There is some chance that snow amounts will be so great that poorly built roofs and structures will collapse.

Snow totals are forecast to be at least 16 inches, with 24 to 30 inches or so possible in some areas, especially north and west of the District. Lesser amounts are likely in southern Maryland where snow is more likely to change to a wintry mix for a time.

This storm shares characteristics of many of D.C.’s greatest snow events, such as Snowmageddon in 2010 and the blizzard of 1996. Snow amounts will probably be comparable to those during Snowmageddon, and winds may be stronger.

This is a serious and life-threatening winter storm. Preparations should begin today and be completed by Friday afternoon. Wherever you happen to be Friday evening, plan to potentially be there into Sunday or early next week.

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Key points

This is long-duration event, with snow forecast for 36 hours or so.

Models are converging on storm onset in the noon-to-5 p.m. period on Friday.

Conditions are the worst after dark Friday continuing through Saturday night.

Thunder snow is possible Saturday.

Winds may gust to 40 mph on Saturday, causing whiteout conditions and some blowing and drifting snow. Strong winds east of the District toward the bay.

Biggest forecast uncertainty is near and especially southeast of the District, where there remains some potential for a dry slot that would interrupt the snow and potentially result in a brief changeover from snow to sleet, cutting down accumulations.

Timeline for immediate metro area.

Noon to 5 p.m. Friday: Snow moves in from southwest to northeast. Temps: 30-35. An inch or less accumulation.

5 p.m. to 10 p.m. Friday: Light to moderate snow. Temps: 25-30. Storm total accumulation: 2-3 inches.

10 p.m. Friday to 7 a.m. Saturday: Snow, heavy at times. Increasing winds. Temps: 25-30. Storm total accumulation: 8-12 inches.

7 a.m. to 5 p.m. Saturday: Snow heavy at times. Possible blizzard conditions. Temps: 25-30. Storm total accumulation: 16-20 inches.

5 p.m. Saturday to 1 a.m Sunday: Snow and blowing snow, gradually decreasing. Temps: 23-28. Storm total accumulation: about two feet.

Answers to frequent questions

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When do I need to be off the roads on Friday? The snow will probably begin between the early and late afternoon. You may be able to sneak in a half-day at the office. However, the wiser move will be to telework as we want to avoid a mass exodus on the roads and a repeat of Wednesday evening. Even if traffic is light and the storm eases in initially, you definitely want to be home by dark Friday as snow will be increasing in intensity, with a few inches possible by around 10 p.m.

Where are power outages most likely? East of the city, but plausible anywhere in the region. The reason we think areas east of Washington and toward the Chesapeake Bay have the highest outage potential is because winds will be strongest there and the snow will have more of a heavy, wet consistency.

West of the District, the snow will have more of a powdery consistency, which will probably lead to to greater amounts but less clinging to trees and power lines. Also winds will tend to decrease the farther west you go in the region.

Will this be a true blizzard? The National Weather Service defines a blizzard accordingly:

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A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer:

Sustained wind or frequent gusts to 35 miles an hour or greater; and

Considerable falling and/or blowing snow (i.e., reducing visibility frequently to less than a quarter mile.

These conditions are possible in the region, particularly on Saturday.

Could this storm underperform? Yes, but unlikely. The signal for a historic storm emerged days ago, and models have not wavered in their forecasts for extreme snow amounts. Capital Weather Gang winter weather expert Wes Junker says the possibility of snow changing to sleet could cut down on totals in some areas:

There is still a chance that the snow could mix with or change to sleet for a period of several hours in the city and especially for locations to the south and east. The 1996 snowstorm, which had a similar evolution, ended up having a period of sleet sandwiched between two periods of moderate to heavy snow. During big D.C. snowstorms, folks east of the city are usually at risk for mixing or even a changeover.

Junker noted that last night’s NAM model simulation introduced the potential for mixing even in the District.

Could this storm overperform? Yes, by a little. The latest model forecasts are generally for at least near-20 inches of snow and up to 30 or so inches. We also must consider that the model snowfall amounts make the assumption that 10 inches of snow will fall for the equivalent of every inch of rain and, in some areas, particularly north and west of the District, perhaps 15 inches of snow will fall for every inch of rain, especially during the second half of the storm as colder air is drawn in. In general, model simulations advertise the equivalent of 2 to 3 inches of rain over the region. Someone in the region is likely to receive more than 30 inches.

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Where will the heaviest snow fall? Some models have placed the snowfall bull’s-eye in west-central and northwest Virginia, while others place it closest to the immediate metro area. Based on D.C. snowstorm history, we favor the heaviest amounts in these western areas, especially near Interstate 81 from around Harrisonburg to Winchester and in the Blue Ridge. Having said that, where localized heavy bands set up is always a wild card, and that could even occur just east of the city.

“The models have a really hard time predicting where one of these bands will set up prior to their formation,” Junker says. “Where such a band sets and persists longest will determine who in the area will get plastered with the heaviest snow.”

What should I do about my flight? Many carriers have announced that you can change your flight plans without penalty. Check your airline. Flights should be able to get in and out of the airports for much of the daylight hours on Friday before conditions really deteriorate after dark. On Saturday, airports may need to shut down temporarily. Air travel is likely to be possible on Sunday, but expect considerable backlash delays and some cancellations.

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What will Amtrak and Metro do? Metro has decided to shut down the rail and bus system for the entire weekend. Amtrak will attempt to continue operating, but consider that many trains were canceled during Snowmageddon in 2010.

What will schools and the federal government do? In past storms of this magnitude, even those that occurred over the weekend (many of them, in fact), schools and government closed up to several days in the week following.

As for Friday, many schools have already decided to close and we expect more will follow suit.

We think the Federal government will likely offer the telework option and may dismiss early on Friday.

Why might there be thunder snow? Yes, on Saturday, Junker says. His technical discussion:

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The forecast atmospheric profile or sounding from last night’s NAM and GFS both argued for a period of thunder snow though they differed on when. The NAM liked early Saturday morning while the GFS early Saturday afternoon.

The GFS sounding below is a classic one trumpeting there is likely to be elevated instability, conducive to thunder. Note that the unstable layer is located above the warm layer due to the east wind off the ocean and below the dry layer. You can tell that the sounding is unstable because the temperature and dewpoint lines are squeezed together and the slope of their line tilts to the left more than the curved dashed line on the sounding (below).