The weak cold front that moved through on Friday will move north as a warm front later Saturday into Sunday.  Showers expected with the warm front Saturday night.  My regular readers know that a tropical system or hurricane near the continental US often plays havoc with the model forecast specifics.


  • Significant and increasing high level cloudiness (cirrus).  Lower level clouds increase during the late afternoon and early evening.
  • High temp 87.0º sd 2.0º
  • Dew points mid 60s.
  • Winds calm to very light from the SE at 0-6 mph


The warm front moves north, but unstable and humid air may result in thunderstorms Sunday afternoon, especially west of Philadelphia.


  • Cloudy in the morning, sun breaks out during the late morning or early afternoon.
  • Showers linger early in the morning.
  • Thunderstorms and showers possible later Sunday afternoon.
  • High temp 91.3º sd 2.6º  (larger than average spread)
  • Very humid!  Dew points near 75º
  • Winds increasing 10-15 mph from the S and SW.  Becoming gusty in the afternoon.

GFS (pre-release version 16) forecast Hurricane Isaias on Tuesday morning at 8 AM, currently the closest forecast approach to our area —

GFS v16 forecast for Tuesday 8 AM  (Click on image for a larger view.)

The model blend (NBM) shows much more rain to the west on Tuesday—

NBM 1 hour rain forecast for Tuesday at 8 AM  (Click on image for a larger view.)


Sat 10:36 PM Update — The relative humidity as expressed as the dew point looks slightly more tolerable on Sunday afternoon than previously forecast.  Still a high  of 93  (Blue Bell)—

Model Blend temperature and dew point forecast for Sunday

A weak frontal boundary will move to our south this evening, ending the showery weather. Somewhat drier air will move in for Saturday with a mix of clouds and sun. It will still be humid.

The boundary will be just to our south and their are signs that it will move back as a weak warm front later Saturday afternoon. An increase in clouds and spotty showers are possible Saturday afternoon from 3 PM  to 7 PM. (Tonight’s models just becoming available downplay the chance of showers.)

It will be very warm and humid on Saturday, but the real heat and humidity returns again on Sunday.

Saturday —

  • A mix of sun and clouds.  An increase in clouds with spotty showers possible after 3 PM.
  • high temp 89.0° sd 1.9°.  Dew points just below 70°
  • winds calm, very light E—> SE—> SSW


  • Mostly sunny.
  • high temp 93° sd 2.0°.  Dew points 69°
  • winds W 6-8 mph


Thu 05:36 PM Update — This afternoon’s models have significant thunderstorm activity in the western suburbs this evening, but the storms dissipate as they move eastward. Most models have them barely making it into Philadelphia this evening.

Below is the NAM NEST at 10 PM; by 11 PM, they’ve dissipated.

NAM-NEST simulated Radar 10 PM Thursday  (Click on image for a larger view.)


This morning’s Canadian HRDPS had more rain in PHL about midnight —

HRDPS 1 AM precipitation rate

Here’s the Model Blend (NBM) 1 hour precip for midnight—

NBM 1 hour precip total midnight

So, there’s low confidence about  the thunderstorm potential this evening in Philadelphia. 

There is a stalled frontal boundary near us today, the remnants of last night’s storms.   Various areas of vorticity and instability near this boundary will trigger showers and thunderstorms.

It will be difficult to exactly predict the timing and location of storms today.

Right now, areas of Philadelphia are having some thunderstorms.    Reviewing last night’s 00z and 06z model runs, no models actually predicted these early morning storms at this time.

However, looking at the same models for triggers of these storms, specifically areas of “vorticity”, the models did show these triggers.

With good degree of success, yesterday’s forecast look at the GFS 500 mb level (~18,000 feet)  vorticity showed yesterday’s storm triggers.

Today we’re going to look at the 700 mb level,  (~ 10,000 ft)

Here’s the Canadian high resolution model (HRDPS) with 700 mb level vorticity shown in pink/red—

Canadian HRDPS (High Resolution Deterministic Prediction System) model 8 AM  forecast vorticity at 700 mb.  The trigger for the storms this morning is shown.  (Click on image for a larger view.)

Other areas of vorticity are visible above, particularly several red (positive) and blue (negative) vorticity couplets in northwestern PA.   These will affect our weather later this afternoon.   These positive-negative ‘couplets’ represent  rapid changes in vorticity (actually a conservation of momentum manifestation), often associated with very dynamic weather.

Here’s the HRDPS 700mb vorticity forecast for 4:30 PM today.   There are plenty of areas of activity—

HRDPS vorticity forecast at 4:30 PM today  (Click on image for a larger view.)

Here’s the model blend forecast (NBM) for showers/storms at 7 PM today—

NBM forecast 7 PM (1 hour accumulated rain)

Expect numerous scattered clusters of showers and thunderstorms anytime, especially between 4 PM and 9PM.



Tonight, our area will see more thunderstorms making it through Philadelphia.

Last night’s storms did hit a wall of stability as they attempted to move into Philadelphia.  Areas north and west, especially upper Montgomery county and parts of Bucks/Lehigh  county saw some heavy storms.

But most of us had just light showers, just remnants of the storms as the thunderstorms ran into a large ‘unfavorable’ environment of “convective inhibition”.

Today, Wednesday, things look a little different as a stronger upper air disturbance approaches from the west—

GFS 5 PM forecast – 500mb Vorticity    (Click on image for a larger view.)

These will likely meet a similar wall of stability, but the strength of today’s dynamics should overcome the wall of convective inhibition.

Very scattered storms may develop ahead of the disturbance as early 2 PM, but the main activity is expected between 5 PM and 9 PM.

NBM  1 Hour precip forecast for 7 PM Wednesday