Fri 09:29 — Forecast Review — The timing for most of the model forecasts was too fast by 2-3 hours. Yesterday’s 18zHRRR 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 12zHIRESW-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.
Update Wed @ 7:51 AM — The current trend for Thursday’s forecast. The thunderstorms likely will come in two main groups. An earlier group, as early as 5 PM but most likely about 7-8 PM may have strong storms. The second group will come after midnight. Both groups can have strong storms, but not likely severe in our area. If there are any severe storms, they would occur in the first group, most likely far northwest of the city.
The models continue to have the storms disconnected from the conditions required for severe storms.
HREF CAPE (1) and Max Upward Helicity (2) at 10 PM. Note that the city is in an area of only moderate (CAPE 300-400 joules/kg ) (Green vs Yellow) and the forecast areas of maximum Helicity (concentric contours) are northwest of Pottstown. CAPE diminishes after 10 PM. (Click on image for a larger view.)
Update Tue @ 10:38 PM — Tonight’s HRRR has temps into the low 70s on Thursday afternoon with strong storms as early as 5-6 PM.
Update Tue @ 5:34 PM — A warm front will move through on Wednesday with significant clouds and possibly some light showers mid-day. Considerable cloudiness should persist through the day Wednesday.
Warm front passages are not as ‘distinct’ as cold fronts and the models have considerable low level moisture and clouds lingering into Thursday morning as it gradually moves north of us.
The big question for Thursday’s weather ahead of the cold front: how much sun will we see. More sun = more intense thunderstorms.
Currently the models have high temperatures only reaching the upper 60s, with little sunshine until late afternoon. If we get more sunshine and temperatures go into the low 70s, expect some severe storms. But, right now, the major severe parameters appear to be separate from the actual line of storms expected to move through around midnight.
Today’s 18z NAM-NEST simulated radar forecast for 9 PM Thursday. The NAM is showing storms breaking out ahead of the main line of storms with the cold front still in central PA at 9 PM. Helicity (brown-red circular contours) is maximized well ahead of the main line of storms. (Notice the area of high helicity near the radar maximum in NY State. That’s what a possible tornadic situation looks like., model-wise) (Click on image for a larger view.)
The specifics are likely to change. Stay tuned.
Update Mon 9:52 PM— Today’s barrage of snow flurries were impressive and predicted by the models. However, I can’t remember a time when so many periods of flurries and snow showers developed for hours on end. It’s what happens when you have an upper cyclonic flow, moisture and very cold, rapidly decreasing temperatures with height along with March sunshine warming the ground interspersed.
While temperatures will be in the teens tonight, we’ll be warming into the high 60s on Thursday!
The next period of interesting weather will be Thursday and Thursday night, as a cold front moves through. While not definite, several ingredients are coming together for possible severe thunderstorms and high winds late Thursday. Currently, the main activity is forecast west of the city, but the exact placement is too early to call. Stay tuned.
Previously Posted Mon 10:58 AM —
The unseasonably cold weather we’re having on Monday is associated with an upper air trough. Temperatures will begin to moderate as we get into Wednesday as an upper air ridge (1) moves over us—
GEFS jet stream wind speed forecast for Wednesday at 11 PM. The upper air configuration will be transitioining to a ridge (1) during the day Wednesday and a surface warm front will move through later Wednesday into Wednesday night. Following warmer and considerable rain on Thursday, another cold upper trough (2) will slide in to replace the ridge for Friday and the early part of the weekend. The trough (2) is expected to flatten out, so it won’t be anywhere as cold as it’s been on Monday. (Click on image for a larger view.)
Until the warm front moves through Wednesday night, we’ll still be in a cooler air mass.
Rain and mild temperatures on Thursday with the warm front followed by a cold front Thursday night. Clearing and cooler for Friday.
Another low pressure system may affect us on Saturday.
