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—

Updated Thu 9:55 PM — Here are the forecast trends for Friday through Sunday—

Light scattered showers/sprinkles early Friday especially southeast of the city, low clouds. Much of the day cloudy but dry. There may be some breaks in the cloud cover especially west of the city. Updated Thu 10:43 PM —Tonight’s high resolution models have some sunny breaks in the afternoon along with some widely scattered showers.

For Saturday, the models (with the exception of the ECMWF) have further reduced the chance of showers except far western suburbs. Brightening skies and some sun expected. Increased chance of some showers late afternoon and evening. Possibly some thunder. Updated Thu 10:54 PM — Tonight’s. NAM-NEST shows a fairly nice day for Saturday

Sunday should be a lot like Saturday, but with more sunshine and warmer. Some showers possible late in the day or evening.

Update Thu @ 9:43 AM —The GFS and other models are in agreement with clouds and showers on Friday as a jet streak associated the closed upper level low affects us.

3 PM Friday—

3 PM Saturday—

The latest ECMWF model, just available, has a different jet flow and forecast than the GFS. The ECMWF continues with a wetter Friday and Saturday.

Updated Tue 11:15 PM — Tonight’s models are showing the chance of clouds and scattered showers as soon as Thursday, with moisture brought inland from the retrograding coastal low.

Updated Tue 7:15 PM — Looking towards the weekend, the same low that gave us the rain last weekend, threatens to rotate back into our area by Friday (see image below).

For Wednesday and Thursday, the same cloud deck that moved in late Tuesday may affect our area at times. Temps in the 70s

As for Friday, there are differences between the ECMWF and the GFS, with the ECMWF showing showers moving in during the day, while the GFS keeps us just with some clouds.

Saturday looks unsettled with clouds and occasional scattered showers but not looking like a rain-out. Again, the ECMWF shows more rain than the GFS. Highs mid to upper 70s.

Sunday may feature warm temperatures and breaks of sunshine ahead of an advancing cold front. Highs may reach near 80º.

Updated Tue 6:53 PM — Well, the clouds from the retrograding coastal low moved in eventually, about 7 hours later than had been forecast last night.

Updated Tue 8:43 AM — The 06z models backed away from the clouds making it into Philadelphia today.

Updated Mon 10:47 PM — Tonight’s models are suggesting a slight forecast change for Tuesday. The retrograding low pressure system off the coast may throw back some low level cloudiness far west enough to make it into the immediate Philadelphia area.

Previously Posted Sun 7:33 PM —

This coming week’s weather will feature a blocked pattern, the other side of the coin of this past weekend’s cold, cloudy, rainy and slow moving upper air low pressure system.

Indeed, the same upper low that gave us the rain will retrograde to our southeast instead of the usual path moving off to the northeast.

This week, upper air winds will come from the north and northeast. We should have dry conditions through Friday.

Meanwhile the same blocked pattern will allow temperatures to build to record levels in the mid section of the country.

Incredibly, the same low is forecast to retrograde westward over Florida and then move back over our area for next weekend with more rain.