Bow-Echo line of storms possible Friday

Forecast Review— The storms that had broken out early, ahead of the main group, were not forecast. These storms stabilized the atmosphere and reduced CAPE. Despite the official ”Tornado Watch”, the storms that moved through our immediate area were not as severe as forecast.

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

Radar composite loop 3:24 PM (courtesy of

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—

12z HRRR just available shows bow echo through Philadelphia. This is the 1 hour accumulation at 7 PM, so the actual storms move through from 5 to 7:30 PM. (Click on image for a larger view.)

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

Last September‘s TornadoesMonday’s Storms RAP 17zToday’s Forecast HRRR 12z
CAPE 3500-4200 Joules/kg22002500-2700
Helicity 800-1100 m^2/s^2130900-1100
Vertical Shear 35-45 15-2022-24
Precipitable Water 2.0″1.6″1.6-1.8”
Lifted Index (minus) 7-9 minus 6-7 minus 6
Peak Wind Gusts 40-5025-3045-50 mph
Aligned Storm Motion and Shear Vectors- YESNOYES
Jet stream- Highly CyclonicCyclonicLocally Cyclonic
500mb – Highly CyclonicCyclonicCyclonic
This profile is SEVERE

Current Radar Composite (courtesy of

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.

Water Vapor with superimposed GFS/NAM potential vorticity. Notice the water vapor X is ahead of the potential vorticity X (Click on image for a larger view.)

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.

06z HRDPS (Canadian High Resolution Deterministic Prediction System) model forecast for 6 PM showing southern extent just south of Allentown. (Click on image for a larger view.)

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.

06z HRRR showing bow-echo simulated radar at 5 PM Friday. Arrows (blue) storm motion vectors Arrows (white) shear motion vectors. (Click on image for larger view.)

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.

18z HRRR simulated radar/ accumulated rain (contours) at 4 PM Friday.

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—

Last September‘s TornadoesWednesday’s NAM-NESTFriday’s Forecast RAP 15z
CAPE 3500-4200 Joules/kg2000-40001900
Helicity 800-1100 m^2/s^2130460
Vertical Shear 35-45 15-2024
Precipitable Water 2.0″1.6″1.6”
Lifted Index (minus) 7-9 minus 6-7 minus 5-6
Peak Wind Gusts 40-5025-3025-28 mph
Aligned Storm Motion and Shear Vectors- YESNOYES
Jet stream- Highly CyclonicCyclonicAnticyclonic**
Jet Stream Speed – HighHighLow **
500mb – Highly CyclonicCyclonicCyclonic
What’s different about this weather compared to Monday – this is a warm front compared to a cold front; the dynamics and vertical structure are very different.
The ** asterisked parameters may be critical here for not supporting severe weather.

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

NBM 12z high temperature forecast for Saturday. Dew points will be in the mid 60s, according to the model blend. (Click on image for a larger view.)

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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