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.
Hurricane Dorian is currently forecast to approach the eastern Florida coastline sometime Monday. The hurricane-specific models and their variants (and there are many of them) have quite a spread with the prediction of landfall locations but landfall somewhere around Cape Canaveral, Florida is currently forecast on Monday.
The model error-tracking done at the NCAR (National Center for Atmospheric Research) in Colorado shows all models to have current errors in predicting the position of this storm of at least 50-60 miles over the past 24 hours. Errors in predicting the location of Dorian over the past 48 hours have been over a 100 miles off. So accuracy is not there yet with this storm track, regardless of model.
The reason I bring up Hurricane Dorian is not because it will directly affect us over the weekend. But there will be indirect effects.
My experience with weather over many years has led me to notice that a strong tropical system in the western Atlantic seems to bring havoc to model forecast accuracy here. I don’t know why that happens but there’s some effect.
As a result, confidence in the model weekend forecast is somewhat lower than usual.
Already, a weekend that had been looking quite beautiful just a few days ago is now looking to have significant high, thin cloudiness, especially on Sunday and Monday.
Saturday appears to be the nicest day, especially from Philadelphia south and east.
Some scattered showers are possible on Sunday. The humidity might be higher than currently forecast.
I’ll take a better stab at the forecast on Friday. Expect changes in the forecast this weekend.
Friday 8 AM update: The front has slowed. Clouds had been expected for much of Friday, but it appears that areas of showers may linger into the afternoon in some areas. This had not been forecast.
It suggests the models don’t have a good handle on the forecast for the weekend. I’ll update this evening.
A cold front will move through tonight (Thursday) and a line of showers and thunderstorms is predicted by most models between 9PM and midnight. Some of the thunderstorms will have heavy rain Thursday night and 2-3 inches of rain are possible in the immediate PHL area. Areas west and north of Philadelphia are showing high shear values, so some thunderstorms will likely be severe.
High pressure builds in for Friday and the weekend. Unfortunately this high pressure center will remain to our north and an easterly or northeasterly flow of air may bring some damp conditions to the Jersey shore on Sunday, according to the latest NAM.
Most of the area will have sunny skies, but periods of some fair weather cumulus clouds are possible as well. A delightful weekend is forecast weather-wise.
High temps Saturday 77, and Sunday 76 with low humidity.
The easterly wind flow needs to be watched. This sometimes results in more cloudiness than the models predict this time of year. Sunday may have more cloudiness than currently forecast.
The slow-moving front that moved through Wednesday with thunderstorms will become a stalled, diffuse frontal boundary to our south; it will serve as a track for weak disturbances (areas of vorticity) to move near our area from west to east over the weekend.
The summer daytime heating combined with these areas of vorticity and moderately high moisture (precipitable water values about 1.5 inches) will result in instability and mostly afternoon and evening thundershowers, some which may be slow moving causing considerable downpours. Many areas will also see a fair amount of sun, especially east of the Philadelphia into NJ. Highs both days near 90.
Currently, the models show areas to the west of Philadelphia to be the most impacted by this setup on Saturday.
The current GFS shows showers to our west Saturday afternoon, but significant sunshine in other areas. (see graphic)
Sunday shows more clouds but fewer areas of showers—
This sort of scenario is impossible to nail down to specifics, despite the pretty graphics; it’s presented to provide an approximation of what to expect.
What will be interesting to see is whether some showers break out around 3-4 PM today (Thursday) in Philadelphia. My view of weather models- if the short range forecast shows to be inaccurate, the longer range forecast should be taken with a grain of salt.