Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx #PhillyWx #Hurricane Erin

Forecast Review —Last night’s second round of thunderstorms brought much heavier rain than had been forecast to some areas—
MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)

Sunday Update

Posted Sunday 08/17/25 @ 5:15 PM — Latest RRFS has likelihood of the strongest storms within the concentric circle -green areas—

19z RRFS Calculated Severe Weather Index > 240 (Click on image for a larger view.)

Much of the city is outside of these areas. Some areas may receive little or no rainfall.

Posted Sunday 08/17/25 @ 5:01 PM — Storms have developed to our northwest.

RADAR 4:48 PM with superimposed RAP model 700 mb wind streams and low level Omega (vertical motion) in yellow. Storms moving into areas of negative Omega (upward motion) should keep them intact. Second line may encounter some decrease in activity. (Click on image for a larger view.)

Highest likelihood of the strongest storms will be south and west of the immediate Philadelphia area.

Sunday Thunderstorms

Posted Sunday 08/17/25 @ 9:54 AM —A cold front will move through the area this evening into tonight. Thunderstorms (scattered) are expected to develop ahead of the frontal passage late this afternoon and especially 6 PM to 9 PM near Philadelphia. Here’s the current situation at 9 AM—

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Water Vapor Imagery reveals emitted water vapor infrared TEMPERATURE, NOT REFLECTED LIGHT from CLOUDS. (Click on image for a larger view.)

The thunderstorms will be scattered with the greatest density of storms southwest of our area—

08-17-25 12z NBM Thunderstorm Coverage density (bluish boxes) and Hourly Probability (color shading) Overall thunderstorm probability is about 18%/1hour or less over most of this map area, and I generally consider 18% or more as reasonable chance. Areas in the bluish boxes, it’s most likely. (Click on image for a larger view.)

Regarding thunderstorm severity, here’s the Severe WEATher index (SWEAT) calculated on HRRR data—

12z HRRR severe weather index (SWEAT). Highest values, greatest color, are in the 250 range, suggesting strong but not severe storms.

As for Hurricane Erin, I’m sticking with the ECMWF-AI model forecast, which continues to show a larger storm with the latest run taking it slightly west of previous tracks. The overall track has been similar over time, with an east-west variation, to slightly westward—

08-17-25 06z ECMWF-AI forecast for Thursday 8 AM. Closest track to NC outer banks so far. (Click on image for a larger view.)

Originally Posted Fri 5:28 PM —Some storms have fired up to our south and west Friday afternoon—

Radar Fri 4:30 PM with superimposed RAP model omega and 700 mb windstreams (10,000 feet) (Click on image for a larger view.)

A weak, slowly approaching cold front will bring the chance of widely scattered thundershowers Saturday and Sunday afternoons. Exact placement and timing still to be determined but mostly far west and north of the city.

12z ECMWF-AI model forecast for 2 PM Sunday. Cold front to our northwest. Tropical disturbance (Hurricane Erin) to slowly move northwestward. (Click on image for a larger view.)

The latest ECMWF-AI model forecast for Hurricane Erin takes it still closer to the coastline of NC late Wednesday. This AI model did very well with last year’s hurricane predictions. It’s been back and forth regarding the track of Erin. Here’s the latest—

12z ECMWF-AI model forecast for late Wednesday night. Closest forecast track so far. (Click on image for a larger view.)

Typical August temperatures and humidity for this weekend.m Afternoon scattered to isolated storms mostly north and west of the immediate PHL area.

Saturday

Partly to mostly sunny. Chance of a widely scattered thunderstorm mainly west of our area. Heat index 91 for PHL

18z Model Blend (NBM) high temp Saturday low uncertainty Contours are 1º increments (Click on image for a larger view.)
Sunday

Partly sunny. Chance of a widely scattered thunderstorm mainly west of our area late afternoon. Heat Index 96º Philadelphia.

18z NBM high temp Sunday. Low uncertainty. Contours are 1º increments (Click on image for a larger view.)