Accumulations will all come down to: Timing, Intensity, Boundary Temps & Surface Temps

When it comes to late March snow events the devil is always in the details.   While the current European forecast model continues to be the most bullish with accumulations,  there are other factors within the European model that need to be studied before we can believe an operational accumulation map (latest operational map is shown below):  So let’s play devil’s advocate:  Could 6 to 10″ of snow actually fall across the Washington Area by Sunday night? 

ecmwf_tsnow_washdc_17

Here are the all important factors to explore:

  • TimingSnow falling during the overnight hours combats the effects of the higher March sun angle which filters through the clouds and warms the earth surface.
  • Intensity – Heavier precipitation acts to pull colder air sitting aloft (a few thousand feet off the ground) down towards the surface helping the snow to accumulate by overcoming warmer surface conditions during the daylight hours. So in effect if you have heavy snow falling during the day, the higher sun angle might not matter as much.
  • Boundary temps – The boundary (layer) of the atmosphere where snowflakes are made need to be cold enough (at freezing or below).  The few thousand feet below the clouds also needs to be cold enough for the flakes to remain frozen or falling as snow.
  • Surface Temperatures – Obviously temperatures hovering around freezing bode well for accumulations to stick on untreated roadways.

So let’s dissect the latest 12z (latest run of the Euro, frame by frame from Sunday morning through Sunday evening) analyzing the factors that we just talked about above:

The following image is showing how much liquid equivalent (either melted snow or rain) is forecast to fall between 7AM and 1PM SundayThe dark blue colors represent liquid totals of nearly half an inch in six hours and if this model run is correct would suggest some heavier snow coming down (typically 1″ of liquid = 10″ of snow).

ecmwf_slp_precip_washdc_14 Boundary temperatures (the essential layers of the atmosphere from cloud base and below where the temperatures have to be at or below freezing for the snowflake to survive the trip without melting into a rain drop): Are they at or below freezing? Yes

ecmwf_t850_washdc_14

Surface temperatures: Here is where it gets tricky …  this latest run of the European forecast model (12z) predicts temperatures to be in the middle 30s Sunday afternoon around 1PMSnow would likely be accumulating on grassy surfaces with most treated roadways away from the foothills/higher elevations being just wet.

ecmwf_t2m_washdc_14

Let’s move to the Sunday time period between 1 and 7 PM:

Image below still shows impressive liquid amounts falling during this time frame (another half an inch possibly on top of what falls between Saturday evening and 7AM Sunday).

ecmwf_slp_precip_washdc_15

Boundary temperatures are still adequately below freezing during this time frame:

ecmwf_t850_washdc_15

Surface Temperatures: By 7PM Sunday, surface temperatures have cooled to near or below freezing!  The only issue is … has the heaviest snow begun to exit to the northeast?

ecmwf_t2m_washdc_15

Let’s move to the Sunday time period between 7PM and 1AM:

ecmwf_slp_precip_washdc_16

By this time boundary temperatures are obviously cold enough as colder air continues to wrap in from the north at all layers of the atmosphere.

Surface temperatures are now forecast to be below freezing at 1AM Monday (Sunday night):

ecmwf_t2m_washdc_16

Now to the GFS & European Ensembles for Snowfall:

The Global Ensemble Forecast System (GEFS) –  The first image shows you the wider view of the Northeast, the second image shows the DC Region (zoomed in view).

gefs_snow_ens_neng_17 gefs_snow_ens_washdc_17

I don’t like the GFS this far out so let’s look at the European Ensembles for snowfall potentialAgain as you can see in the images below… more bullish on at least a 1 to 3″ snowfall for downtown DC with the potential for a lot more as you head north and west.

eps_snow_50_washdc_43 eps_snow_25_washdc_43

Averaging out the Ensembles above:

eps_snow_m_washdc_18

Then there are the SREF models (Short Range Ensemble Forecast) that update more frequently.  The following are projections for DCA (National Airport), IAD (Dulles) and Frederick Maryland.  Keep in mind these SREF models are American engineered models (not the European, so I read them with a grain of salt this far out).

Currently the latest SREF is forecasting between 1 to 3″ for DCA; 2 to 4″ for Dulles; and up to 5″ in Frederick… subject to change.  *The graphs below show snowfall amounts (in inches) on the left side and time period (hours in the future from now) on the bottom with the vertical blue lines predicting snowfall accumulation*

KDCA_2016031715_6snow KIAD_2016031715_6snow KFDK_2016031715_6snow

Ever wonder why the snowfall forecast changes so frequently on many of the apps out there on your smart phones… this is why… they are typically blending the various models that I have just shown you…. until consensus builds just before the event.

In Review: 

  • The best part of the upcoming weekend for getting that morning jog in looks to be Saturday morning.  Light precipitation in the form of rain and snow may start as soon as the early to mid afternoon.

 

  • Exactly how much snow accumulates will depend on precipitation intensity, surface temperatures, track and timing.  A general one to four inches of snow is currently my best early guess on grassy surfaces.  But I can’t discount that heavier snow may fall Sunday causing temperatures to dip into the lower 30s with some locations picking up a heavier snowfall (as the European forecast model continues to suggest).  Stay Tuned.

 

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