Radar & lightning: Latest regional radar shows movement of precipitation and lightning strikes over past two hours. Refresh page to update. Click here or on image to enlarge. Or see radar bigger on our Weather Wall.

* Winter Storm Warning today for entire region | Wind Chill Advisory tonight thru Wed. noon | Local closings *

10 a.m. update: We have started a new post with live updates here: Snow moves into area, worst conditions this afternoon and evening (LIVE UPDATES)

9:30 a.m. update: Snow has now advanced into Bethesda and Takoma Park in Maryland, and Vienna into Virginia, according to reports. It’s rapidly accumulating in western Montgomery and Frederick counties, where some locations already have close to one inch. Flakes will move into the District by 10 a.m. It may take until 11 a.m. or so for snow to reach the south and east suburbs.

A lot of snow lovers are concerned about above freezing temperatures. Here’s the deal: when the snow starts steadily, temperatures will fall below freezing. Take a look at temperatures around the region at 9 a.m. below. Notice how it’s mostly in the upper 20s to near 30 north and west of the black line – that’s also where it’s snowing. As the snow advances south and east, so will the freezing line. There is plenty of cold air available for this storm, and temperatures will fall steadily once the snow begins.

Temperatures at 9 a.m.

9:00 a.m. update: Reports indicate the first flakes falling in Ellicot City and Shady Grove in Maryland, and Ashburn and Reston in Virginia. In Frederick County, Maryland, the snow is coming down at a good clip and accumulating. Over the next hour, snow should creep south and east inside the Beltway.

Radar – which is showing steady snow falling from northwest Virginia across northern Maryland – offers a clue as to where models expect the heaviest snow to fall today. On the graphic below, on the left is the NAM computer model simulation of total precipitation through 10 p.m. tonight. On the right, is a radar screenshot from 8:45 a.m. You’ll notice a lot of the greens on radar in Maryland correspond with where the model shows the most precipitation. Both the model and radar fit in with our ideas of a band of 6-10 inches of snow generally north of District (scroll down for our accumulation map), with somewhat less amounts to the south (4-7 inches).

(Left) Storm total liquid equivalent precipitation simulated by NAM model. (Right) Radar view at 8:45 a.m. (National Weather Service)

8:30 a.m. update: Snow has pushed into central Loudoun County and western Montgomery county, with reports of flakes as far east as Germantown.  Already, we’re seeing the development of heavy bands north of the District, which fits in with our expectation that’s where the heaviest snow will fall.  In the next hour, flakes will edge closer to the immediate metro area, moving into the I-270 corridor and western Fairfax County. We’re still looking at an estimated time of arrival of around 10 a.m. inside the Beltway.

Neighborhood streets dusted in New Market. @capitalweather @alanauglis @FredScanner pic.twitter.com/yqq2pXdCn3

— James McCoy (@jamesmccoy) January 21, 2014

@capitalweather @dougkammerer @JimCantore looking good in purcellville va. pic.twitter.com/zsNKii1Tv3

— puckpapi (@PuckPapi) January 21, 2014

7:30 a.m. update: Around D.C., it’s the calm before the storm.  To our west the snow is getting going.  Flakes are starting to fall around Frederick, and Martinsburg already has a dusting. Radar also shows snow sneaking into western Loudoun County. Expect snow to reach eastern Loudoun and western Montgomery counties in the next hour.

@capitalweather sticking in Martinsburg, WV. pic.twitter.com/KNaTEhCce4

— Giles Snyder (@GilesSnyder) January 21, 2014

Also, this almost slipped through the cracks, but there is a Wind Chill Advisory in effect from 6 p.m. tonight until Wednesday at noon for wind chills between -5 and -15 in the region. Temperatures tonight will drop through the teens and single digits and winds will be sustained at 15-25 mph with gusts to 40 mph.

6:50 a.m. update: Reports are coming in of snow around Martinsburg, Hagerstown and Front Royal. Along the I-81 corridor (western Va., and west central Md.) is where the action will be for the next couple hours. NOAA’s Storm Prediction Center says snowfall rates of 1-1.5″ per hour will be developing there, meaning travel will rapidly deteriorate in that area. Expect to see reports of snow creeping into Loudoun and Frederick counties over the next hour.

(National Weather Service)

6:25 a.m. update: After reviewing the latest models, radar and temperature trends, we’re slightly lowering the expected snowfall for the immediate metro region and points south (from 5-8″ to 4-7″, which was our original forecast yesterday). We’ve also slightly increased the bust potential given these changes. Models have shifted the heaviest simulated snow just north of D.C., and temperatures are running a bit warmer than expected.

There is little doubt temperatures will quickly fall below freezing once the snow begins, but the warm ground and snow lost to melting initially could reduce totals a bit, especially around downtown. It still looks like the worst conditions will start in western areas late morning and then spread into the immediate metro area around midday. The morning hours around town shouldn’t be that bad, although we expect snow to begin by around 10 a.m. (perhaps very briefly as some rain along and east of I-95) if not a little sooner.

Capital Weather Gang snowfall forecast for January 21 issued at 6:30 a.m. January 21

*** See below for detailed forecast through the weekend ***

FORECAST IN DETAIL

An Alberta Clipper area of low pressure transforms into a coastal storm today, almost certainly delivering the biggest snow in almost three years to the D.C. metro area.  Look for snow to start flying by mid-morning and get heavier during the afternoon. We may get much of the morning commute in before too much snow, but driving conditions should quickly deteriorate thereafter, with snow likely lasting into the evening, before the real cold air arrives. Tomorrow aims to be our coldest day since that Polar Vortex blast on Jan. 7., with highs only in the teens. We may have a few light snow risks late this week or weekend, but otherwise we’re mostly dry after today with temperatures below freezing through Friday.

Today (Tuesday):  Snow expands from west to east during the morning hours, likely reaching the immediate metro area by ~8-10 a.m. While temperatures were warm yesterday, it should only take an hour or so of snow before roadways start to see accumulations (maybe a bit longer in urban D.C.). Snow should be heavier at times for western areas by late morning, and then from around D.C. toward points east during the afternoon. Early-morning temperatures may still be in the 30s in some spots, before collapsing into the 20s as the snow intensifies. Winds pick up in the afternoon, reaching 15 to 20 mph from the east and northeast. That means very cold wind chills in the single digits by the late afternoon into evening.  Confidence: Medium-High

Tonight: Snow should fade out by around 7-10 p.m., with total accumulations around 5-8″ in the metro area with some higher totals possible especially to the north (see map above). Winds up to 15-25 mph through the evening and into the overnight, with gusts to near 35 mph, produce some drifting snow and biting wind chills in the single digits down to near zero. Winds should relent a bit toward dawn as skies start to clear and temperatures plummet. Lows sink to the low single digits out near Dulles and in the northwest suburbs, to the upper single digits around the District itself toward points south and east.  Confidence: Medium-High

For related traffic news, check out Dr. Gridlock. Keep reading for the forecast through the weekend…

Tomorrow (Wednesday):  Our fresh new snowpack is not going anywhere anytime soon. Despite partly to mostly sunny skies, temperatures remain frigid with highs only in the mid-teens to low 20s. Winds are lighter than this evening, but 10-15 mph breezes from the northwest are still enough to make it feel like the single digits for much of the day.  Confidence: Medium-High

Tomorrow night: Partly cloudy and still cold with lows in the single digits (outer suburbs) to low-to-mid teens (city). At least winds are light. Confidence: Medium-High

A LOOK AHEAD

Thursday looks to be partly sunny and still cold as temperatures remain below freezing.  We expect highs no better than the mid-20s in most spots, along with light winds. Can’t rule out a flurry or snow shower as well. Thursday night is partly cloudy, still with a chance of a flurry, with lows ranging from the single digits to low teens. It’s a bit breezy, too, with wind chills near to below zero.  Confidence: Medium-High

Friday is our third day in a row with temperatures solidly below freezing. We may have a few flurries around in the morning associated with a reinforcing cold push. Morning temperatures probably hold in the teens before highs reach for the 20s, though they may not get there until mid-to-late afternoon, or even toward evening as winds start to come from more from the southwest.  Friday night is cold again as lows drop into the teens.  Confidence: Medium

The last weekend of January should be warmer, but it still feels like winter with highs in the 30s and lows in the teens and 20s. Daytime temperatures should exceed freezing in all areas so that the snow may start melting more significantly.  However, we have a chance at some light snow late Sunday before another cold shot early next week. Confidence: Medium