Yesterday, the short range models did well; the NAM over-stated the chance of showers. In the immediate PHL area, there were a few widely scattered thunderstorms, mostly far north and west.
Despite strong lifted indexes, vertical velocities and even some low level convergence, the atmosphere was “capped” with hot air aloft, basically negating any instability in the lower levels.
Today, the chance of thunderstorms is on par with the usual 30 percent background level seen in hot humid air masses.
The HRRR shows a weakening line of shower and thunderstorms moving through between 5 and 7 PM. Not sure if this will hold together.
The models keep the temps in the low-mid 90s with dew points in the low to mid 70s.
The short range models (HRRR and LAMPS) show a lower probability of thunderstorms today compared to Friday.
Over the past week, the NAM lifted index and vertical velocity parameters have been highly correlated with thunderstorm development.
The NAM model shows positive vertical velocities and very unstable lifted indexes for much of the day, all parameters conducive to thunderstorm development. These parameters are more conducive to thunderstorms than yesterday. The NAM cranks out some precip during the evening hours, after 8 PM.
So, if we go with the short range models, look for scattered thunderstorms between 5 and 7 PM with a probability of only 35%.
If we go with the NAM parameters, look for scattered thunderstorms possible anytime late morning through afternoon and especially late evening.
Trying to nail down a specific predictive parameter has been a challenge with this air mass.
A strong southwesterly flow of hot, tropically humid air will remain over our area this weekend.
The models have not been doing a stellar job in predicting the placement or timing of showers or thunderstorms that develop in the unstable atmosphere. The thunderstorms over the past week have been developing principally in areas of low level moisture convergence, a parameter difficult to predict in advance.
The pattern will be very hot and humid both Saturday and Sunday. Temperatures in the mid 90s with dew points in the mid 70s will make for oppressively hot conditions.
Regarding thunderstorms, most of the activity is still predicted to occur far north and far west of Philadelphia, but the models show scattered storms developing near PHL late afternoon and evening both Saturday and Sunday. Too difficult to predict in advance– I’ll update Saturday and Sunday mornings as to the best guess for thunderstorm development.
