Major Snowstorm looming

1:00PM UPDATE:
Snow totals are in...

...MONTGOMERY COUNTY...
   3 NNW GERMANTOWN      19.0  1031 AM  2/13  NWS EMPLOYEE            
   DAMASCUS              19.0  1030 AM  2/13  NOW GLAZE OF ICE ON TOP 
   1 ESE DAMASCUS        17.0  1000 AM  2/13  PUBLIC                  
   2 NNW BOYDS           16.2  1111 AM  2/13  PUBLIC                  
   1 NNE WASHINGTON GRO  15.0  1100 AM  2/13  TRAINED SPOTTER         
   1 ESE NORBECK         14.4  1048 AM  2/13  TRAINED SPOTTER         
   1 WNW OLNEY           13.9   943 AM  2/13  TRAINED SPOTTER         
   N ROCKVILLE           12.5  1023 AM  2/13  NWS EMPLOYEE            
   2 SSW ROCKVILLE       11.0  1024 AM  2/13  TRAINED SPOTTER         
   1 ESE TAKOMA PARK      9.2  1000 AM  2/13  TRAINED SPOTTER         

7:00AM UPDATE:
Widespread totals of in excess of a foot throughout the area due to persistent mesoscale banding over the region last night resulting in heavy snowfall rates of 1-3 inches/hr, The snow/sleet line has worked its way into DC and southern Moco. How far west it goes before retreating has yet to be seen. It is likely that it will not move past southern Moco and begin to retreat in the coming hours.

Blue is rain sleet line. Green is sleet/rain line possibly.

3:30AM UPDATE:
The changeover to sleet is beginning. some spots in DC are reporting sleet though this could be h=just an isolated occurrence. In addition, the NWS has extended the winter storm warning due to the possibility of heavy snow as the backside of the storm swings through thursday evening.

1:20AM UPDATE:
The NWS has issued a special weather statement for the DC area for snowfall rates up to 3 inches an hour associated with a heavy snow band in the area.

...HEAVY SNOWFALL EARLY THIS MORNING...

A BAND OF HEAVY SNOW OVER THE VIRGINIA PIEDMONT IS EXPECTED TO
OVERSPREAD THE WASHINGTON METROPOLITAN AREA BY 2 AM THIS MORNING.
THE HEAVY SNOW WILL OVERSPREAD THE BALTIMORE METROPOLITAN AREA
BETWEEN 2 AM AND 3 AM THIS MORNING.

HEAVY SNOW WILL PERSIST FOR A COUPLE HOURS WITH VISIBILITY REDUCED
BELOW ONE-QUARTER MILE. SNOWFALL RATES OF ONE TO AS MUCH AS THREE
INCHES PER HOUR ARE EXPECTED UNDERNEATH THIS BAND. TRAVELLING WILL
BE VERY DANGEROUS DURING THIS TIME SINCE ROADS WILL BECOME QUICKLY
SNOW-COVERED.

In addition, it has raised snowfall totals in the warnings from 6-10 to 7-12. We could be looking at an over performing storm. There is a possibility of the rain during the daytime changing back to snow thursday night.

...WINTER STORM WARNING REMAINS IN EFFECT UNTIL 5 AM EST FRIDAY...

* PRECIPITATION TYPE...HEAVY SNOW.

* ACCUMULATIONS...7 TO 12 INCHES...WITH UP TO 15 INCHES AT
  ELEVATIONS ABOVE 800 FEET.

1:00AM UPDATE:
The models are right on the money. Giant snow band is heading for DC with 1-2 inch+ snowfall rates. It just keeps building back south. This is going to be fun to watch.

Radar @ 1:00 snow huge 35-50 dbz snow band to the south of DC heading northeast

10:40 PM UPDATE:
The hi-res models have come in with INSANE amounts of snow. For example, the RAP gives us a foot of snow through 10:00AM and the HRRR also gives us a foot through 9:00AM. These snow totals do not include compaction or any melting that may occur so the actual snow depth will be lower. However, these models also indicate high instability which is responsible for these snowfall rates. Richmond, Va has already reported thundersnow. Just for fun, I will issue a Thundersnow Watch (not something that actually exists) for Moco from 12:00 AM through 6:00AM. Although this does not guarantee thundersnow, this does mean that the potential for it is there.

8:15 UPDATE:
Light snow has broken out across the region and will increase in intensity throughout the night. Cold temps will make sure the snow accumulates immediately. The SPC has issued a mesoscale discussion about the possibility of heavy snow.

AREAS AFFECTED...PARTS OF THE MID-ATLANTIC STATES

   CONCERNING...HEAVY SNOW 

   VALID 122341Z - 130515Z

   SUMMARY...A LARGE SWATH OF MODERATE TO HEAVY SNOW WILL DEVELOP NEWD
   THIS EVENING WITH RATES OF 1-2 IN/HR. THE MOST PERSISTENT BAND
   SHOULD EXIST ALONG THE BLUE RIDGE MOUNTAINS AND ADJACENT PIEDMONT
   FROM NWRN NC TO CNTRL VA THROUGH ABOUT 03Z (10:00PM EST) AND INTO THE WASHINGTON
   DC METRO AREA BY 06Z (1:00 AM EST).

   DISCUSSION...23Z SURFACE OBSERVATIONS CONFIRM A BROAD AREA OF
   MODERATE TO HEAVY SNOW AS FAR N AS THE I-64 CORRIDOR IN VA/WV. 12Z
   CONVECTION-ALLOWING WRF GUIDANCE GENERALLY APPEARS TOO SLOW WITH NRN
   EXTENT /OUTSIDE OF THE NSSL-WRF/ COMPARED TO RADAR/SURFACE
   OBSERVATIONS. CONSENSUS OF CONVECTIVELY-PARAMETERIZED GUIDANCE
   APPEAR CLOSER TO REALITY...WITH 15Z SREF MEAN AND 18Z NAM SUGGESTIVE
   OF SNOWFALL RATES AVERAGING 1-2 IN/HR. THIS APPEARS REASONABLE GIVEN
   INTENSE FRONTOGENESIS/ISENTROPIC LIFT DOWNSTREAM OF A VIGOROUS AND
   STILL AMPLIFYING SHORTWAVE TROUGH OVER THE SOUTHEAST. THE MOST
   PERSISTENT BAND OF HEAVY SNOW WILL LIKELY EMANATE NEWD FROM THE NWRN
   NC/SWRN VA AREA TOWARDS THE WASHINGTON DC METRO AREA BETWEEN 03-06Z.

ORIGINAL TEXT:
Let me put it simply. A major storm is now inevitable expect 4-8 inches of snow by sunrise, possibly more. snowfall rates will be in excess of 1 inch per hour for much of the night as the storm blows up off the NC coast.

IR satellite

Radar @ 7:15 PM 2/12/14

Tonight's forecast is final. temps in the mid to upper 20s and heavy snow. 4-8 inches expected.
The heaviest snow should be in the early morning hours. Snowfall rates then could approach 2 inches/hr and even 3 inches/hr in the heaviest convective bands. Thundersnow is definitely possible in the heaviest of snows. In addition, winds will pick up to gust near 25 mph at times.

HRRR 1hr snowfall from 3:00AM to 4:00AM  shows widespread heavy 1 inch per hour rates.

HRRR simulated radar, precip type and freezing line

 Now that we are in the short term, I have turned to high resolution models such as the HRRR model, which can do better to sniff out convective bands and other small scale features. The HRR shows heavy snow for DC throughout the night and into the morning. Although this may be overdone by 2-4 inches, it is possible. The HRR shows around 10 inches of snow by 6:00 AM

HRRR snowfall by 6:00 AM Thurs morning.

Tomorrow during the day is less clear. When the changeover to rain is the issue. The changeover near DC is expected around mid morning just as the heaviest precip heads east, dryslotting us. Places to the north and west may not change over at all while places to the south and east will change earlier. Light rain or rain snow mix is then likely for the rest of the day with temps in the low to mid 30s. In the afternoon, a second area of snow might swing through, mixing with rain and snow around the area. Depending on the strength of this "deformation zone", We could get another few inches of snow with the highest amounts, as usual, to the north and west. When all is set and done, we will range from 7 to 14 inches of heavy, wet, snow across the county.

DC is in the dry slot with snow to the west at 10AM

Temperatures are just above freezing. at 10 AM

Maryland is under a state of emergency and Montgomery County Schools have closed for Thursday. I will continue to have updates throughout the night. Any snowfall reports are appreciated and can be placed in the comments section.

Thanks for reading and enjoy the snow!

-Alex