Monthly Archives: January 2019

Philadelphia Weather Outlook

WEEKEND WEATHER OUTLOOK

A strong cold front moves through Thursday and cold, high pressure builds in for the weekend. While high pressure usually brings fair, sunny skies, this high pressure system will have embedded vorticities (disturbances, or eddy currents) that may bring periods of clouds with the sun.

There’s even a chance of an occasional snow flurry, especially north of the Philadelphia area.   Timing of these fast moving vorticities and cloudy periods is too difficult days in advance.

Temperatures will be cold, with highs Saturday near 30 and highs on Sunday nearer 40.

Another strong cold front moves through Tuesday into Wednesday.  Light rain possibly changing to snow is possible.  Models don’t show low pressure development at this time, but it’s a possibility.

Commentary

WHAT THE MODELS GOT WRONG…

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.

[su_note note_color=”#d9f2da”]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.[/su_note]

Winter Storm Outlook

WINTER WEATHER UPDATE – SATURDAY

[su_box title=”Winter Weather Update Sat 11:30 PM” box_color=”#defcdc” title_color=”#000000″]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. [/su_box]

[su_box title=”Winter Weather Update Sat 5 PM” box_color=”#defcdc” title_color=”#000000″]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. [/su_box]

[su_box title=”Winter Weather Update Sat 11 AM” box_color=”#defcdc” title_color=”#000000″]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. [/su_box]

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?”