THIS WEEK’S WEATHER

Update Fri @ 9:29 AM

Fri 09:29 — Forecast Review — The timing for most of the model forecasts was too fast by 2-3 hours. Yesterday’s 18z HRRR did well in forecasting the strongest storms in the western suburbs of Philadelphia and having the line of storms diminish as it approached the Delaware river, as depicted in my graphic below. The 12z HIRESW-2P5 did the best with timing and the storms falling apart as they approached the Delaware River.

I’ll have my regular “Weekend Weather Forecast”— this evening.


Update Thu @ 9:14 PM — Did any of the models get the timing right? Well, all the afternoon models had the line of storms coming through by now. Looking back at all models, only this morning’s HIRESW had it close. (The HIRESW is only run twice daily.)

This morning’s 12z HIRESW-2P5 radar forecast for 9 PM tonight. It probably did the best with the timing. More interesting is the HIRESW-2P5 predicts the storms to fall apart as it goes through Philadelphia between 11-12 midnight. It will be interesting to see if that happens too. (Click on image for a larger view.)

Update Thu @ 5:55 PM — Looking at current radar, I’m thinking the lack of high CAPE is taking a big bite out of the strength of line of storms. I’m not all that confident that high vertical shear and helicity will make up for lack of high CAPE. I guess we’ll see.

NEXRAD radar 5:50 PM (Click on image for a larger view.)

Update Thu @ 4:35 PM — This afternoon’s NAM-NEST and HRRR continues the earlier forecast: The main line of storms is expected about 7-9 PM approaching the immediate PHL area and continues past the area through 11 PM to 1 AM.

As forecast, there has been little sunshine and CAPE values are not expected to be in the danger zone.

Nonetheless, there’s plenty of kinetic energy to feed the storms, the windy conditions ahead of the front being a good indication. Hail and strong gusty winds near 50 mph are possible.

Based on review, here’s where I think the most significant storms will be—

Today’s 18z HRRR showing maximum shear, helicity, highest CAPE and simultaneous rainfall in the white boxed area. This area will progress into NJ but not far between 9 and midnight. The models show a decrease of storm strength into NJ. ALWAYS DEFER TO THE OFFICIAL NWS WARNINGS FOR UP TO DATE CONDITIONS. (Click on image for a larger view.)

Update Thu @ 11:31 AM — This morning’s high resolution models have become available.

As forecast last night, some scattered thunderstorms could break out as early as 3-4 PM. The main line is expected about 7-9 PM in the immediate PHL area , although some models now forecast it closer to 11 PM.

The one thing that has been prominent and consistent about this forecast time frame is the lack of high CAPE, especially high CAPE immediately preceding the line of thunderstorms. CAPE is a measure of available thermodynamic instability/energy for severe storm formation. CAPE values are only forecast to be in the 300-700 joules/kg range.

In place of high CAPE is actual kinetic energy in the form of ascending wind shear, specifically something called “vertical wind shear” which is a measure of increasing winds with height. The vertical wind shear forecast by this morning’s models is not forecast to be as high as previous model runs.

For high vertical wind shear to have maximum impact on thunderstorm development, the shear direction must be the same direction as the storm motion of the thunderstorm cells themselves. That alignment is also not as optimized as previous forecasts had shown.

Today’s 12z HRRR Vertical Wind Shear (white streamlines) Storm Cell Motion (Blue arrows) and simulated radar. Notice there is maximum alignment between storm motion and shear vectors in area (1) but not so much in area (2) This suggests that storms that break out ahead of the main line may be more severe than the storms associated with the line of storms. (Click on image for a larger view.)

Based on the low CAPE, it appears that thunderstorms will be strong, but not necessarily severe. Tornadoes are unlikely (based on lower helicity forecast) and separation of maximum helicity from the actual line of storms.

The big issue will be wind gusts, which are forecast to approach 50 mph ahead of and with the storms.

As I had written last night, if we get more sun this afternoon than forecast, things could get more severe.




Update Wed @ 8:54 PM — What a difference a day makes with weather forecasts. This afternoon’s models have moved towards the likelihood of very strong and even severe storms Thursday afternoon and evening.

The HRRR is showing the possibility of scattered storms as early as 3 PM in some areas. The HRRR shows areas of high helicity with these scattered storms. While helicity values are less than the 1000m2/sec2 of last September’s tornadic outbreak, they are on the high side and an isolated tornado can’t be ruled out.

Today’s 18z HRRR simulated radar forecast 3PM Thurs with Helicity (concentric contours). Helicity values approach 600 m2/sec2 which is towards the high range. (Click on image for a larger view.)

Most models have the main line of storms starts moving through between 7 and 8 PM and goes through at least 11 PM.

Today’s 18z HRRR simulated radar forecast 7PM Thurs with Helicity (concentric contours). Helicity values approach 850 m2/sec2 (Click on image for a larger view.)

While CAPE values are still in the lower range, very strong wind shear is expected to provide the fuel to energize these storms. CAPE is the thermal fuel. (Wind shear is the change in wind velocity with height. Expected Wind shear is forecast to be 35 m/sec which is very high.) Wind Shear is the kinetic fuel.

HRRR forecast Wind Shear (shading) and Helicity (Contours) at 7 PM. Scale is for Wind Shear. (Click on image for a larger view.)

While overall rainfall is not expected to be extreme, WINDY conditions and high wind GUSTS from severe storms is expected.

I’ll update tomorrow.


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Another low pressure system may affect us on Saturday.

WEEKEND WEATHER FORECAST

Update Sun 8:51 AM— The clouds from that predicted upper air disturbance have started moving in. The cloudiness mid-day may be considerable and last into mid to late afternoon today.


Saturday Forecast Review —
HRRR 8 AM (12z) Saturday forecast for 11 AM Saturday -Hail parameter showing 0.9 inches (red/orange)  (Click on image for a larger view.)

Stan in Collegeville had commented that he had some brief hail late Saturday morning.  I was curious if the models had predicted this. Indeed, yes.  Here’s the  HRRR model “hail parameter” from Saturday’s 8 AM model run (12z). 

FYI, NOAA’s Severe Storm Site defines hail as being greater than 0.2″ in diameter.  Otherwise it’s still ‘sleet’ or ‘ice pellets’, even if it’s created from low level convective or thunderstorm activity.   

 

Update Fri 10:49 PM— Based on tonight’s early models, HRRR, NAM-NEST, I’ve updated Saturday’s forecast below. It’s more optimistic about some sunny breaks. Sunday’s forecast unchanged.


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Previously Posted Fri 4:56 PM —

Sunday

Mostly sunny, some periods of instability cloudiness late morning into early afternoon. WINDY and chilly. High 45.5º ± 2.6º NBM model Blue Bell)

Winds

NBM wind forecast (Click on image for a larger view.)