Tag Archives: Philadelphia area Weather Forecast


Happy New Year!

I got up this morning and saw the clouds and even some very light snow flurries.   Last night’s models did not show these clouds or flurries.   Nor did the higher resolution just-run hourly models like the HRRR or RAP show it.

Did the models capture anything that could explain the weather this morning?

They did, but apparently the model’s algorithms didn’t translate a strong upper air disturbance and moisture into the clouds and flurries that we have.

Last night’s GFS shows a strong area of vorticity (“an upper air disturbance”) passing through along with increased moisture in the upper and mid levels.

GFS 500 mb vorticity 8 AM Wed.   Click for larger image
GFS humidity – several upper levels  8 AM Wed

Add the humidity, vorticity and upper air cyclonic flow together and you get the weather we have this morning.

If the models were somewhat better, this combination of parameters should have triggered its cloud algorithm to show the clouds we have.   But it didn’t.

Based on the same thinking and parameters, the GFS shows humidity and vorticity move off between 12 – 2:30 PM.  We should see more sun by that time.

[su_note note_color=”#defcdc”]Update Wed 10 AM: This morning’s high resolution NAMNEST finally shows the light precip and clouds we already know about.

NAM NEST precip and cloud forecast for 11 AM Wednesday




The warm front clearly did NOT make it north of the Philadelphia area, as had been previously predicted by the GFS.  It’s in the 40s.  The short range models got this one correct yesterday.  Not a surprise; the new GFS has been biased to the warm side.

Today’s GFS has caught up with short range models.  Here’s the latest GFS below.  We don’t get out of the 40s north of the city.

GFS forecast temperatures for 3 PM Monday. Click for larger image


Speaking of temperatures, here’s the latest NBM (National Blend of Models) forecast for New Years Eve- Midnight.  Clear skies.   Winds 7-9 mph.

NBM 12:01 Jan 1, 2020    Click for larger image


[su_note note_color=”#defcdc”]Update Sun 12 PM: So, the current radar shows precipitation moving in sooner than any models had predicted.  Rain may start as early as 1 PM.[/su_note]

Most of last night’s models continue with the idea that rain begins in the general Philadelphia area around 3-4 PM today, Sunday. A remaining exception is the high resolution Canadian (HRDPS) which waits until 7 PM for the rain to start.

Rain may be heavy overnight and into Monday morning, then more light and scattered.

A warm front will try to make its way into Philadelphia and areas on a line to its south on Monday morning.  If it does, temps will rise into the low 60s.

However, only the GFS has it making its way as far north as Willow Grove.  The NAM, NAMNEST and the Canadian HRDPS, GDPS  keep it further south.  Areas that remain north of the warm front will remain in the low 50s. (Warm fronts are not as distinct as cold fronts.) So it may not get as warm as previously though on Monday.

NAM warm front position 12 PM Monday.  (NAM Temp, wind streamlines)

Most models have a cold front moving through about 2-3 PM, dropping temperatures.


[su_note note_color=”#defcdc”]Update Fri 11 PM: Tonight’s NAM and GFS have moved the rain start time to 3- 6 PM on Sunday.[/su_note]

An upper air ridge will continue to bring us mild temperatures. The upper ridge will break down as low pressure approaches and moves up through the Great Lakes from Sunday into Monday.  A secondary coastal low will develop on Monday

Saturday will be sunny in the morning, but throughout the day, high cirrus clouds will increase in sky coverage.  High 54.  Light winds.

Low pressure will gradually affect our weather on Sunday.  Sunday will be cloudy and increasingly windy.  As discussed in previous posts, rain should hold off until late afternoon and evening. (5-7 PM). High 52.

The rain should become heavy late Sunday night and continue into Monday. Monday was expected to be very warm with temps near 60º, but the models are suggesting a coastal low formation which may bring winds in from the northeast, lowering the high temperatures.