Let the Games Begin

First - a Winter Storm Watch has been put in place for our region for Wednesday night through Thursday.  By strict National Weather Service definition, a "Winter Storm" is one that has the potential to produce 5 or more inches of snow.

So what do we have here?

I'll start with what we don't have:  guidance model consensus.  The solutions range as follows:

GFS:  a smaller shot of snow - less impact

ECMWF (Euro): big hit, "warmer" solution - possible snow/sleet/rain mixing issues on Thursday

CMC, UKMET, NAM: some variation of "keep the dog inside lest it disappear beneath the snowfall"

I'd really like to have better model consensus about 60 hours out from the storm but that's why this is such a difficult area for which to make a forecast.

Right now, my instinct tells me that we're going to get snow - quite a bit of it - but that where you are might make a big difference as to how much you see.  I'm reasonably confident at this point that the western suburbs will see a lot but I worry about DC and points east.  If the Euro verifies as-is, it could be too warm to snow on Thursday.

If the GFS verifies as is, we're all out of the huge snow ballgame.

Here's a bit of the GFS run from this afternoon.  (Note:  the afternoon run of the GFS is considered less trustworthy because it is not initialized from upper air data - it uses physics to make a forecast based on the now-6-hours-old upper air data from the morning)

This would be Thursday afternoon.  The low is positioned off the coast of Norfolk and is throwing back precipitation into our region.  This is a "colder" solution which gives us no precipitation-type problems but gives us overall precipitation amount problems.

The storm is gone in the next frame working its way offshore and up the coast.  The GFS predicts the following as snowfall totals:

As you can see, this guidance suggests our region misses the jackpot zone and we find ourselves in a VERY tight precipitation gradient with almost nothing to the northwest and heavier precipitation to the southeast.

At this point, however, the GFS is an outlier.  It has, by far, the most eastward track for the surface low and the least impact to our region.

On the other hand (hide the dogs!), this is what the NAM model guidance suggests for the same storm period:

which is a monster hit for the DC area and really no precipitation gradient problems to worry about.

The ECMWF (Euro) forecast for our area is a little more problematic.  Here's the raw model output for National Airport:

ECMWF Deterministic FORECAST FOR: DCA    LAT=  38.85 LON=  -77.03 ELE=    16

MON 12Z 10-FEB  -4.0   -10.1    1023      65      64    0.00     543     526  
MON 18Z 10-FEB  -0.9   -11.3    1024      49       7    0.00     543     525  
TUE 00Z 11-FEB  -5.4   -11.5    1025      59       4    0.00     544     524  
TUE 06Z 11-FEB  -5.4   -12.5    1028      52       7    0.00     543     522  
TUE 12Z 11-FEB  -7.1   -13.4    1030      49       9    0.00     546     523  
TUE 18Z 11-FEB  -2.2   -12.6    1030      39       9    0.00     547     523  
WED 00Z 12-FEB  -5.0   -11.5    1031      51       9    0.00     548     524  
WED 06Z 12-FEB  -7.0   -10.8    1033      55       3    0.00     550     524  
WED 12Z 12-FEB  -7.9    -8.1    1034      57       3    0.00     553     527  
WED 18Z 12-FEB  -1.9    -6.6    1032      40       9    0.00     555     530  
THU 00Z 13-FEB  -2.9    -6.3    1027      62      85    0.00     555     534  
THU 06Z 13-FEB  -2.4    -2.3    1018      92      98    0.30     552     538  
THU 12Z 13-FEB   0.6    -0.6    1005      93      98    0.66     549     545  
THU 18Z 13-FEB   1.1    -3.8     995      92      42    0.45     537     541    
 FRI 00Z 14-FEB   1.6    -4.1     996      89      89    0.30     533     537  
FRI 06Z 14-FEB  -0.2    -2.3    1002      73      20    0.04     536     534  
FRI 12Z 14-FEB  -6.2    -1.2    1007      79      54    0.00     540     535

I have bolded the two problem times.  During the heaviest precipitation, two not-so-good things happen.  One, the 2M temperature goes positive (0.6 and 1.1).  That means surface temperatures are expected to be above freezing.  We've seen a couple of times recently where that made a big difference in snow acummulation.

The second thing is that the 1000-500 mb thickness rises above 540 (545, 541) which is the "benchmark" for snow vs sleet or rain.  The Euro is indicating that a little over an inch of liquid precipitation (normally around 10 inches of snow) is going to fall with marginal - at best - temperatures aloft and at the surface.

This type of thing can bust a forecast like a piñata.

The evening model runs will be very interesting to watch.  I'll let you know what things look like as we get closer.

For now, prepare for a significant winter storm from Wednesday night into Thursday night with snow and wind.  Amounts, timing and track to be determined as we get closer to the event.