UPDATE: NAM, GFS are in - Still No Consensus

Here we are, less than 24 hours before the onset of this storm and this is what I have to work with from the NAM and GFS so far:

NAM: heavy, early burst of snow most of which falls in the 12 hours between around 8 PM Wednesday and 8 AM Thursday (and especially in the early morning hours of Thursday).  A lot of precipitation is simulated (which is a known NAM bias).  The wettest, coldest of the models simulates at least 10 inches of snow for most of the region DC and westward with few snow to sleet/rain changeover problems except for southern Maryland.

GFS: longer, significantly drier snow event which extends from 8-9 PM Wednesday into Thursday night.  Half (or less) of the precipitation simulated by the NAM and it's a bit warm as well which causes precipitation-type issues.  Widespread 4-6 inch snowfall from DC westward.

Here's a comparison of the forecast storm position for the same time from the GFS (top) and NAM (bottom):

You can see that the NAM is much faster with a low off Cape Hatteras at this hour while the GFS still has the low off the coast of the SC/NC border.

I'm still fairly worried about mixing on Thursday and I would base my forecast on that except that the NAM (and some other models as well) keep coming up with these solutions where the bulk of the precipitation falls before the temperatures become an issue.

Right now I feel that 4 inches is something we can count on.  From that point, I'm less certain.  My gut tells me 6-10 is where this will end up but without better guidance consensus, I'm relying on forecasting skills which haven't been professionally practiced in many years.  For what it's worth, the official forecast from the National Weather Service is as follows:

Afternoon NAM run is a big hit...

Widespread 10+ inch amounts throughout the area according to the NAM.

I'm taking a closer look.

Significant Snow Likely - But How Significant?

Things are still not as clear as I'd like them to be.

The Euro is still the westernmost track and the Canadian model (CMC) has come into line with that   Their tracks are quite close to the coast which create precipitation type issues in the forecast even west of DC and cuts down on the overall amounts.

The NAM, on the other hand, is a much colder solution and keeps us in the heavy precipitation but now it seems like the outlier.    The GFS is somewhere in between.

Here's the ECMWF deterministic output:

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

TUE 12Z 11-FEB  -7.6   -12.6    1030      60       9    0.00     546     523  
TUE 18Z 11-FEB  -2.0   -12.8    1030      43       9    0.00     547     524  
WED 00Z 12-FEB  -4.6   -11.8    1031      58      13    0.00     547     523  
WED 06Z 12-FEB  -6.3   -11.3    1033      51       4    0.00     549     523  
WED 12Z 12-FEB  -7.4    -9.5    1034      53       3    0.00     553     527  
WED 18Z 12-FEB  -1.8    -7.7    1032      39       3    0.00     555     530  
THU 00Z 13-FEB  -2.9    -6.7    1028      57      44    0.00     556     534  
THU 06Z 13-FEB  -3.2    -3.9    1021      88      98    0.18     554     537  
THU 12Z 13-FEB  -0.5     0.0    1010      93      99    0.55     552     544  
THU 18Z 13-FEB   0.9     2.8     998      91      76    0.21     546     547    
FRI 00Z 14-FEB   1.2    -1.2     992      89      53    0.15     533     540  
FRI 06Z 14-FEB   1.8    -2.7     996      87      90    0.25     532     536

 The same two highlighted times are still problematic - maybe even a bit more than they were last night.  The surface temperatures rise above freezing in the afternoon Thursday (0.9) and the thicknesses go well above the 540 benchmark (544, 547) meaning that mixing issues will almost certainly be present for areas east of I-95, DC included.

Looking at the GFS, I don't see as many issues.  This frame from the GFS shows heavy snow in our area Thursday afternoon:

The NAM shows a similar story but the precipitation is heavier.  However, this is a common bias with the NAM model so we can't take it too literally.

I'll be looking carefully at the upper level low as it moves into position over the next 12 hours.  If the storm "wraps up" too quickly it will make for a more potent storm but a more westward track.  The further west the track, the more mixing issues we have.  You really get a sense here of the precision necessary to produce big snows in the DC area.  The model data supports this.  There are SREF members forecasting everything from 2 inches to 20 inches for the DC area.  Clearly, that's not useful basis for a forecast.

I think we're in for some snow.  I think 4 or more inches is a good bet areawide with the first 3-5 accumulating overnight Wednesday into Thursday morning.  After that, it gets tricky.  Where will the storm track?  How close to the coast?  Does banding set up in our region?  Does the amount of lift in the atmosphere counter warming temperatures?

I think all of those questions are left to be answered still and will make the difference between a good storm, a big storm and a major storm.

UPDATE: Euro Puts Bullseye on DC

The early morning run of the ECMWF (Euro) model does nothing to dissuade snow forecasts and, in fact, paints a bullseye over DC.

Still some disagreement in the models - particularly the GFS and the Euro is still a bit too close to the coast for comfort (precip mixing issues).

I'll write more later this morning.

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.

Keeping an Eye on Wednesday/Thursday

This past weekend was a perfect example of what happens when two separate storms fail to phase.  The result is two rounds of snow showers and that's it.

This Wednesday-Thursday is starting to look much more interesting in the last few model runs.  There are still some crucial differences - particularly with storm track.  The GFS is currently the outlier showing a storm that phases too late and ends up out to sea, barely brushing the area.

The Euro and other models show a storm moving up the coast - maybe too close (which would draw warmer air from the Atlantic and give us precipitation type issues).  But trends in the models seem to be pointing towards a colder solution and more snow.

This could be the big one of the winter but it's still too early to tell.  Once I've had more time to look at the guidance, I'll post my thoughts but, for now, keep an eye on this time period.

Storm Potential for This Weekend

I'm keeping an eye on models and data suggesting the potential for a significant winter storm in our area Saturday into Sunday.  Details are a bit sketchy and the models don't agree so for now, the signal is there for a big storm and some models are trending colder - suggestive of all snow - while others keep us in mixed precipitation.

Storm track is everything and I suspect the models won't have a good grasp until after this ice storm gets through here tonight and tomorrow.

I'll post more once the situation clears a bit.