Snow to taper off in most of D.C. area by 4 a.m. Saturday

This will be our last update. We’ll have a new forecast posting at 5 a.m.

1:15 a.m. — Snow for the next few hours before tapering off between 4 and 6 a.m.

Precipitation on radar has filled in and snow is falling in many parts of the region; in some areas — especially from the District southward, some areas are seeing the precipitation start as rain or a rain-snow combination.

Over the next hour, most places should see a transition to all snow (except parts of southern Maryland). Through until 4 or 5 a.m., the snow could fall heavily at times, reducing visibility and coating roads — especially north of the Beltway. Between 4 and 6 a.m., snow should rapidly pull away to the east, although flurries could linger until a couple hours after sunrise.

Late night trends — examining radar and short-term models — are continuing to suggest overall snow amounts may fall short of anticipated levels in a number of areas. We’ll recap totals and assess the forecast after sunrise.

With the snow ending relatively early Saturday, there should be time for crews to treat major roads in snow-affected areas and for folks to get out and about as the day wears on.

12:15 a.m. — Light rain and snow entering D.C.'s western suburbs. It will spread east and should turn to all snow quickly.

Snow has crossed Interstate 81 and the ground is covered in Winchester, according to reports. Snow has also begun around Frederick, Olney and Clarksburg in Maryland. The reports from these locations to the west indicate that, once snow begins, it picks up in intensity quickly.

However, temperatures are mild enough (still near 40) in the immediate metro area that precipitation may begin as rain for a brief time before quickly changing to snow as temperatures quickly fall through the 30s.

Radar does show some precipitation expanding over the area already but, because the air is still somewhat dry, it’s evaporating before reaching the ground. We expect the air to moisten over the next hour and for precipitation to be falling in most places by betweem 1 and 1:30 a.m.

Prime time for snow accumulation will be between approximately 2 and 5 a.m. when snowfall rates may reach 1 to 2 inches per hour for a time.

10:30 p.m. — Snow approaching Interstate 81, spills into area starting around midnight

The leading edge of precipitation is approaching the Interstate 81 corridor. It may briefly start as rain before changing to snow. Petersburg, in eastern West Virginia, had about 30 minutes of light rain before flipping.

We’re still a go for snowflakes to push past the Blue Ridge near and after midnight, then they rush across our region through the pre-dawn.

The heaviest snow will probably come over a two-to-three-hour period, focused roughly 2 a.m. to 4 a.m. locally. Snowfall rates up to an inch or more per hour are still a decent bet during this time. Where on the “near freezing” spectrum temperatures fall during the heaviest activity will be critical to overall totals.

Readings have fallen off a few degrees in recent hours, now mainly in the upper 30s and low 40s locally. With the onset of precipitation, temperatures fall into the low and mid-30s fairly rapidly.

9:35 p.m. — We’re still expecting snow, heavy at times, but are lowering predicted totals

Since this morning, when model simulations really increased predicted precipitation, the snowfall forecasts have backed off considerably. Some reliable models have cut totals in half, and the average of all predictions has lowered considerably. Most models also now show snow will fall over a relatively short duration — moving in and out within about four hours.

The other wrinkle is that temperatures are still well above freezing, in the upper 30s and low 40s. When the precipitation (it could briefly begin as rain or a rain-snow mix in some areas) first moves in between midnight and 1 a.m., it will cool the air, but some snow will be lost (or not stick) because of melting.

For all of these reasons, we’re reducing predicted snow totals by an inch across the board (see revised map above; for transparency, our map produced earlier Friday is still provided below).

We still think there’s a chance some of the hilly and colder areas of northern Montgomery, northern Loudoun, Frederick, western Howard and Carroll counties see up to 4 to 5 inches. But we think 1 to 3 inches will be a lot more common around the close-in metro area.

Our next update will post around 10:30 or 11 p.m.

7:30 p.m. — Snow timing on target, with questions as to intensity and temperatures

The leading edge of snow continues to advance toward the area. In the last hour snow was reported as close as Johnstown and Altoona in Pennsylvania. More precipitation is barreling through West Virginia on course to hit the area. A bulletin from the National Weather Service called special discussion to this snowfall, which is expected to be heavy as it spreads east.

The onset time for accumulating snow remains mostly on target. It may have slowed a hair, probably spilling over the Blue Ridge and into the local area between around midnight and 1 a.m.

Temperatures range from the upper 30s to mid-40s. The onset of snow will bring readings down rapidly, but there is some question as to where they bottom out.

Whether temperatures end up above or below freezing will matter for accumulation given how fast this system is moving. The latest short-range models continue to indicate snowfall totals may well end up nearer the lower end of our forecast ranges.

4:25 p.m. — Precipitation approaching West Virginia, Pennsylvania; snow should reach D.C. region around midnight

Radar shows our storm system progressing through the Tennessee Valley and it’s spreading snow from southern Illinois through Ohio. Flakes should start to fly in western Pennsylvania and West Virginia over the next hour or so.

The snow should reach the Interstate 81 corridor around 11 p.m. and then sweep into the Washington region over the following two hours or so from west to east. This onset time is a little later than some earlier forecasts but should allow anyone out this evening to safely return home before the serious snowfall begins.

Models still show the potential for a very heavy burst of snow between about 1 and 5 a.m. before the storm exits to the east.

Because of the limited duration of snow, some of the newer model information suggests amounts may err toward the lower end of our predicted ranges, especially from the District and to the south, but we make that statement with low confidence.

Original article from midday

The holiday weekend is likely to get off to a snowy start in the D.C. area Friday night.

We have increasingly high confidence in a quick-hitting, accumulating snowfall starting Friday evening and winding down before sunrise Saturday. The overnight timing and the likelihood of a few hours of moderate to heavy snow means that roads could be snow-covered and hazardous, even though temperatures will start out above freezing. However, any travel disruptions should ease by mid- to late Saturday morning.

The National Weather Service has issued a winter weather advisory for most of the region Friday night and says to “plan on slippery road conditions.” A more serious winter storm warning is posted for northern Fauquier, northwest Prince William, Loudoun, Montgomery, Howard and Baltimore counties and points west. The Weather Service says “travel could be very difficult.”

The entire region could see snowfall rates of 1 to 2 inches per hour for a time between midnight and 5 a.m. under the heaviest snow bands, and visibility could be reduced to less than half a mile at times, the Weather Service said. There’s even an outside chance of thundersnow — when thunder and lightning briefly occur as flakes pour down.

Accumulation forecast

We’re forecasting 3 to 5 inches across our northern and western zones, including Northwest Washington. Precipitation of mostly or all snow and slightly colder temperatures in these zones should enhance snow accumulation. A few spots could top out around 5 to 6 inches.

We’re forecasting 2 to 4 inches around downtown D.C. and to the south and east, except only a coating to 2 inches in southern Maryland. In these zones, precipitation could start as rain or a rain-snow mix, and slightly warmer temperatures should keep accumulations down a bit.

Slight shifts in the forecast are still possible as new information comes in.

Storm timeline

9 p.m. Friday to midnight: Snow arrives from west to east, possibly starting as rain or a rain-snow mix around downtown Washington and to the south and east, and becoming moderate to heavy to the west. Temperatures: mid- to upper 30s.

Midnight to 4 a.m. Saturday: Snow could be moderate to heavy at times across the entire region, accumulating on roads. Temperatures: upper 20s to mid-30s.

4 a.m. to 6 a.m. Saturday: Snow tapers from west to east. Temperatures: upper 20s to low 30s.

6 a.m. to 10 a.m. Saturday: Improving road conditions on main roads and highways. Side roads could still be snowy or slick; lingering snow shower possible. Temperatures: low to mid-30s.

Temperatures described above will be lowest in areas north and west of downtown Washington and highest to the south and east.

Storm impact

The storm rates as only a low-end Category 2 out of 5 (and a Category 1 in our far-southern areas) on our Winter Storm Impact Scale. It’s occurring at one of the least disruptive times possible — in the middle of the night before a long weekend. And, outside of a lingering snow shower or two, it will be over by the time most people are waking up.

If you’re among the few to be on the roads between midnight and 5 a.m. Saturday, however, driving will be very hazardous because of the snowfall rates of 1 to 2 inches per hour and low visibilities. Roads will quickly become snow-covered and slick.

If you have Saturday morning plans, they may be disrupted until crews can treat and plow neighborhood roads, assuming you’re not comfortable driving on a bit of snow. We imagine major roads will be cleared pretty quickly Saturday morning. There could be delays and some cancellations at airports, so check your flight status before heading out.

By Saturday afternoon, snow will be steadily melting, and the area should be able to return to normal pretty fast. Lingering wet and slushy areas will refreeze Saturday night, so watch out for icy patches overnight into early Sunday. With highs well into the 40s on Sunday and sunshine, a lot of the snow will be gone by Sunday night.

How could the forecast go wrong?

There’s a pretty solid model consensus that most places, except areas south of Prince William and Prince George’s counties, should see at least 2 to 3 hours of moderate to heavy snowfall. However, amounts could end up on the low end of forecasts or in our “bust” range if precipitation intensity is a little lighter than simulated and temperatures are on the warm end of predictions. The risk of a bust is highest around downtown Washington and to the south, where above-freezing temperatures could eat into amounts. Also, if the snow moves through more quickly than forecast, that would also reduce totals. Some models show just a couple of hours of moderate to heavy snow.

On the flip side, amounts could end up at the high end of forecasts or in boom territory if the snow is on the heavy side of model predictions, which will cause temperatures to fall faster. Also, if the steady snow manages to linger a little later than 4 or 5 a.m. — when some models cut off — that could add an inch or so above the most probable amounts.

Expert analysis

“A quick-moving storm is likely to produce a narrow zone of particularly heavy snow Friday night. Where this band develops and stays in place the longest is where we could see amounts toward the high end of our snowfall forecasts, especially if it occurs in our typically colder locations north and west of the city,” said Wes Junker, Capital Weather Gang’s winter weather expert. “Areas that miss out on such a heavier band could see accumulations on the lower end of the forecast range, especially in our typically warmer locations from downtown D.C. to the south and east.”