The latest NBM continues to show the highest probability of thunderstorms from Philadelphia eastward at about 4 PM.
Updated Tues 10:35 PM with HIRESWhighlighted below
A cold front will move through Wednesday afternoon and evening. The actual upper trough is not very sharp and the energy of the front will come through not as a single punch.
Prior to today, the models were showing very little in the way of thunderstorms. Today’s models have changed somewhat with some activity being forecast by most models.
The HREF shows scattered areas of strong vertical motion between 12 PM and 4 PM. Peak time as early as 2 PM Main activity currently appears to be north and east of the city.
Precipitable water (1.4″ to 1.7″) is not as high as recent past thunderstorms and CAPE levels are in the 1100 J/Kg range. Enough for some strong storms but right now, it doesn’t look like a major storm event for most areas.
Fri 08:18 PM Forecast Review — The storms developed and moved just south of Philadelphia as picked up by the afternoon HRRR.
Updated Fri 4:25 PM, highlighted below, based on new HRRR Updated Fri12:30 PM, highlighted below, based on new HIRESW
Strong thunderstorms expected early this evening. The latest HRRR shows all the ingredients coming together about 4:30 -8:30 PM 5:30 – 9 PM(peak 7-8 PM) in Philadelphia.
Update: The latest HRRR has the heaviest storms just to the south of Philadelphia
CAPE values in the 1600-2200 J/Kg range. Lifted Index moderately negative at -5. Very strong upward vertical motion is predicted to develop (blue shading/white arrow below) along with cyclonic upper air contour (blue arrows).
Heavy rain expected, especially from Philadelphia south and east.
Updated Wed 8:25 PM: This afternoon’s GFS has heavy rain blossoming around Philadephia between 8-10 PM, while the HRRR and the RAP do not. Current radar seems to support the GFS. We’ll see soon enough.
Wednesday’s thunderstorm outlook is looking a bit like Tuesday’s, but with some differences. These storms will also be slow moving and will develop like today’s but will be moving southward instead of northeastward.
CAPE values only about 2000 instead of Tuesday’s 2800 J/Kg. Precipitable water still high at 1.9-2.0″ water, but also a bit lower than Tuesday. Instability values also lower.
Storms are possible in as early as 2-4 PM but eventually blossoming in the immediate Philadelphia area by 4 -7 PM and continuing into the late evening. There is some suggestion that areas south of the city and in NJ may be more affected, especially in the evening. Last nights’s models continue with a timeframe of 4-7 PM for the onset in the immediate PHL area, then continuing southward.Highest probability is 6 – 8 PM