An easy forecast to make for this weekend.   High pressure will bring a southwesterly flow of very hot, humid air into our region.  Both Saturday and Sunday will be sunny with highs on Saturday 95 and 99-101 on  Sunday. ( It’s July and having a heat wave is what happens in the summer!)

On a positive note, the latest model runs have a bit less humidity than previously forecast.  Dew points in the mid to upper 60s during the daytime hours.  This is a slight improvement over what had previously been predicted by the models; they had been showing dew points in the low 70s during the day.    It’s still going to be hot!

GFS model forecast
GFS Model forecast for Sunday afternoon.

The latest models have highs in the 90s for the next 7+ days!


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.


I’ve been reviewing the latest runs of the NAM and GFS models.  Both models have a warm front moving up from the south and through Philadelphia  between 8 AM and 10 AM Saturday morning.

There are large differences between the models regarding the precipitation.  Specifically, the NAM cuts off the precipitation following the frontal passage, allowing things to dry out after 10AM until late afternoon, when additional thunderstorms may form.   QPF values are less than 0.30 inches water.

However, the GFS maintains enough dynamics for showers and thunderstorms throughout Saturday, especially early afternoon Saturday.   QPF values are over 1.20 inches water!

I’ve been looking for clues as to which model forecast is correct.  For some reason, the non-operational NBM and EKDMOS blended models have not been available for the past day. Other models (CMC, ICON) support the wetter GFS.

My own experience with warm fronts in summer is that things tend to clear out following a warm frontal passage.  So I’m inclined to go with a more optimistic forecast.

So… after early morning showers, expect precipitation to end by 10 AM with mostly cloudy conditions, but skies may brighten considerably.   Expect scattered thunderstorms to re-develop late afternoon into evening.  High 81.   (A lower confidence forecast than usual.)

For Sunday, a mix of sun and clouds.  Warm and humid.  High 88.  Chance of thunderstorms late afternoon.

I’ll update Friday evening, but it may be late.

Fri 10:30 pm update: Tonight’s NAM continues with a break in the showers between 10 am and about 5pm Saturday.  So staying with the optimistic forecast.