Updates: Snow continues through the afternoon, tapers off this evening

* Winter weather advisory for much of area until 4 p.m. Saturday | Winter storm warning for St. Mary’s County *

Key points

• One to four inches is possible. Based on the latest information, we favor the lower end of that range.
• Snow is sticking on grass and elevated surfaces, but roads are generally fine. Where the snow gets heavy, road conditions could deteriorate rapidly.
• Snow will persist into early evening and then begin to taper off.
• This is the first measurable December snow in D.C. since 2013.

3:26 p.m. update: The heavier snow is focusing north and west now, instead of to the southeast. The colder temperature at higher elevation is taking over in this event.

Our next concern is the drop in temperature that’s certainly on the way when it starts to get dark. At that point, roads (mainly side streets and others less-traveled and un-treated) could become slippery. Temperatures are holding steady at 32 degrees in the Beltway and 30 degrees in the northwest suburbs.

A few snow totals that have been reported since 2 p.m.:

Clarksburg, Md. — 3 inches
Damascus, Md. — 3 inches
Laytonsville, Md. — 2 inches
Leesburg, Va. — 1.9 inches
Falls Church, Va. — 1.3 inches

1 p.m. airport totals:

DCA — 0.5 inches
IAD — 2.1 inches
BWI — 0.9 inches

The next airport updates will be at 7 p.m.

2:21 p.m. update: CWG meteorologist Greg Porter was looking at the jet stream a little while ago and shared this analysis with us:

We could see one more increase in snowfall intensity sometime after 3 p.m., before the snow tapers off early this evening. Why the final burst? For that answer, we need to look up 30,000 feet, a level where the jet steam resides.

In general, faster winds at this level (depicted below) corresponds to a more favorable environment for snowfall intensity at the surface. Over the next few hours, the tail edge of a wide swath of fast jet stream wind speeds (over 180 mph) will slide over our area. During that brief period, we can expect an uptick in the snowfall rate intensity.

Upper level winds, color-coded by speed. Purple is the fastest speed. (National Weather Service)

See bottom of post for earlier updates, and immediately below for the entire forecast from 5 a.m.

TODAY’S DAILY DIGIT
A somewhat subjective rating of the day’s weather, on a scale of 0 to 10.

7/10: The first snowflakes of the winter can certainly make the day better than it would otherwise be!

EXPRESS FORECAST

Today: Snow developing, occasionally moderate. Highs: Mid- to upper 30s (before snow).
Tonight: Clearing, breezy. Lows: Near 20 to upper 20s.
Tomorrow: Partly to mostly sunny. Highs: Mid-30s to near 40.

View the current weather conditions at The Washington Post headquarters.

FORECAST IN DETAIL

After two Decembers in a row that were among the top five warmest, a return to a more wintry version of the month is here. We’re likely to all get on the board for this season’s snow tally with our first event today. When it comes to snow around Washington, it’s unusual to have high certainty. As is the typical case, we go into this one with a few unanswered questions.

Listen to the latest forecast:

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Today (Saturday): Snow is developing across the area this morning. This is a case where delayed snow is not necessarily denied snow.

The anticipation has been that the best accumulation risk is likely to come somewhere from about mid-morning into mid-afternoon. Any more or less on either end of that period matters with the final tally, but this doesn’t necessarily have the all-or-nothing feel of a later-season snowstorm around here. Once snow is underway, temperatures are near freezing, but they rise above freezing when it’s not snowing. Confidence: Medium

Tonight: If any snow lingers through sunset, it is on very borrowed time. Clearing and breeziness are the stories of the night, along with a deep chill made deeper by a wintry-looking environment. It’s a hard freeze across the region, with lows ranging from near 20 to the upper 20s. Breezes may stay up near 10 mph, which adds an extra shiver at times. Confidence: Medium

Follow us on Facebook, Twitter, and Instagram for the latest updates. For related traffic news, check out Dr. Gridlock. Keep reading for the forecast through the weekend…

Tomorrow (Sunday): It’s feeling like full-on winter at this point, especially in places that have anything resembling a snow-pack when today’s system is said and done. December snow likes to stick around, thanks to the nonexistent sun angle. It’s blustery as well, in the wake of the weak coastal that wraps up a bit to the northeast. Highs are in the mid-30s to near 40s. Some melting is likely, and it’ll need to be watched for refreeze as the sun goes down. Confidence: Medium

Tomorrow night: It’s a mostly clear and seasonably cold night. Lows are in the low 20s to about 30. It might be even colder, if not for the wind. It decreases from the day, but gusts of about 5 to 10 mph remain through the night. Confidence: Medium

A tiny snowman from our last December with snow of note, back in 2013. (James0806 via Flickr)

A LOOK AHEAD

It’s a cold and tranquil start to the work week Monday. Skies are mostly sunny early in the day, but clouds probably increase with time. They may mainly be high-level as a system prepares to pass to our north through the Great Lakes. High temperatures are below normal, in the near-40 to mid-40s zone. Confidence: Medium

There’s no major change in the pattern, with a big upper-level low still parked over the northeast on Tuesday. A ripple within that low, in the form of a clipper, will pass to our north. That gives us a temporary reprieve fromthe coldest of air. In this case, we’re talking highs in the mid-40s to near 50. We actually spend much of the day behind the front with this system, but the cold air lags a bit. This cold is the real deal, with highs near freezing Wednesday. Quick hitter with that level stuff. Confidence: Medium

SNOW POTENTIAL INDEX
A daily assessment of the potential for at least 1 inch of snow in the next week, on a 0-10 scale.

7/10 (↑): Many signs point to accumulation with our first winter storm of the season. Snow is rarely simple, though.

Earlier storm updates…

1:37 p.m. update: Reagan National Airport reported 1/2 inch of snow at 1 p.m., which makes this the first accumulating storm in December since 2013 in Washington, D.C.

Although it’s been snowing since early this morning, the ground has been warm and the snow is quick to melt once it reaches the ground. Now that it’s been coming down for a few hours, temperatures have dropped and ground temperatures are cooling.

1:03 p.m. update: For most of the morning, areas around the Beltway were not seeing snow accumulate much on the grass, due to the warm ground temperatures. But that seems to be changing, now, after a couple hours of persistent snow. But the roads still look good, thanks to pre-treatment.

The heavier bands of snow that had been hovering over the northwest suburbs and eastern areas around Annapolis are fading. We may see the snow lighten up over the next couple of hours, but it looks like it will continue through sunset.

Given that, our next concern is the drop in temperature that’s certainly on the way when it starts to get dark. At that point, roads (mainly side streets and others less-traveled and un-treated) could become slippery.

As it stands now, temperatures are hovering around the freezing mark across the region.

11:40 a.m. update: This event is hitting its stride locally. Much of the area is seeing moderate snow and some bands of heavy snow have set up as well. As of the time of this update, the most intense snow activity was occurring in bands west and east of the main Interstate 95 corridor. These bands will continue into the early afternoon and are likely to shift around a bit with time.

Radar snapshot as of 11:35 a.m. shows widespread snow along with heavier bands.

Temperatures are now near and below freezing in most of the area. Any heavier snowfall may have a quick impact on roads, and patchy iciness remains a possibility otherwise.

Some snow totals are starting to trickle in. Winners thus far include a location near Damascus with one inch, and the same at a location near Dulles and near Herndon. The highest totals in the region include numbers around four inches in St. Mary’s County, much of that from last night.

10:05 a.m. update: The winter storm warnings in the region have been replaced by winter weather advisories, except in St. Mary’s County in Southern Maryland. The National Weather Service has trimmed expected totals this morning. The latest numbers from them are below.

NWS forecast as of 9:30 a.m.

9:35 a.m update: Snow is now falling across much of the area, and it is sticking as well. Temperatures have responded to the onset of snow by falling to near freezing or a little above. They should stay around there, or drop a little more, as snow continues into late this morning.

A large band of moderate snow has developed across the Interstate 95 corridor, and it extends into the northwest suburbs. This area of snow appears likely to continue to develop and move over the area in the hours ahead. We may also see stronger but more concentrated zones of moderate to heavy snow develop within the larger shield as we head through the midday.

For now, roads seem generally passable with little concern. As temperatures remain near freezing, some patchy slick spots are possible. If heavier snow develops, road conditions could go downhill relatively quickly.

8:05 a.m.: We remain in wait-and-see mode, but we are transitioning into snow event mode. Snow continues to blossom across the area, and reports (thank you!) are rising. Temperatures hovering near the mid-30s should still tend to fall back as snow becomes more consistent.

6:25 a.m. update: There’s nothing like waiting for snow to fall. Late last night it became increasingly apparent that this morning’s snow could be somewhat delayed. But as noted below, that doesn’t necessarily mean it is denied. We continue to see a snow shield develop across the area early this morning. Heavier bands are also showing up, with the most notable to the south and east of the city so far.

Given the slow start, temperatures are still somewhat mild. As snow get underway, temperatures should cool toward freezing where it falls readily.