Possible Major Storm

10:45PM Update:
I just noticed something interesting, the temps stay in the low to mid 20s throughout the storm according to the NAM only to rise above freezing after the storm passes. These lower temperature would promote higher snow ratios up to 15:1 at the start to 10:1 near the end. The NWS also agrees:

00Z NAM PUTS LOW PRES IN A PRIME [POSITION] FOR SNOW PRODUCTION IN THE 
MID ATLC. 850-1000 THCKNS IN NAM GO THRU 12Z THU...AND ARE SUB 
1300 [ACROSS] THE CWA W/ THE XCPTN OF LWR SRN MD.

10:25 PM Update:
The GFS should be coming out soon and also, I got some pretty NAM maps for you. :)

10:1 ratio, likely not accurate

Gives DC an inch of liquid equivalent precip so depending on the temps and snow to liquid ratio, snow totals could vary with higher ratios to the west where it is colder and lower ratios to the east where it is warmer.

10:00PM Update:

The 00z NAM has come out and has a mixed bag. It has come back to earth with less insane snow totals, but it has the low going east, which cuts down even more on precip, with only 10-11 inches for the DC metro with less going North and West. It also delays the timing to early Thursday morning. But being a short range model, such tiny differences should not be worried about. Let's wait for the GFS and euro to come out to see if there are any clear trends. Remember, the models will keep flipping back and forth, so the snow amounts will not be clear until about 24 hours before the storm (we are about 48 hours out) and not certain until after the storm is over.

ORIGINAL TEXT:

The NWS has issued a Winter Storm Watch for its whole forecasting region for an excess of 5 inches possible wednesday night through friday.

THE NATIONAL WEATHER SERVICE IN BALTIMORE MD/WASHINGTON HAS
ISSUED A WINTER STORM WATCH...WHICH IS IN EFFECT FROM WEDNESDAY
EVENING THROUGH THURSDAY EVENING.

* PRECIPITATION TYPE...SNOW AND SLEET

* ACCUMULATIONS...THE POTENTIAL FOR 5 OR MORE INCHES OF SNOW AND
  SLEET.

* TIMING...SNOW IS EXPECTED TO MOVE IN FROM THE SOUTH WEDNESDAY
  EVENING. SNOW MAY MIX WITH SLEET AND RAIN LATE WEDNESDAY NIGHT
  AND THURSDAY. THE SNOW COULD BE HEAVY AT TIMES WEDNESDAY NIGHT
  AND THURSDAY.

* TEMPERATURES...IN THE MID TO UPPER 20S 20S WEDNESDAY NIGHT...
  SLOWLY RISING INTO THE LOWER AND MIDDLE 30S THURSDAY AFTERNOON.

* WINDS...NORTH 15 TO 25 MPH WITH GUSTS UP TO 35 MPH.

* IMPACTS...ROADS MAY BECOME SNOW AND SLEET COVERED AND SLIPPERY.
  TRAVEL MAY BE DANGEROUS WEDNESDAY NIGHT AND THURSDAY. HEAVY WET
  SNOW COULD LEAD TO SOME POWER OUTAGES.

The 18z models are out I will not be including the snow maps for the sake of time:

               North/West of DC  South/East of DC                          Notes:

GFS 18z            3-5                     6-10               Weakest solution, more out to sea

NAM  18z        10-20                 20-30            Insane, too wet, ideal low track, inaccurate at this range.

CMC 12z         10-14                 10-14             DC bullseye, weaker low track close to the coast

ECMWF 12z    14-24                 6-14              Shows freezing line west of I-95, low tracks                                                                                            farthest west bringing in warm air, strongest low

I would give a preliminary forecast of 6-10 inches of snow. Temperatures will be an issue and the snow could possibly mix with and change to sleet and rain as the low makes its closest approach. The disturbances that are responsible for the storm have not been fully sampled for data yet because it was out at sea for the 12z and 18z runs. The first fully sampled run will be tonight's 00z run which will come out in the 10:00-11:00PM timeframe. I will probably have an update then.

 Thanks for reading!

-Alex