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Fri 11:30 PM – The two versions of the WRF model just available show showers as early as 1PM Sunday and more sun than the GFS or NAM. High temp 86-88 on Sunday.

The WRF model forecasts have been really impressive lately. The downside is the model comes out late, only forecasts out to 48 hours and is only run twice daily. But accuracy is worth the wait.  

Fri 10:45 PM – Tonight’s NAM has more sun, less cloudiness on Saturday afternoon, more clouds on Sunday.

Saturday will begin with the incredibly nice weather we had today (Friday).

As the crisp, unseasonable airmass over us moves off to the east, weak low pressure from the Midwest will pass to our north on Sunday. A southwesterly flow of increasingly humid and unstable air will move in for Sunday.

Saturday will start sunny and delightful.  High temperature 82-84. During the afternoon a very weak “pseudo warm front” will approach.  An increase in cloudiness expected with mostly cloudy skies by late afternoon.

Sunday will be mostly cloudy (some bright spots possible), warm and increasingly humid. As the warm front slowly moves to our north Sunday, light showers  are possible in the morning, mostly far north and west, although a sprinkle or quick shower can’t be ruled out here.  With increasing heat and humidity and an upper air disturbance moving through, expect showers and thunderstorms to break out in the afternoon.

The FV3-GFS I spoke of earlier this week has fully replaced the older GFS. The “FV3-GFS” after this past Wednesday IS the “GFS” now.

The GFS has the chance of light showers as early as 1 PM.  Most of the shower and thunderstorm activity is expected during the late afternoon and evening hours according to both the NAM and GFS.

I’ll update over the weekend to give a more specific timing and indicate whether some areas will have any severe weather.

What’s with this cool weather? Remember, we’re just coming off the 10 year sunspot cycle minimum. Back in the summer of 2009, we had fewer days in the 90s and many similar very cool spells. I’ve talked about this in the past.


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Thurs 7PM Update : Saturday is looking to be the better of the two days this weekend. Saturday will be dry, but the models are suggesting periods of cloudiness, just north and west of the city.

Sunday looks to be unsettled with considerable cloudiness and a chance of showers, both in the morning and especially in the afternoon. It will be warm and humid.

Speaking of the FV3-GFS, the model shows a disturbance moving through on Saturday afternoon, a pseudo-warm front.   Saturday starts sunny, but considerable cloudiness develops in the afternoon.  High 84.    Low dew points to start, but it becomes more humid later in the day.

Sunday looks to be cloudy very early, then hazy sunshine, very warm and somewhat humid.  High 88.   Thunderstorms originally expected in the afternoon now are forecast to move through about 7-8 PM.


Today, with its morning model run, the National Center for Environmental Prediction (NCEP) will have upgraded the GFS model (version 14) to the new and improved FV3-GFS (GFS version 15.1.1). The FV3-GFS becomes the main global model of the National Weather Service today.

The FV3-GFS (Finite Volume Cubed Sphere) has been in development for well over 10 years and has been in testing for the past three. It uses a different three dimensional geometry to reduce errors inherent in all numeric weather models. It includes different modeling physics and new parameters.   Significant information on the FV3-GFS can be found here.  Background information is here.

The model data has been available on university web sites in a very limited fashion for awhile.  Over the past two weeks, the NCEP made the data available on the main weather model download server, “NOMADS”.

The FV3-GFS has been heavily evaluated and is considered “equal or better” than the current GFS.


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Sun 8 AM : The short range high resolution models have the clouds coming in somewhat earlier than the FV3-GFS and GFS. About 2-3 PM. The FV3-GFS still about 7 PM. It will become quite windy today.  Areas to the north of Philadelphia will be sunny most of the day.

Current RAP (Rapid Refresh Model) cloud composite for 2:30 PM Sunday

Tonight’s NAM keeps the main cloud deck just to our south for much of  Sunday.  High clouds at times, but lower clouds aren’t forecast to move in until 4-7 PM. High 76-78

The WRF model, just available, has the clouds moving in about 1-3 pm, earlier just south of the city.

Storm Forecasts & Other Things "up in the air".