Updated Mon 11:10 PM— Tonight’s GFS has a significant snow for us. A large change from its previous runs. Details Tuesday morning.

Updated Mon 07:40 PM— The general trend continues with the Saturday storm side-swiping us to our east.  Here’s the statistical version of the GFS  (GEFS) showing the range of positions of the low pressure system late Saturday afternoon.  Notice that there’s a difference in position but all the tracks/positions are off-shore:

GEFS pressure statistics: 90, 50, and 10 percentiles sea level  pressure.

Monday afternoon’s operational GFS has light snow making it into Philadelphia, but has just a coating.  Temperatures above freezing. Still too far in the future to make a meaningful forecast.

GFS Precipitation forecast 5PM Saturday

I heard on the news/weather tonight that “whether we get some snow depends on the track of the storm”.  Isn’t that the purpose of a forecast?  I can’t make these things up.

Updated Mon 08:10 AM— Last night’s models have not clarified the forecast for next weekend. The trend has been for the storm to be further off the coast, but the track and development is still considered to have above average uncertainty.

…from Sunday—

This afternoon’s clouds were expected and forecast, as an upper air disturbance rotates over us from the upper low pressure system associated with the rain that passed through yesterday.

As mentioned in last Thursday’s post, some of the extended range models are suggesting the development of a coastal low this coming weekend.  (The extended range forecasters at the NWS have described the coming weekend’s weather as having higher than usual uncertainty. )

That said, this morning’s GFS, GEFS, CMC and ECMWF models are showing some sort of coastal low for us in an unclear time frame ranging from Saturday afternoon into Sunday.  The Canadian model shows the most developed snow storm for us and isn’t necessarily correct:

CMC forecast 1 AM Sunday morning

The GFS shows a low with less development and faster moving:

GFS Saturday 5 PM

The latest GFS Ensemble forecast (GEFS) (statistical) shows a low that is least developed and slowest:

GEFS forecast Sunday 7 AM

I wouldn’t go out and replace your snow shovel just yet, but it’s something to keep an eye on.

Amazingly, this is the first time this entire winter season where anything resembling a coastal snow storm presented even as a possibility.

Last year, we had multiple ‘potential’ snow storms that all ‘evaporated’ as we got closer to the time of the actual event. This may be no different, especially with little model agreement.  Stay tuned.



Update Sat 05:08 PM —Forecast Review: The rain ended as predicted but the clearing forecast by the models didn’t occur.  We had 1.92 inches rain.  The high temp was about 46º, much less than the 53º predicted or my forecast of 57º.

The latest NAM NEST shows rain ending about 2-4 PM this afternoon. Cloud cover depiction (white) shows some clearing about 4 -5 PM.

NAM NEST simulated Radar, wind barbs and cloud cover depiction

The latest hourly HRRR (High Resolution Rapid Refresh) still shows rain until 3-4 PM today.

HRRR Saturday forecast hourly rain accumulation- Click for larger image.


Updated Fri 11:15 PM — Forecast from earlier still holds.  Here’s the latest simulated radar depiction from the HIRESW NMMB2 showing rain exiting at 2 pm Saturday:

HIRESW NMMB2 model Simulated  Radar 2PM  Saturday 

The latest models have come in and heavy rain is still forecast for the Philadelphia area.  Depending upon the model, anywhere from 1.5-2.0 inches of rain will fall over most of the area between 7 AM and 2-4 PM!

But there are changes— the models have the rain ending between 2 and 4 PM on Saturday.  There may be some clearing skies very late in the afternoon. (Previously, the rain was expected to end later.)  It will be mild on Saturday, with high temps near 57º!

Sunday will be sunny in the morning, with cloudiness moving in during the mid afternoon as an upper air disturbance rolls through.  It will be mild for January, with high temps near 49º.


The models have been consistent in their forecast of heavy rain for Saturday.  The latest GFS model cranks out almost 1.8 inches of rain  right over us during the daytime hours, starting daybreak and ending early evening.  Mild with high temps in the 50s!

Here’s the latest PRATE (precipitation rate) forecast from the GFS—

GFS Forecast PRATE forecast GIF for Saturday. Click for larger image.

(If a picture is worth a thousand words, a .GIF file is worth a bit more!)

Sunday looks to be sunny, but some upper air disturbance may bring some clouds in the afternoon.

Things still look quiet, snow-wise.  There’s a possibility next weekend, but the models aren’t all-in on this.