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 TornadoesMonday’s Storms RAP 17zFriday’s Forecast RAP 15z
CAPE 3500-4200 Joules/kg22001900
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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Interesting Weather for Monday

Forecast Review Mon @ 7:26 PM — So, in my book, these were typical thunderstorms with typical outflow wind gusts. Strong, yes. Severe, hardly. (I’m sure there will be some TV footage of a tree down somewhere.)

As mentioned the HRRR, HIRESW and the NAM-NEST did very well with timing and the handling of the front. The RAP was less impressive. Here are a few takeaways for the lack of severity—

•Temperatures were in the 70s, not the 80s or 90s
•Shear values were in the regular range one sees before thunderstorms. They weren’t over 30.
•Helicity values were relatively low. My software settings don’t even show helicity contours until they reach 400+.
•Shear and storm motion vectors were not aligned.
•Barometric pressure tendencies were flat before the frontal passage
• Jet streak position was west of the front and PHL was not in the entrance or exit regions to enhance the storms.

Radar loop showing storms diminishing as they proceeded eastward.

Update Mon @ 5:46 PM — Well, some severe strong activity has popped in upper Montgomery County. The NAM-NEST looks to be correct, as did the earlier HRRR. Expect some strong storms about 6-7PM

Update Mon @ 4:57 PM — The latest HRRR model has really backed off on the extent of [severe] storms coming. I can understand how it’s difficult for TV weather and the NWS to back off on a [hyped] forecast, especially when people’s safety might be at risk.

Nor did I want to stick my neck out too far with my post earlier today with a forecast saying these are going to be just the usual thunderstorms. But with my commentary on the overuse of word ‘severe’, it was my way of hinting at the possibility.

Anyhow, the latest NAM-NEST is still predicting a significant line of storms about 6-7PM . The latest HRRR not so much.

Any barometer fans out there? Maybe you’ve noticed the barometric pressure is already rising flattened? The front has already close to moving through!

Update Mon @ 3:01 PM — Newest models have the front moving through earlier- 4-5:30 PM (RAP Model) 5-7 PM (HRRR model)

Greatest shear is in northern NJ, not in this area.

CAPE is in the 1500-2200 range which is reasonably high. Latest HRRR has helicity in the 100-160 range in some areas.

There are differences in the two short range models. The RAP has several areas of storms moving through this afternoon into this evening The latest HRRR has storms ahead of the front with the main storm line moving through 5:30-7:30 PM (Philadelphia).

Updated Severity Parameter Table: (17z = 1 PM EDT)

Last September‘s TornadoesToday’s Weather
HRRR 06z
Today’s HRRR 12zToday’s HRRR 17zToday’s RAP 17z
CAPE 3500-4200 Joules/kgCAPE 1500-2400 Joules/kg20001500-20002200
Helicity 800-1100*Helicity 150-250*100100-160130
Vertical Shear 35-45Vertical Shear 15-2510-1525-3015-20
Precipitable Water 2.0″Precipitable Water 1.6″1.5″1.6″1.6″
Lifted Index 7-9 (minus)Lifted index (minus) 6-7 minus 6-7minus 6-7minus 6-7
Peak Wind Gusts 40-50Peak Wind Gusts 3535 mph35 mph25-30

Plenty of ingredients available for strong storms. However, shear vectors and storm motion vectors not aligned, suggesting difficulty in creating/maintaining severe storm cells. I guess we’ll see.

Update Mon @ 11:57 AM — Some storms have broken out ahead of the main front, as mentioned previously.

Of interest is the current water vapor image with superimposed NAM and GFS potential vorticity. There’s poor line up of these features, suggesting that these current models aren’t capturing reality that well.

11:30 AM Water Vapor with superimposed potential vorticity contours.

The RAP model has a better conformity to current water vapor imagery. The RAP has significant showers/thunderstorms during the afternoon, breaking out ahead of the main front that moves through at 6 PM—

14z RAP model shows better conformity to water vapor image. The RAP has significant showers moving in during the afternoon, ahead of the main front.

This morning’s HRRR and HIRESW show the main area of thunderstorms moving through between 4-6 PM. Here’s the 12z HRRR at 6 PM—

HRRR 12z showing simulated radar and cloud levels at 6 PM today. (Click on image for a larger view.)

I’ve also updated the table below with the latest HRRR representative values. Notice that the severe parameters have decreased, especially shear values.

Last September‘s TornadoesToday’s Weather
HRRR 06z
Today’s HRRR 12z
CAPE 3500-4200 Joules/kgCAPE 1500-2400 Joules/kg2000
Helicity 800-1100*Helicity 150-250*100
Vertical Shear 35-45Vertical Shear 15-2510-15
Precipitable Water 2.0″Precipitable Water 1.6″1.5″
Lifted Index 7-9 (minus)Lifted index 6-7 (minus)6-7
Peak Wind Gusts 40-50Peak Wind Gusts 3535 mph

One parameter that stands out today is the hail parameter, unique to the HRRR model. It’s a parameter that can’t be always taken literally, but today’s hail parameters show the possibility of large size hail—

HRRR hail forecast for 6 PM. Black is likely hail greater than 1.6″ diameter possible (Click on image for a larger view.)

Update Mon @ 8:37 AM — Just a few thoughts about today’s weather. Can anyone remember an outbreak of thunderstorms where the term ‘severe’ wasn’t used?

From a liability point of view, I think the NWS and TV weather people use the term just a bit too frequently. (But who’s not going to say when there’s any lightning and wind involved that somewhere, it might be severe?) I think you get my point. Let me be more specific regarding today’s forecast.

I see a high probability of strong storms and maybe even ‘severe’ weather today. But let’s compare a few severity parameters to those from the relatively recent tornadic outbreak of this past September 1st.

Last September‘s TornadoesToday’s Weather
CAPE 3500-4200 Joules/kgCAPE 1500-2400 Joules/kg
Helicity 800-1100*Helicity 150-250*
Vertical Shear 35-45Vertical Shear 15-25
Precipitable Water 2.0″Precipitable Water 1.6″
Lifted Index 7-9 (minus)Lifted index 6-7 (minus)
Peak Wind Gusts 40-50Peak Wind Gusts 35

As you can see, there’s ‘severe’ and then there’s SEVERE of last September. Today will be different.

It’s sorta like the radio world where all children are ‘above average’ and all thunderstorms are severe.

I’m sure there’s going to be some big storms today. But I think it’s important to keep it all in perspective. While anything is possible with thunderstorms, based on today’s current forecast parameters, I don’t think there will be any tornadoes. This will likely just the first of many summertime ‘severe’ thunderstorm days. I’m glad I got this off my chest. 🙂

Updated Mon 7:36 AM —Unstable air and a cold front will combine to form strong thunderstorms today, Monday.

Additionally, low pressure systems are expected to form ahead and along the front, enhancing the duration of the rain. While the main front is expected to pass through about 5:30 – 6:00 PM, low pressure will develop causing thunderstorms to break out as early as 1-2 PM; additional rain may last until 8 PM. (This event will not be a single thin line of storms, based on current models.)

The morning high resolution shorter range models 24 hour forecasts will available shortly after 10:30 AM. I’ll update this post later this morning.


Update Sun @ 9:48 PM — Latest radar and latest RAP and HRRR have the storms not making it into our area tonight. I can’t account for the GFS and HREF which had forecast this disturbance to move in.

Update Sun @ 8:41 PM — The line of storms expected earlier this evening did fall apart, as many models (but not all) had predicted. Looking closely, the shear vectors and storm motion vectors were at 90º to each other.

There is another strong upper air disturbance that is very much blossoming on water vapor imagery this evening (see the live water vapor image on this site).

Water vapor image. The yellow ‘river’ of air in southwest PA is warm and unstable as it moves against colder air (blue) (Click on image for a larger view.)

Here’s a current superimposed radar with water vapor image at 8:40 PM —

Current Radar and Water Vapor image 8:40 PM (Click on image for a larger view.)

Storm motion and shear vectors align better about 11 PM. These storms may make it into Philadelphia between 10PM to 1AM. It will be interesting to see if it happens, as it is forecast by the latest GFS.

Monday will be an active day here weather-wise as a strong cold front moves past. Isolated storms may break out in the morning. Available CAPE values are expected to be very high, exceeding 2500 joules/kg. Heavier storms in the 3-6PM time frame. More on that tomorrow.

Update Sun @ 6:11 PM —More models show some storms moving into the Philadelphia area between 9 PM and 1AM. Monday looks to be an active weather day. Stay tuned.

Update Sun @ 11:37 AM — Last night’s models had any rain dissipating before making into our area this evening. However, some of this morning’s models show the showers and thunderstorms making it into the Philadelphia area this evening, between 6 PM and 11 PM. (Not all are on-board: the HRRR has them missing us; the Canadian RGEM has them falling apart just west of us.)

Here’s the latest high resolution HIRESW forecast for 7 PM—

12z HIRESW-2P5 (ultra high resolution) simulated radar/accumulated rain/clouds (black) forecast for 7 PM Sunday. (Click on image for a larger view.)

Updated Sat 10:41 PM — Sunday Forecast below unchanged. Fog lifts and clouds break for sunshine around noon. Showers and thunderstorms look less likely late Sunday and Sunday evening; they will weaken as they approach Philadelphia from the west. Western suburbs may see some activity after 6 PM.

Monday may be an active day for afternoon thunderstorms.

Update Sat @ 9:42 AM — The latest HRRR radar/rain/clouds forecast for 2 PM Saturday—

Today’s 12z HRRR with radar/rain (green contours) and clouds (black/grey) forecast for 2 PM Saturday. The arrow shows the direction of movement of these showers, from southeast to northwest. (Click on image for a larger view.)

Updated Sat 7:34 AM — Last night’s models have come together with the forecast of showers moving in from the southeast as early as 11- 1 PM Saturday. About 0.2-0.3 inches of rain expected.

For Sunday, the forecast is mostly the same- cloudy in the morning, becoming sunny in the afternoon. Chances of thunderstorms moving in, especially western suburbs late afternoon Sunday.

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Cloudy early, then by late morning sunny, hazy warm, and a bit humid with dew points in low 60s. Showers move in late in the afternoon. Thunderstorms possible late afternoon and evening, especially western areas. High temp 80.0º ± 3.0º NBM model, Blue Bell.

Weather… and Other Things 'Up in the Air'

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