forecast for Saturday.

The ICON model continues its forecast for mostly sunny for Saturday through Monday.

With three major models showing rain on Saturday, I can’t explain why the NBM (Model Blend) and shows no rain and little in the way of cloudiness Saturday through Monday.

I think the GFS has performed the best over the past week in predicting the upper closed low with lingering clouds and showers. I would expect some rain on Saturday and improving conditions Sunday and Monday.

Today’s 18z NAM cloud/precipitation forecast for 2 PM Saturday. (Click on image for a larger view.)

Update Wed @ 8:06 AM — The weekend forecast continues to be low confidence as the GFS has reverted back to an upper level closed low lingering in the area (now to our south) through Monday while the ECMWF has the closed low opening and lifting away by late Sunday.

The new GFS model has the low moving east southeast instead of northeastward, bringing us a cool northeasterly flow.

We’re talking about occasional/intermittent showers, some sun, significant periods of cloudiness and cool temperatures, not a soaking rain.

Monday looks to be the best day.

GFS forecast for 5 AM Monday showing closed upper low (Purple L) and weak surface low (-L black isobars) off Carolina coast. (Click on image for a larger view.)

Updated Tue 5:42 PM — There’s been an increase in uncertainty around the weekend forecast with the major models doing a reversal of their forecasts from yesterday. The ECMWF now has the upper low pressure system lingering through at least Saturday, while the GFS moves it out faster.

Regardless of the model, most of Sunday and especially Monday, appear to be very nice days. Saturday has the most uncertainty in the forecast with a possibility of lingering showers.

The ICON and GFS are currently the models with the most optimistic forecasts at this time, but a lack of forecast continuity still makes it low confidence.

Here’s the ECMWF forecast for late Saturday showing lingering rain —

Today’s 12z ECMWF forecast for 5 PM Saturday showing the upper low over eastern Pennsylvania/Nj with lingering showers and clouds. (Click on image for larger view.)

Here’s the latest GFS forecast showing clear skies for Saturday—

Latest GFS (12z) showing sunny skies for Saturday. (click on image for larger view.)

We’ll likely have to wait at least another day or so before the forecast is more predictable.

Previously Posted Mon 8:05 PM —

The models are showing an interesting narrow upper air trough in the middle of the country mid week that may or many not close off into closed low over the Ohio Valley.

Most of the shower activity through Thursday will remain south of our immediate area. But some degree cloudiness will affect us Tuesday through Thursday before rain moves in Friday.

Today’s 18z GFS jet stream (250 mb) forecast showing deep, closed jet level low pressure system over the the Mississippi Valley. (Click on image for a larger view.)

For the weekend, there’s a lack of agreement between the GFS, ECMWF, ICON, and CMC regarding the upper trough and closed upper level low. The GFS has a slow, closed low forecast which would adversely affect this coming weekend’s weather, while the other models open up the low and move it off the coast. It’s too early to tell which is correct and we’ll have to follow the model trends for the next few days.

The most optimistic forecast right now: Friday looks to be rainy, sunny Saturday, Sunday into Monday—

Today’s 12z ECMWF optimistic forecast for early Saturday morning. (No closed upper level low) (Click on image for a larger view.)

Unfortunately, the latest GFS maintains that closed low through the weekend with plenty of showers. Again too early to hang our hats on the pessimistic GFS—

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PERIODS OF SHOWERS AND STORMS FRIDAY

Weather… and Other Things 'Up in the Air'