Category Archives: Winter Storm Outlook


Friday Storm Update

Update Fri 12/23 @ 10:44 AM — Latest NBM shows virtually no snow accumulation in our area. For those holding out hope for a white Christmas, here’s the latest HRRR

Today’s 12z HRRR snow totals (Click on image for a larger view.)

Update Fri 12/23 @ 10:15 AM — Additional rain now falling as the front moves in at 10 AM.

Storm rainfall measurements via MRMS through 9 AM—

MRMS calculated rainfall at 9AM for the previous 24 hours. Contours are mm. 25.4 mm= 1 inch (Click on image for a larger view.)

Current Temperatures at 10 AM—

RTMA temperatures at 10AM Surface Cold Front is moving through Downingtown and Birdsboro (Click on image for a larger view.)

Cold Front Passage

Update Thu 12/22 @ 9:47 PM — Latest NBM (00z) shows the rain to snow transition in the city about 11 AM.

00z NBM shows rain to snow transition at 11 AM. A very light coating of snow possible in some areas. (Click on image for a larger view.)

Some breaks in the clouds expected mid to late afternoon.

Temperatures drop from 58º to 32º by noon and into the 20s by afternoon—

00z NBM meteogram for Blue Bell PA (temperature and dew point with standard deviations) (Click on image for a larger view.)

Areas that may see a light coating (less than 0.3 inches) —

CMC HRDPS snow totals (Click on image for a larger view.)

Storm Update

UpdateThu 12/22 6:31 PM — Forecast essentially unchanged. Cold front moves through as early as 9 AM according to latest GFS. Highest winds after midnight and just ahead of the frontal passage.

Update Thu 12/22 @ 11:36 AM — Latest 12z GFS available. GFS has closer to 1.5 inches of rain instead of the higher totals posted from the HREF. Probably somewhere in between these two.

The GFS also has the cold front moving through earlier, about 9 AM at Blue Bell and the changeover to snow squalls/flurries. The GFS has a slight coating of snow in some northwest suburbs, but within the range of model noise.

Storm Update

Update Thu 12/22 @ 10:33 AM — This morning’s models are coming in. General forecast from yesterday still holds.

Here’s some changes with the 12z models—

  • Rainfall has increased and will total from 2 to 2.5 inches according to the HREF!
  • Highest winds and wind gusts after midnight tonight and especially during Friday morning.
  • Changeover to snow squalls and flurries may occur earlier, from west to east, from 9 AM to 11 AM.
  • The latest NBM has little to no accumulation, even in western suburbs.

Wind Meteogram for Blue Bell PA

Today’s 13z NBM Wind Meteogram for location Blue Bell PA (Click on image for a larger view.)

Forecast Update

Update Wed 12/21 10:56 PM — No change in the forecast summary from this afternoon below. (The NBM joins the HRRR with an 11 AM start tomorrow.)

Current Storm Status on Wed Evening

Update Wed 12/21 @ 8:25 PM — The pieces are coming together for the forecast. So what does it all look like on satellite and radar?

Water vapor image (at this specific wavelength-) shows the mid-atmosphere jet stream components—

Water Vapor at 8 PM Wednesday shows the subtropical jet bringing abundant moisture from the south. The main energy for the storm is just north of this image approaching Vancouver. This jet stream has winds of 200 mph and incredibly will be here on Friday.(Click on image for a larger view.)

Here’s the current composite radar—

Radar showing rain associated with advancing warm front from the south (red). This will move over us Thursday bringing heavy rain. Cold front well ahead of jet flow (blue) will move through here on Friday. (Yellow lines are Theta -E contours from the RAP model and clearly show the current position of the warm front. (red) (Click on image for a larger view.)

Update on Thursday -Friday Storm

Update Wed 12/21 @ 4:50 PM — This afternoon’s models just becoming available. Not much change from the previous two updates and a generally good consensus among the models. Here’s the current summary:

  • Rain starts in earnest between 11 AM Thursday, (HRRR model) and 2 PM (NBM model) although some light sprinkles can’t be ruled out as early as 7 AM.
  • Precip no longer expected to start as a snow mix here. Just rain.
  • Heaviest rain: late evening Thursday and Thursday night and also a burst of heavy rain as the front moves through Friday morning.
  • Rainfall will be heavy: 2-2.5 inches total rainfall
  • Cold front comes through Friday as early as 10 AM (HRRR) or 11 AM (NBM) with heavy rain and rapid drop in temperatures. At or below freezing by noon.
  • Wind gusts in the 45 mph range, possibly 50 mph after midnight Thursday and especially with the frontal passage Friday morning.
  • With the frontal passage, rain changes to snow showers between 11 AM and noon Friday.
  • No accumulation in the immediate PHL area. A thin coating possible northwestern Montgomery/Bucks/Chester counties.

More updates and fine-tuning as necessary. Stay tuned.

Update on Thursday -Friday Storm

Update Wed 12/21 @ 11:06 AM — Quick update. Latest GFS just became available. Rain moves in earlier, about 11 AM Thursday.

Friday: The front moves through about one hour earlier on Friday: 10 AM (Blue Bell PA) . Highest winds at that time. Change to snow squalls/flurries by 11 AM

Updates later this afternoon.

Update on Thursday- Friday Storm

Update Wed 12/21 @ 9:06 AM — After review last night’s models, I’m going to lean on the new NBM model (version 4.1) just released last week. Using this morning’s 12z run just available.

Thursday: While some sprinkles and even snow flurries will move in as early as 7 or 8 AM, the NBM shows accumulating rain moving in late morning on Thursday

Early Thursday morning—

Today’s 12z NBM 4.1 model Precipitation type at 7:30 AM (Click on image for a larger view.)

The rain moves in late Thursday morning and early afternoon—

NBM accumulating rain at 12 noon Thursday. Just starting and very light here. (Click on image for a larger view.)

Heavy rain Thursday late afternoon through night.

For Friday, the front is moving through just a bit earlier, about 11 AM. Wind gusts 40-50 mph expected.

12z NBM 4.1 meteogram for Blue Bell PA (wind and wind gusts with standard deviations) (Click on image for a larger view.)

A rapid mix to snow squalls and flurries. Here’s the NBM PTYPE—

NBM (12z) PTYPE at 11 AM Friday. (Click on image for a larger view.)

A coating or dusting possible by late afternoon Friday, but probability is is only 40% for Blue Bell.

Check back later for updates.

Differences in the Thursday Start Time

Update Tue 12/20 @ 9:03 PM — Latest NBM (00z) just available shows a light spotty mix at 8 AM Thursday. All to rain by 9 AM.

NBM version 4.1 (new model version) with improved PTYPE shows a spotty mix of very light rain and snow at 8 AM Thursday. (Click on image for a larger view.)

Update Tue 12/20 @ 8:26 PM — There are large differences in the precipitation start time on Thursday.

The GFS shows the precipitation not arriving here until about 2 PM Thursday.

The GFS at 8 AM Thursday shows precip well to our southwest—

GFS forecast (PTYPE) 8 AM Thursday Precip is well south of our area. (Click on image for a larger view.)

Several of this afternoon’s higher resolution models show light precipitation starting in the morning. The NBM is in this camp.

The Canadian RGEM even has some very light snow or frozen mix on the onset early Thursday morning.

The Canadian RGEM forecast for 8 AM Thursday morning (below) can’t be disregarded and seems to be supported by the latest ECMWF just available —

RGEM at 8AM Thursday. Last winter, the RGEM was remarkably good with Precip Types. Be advised that the RGEM PTYPE graphics always suggest much more precip than actually falls. (Click on image for a larger view.)

Of course it quickly changes to all rain on Thursday.

Thursday to Friday Storm Update

Update Tue 12/20 @ 5:41 PM — (added NBM snow accumulation below)

The latest models have stayed with the earlier timing of the storm and the cold front passage.

Thursday: This afternoon’s HRRR suggests that light rain moves in during the morning hours Thursday with rain becoming heavier about 2 PM. (The latest ECMWF and NBM have the rain moving in by late morning Thursday.)

Rain becomes increasingly heavy later Thursday and Thursday night. Winds increase after midnight Thursday. 1-2 inches of rain expected.

Friday: Here’s the latest NBM wind meteogram for Wings Field, Blue Bell showing wind gusts 45-50 mph from 6 AM to 6 PM Friday —

Today’s 19z NBM meteogram for Blue Bell PA. Frontal passage between 11 AM and 1 PM Friday. Wind gusts 45-50 mph! (Click on image for a larger view.)

I don’t see any snow squalls in any model’s forecast PTYPE, but the ECMWF has all three critical thickness lines east of us by 1 PM with some precip to their west. It could be the atmosphere dries out rapidly with the front, but I wouldn’t be surprised to see some non-accumulating snow squalls. (Hold your horses…latest GFS DOES SHOW SNOW SQUALLS)

12z ECMWF forecast for 1 PM Friday. The violet, magenta and red critical thickness contours are all east of us. Any precip will fall as snow even if temps haven’t fallen to 32º at the surface. (Click on image for a larger view.)

Latest GFS Just Available

Today’s 18z GFS shows PTYPE snow following surface 32º passage (white line) at 2 PM Friday (Click on image for a larger view.)

Stay tuned.

NBM snow accumulation by Friday evening—

19z NBM snowfall by Friday afternoon. Far northern and western suburbs may have a residual dusting of snow. (Click on image for a larger view.)

Update Tue 12/20 8:44 AM — A brief update with the current trends for the storm. The cold front continues to come through earlier and faster on Friday. The dry slot forecast by last night’s NAM has disappeared with its early morning 06z forecast.

The actual cold front moves through about noon to 1 PM Friday with thunderstorms and high wind gusts, possibly followed by plummeting temperatures.

Last night’s ECMWF suggests a quick period of non-accumulating snow showers/squalls following the frontal passage.

Clearing skies mid to afternoon on Friday. Very windy.

Updates later.

Update Mon 12/19 11:00 PM — Tonight’s NAM forecast range extends to 7 AM Friday.
Tonight’s NAM suggests that the heavy rain Thursday night departs early with a possible dry slot Friday morning. The sharp cold front comes through in the early afternoon with high winds perhaps some thunderstorms, another brief period of rain, followed by plummeting temperatures.

Storm Exits earlier on Friday

Update Mon 12/19 @ 7:59 PM — This afternoon’s models are leaning towards faster storm movement and an earlier cold front passage on Friday. (Previous models showed the front moving through late afternoon to evening.)

The NAEFS and GFS shows the front moving through about 12 PM to 1 PM Friday with the rain diminishing early to mid afternoon Friday. Temperatures start to drop with the frontal passage. High winds expected with the frontal passage.

Today’s 18z GFS simulated radar shows the front has moved through at 1 PM Friday. (Click on image for a larger view.)

Posted Mon 5:10 PM —

The big weather story for this week is the huge storm expected set up near the Great Lakes. For several days last week, it looked like we might have quite the snowstorm here; so much so that I thought it was prudent to set up a separate Winter Storm page.

Over the weekend, things have changed dramatically and a separate coastal storm is not expected to form with this system. A deep upper low pressure system will spawn a surface low west of us. Virtually no snow is currently forecast in our area by any model from this storm. Indeed, temperatures will rise into the 50s before falling rapidly below freezing Friday evening and night.

Nonetheless this will be a monster of a storm . Central pressures are forecast to rival recent hurricanes here. Heavy rain and high winds are expected.

The impulse that will set off the large storm is not yet over the continental US. Leading jet flow energy is becoming visible below—

Water Vapor Image (Satellite) with RAP model 300 mb wind streamlines (orange lines) Storm (1) heads south of us over the next day or so. The leading edge of the impulsed that will form the huge storm for Thursday and Friday is just becoming visible (2) The main energy is over the Pacific ocean. (Click on image for a larger view.)

Currently rain from this storm is expected to move in Thursday afternoon and heavy rain will exit late Friday afternoon or early evening. Stay tuned.



Fri 10:05 AM Forecast Review — So, we had even more ice accumulation than I would have guessed and temperatures did fall at or below freezing well into NJ, as forecast by last night’s NAM-NEST and HRRR.  The URMA shows temperatures at 2 AM—
URMA from 2 AM Friday morning  (URMA= “UnRestricted Mesoscale Analysis”)  The URMA represents actual temperatures and conditions and is used for model verification.  Since it represents corrected observations, the URMA is is released with a six hour delay to allow for corrected data input. (Click on image for a larger view.)

While the changeover to rain occurred, there was continued ice accretion past the rise in air temperatures above 32º.   Most of the models over-predicted the rate of temperature rise.  The major global models (GFS, ECMWF) never showed the temperatures drops into NJ. 

Show More