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.

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.

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.

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

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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.

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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.

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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.

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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.

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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 —

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

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

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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.

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—

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—

Bow-Echo line of storms possible Friday

Update Fri @ 3:31 PM — Latest radar shows storms have broken out ahead of the main line (not forecast by the models) and the main line of storms will move in about 4 PM

Updated Fri 11:20 AM — Mobile update: Latest HRRR 13z has the storms closer to 6-7 PM and has reduced shear and helicity values, but still high. Still affecting PHL area.

Update Fri @ 10:12 AM — One more thing…the latest NAM just available keeps the severe weather near Allentown. So quite a bit of uncertainty here with the exact placement of the storms.

Update Fri @ 9:41 AM — Latest HRRR just available—

These storms DO have the potential to be fast moving and SEVERE.

I’ll be out the rest of the day. I’ll try to update via mobile.

Update Fri @ 9:20 AM — Some additional showers have developed in western suburbs this morning, showing the answer to the ? in the graphic below. The NAM-NEST shows these as weakening as they approach the city.

Update Fri @ 9:12 AM — The current water vapor imagery shows a lack of agreement between the real world (water vapor) and last night’s GFS NAM (contours) superimposed.

This suggests the models may not be accurately capturing the storm. The water vapor image suggests that the storm is moving faster than forecast.

Update Fri @ 9:01 AM — For this storm, I decided to dust off the recently updated Canadian high resolution model (HRDPS), a model I don’t routinely download because it takes a lot of post-processing time. Anyhow, it continues with several other model’s northern track of this storm line. So the northern track, missing Philadelphia, appears to be the model-preferred track except for the HRRR.

I’m waiting for the latest 12z HRRR and RAP models to get a better handle on the likely track of this mesoscale complex.

Updated Fri 7:34 AM — A quick update. Last night’s models are suggesting the possibility of a fast moving line of strong to severe thunderstorms between 3:30 PM and 6 PM tonight. The latest HRRR and RAP forecast a ”Bow Echo” mesoscale complex associated with a derecho type clusters, a long-lived line of severe storms in bow formation.

I’ll update later with the 12z HRRR that becomes available about 9:35 AM.

Updated Thu 9:53 PM — The term ‘severe’ may actually become appropriate to describe the thunderstorms now forecast to move through late Friday afternoon 4 PM -6 PM. (The timing may change with this sort of system.)

The HRRR model graphic below still captures the current forecast track, with the most intense weather north of the immediate PHL area.

Tonight’s early models, along with the ECMWF, take a disturbance, currently in Illinois, through northern Montgomery, Lehigh and northern Bucks counties.

Thu 10:59 PM — Tonight’s HIRESW has the line closer to Philadelphia

I’ll update tomorrow morning.

Updated Thu 3:11 PM — What had been expected to be an uneventful warm front passage on Friday afternoon now appears to possibly be a little more noticeable: the latest models show clouds and even some showers/thunderstorms late afternoon Friday with the warm front. The heaviest activity (if any) will be far northern suburbs.

Continuing with my earlier theme of the week regarding thunderstorm severity, it will be interesting to see if ‘severe’ thunderstorm watches will be issued. (I expect it won’t be issued or needed.)

Here’s representative severe weather parameters from today’s 15z RAP model for Friday late afternoon compared to Monday and the superstorms of 9/1/21—

Update Thu @ 9:54 AM — Latest NBM high temperatures Saturday—

The ICON model was the first to show these temperatures. It has highs in the 97º range.

Updated Wed 8:09 PM — Some rain expected tonight which should end by mid morning Thursday.

An uneventful warm front moves through mid day Friday, possibly causing a short period of cloudiness. An increase in humidity will be noticeable Friday afternoon.

The big story will be the very hot temperatures for the weekend, especially Saturday. Highs look to be 93-96° with dew points approaching an uncomfortable 68-70°. Hot, even if this were July!

Thunderstorms are forecast to stay to our west on Saturday, but clouds and thunderstorms may move in late afternoon Sunday

Updated Tue 10:46 PM — The latest NBM model shows a high temperature of 93° on Saturday.

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Posted Tue @ 8:18 PM —A complex jet flow to our north will spawn weak surface low pressure systems.

Some showers are expected Wednesday night ending Thursday morning, but another weak disturbance may bring some widely scattered showers Thursday afternoon.

By Friday, the jet flow is well to our north and very warm air (80s-90s) will move up from the southwest.

By Sunday, low pressure with plenty of moisture from the Gulf of Mexico moving northeastward. Earlier model runs showed rain for Sunday, but this latest 18z GFS has the rain holding off. There’s uncertainty with the Sunday forecast—