Fri 11pm Update: Tonight’s NAM shows showers before 8am Saturday, then dry until later in the afternoon with  showers  from PHL and south. Tonight’s NAM also keeps Sunday dry until Sunday evening. 

This weekend’s weather is going to depend greatly on your location— a weak front will move south through PHL late Saturday and will orient west to east just south of Philadelphia.  Like the past few weekends, high pressure will nose in from the northeast on Sunday— an easterly wind will keep temperatures cool on Sunday.

Currently the models are in somewhat reasonable agreement that most of the disturbances riding along the boundary this weekend, especially Saturday, will be in Delaware and Maryland and may just reach into extreme southern Pennsylvania and southern NJ.  Those are the areas where widely scattered showers and thunderstorms are expected.  Areas north of PHL—  Bucks County and Upper Mongomery county look to be relatively rain-free.

So Saturday will be a mix of sun and clouds;  showers will develop late afternoon, mostly to our south.  High 83.


NAM Forecast
NAM Forecast for 5 PM Saturday.   

For Sunday, the NAM keeps our area dry with a mix of clouds and sun; the north-south dichotomy will remain.  An easterly wind will keep temperatures cool; with a high of 74.

BUT, there are large differences with the model forecast for Sunday. The GFS pushes more precipitation into our area on Sunday into Sunday evening while the NAM shunts it south into Delaware and Maryland. With such LARGE differences, we’ll have to wait to make a better call.


Fri Morning Update: The latest models have shifted the showers to late Saturday and most of Sunday into Sunday night.  So Saturday is looking like a better day, especially from PHL and northward. I’ll do an updated weekend forecast Friday evening.

The weather for this weekend is again looking unsettled.  There are huge differences between the NAM and GFS models for Saturday with the NAM being much wetter than the GFS.

Basically, there are timing differences with the disturbances that are expected to move along a frontal boundary. The position of the front is also in question.

Perhaps there’s insight in the fact that the new GFS model under development (referred to as the GFS FV3- “Finite Volume Cubed Sphere” ) has a similar forecast for Saturday as the NAM.  By the way, I’ve been looking at the GFS FV3 more and more.

That would mean that Saturday would be the wettest and Sunday would be dry.  Too soon to be sure.

As for the cool weather so far this season, it’s not been a surprise— the nadir in the solar cycle is known for cool weather.  The secondary effects of a solar cycle nadir results in  a lower solar wind  causing more cloudiness.


With all the thunderstorms that have moved through Philadelphia this afternoon and evening, there are areas to the immediate northwest that have barely gotten anything.

Showers and thunderstorms should continue tonight, ending before daybreak.

Sunday should look similar.to today, with a mix of clouds and sun. Essentially dry for much of the day.  An easterly wind will make things much cooler with a high of about 67.

Clouds increase late in the afternoon with showers developing late afternoon.


I thought it was going to rain?  Me too!  So what’s with the forecast?

Last night’s NAM data (which comes out later, between 10 and 11 PM due to daylight savings time), showed some changes with the forecast and I tried to patch my forecast with a quick update at 11 PM.  I failed to fully examine the changed humidity fields— which showed this sunshine for the morning hours today.

So the models continue with some sun and essentially dry conditions for this morning .  Things change after 12PM  today,  as low pressure develops to our south and things become more unstable.  Scattered thunderstorms can form almost any time after 12 PM today and the probability increases considerably during the late afternoon.

Afternoon simulated radar
HRRR 4PM Saturday simulated radar forecast

Like last Saturday, these storms will not move in as a line, but will develop in place and be slow movers.  Most areas will have had showers and thunderstorms by 6 pm, although the areal coverage won’t be large at any one time.

The chance of showers persists into the evening.


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