The National Weather Service has raised its winter-storm-threat scale to its highest level. It warns there is potential for significant travel delays, closures, and threats to life and property, and is urging residents to start planning ahead.

Snow is forecast to begin between Friday morning and afternoon. The heaviest snowfall and most difficult conditions are likely to start late Friday afternoon into Saturday.

In addition to heavy snow, the combination of wet snow and high winds are possible Friday night through Saturday, which could lead to power outages.

Exactly how much snow falls and where is sensitive to the exact storm track, which will invariably jump around a bit.

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As areas along and east of Interstate 95 will be close to the snow-sleet-rain line, these shifts could well have profound implications on specific amounts and the overall storm impact.

There is still a small chance the storm tracks far enough to the southeast that this is more of a moderate snowstorm rather than a blockbuster.

As such, we are not yet in position to forecast specific snow amounts, but rather provide percent likelihoods of different totals.

D.C. metro-area snow probabilities:

Chance of at least 1 inch: 90 percent

Chance of at least 4 inches: 75 percent

Chance of at least 8 inches: 65 percent

Chance of at least 12 inches: 50 percent

Chance of at least 20 inches: 15 percent

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For amounts of greater than 4 inches, add 10 percent to these probabilities north and west of the Beltway, and subtract about 10 percent east and southeast of the Beltway.

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Confidence is high and growing higher that areas north and west of a line from around Warrenton to Fairfax to Rockville to Columbia should see mostly snow and very heavy amounts. Twenty inches or more — as simulated by many models — are not out of the question.

Read below for the range of scenarios in the context of the computer model forecasts.

Storm scenarios

Scenario 1: Severe snowstorm. Most likely, 55 percent chance.

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This scenario offers a low tracking across the South and then strengthening off the Mid-Atlantic coast. Such a track helps facilitates the funneling of cold air southward east of the mountains, which keeps the precipitation snow across the area except for parts of southern Maryland.

This scenario would produce a crippling snowstorm with accumulations of 1 to 2 feet across much of the area with near-blizzard conditions possible, especially northeast of the city. The strong winds and possibility of wet snow near the rain-snow line also raises the possibility of power outages.

Last night’s Canadian, GFS and European models and the majority of European ensemble members offered such a forecast. There are still questions about whether the snow would start by Friday morning’s rush hour. Last night’s GFS argued that it might start across the western suburbs early enough to possibly affect rush hour, while last night’s European model and today’s NAM would argue for a later start. A start time mid-morning to early afternoon seems more likely than the GFS’s early start.

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Regardless, Friday night’s rush hour would probably be a mess.

The city would probably grind to halt Saturday with snow continuing throughout the day and into the night. According to some models, light snow could linger over some locations into early Sunday morning.

Unfortunately, this severe scenario seems the most likely.

Last night’s European ensemble model does a good job of illustrating how bullish the majority of the models are concerning substantial snow (see below).

The figure above shows the percentage of the 50 European ensemble members (models with slight differences to their initial conditions or physics) that were predicting greater than 12 inches of snow across the area. The European snow product tends to be a little overly bullish but still, it’s an impressive display of how much potential this storm has to produce over a foot of snow.

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The one important caveat to note is that the upper-level disturbance that is supposed to produce the storm is still over the Pacific, where the data are not as good as over the United States, so there still is potential for change to the forecasts.

Scenario 2: Snow to mixed precipitation. Somewhat likely. 35 percent chance.

In this scenario, the storm produces significant snow west of the city but mixes with or changes to rain after a period of snow. The rain or mix would then change back to snow depending on how quickly the storm deepens as it moves away from us.

In this scenario, the initial low tracks to Tennessee or Kentucky and then either is slow to deepen east of the mountains or the emerging coastal storm tracks a little too far west for precipitation to stay all snow. As of last night, all the high-resolution operational models had moved away from this solution, but such a scenario still is possible.

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A few members of the European and American (GFS) model ensemble were predicting a snow to sleet or rain event. Such a scenario would give the District accumulating snow, but how much would depend on how quickly the changeover occurred and whether the rain or mix changed back to snow.

How much snow falls in such changeover situations is always tricky. Such a solution still could produce heavy snow on the order of 6 to 12 inches for the city and lesser amounts to the south and east.

In such a scenario, there would probably be some travel problems on Friday into Friday night, but rain could alleviate those problems by later on Saturday along and east of I-95. Areas west of I-95 and especially north and well west and north of the Beltway would still experience a heavy snowstorm in this scenario.

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Scenario 3: Moderate snowstorm. Unlikely. 10 percent chance.