Category Archives: Updates


Saturday morning’s NAM has backed off on the showers in eastern PA but has a slight chance for widely scattered showers in central NJ Saturday morning. Lingering periods of cloudiness in the morning still expected.  

Tonight’s NAM data is just becoming available and it has the front lingering over our area during Saturday morning.  This is a large change in the forecast. So there’s a chance of lingering light, scattered showers during the morning hours tomorrow, Saturday. Skies improve in the afternoon.

(GFS data not available until much later due to daylight saving time, so I can’t compare models at this time)

A weekend forecast  that appeared high confidence, now appears to have some new uncertainty.


This morning’s models haven’t changes significantly. Widely scattered showers are now in Delaware.   The short range models have the rain moving into the immediate  Philadelphia between noon and 2 PM, earlier to the south.   The rain continues into the late evening, heavy at times.

The axis of the heaviest precip shows some variation among the models, with areas just to our south and areas of NJ having the heaviest.   It’s very difficult to pin point the exact placement of the heaviest precipitation.

Sunday is currently looking relatively dry, with only very widely scattered showers; most of the area rain free.


The models did poorly with yesterday’s forecast; I guess the biggest clue that things weren’t going to go as forecast were the large differences in QPF forecast by the NAM and GFS.  The temperatures remained about 69 instead of the forecast 81; essentially,  the warm front never moved north as had been forecast.

I am somewhat more confident about today’s forecast, at least based on the short range models (GFS LAMPS;  NAM-based HRRR).  Both show a mix of clouds and some sun due to an occluded front that has stalled to our west.

Frontal positions
Current Frontal Positions at 8 AM Sunday.

Both models currently show a moderate chance of thunderstorms moving in between 5PM and 8 PM, with highest probability about 7 PM.    Still, the overall chance is only about 50%, so it’s  possible that not all areas will get the showers and storms today.

High temperatures 87 with dew points in the somewhat uncomfortable upper 60s.




Noon Update: My optimism about clearing after the warm front passage was apparently misplaced.  This was a “low-confidence forecast” for a reason.  The warm front hasn’t really made it past Philadelphia and it won’t until later in the afternoon.  A light drizzle has also developed. The GFS model was apparently correct.  Showers are beginning to develop to our south and may move in earlier than expected.  
Yet Another Update 5pm: The FV3-GFS shows lower chance of thunderstorms this evening. The warm front never made it north of Philadelphia!  Poor model performance today!

The showers have pretty much ended after 8 AM here, so yesterday’s NAM forecast has verified.

The question is – how much clearing  and brightening skies will we get?   The NBM (National Blend of Models) has things brightening considerably about 1 PM.

Current upper air analysis shows dry air punching in from the south (see 300 mb humidity graphic below)

But lower levels of the atmosphere are pretty saturated.  So I’m not sure if the drab, cloudy conditions will improve, but I think they may.

Current analysis shows much of the dynamics for thunderstorms remain to our south:

lifted index
Current radar and lifted index

Those dynamics supporting thunderstorms are expected to move northward by late afternoon giving us scattered thunderstorms.

So brightening skies (maybe) for much of Saturday with scattered showers and thunderstorms re-developing around 4-5 PM.