Monthly Archives: September 2019

Philadelphia Weather Outlook

WEEKEND WEATHER OUTLOOK

High pressure will build in to our north Friday and an easterly flow will bring cool temperatures and a significant amount of cloudiness on Friday.

Even more cloudiness is expected on Saturday as the high pressure system moves off to our east and the winds become southeasterly ahead of a warm front.  Some showers, widely scattered, are possible on Saturday as well, although much of the area will be dry.

A weak cold front moves in later Saturday and Saturday night with some showers.  Another high pressure system moves in for Sunday bringing more sunshine, although there are model differences about the amount of clearing we get on Sunday.

Complicating the forecast  is another tropical system that may form near Florida over the weekend and follow a track somewhat similar to Dorian.  This system is not expected to intensify anywhere near the level of Dorian.  The model forecast reliability often seems to be negatively impacted  by a tropical system in the area.  Stay tuned.

Commentary

TODAY’S UNEXPECTED RAIN IN PHILADELPHIA

I find it fascinating that most of today’s models didn’t predict the rain that developed in a narrow, persistent band over Philadelphia. Interestingly, little rain developed at the shore to this point.

Radar 4:30 PM Friday, courtesy of weathertap.com

Going back to last night’s models, the WRF-NMMB and to a lesser extent the WRF-ARW were the only models to predict this rain last night.   However, the same models run this morning didn’t show it as much and the GFS, NAM really were off about this.

The unexpected rain was the result of some vorticity and vertical lift in the mid levels of the atmosphere, captured on this afternoon’s 2PM RAP model analysis—

RAP Model 2 PM Friday Analysis showing lift and vorticity at the 500 mb level (~18,000 feet)

I don’t know why, but as I mentioned last week, hurricanes seem to throw off the model forecasts, often in big ways.

This rain is expected to clear out later today.  The weekend looks to be beautiful with sunny skies and comfortable temperatures in the 70s.  I’ll do a brief update later this evening.