* Winter storm warning 3 a.m. Thursday to 7 a.m. Friday for much of area, except winter weather advisory in Calvert and St. Mary’s counties *

9:30 p.m. — Models continue to support forecast but questions about how much snow versus sleet

After reviewing the latest model forecasts, it looks like the immediate D.C. area will be right on the snow/sleet line and whether we see more snow or sleet will depend on temperatures a few thousand feet high and just one degree will make a big difference.

Here’s the rub:

  • If we end up with more sleet than snow, the total accumulation of snow/sleet would tend toward to the low-end of our forecast, but would form a cement-like 1 to 2 inch coating.
  • If we end up with more snow than sleet, total accumulations will be on the high end of our forecast, or even boom, with at least 3 to 5 inches or so of snow with a bit of sleet likely mixed in.

No matter which scenario plays out, because of the cold temperatures and heavy precipitation, we’re pretty confident in snow/sleet covered roads and lots of cancellations of in-person activities.

Our coverage will begin between 5 and 6 a.m. in the morning on Thursday.

5:15 p.m. — Forecast on track. Here’s what other weather teams are predicting.

Reviewing the afternoon models, we think our forecast described below still holds and wouldn’t change anything at this time.

For those curious, here’s how much snow other weather teams are calling for in Washington:

Like the Capital Weather Gang, these teams call for increasing amounts as you head northwest. Forecasts are generally consistent plus or minus an inch or two.

Original forecast from early afternoon

The most significant winter storm this year is set to sweep into the Washington region early Thursday, producing substantial amounts of frozen precipitation.

The combination of snow, ice and freezing temperatures will create hazardous conditions for motorists and pedestrians, especially Thursday morning when the precipitation may be heavy. Expect roads, even some well traveled and treated, to become covered in snow and/or sleet and for visibility to be impaired.

Freezing temperatures mean that snow and sleet will stick.

While the heaviest snow and ice are expected in the morning and into early afternoon, this will be a long event, with intermittent mixed precipitation continuing Thursday afternoon and night and perhaps into the morning hours Friday, before tapering off.

Travel conditions may improve some by mid- or late Thursday afternoon, but scattered light mixed precipitation means that untreated roads will remain slippery.

Precipitation should begin as snow early Thursday, possibly mixed with sleet, before changing to sleet and freezing rain between midmorning and afternoon. In our western and northern areas, snow could last longer before a transition to icy precipitation.

The Washington, D.C., area is expected to get a mix of accumulating snow, ice and freezing rain for much of the day on Feb. 18. (Amber Ferguson/The Washington Post)

The timing of the transition from snow to sleet will help determine how much accumulation occurs and where. In our north and northwestern areas, where snow holds on longest, as much as 3 to 8 inches are likely. In the immediate D.C. area and to the south and southeast, 2 to 4 and 1 to 3 inches of snow and sleet are most probable.

We have lowered our predicted snowfall slightly in the District (from 2 to 4 to 1 to 3 inches) and to the south and southeast since Tuesday, because of more anticipated mixing with sleet, which accumulates more slowly than snow and packs it down. We increased predicted amounts in our far northwest areas toward the mountains (to 4 to 8 from 3 to 6 inches) where some model projections have consistently shown increased snow potential.

In areas that see a faster transition to sleet and especially freezing rain, the potential for ice buildup increases. Especially south and southeast of the Beltway, toward southern Maryland, we could see up to 0.25 inches, possibly more, of ice. In addition to slick, hazardous untreated roads and walkways, this could lead to pockets of tree damage and power outages.

Storm timeline

The steadiest and heaviest precipitation and most snow and sleet accumulation are expected through midafternoon Thursday. Some additional accumulation of mostly sleet and freezing rain are possible into the first half of Friday, but may be more intermittent and generally not as heavy. The temperature range indicates that the coldest temperatures will be in our northwest areas and mildest from downtown Washington to the southeast.

2 to 6 a.m. Thursday: Snow develops. Sleet-snow mix possible southeast of D.C. Temperatures 27 to 32.

6 to 10 a.m.: Snow, heavy at times, possibly mixing with and changing to sleet, especially from D.C. to the southeast. Accumulation likely. Temperatures 25 to 30.

10 a.m. to 2 p.m.: Wintry mix, possibly heavy at times. More snow and sleet northwest of D.C., more sleet in immediate area, sleet and freezing rain to southeast. Accumulation likely. Temperatures: 26 to 32.

2 to 10 p.m.: Intermittent wintry mix, generally lighter in intensity (an evening period of steadier precipitation is possible). More sleet northwest of D.C., sleet and freezing rain elsewhere. Light accumulation possible. Temperatures 26 to 32.

10 p.m. Thursday to 6 a.m. Friday: Intermittent wintry mix, lighter in intensity. More sleet northwest of D.C., sleet and freezing rain/drizzle elsewhere. Light accumulation possible. Temperatures: 26 to 32.

6 to 10 a.m.: Periods of wintry mix, gradually tapering off west to east. More sleet and snow to northwest of D.C., sleet and freezing rain/drizzle elsewhere. Little or no additional accumulation. Temperatures: 30 to 34.

Expected impact

On Capital Weather Gang’s winter storm impact scale, this event rates a Category 3, or “significant.” The combination of snow and ice, falling heavily during the first half of Thursday, will make travel very difficult. Thursday morning’s commute will be disrupted, and we expect that many if not most in-person schools will close or switch to distance learning. Flight delays and some cancellations are likely.

In addition, the duration of the event coupled with freezing temperatures, lasting 24 to 36 hours, means that roadways and walkways, especially the untreated ones, will remain slick for an extended period.

Although less snow will fall southeast of Washington, the impact of this event will be no less significant because of ice. The buildup of ice will make walking and driving on untreated surfaces challenging. Bridges, ramps and overpasses will be particularly hazardous for motorists. In places where ice accumulates beyond a quarter of an inch, most likely in southern Maryland, tree damage and power outages are possible.

What we know

  • Temperatures will be in the 20s to around 30 during the morning rush as snow and sleet spreads over the area.
  • Roads will probably be covered quickly with snow and ice, as sleet can even coat treated roads because it is denser than snow and harder to melt.
  • The heaviest snow and sleet will occur in a window between about sunrise and early afternoon Thursday. Then precipitation should become scattered and light as a dry slot moves in.
  • Cold air being supplied from an Arctic high-pressure zone will keep temperatures at or below freezing for the duration of the event, except perhaps in far southern Maryland.
  • The best chances of heavy ice accumulations (greater than 0.25 inches) from freezing rain will be over southern Calvert and St. Mary’s counties in Maryland and adjacent areas into Virginia’s northern neck and the eastern Delmarva.

What we’re less confident about

  • How quickly the precipitation will arrive Thursday morning and how rapidly the snow will change to sleet. The faster the precipitation arrives, the snowfall potential increases, because the very strong southwesterly winds aloft are going to draw in milder air that will turn the precipitation into a mix. If the snow start earlier, it allows more time for it accumulate before the changeover.
  • How much precipitation we will see after the initial period of heavier snow and sleet in the morning and early afternoon Thursday. We may see precipitation ease or stop at times late Thursday afternoon into Friday morning, when it should finally taper off. Some models indicate that we could have another period of steadier mixed precipitation Thursday evening for a time; others show very little, with just some patchy freezing drizzle after the initial wave.