6:10 p.m. — Winter storm watch issued from Saturday morning to Sunday morning

Due to the potential for sleet, freezing rain and “heavy icing” on Saturday and Saturday night, the National Weather Service has issued a winter storm watch for the District, and Prince George’s, Fairfax, Prince William, Anne Arundel, Charles, Calvert and southern Fauquier counties. It predicts up to an inch of snow and one-quarter inch of ice, which could result in tree damage and power outages. “Travel could be nearly impossible,” the watch says.

The watch does not include Loudoun and Montgomery counties along with counties to the north and west due to uncertainty as to how far north and west substantial precipitation will extend. We’ll have additional information on this threat in updates on Friday.

Original article from Thursday afternoon

A parade of winter storms has been marching through the D.C. area and, depending on where you live, you may have seen a blanket of snow, a wintry mix or just cold raindrops. The stormy pattern shows no signs of stopping, with four more chances for wintry precipitation over the next week.

The first of the four arrives Thursday afternoon or evening, when light snow is expected mainly in areas south of the Beltway. It could produce a coating to a couple of inches.

Then on Saturday, a wintry mix of precipitation is possible throughout the region. With temperatures predicted to remain below freezing, icy conditions could develop. However, there are questions about how widespread any icy precipitation may become, with some model projections keeping more of it to the south.

Next week, winter storms could ride through the area Monday into Tuesday and again Thursday into Friday. Both could generate a mixed bag of precipitation, and these would be carrying more abundant moisture all the way from the Gulf of Mexico.

Let’s examine each of these winter weather possibilities and also look back at how much snow fell from the first wave on Wednesday night.

Snow potential Thursday afternoon into early Friday

Patchy light snow may develop over Washington’s southern suburbs Thursday afternoon before becoming steadier this evening and tonight. It’s possible that light snow will extend into areas inside the Beltway for a time this evening.

The potential for accumulation increases as you head south. We can’t rule out a coating inside the Beltway, but the best chance for an inch or more is from around Fredericksburg and to the south. Even in our southern areas, the snow will not last that long, tapering off in the predawn hours Friday and mostly over by sunrise.

That said, some spotty light snow showers or flurries are possible anywhere in the region during the day Friday, but accumulating, steady snow is not expected.

Snow that falls initially in our southern areas Thursday afternoon and early in the evening may have trouble accumulating except on grassy areas with temperatures initially a bit above freezing. However, temperatures should fall below freezing not long after dark, creating some slick spots on untreated roads and walkways.

Saturday wintry mix potential

Saturday brings the potential for snow and ice to the region as a low-pressure area forms off the coast of the Carolinas, and relatively warm air rides up and over a dome of cold air. The precipitation is most likely to be relatively light, but this can be deceivingly dangerous. Even light ice accumulations can cause significant travel disruptions, as with the deadly 100-car pileup in Fort Worth on Thursday morning. The chance of ice increases in the afternoon and evening.

While it’s not clear how widespread or steady precipitation will be, there is a consensus that surface temperatures will be below freezing, which means any frozen precipitation we see should stick on untreated surfaces.

Just a small amount of sleet or freezing rain could lead to slick and possibly hazardous roads and walkways.

What we know

  • A strong, cold high-pressure system will be located to our north while winds aloft will cause relatively mild air to be lifted over the cold dome east of the mountains. That could yield some frozen precipitation in the region.
  • The southerly winds will warm temperatures enough to prevent us from getting much snow, and instead suggest more sleet and freezing rain.
  • The extended period of overrunning potential gives this system the ability to produce periods of light frozen precipitation from Saturday morning into early Sunday, although the most likely time frame will be Saturday afternoon and evening.
  • Computer models have been trending toward lower amounts of precipitation with this storm. The heaviest precipitation may focus to our south and east. That’s fortunate, since it reduces the potential for a damaging ice storm.

What we’re less confident about

  • What the predominant precipitation type will be. Very small changes to the maximum temperature in the warm layer aloft could mean the difference between sleet and freezing rain. Freezing rain is more dangerous because the icy glaze it leaves behind provides little friction, making streets and sidewalks much more treacherous, whereas sleet tends to be crunchy and offers more traction.
  • The exact timing of precipitation Saturday into Sunday morning, because it could be spotty and intermittent. Anytime you see light rain falling, assume roads and sidewalks might be slick.
  • Whether an area of moderate freezing rain might develop somewhere in the area to cause more widespread and significant icing problems.

Storms next week

While an Arctic high-pressure zone continues to feed cold air into the region, two more storms are expected to approach the area from the southwest. Model projections indicate both storms may cut to our west, which would allow temperatures aloft to warm enough to change any snow to an icy mix.

Of the two storms, the first — Monday into Tuesday — may have more cold air to work with and more potential to produce accumulating frozen precipitation. The second storm, Thursday into Friday, may initially start as a wintry mix but flip to rain as more warm air is drawn northward.

However, as abundantly obvious given recent fluctuations in model projections more than a couple of days in the future, the specifics of these forecasts are likely to change. Both storms, drawing moisture from the Gulf of Mexico, have the potential to produce a substantial amount of precipitation.

How much snow fell Wednesday night and how did it compare with forecasts?

Snow totals from the wave of precipitation that traversed the region between Wednesday afternoon and Thursday morning varied considerably from north to south.

Just a coating of snow fell in the immediate area (or no accumulation near downtown Washington), an inch or two accumulated just north of the Beltway, and then 3 or 4 inches fell from Interstate 70 northward, with localized totals of 5 to 6 inches (detailed list of snow totals). Here is a summary of totals:

  • Frederick: 3.5 to 5.5 inches
  • Ellicott City: 4 inches
  • Damascus and Clarksburg: 4 inches
  • Gaithersburg and Germantown: 2 to 3 inches
  • Columbia: 3 to 4 inches
  • BWI Marshall: 2.6 inches
  • Annapolis: around 2 inches
  • Laurel: 1.5 to 2 inches
  • Olney: Around 1.5 inches
  • Potomac and Silver Spring: Around 1 inch
  • Loudoun County: A coating east to 1 to 3 inches west
  • Fairfax County: Mostly a coating

When you consider Capital Weather Gang’s revised snowfall forecast, issued midday Wednesday, there is a reasonably good match between the forecast and how much fell. Our forecast was actually a little too low in some areas north and northeast of the Beltway, particularly in Howard County and around Baltimore.

Of course, our original snowfall prediction made Tuesday, was farther off in many areas.

As we explained on Wednesday, when we revised the forecast, model projections simply shifted the zone of heavier snow about 60 miles north. That had big implications for the immediate D.C. area.

We strive to provide the best possible information based on the best tools we have, while explaining the range of possibilities. Even when we incorrectly called for 1 to 4 inches of snow in the immediate area on Tuesday (from the initial wave), we mentioned the possibility things could shift north.