The computer modeling of this storm had many aspects go wrong…

  • The original track, consistently a near coastal storm for many days, became an inland storm.
  • The precipitation start time in Philadelphia, shown by most models to be 3-5 PM, didn’t start until 6-8 PM.
  • Thermal profiles, due to dynamic cooling and modeling error, allowed more snow than predicted before the changeover.
  • Temperatures that were expected to rise rapidly into the 40s shortly after midnight didn’t reach 40 until 6 AM today.
  • High temperature today of 49-50 (GFS) was never reached.
  • Winds that were supposed to pick up during the night and this morning, didn’t happen (yet.)
  • Winds are so light now that fog has developed in many areas, also unpredicted by the models.
  • Temperatures, predicted to rapidly drop to freezing by 1-2 PM, are now predicted to be in the 40s until noon!

With this storm, it’s not that one model did better than another. The GFS, NAM and NBM (the NBM is a composite of many models, including the European, Canadian, GFS, NAM, their statistical analogs, and the HRRR) all got it wrong.

Flash freeze?   I don’t think so.  “Flash Freeze” warnings became its own TV weather reality show when it was realized that this was going to be just a rain storm.  Originally based on model runs from early Thursday and predicated on a originally predicted temperature drop from about 49 to 32 in just a few hours, current short range models now show an unremarkable drop in temperatures during the afternoon today.  But the Flash Freeze likelihood has been dispelled by the model runs for two days.

The models are still showing high winds (likely) and a cold snap for later this afternoon and tonight.

Current temperatures are running warmer than even this morning’s models. It’s 43 northwest of the city at 11 AM, but the NAM run from this morning predicted 36.

The NBM is looking the best right now- temperature drops to 32 about 3-4 PM and 5 degrees by Monday morning.


Winter Weather Update Sat 11:30 PM
The precipitation started a bit later than the morning models had forecast and some dynamic cooling allowed snow to fall for the first two hours. It’s changed to rain and the rest of the forecast remains intact.
Winter Weather Update Sat 5 PM
No accumulating snow expected here. Some timing changes. The short range models now show rain doesn’t start in and around Philadelphia until 6-8 PM. Rain ends about 10 AM Sunday. Temperatures drop below freezing about 2-3 PM on Sunday.
Winter Weather Update Sat 11 AM
This morning’s GFS comes in warmer and a more westward track. The severe weather forecasts were possibly justified about two days ago, but the models yesterday and today makes this a heavy rain storm with a sharp cold front for Philadelphia and even areas approaching Allentown.

Any snow flakes will be insignificant with surface temperatures in the mid 30s rising into the 40s.

The latest model NAM just becoming available.  Here are the trends for Philadelphia and immediate suburbs –

  • Last night’s GFS and NAM runs were similar. This morning’s NAM is warmer and faster.
  • There’s a trend towards a warmer storm, partially due to track change but also due to less intensification, less phasing of jet streams.
  • Earlier start and earlier end of precipitation.
  • While I have no expertise, I think the “flash freeze” hype has been just that, and it was likely hype as early as yesterday’s model forecasts showing the precipitation ending before the frontal passage.
  • Rain ends about 8-9 AM Sunday.
  • Temperatures drop to freezing about 1 PM and continue to drop

Here are the specifics:  Precip starts 3-5 PM.   Some light snow possible at start, but a very quick change to sleet and then heavy rain.  Little or no accumulation.

Heavy rain through 8 AM Sunday.  Temperatures rise into the 40s or near 50.    Rain ends 8-9 AM Sunday.  Cold air delayed and moves in several hours after rain  ends, significantly reducing the chance of a flash freeze.  Very windy!   Temperatures drop to freezing about 1-2 PM and continue dropping into the 20s.   Near 10 at night.

Regarding the “flash freeze”, I think it became its own reality when the models predicted no frozen precipitation falling with when the front moved through.  In my mind, a flash freeze occurs where there’s still active precipitation when the temperatures drop rapidly to below freezing.  Sure, any standing water will freeze.  Is that a “flash freeze?”




Winter Weather Update

The latest NAM data has become available.  Here are the trends:

The precipitation moves in earlier,  now beginning between 3 and 5 PM.  The NAM is showing a warming trend in all levels of the atmosphere that will extend further north than previously predicted.

Precipitation possibly starts as snow but quickly mixes with sleet and rain and changes to all rain by 7 PM in Philadelphia and by 9 PM in the northern and western suburbs.  A coating to less than an inch of accumulation in and around Philadelphia followed by heavy rain Saturday night.  Total QPF values about 1.90 inches water.

The arctic front moves through about 8 AM Sunday morning. Some changeover to snow showers possible, but little or no accumulation in and around Philadelphia.  High winds and gusts.  Temperatures rapidly plunge to below freezing by 1 PM and into the teens by evening.

NAM total snowfall prediction

With the high winds, open roadways may dry but areas of pooled water or drainage will quickly freeze by afternoon.

So this is really looking like just a heavy rain storm and a sharp cold front for Philadelphia and the immediate areas.


Winter Weather Update Fri 5 PM
Another quick update. This afternoon’s NAM and GFS show a faster storm onset. Precipitation starts about between 3-5pm Saturday afternoon. It’s likely to start as snow, with 1, possibly 3 inches before a rapid changeover to sleet and then rain.

Heavy rain by early evening Saturday. (Freezing rain likely far northwest in Allentown up until midnight.) Rain tapers in the morning Sunday as temperatures plummet and winds pick up. Minimal accumulation expected in the immediate PHL area when all is said and done.

Temperatures are forecast to drop from 49 (GFS) or 42 (NAM NBM) to 30 by 1 PM, then falling further during the afternoon to 20 by 7 PM and single digits at night. Windy conditions may dry some open roadways, reducing icing, but standing areas of water or draining areas from the side of roadways will freeze.

The next model runs come out between 9 PM (NAM) and 10:45 (GFS). I’ll update before 11 PM.

Winter Weather Update Fri 10 AM
A quick update. This morning’s NAM shows a faster storm onset. Precipitation starts about 4pm Saturday. A very fast changeover to sleet and then rain. Freezing rain possible far northwest. Heavy rain by early evening Saturday. Rain tapers in the morning Sunday as temperatures plummet and winds pick up. Minimal accumulation expected in the immediate PHL area when all is said and done.

NAM accumulations
NAM forecast total accumulations by Sunday noon.

Last night’s models did very well with the light snow that should be ending within the next hour.

Here’s the latest on the Saturday to Sunday storm. For Philadelphia and its immediate suburbs, a total storm QPF is about 1.95 inches water. Most of this precipitation will be rain, but the beginning and end of the storm will have snow or mixed precipitation.

Based on the latest 06Z run of the NAM and GFS models (1AM last night), the NAM is holding off much of the precipitation until temperatures rise enough when the precipitation begins towards evening. Temperatures in all levels of the atmosphere will minimally support snow, so between 5 and 7 PM Saturday, expect some light snow rapidly mixing with sleet and rain and then all rain by 9 PM. In the far northwestern suburbs, an extended period of sleet and freezing rain may be occur and areas near Allentown may remain a frozen mix until midnight.

The GFS also has the atmosphere warming, but it has more precipitation starting somewhat earlier in the late afternoon, with greater mixed precipitation accumulating before the changeover to rain about 9 PM.

The Arctic front moves through about daybreak Sunday. Temperatures plummet from about 40 at 7 AM to 28 at 1 PM and 17 by 7 PM. Little moisture is left with this drop in temperatures. A change back to snow is expected Sunday morning, but accumulations in the immediate PHL area and surround suburbs will be minimal Sunday, according to latest models.

Stay tuned for more updates. We’ll be in better range for specifics tonight and tomorrow morning.