Wed 09:39 PM — The westward path of Elsa, as indicated in Monday’s post (“If-Else(Elsa), then Rain”) continues to hold. Latest models have 1.5-2.5 inches of rain for areas in Philadelphia and NJ. More info tomorrow…
Wed 05:55 PM Forecast Comment — So why little thunderstorm threat today vs yesterday? Despite high predicted CAPE and hugely negative Lifted Index values, there is little vertical upward motion trigger today in the upper atmosphere. Wind flow in the upper half of the atmosphere is slightly anticyclonic today, resulting in sinking airflow compared to yesterday’s cyclonic flow. Some widely scattered storm activity is possible, but nothing like Tuesday night.
Updated 1:08 PM highlighted. This morning’s models have the strongest vertical motions and dynamics from Allentown northward. While some scattered storms are possible in this area, the heaviest activity will be in the Allentown area.
Another quick post- Last night’s models show significant instability and CAPE values between 5 PM and 11 PM Wednesday evening. Thunderstorms, heavy to severe, will likely develop again this evening from 5 to 11 PM. (GFS timing shows peak about 10 PM)
The NBM and GFS keep the heaviest activity to the north and west, but the higher resolution models forecast some impressive numbers in our area. Here’s the latest RAP model—
RAP model CAPE at 9 PM. (>3900!!!!) (Click on image for a larger view.)RAP model Lifted Index at 8 PM (-9!!!) (Click on image for a larger view.)
Wed 08:17 AM Forecast Review — The storms did re-develop dynamically last night between 10-11 PM right around the PHL area. Some were very strong (we lost our power). That’s what you get when CAPE values are >3000 and the Lifted Index is in the -8 range.
For today, Wednesday, most of last night’s models have the major dynamics staying to our far north and west. Some thunderstorms possible 5-11 PM , but not as severe as last night.
Updated on Tue 7:26 PM highlighted below
The storms popped early due to high instability, but luckily, not the severity that had been forecast. The intense storms formed far north, in NYC.
As post-storm forensics, I guess there were very low jet stream winds to mix in in this area.
The latest GFS and RAP models have another disturbance moving through between 10-11 PM tonight. The earlier storms have stabilized the atmosphere. We’ll see what happens.
Updated on Tue 5:28 PM highlighted below
Radar and Water Vapor imagery @ 5:20 PM with RAP model upper air wind flow superimposed. The storms are developing a littler earlier than I had forecast. They are expected to develop just west and over Philadelphia and strengthen as they move rapidly eastward. They are forming in response to an upper air and surface trough that is forming dynamically over our area. Additional storms may develop later this evening. (Click on image for a larger view.)
A quick mobile post. Models all have thunderstorms, likely severe, moving through between 6 PM and 10 PM this evening. CAPE values are very high at 3000 J/Kg and instability Lifted Index very high at -8. Storm motion and shear vectors nearly parallel providing an environment for strengthening.
