Overnight snow could drop 5-8 inches

"WINTER STORM WARNING IN EFFECT FROM 9 PM THIS EVENING TO 7 AM EST TUESDAY... PRECIPITATION TYPE...BRIEF PERIOD OF SLEET...CHANGING TO SNOW AROUND MIDNIGHT WHICH WILL BE HEAVY AT TIMES OVERNIGHT. * ACCUMULATIONS...4 TO 8 INCHES OF SNOW. WITHIN THIS RANGE...THE HIGHER AMOUNTS WILL BE IN THE NORTHERN PART OF THESE COUNTIES."

UPDATE, 10 a.m.: A Winter Storm Watch has been issued beginning at 9 p.m. for the northern counties, including Frederick, Carroll, northern Baltimore and Harford counties. Forecasters say 4 to 6 inches of snow are possible overnight in the Watch area.

"A WINTER STORM WATCH MEANS THERE IS A POTENTIAL FOR SIGNIFICANT
SNOW AND SLEET ACCUMULATIONS THAT MAY IMPACT TRAVEL. CONTINUE TO
MONITOR THE LATEST FORECASTS."

Here's the updated (clickable) forecast snow map:

Weather Service had predicted 1 to 3 inches of snow overnight at BWI-Marshall Airport as a pair of low-pressure systems move through the region. The precipitation will start as rain, become mixed in the wee hours of Tuesday as temperatures drop behind the first low, then change to snow before ending during the rush hour.

Here's how Eric the Red saw it last night:

"Over the weekend, models did everything imaginable... no storm, all rain, all snow, and now back to a north to south changeover.  I think I'll stick with 2 to 5" on grassy surfaces for central and northern MD... and once you get down toward Annapolis and DC, the question of when (and if) the changeover occurs becomes fuzzier.  I would suspect that unless the cold air really pushes in fast, locales south of Baltimore will struggle to get more than an inch or two."

Here, on the jump, is how he sees it this afternoon:

"It appears the near-perfect timing necessary for a good snowfall tonight will in fact play out, so I think I better up the totals... esp across nrn MD.  For much of central and northern MD, 4 to 8 inches of snow are liklely tonight.  Totals will be lower farther south into northern VA, but the changeover to snow (if any sleet or rain is falling initially) should be quick enuf to get even the southern portions of the region a decent 2-4" snowfall.

"The models are in excellent agreement that a storm system coming in from the west will collide with the timely arrival of cold air from the north.  I was figuring that recent warmth and time of year would cut into snow totals, but there are a couple things at play here that will make that original thought null and void.  First, altho it may not feel like it just yet, plenty of cold air is pushing south out of New England into the Mid Atlantic...and temps at the surface are expected to plunge into the low to mid 20s as the snow arrives.  That will up the liquid-to-snow ratio.  Also, this event will be at night, removing solar insolation from the equation. 

"Timing... snow or mixed precip will arrive from the west between 7 and 10 pm.  What ever sleet or rain there is will quickly change to snow as evaporative cooling kicks in.  A period of heavy snow is likely across central and northern MD late tonight, and snow will linger into the morning commute.  Temps tomorrow will not get above freezing, so this will definitely stick around for a little while.

"I should note that I am being conservative here.  If the WRF has this right, then snowfall will be in the 7 to 14" range across central and nrn MD, due to higher pcp totals and cold air pushing up the snow ratio.  The RSM would suggest 8 to 16" for the same reasons.  The GFS is a bit lower (and the model I'm leaning on for this one), with a liquid total of ~ 0.50"... and that would get us in the 4 to 8" range.  Anyway, here we go..."