A powerful Nor’easter developing tonight in the Mid-Atlantic promises to deliver every type of winter precipitation imaginable across the metro region. Heavy snow is likely to plaster our far north and west areas. A mix of accumulating snow, sleet and rain will hit the District and close-in suburbs, while our southern and eastern areas see more cold rain with some sleet and snow mixed in.

Precipitation will move into the region between about 6 p.m. and 9 p.m. but should not cause problems for the evening commute, given temperatures above freezing.

Overnight, conditions will vary considerably over short distances. By dawn, you may only have to travel 10 miles to go from 5 to 10 inches of snow to very little. Tuesday morning’s commute is likely to be a mess, with heavy snow in our colder areas and a sloppy mix in the immediate metro.

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The snow amount forecast is extremely complex as, after a period of snow this evening, the rain-sleet-snow line is likely to set up right along or just west of Interstate 95 in the middle of the night.

The Weather Service described the uncertainty in the exact location of the rain-sleet-snow transition zone as “tremendous” in its morning discussion.

Areas that are likely to fall north and west of the rain-snow line should experience at least 6 inches of heavy snowfall. This includes much of Loudoun and Frederick counties and northern Montgomery County.

Snowfall amounts may then decrease rapidly as you go farther south. We have bumped down snowfall totals in the immediate metro area for the second time in the past day as the likely storm track has shifted closer to the coast, which will draw in more mild air.

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Inside the Beltway, 2 to 4 inches of snow and sleet is most likely, with amounts quickly increasing to the north and west, and decreasing to the south and east.

In our colder north and west areas that see heavy snowfall, the weight of the snow on trees that have flowers and are leafing, combined with strong winds late at night, could lead to power outages.

Timing and storm evolution

6 to 9 tonight

Precipitation should begin to develop in the region between 6 p.m. and 9 p.m., from south to north. It may start briefly as rain but should change to snow quickly in most areas. With temperatures above freezing initially, we don’t expect travel problems for the evening commute.

9 p.m. to midnight

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Once the snow gets going, temperatures should quickly fall to near freezing between about 9 p.m. and midnight, except near the Bay and Southern Maryland. Some snow accumulation could begin in our colder areas by around 10 p.m. and probably closer to 11 p.m. or midnight in the immediate metro region.

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Midnight to 3 a.m.

Between midnight and 3 a.m., an area of mixed precipitation (sleet and some rain) will try to advance toward the metro region from the southeast as the snow is really picking up in intensity. The big question is how far northwest it moves. It is likely that areas inside the Beltway will mix with or change to sleet, rain, or freezing rain for a period of time. But before any changeover, a quick one to three inches of snow is possible. Temperatures hover around freezing.

Farther to the northwest, the changeover to sleet and freezing rain overnight is less certain, although some mixing with sleet is a good bet even into upper Montgomery County and eastern Loudoun County. In areas well south and east of Washington, mixed precipitation becomes all rain in this midnight to 3 a.m. window.

3 a.m. to 6 a.m.

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Between 3 a.m. and 6 a.m., mixed precipitation is likely in the immediate metro area, rain in Southern Maryland and along the Bay, and heavy snow from northern Fauquier County through central Loudoun County and northwest Montgomery County (and to the north and west). The snow may really pile up in these colder areas during this window, with snowfall rates of one to two inches per hour.

In the mix zone in the immediate metro region, accumulating sleet is possible — although some areas could change to freezing rain or plain rain while others see snow. Temperatures will be near freezing, so it’s going to be a sloppy mess.

6 a.m. to 9 a.m.

In the immediate metro area, heavy mixed precipitation may change back to snow. Some additional accumulation of an inch or two is possible — although this is low confidence. This window coincides with the morning commute and slick roads and reduced visibility are possible. Our far southern and eastern areas may also see the rain switch to mixed precipitation, but accumulation, if any, should be minimal.

Snow will continue in colder areas to our north and west, where roads are likely to be snow-covered and travel difficult.

9 a.m. to noon

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Steady snow and/or mixed precipitation should taper off from southwest to northeast. Not much additional accumulation is likely in this window due to temperatures edging upward and the high March sun angle.

The afternoon and evening

Widely scattered, intermittent snow showers may cycle into the region from the north. They may briefly reduce visibility and/or add on a coating in some areas, but generally shouldn’t amount to much.

Afternoon temperatures should rise above freezing except in our colder areas, but will drop back below freezing at night — so wet and slick spots could turn icy.

Storm impact analysis

Using Capital Weather Gang’s Winter Storm Impact Scale, which rates storms on a scale from 1-5, we see a wide range of classifications in the D.C. region because conditions are going to vary over such small distances.

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Immediate metro area rating

Inside the Beltway, where we’re predicting roughly 2-4 inches of sleet and snow, this storm rates as a Level 2 “disruptive storm.” The amount of snow we’re expecting isn’t huge, but as temperatures will be near freezing and wintry precipitation is likely to impact Tuesday morning’s commute, this storm is likely to cause enough complications in the region to merit this rating.

North and west suburbs rating

In northern Fairfax County, southern and central Montgomery County, and northern Prince George’s County, where temperatures will be colder and 3-7 inches of sleet and snow are likely, this storm rates slightly higher as a Level 3 “significant” storm. Some roads are likely to be snow-covered and slick and school closures are likely in this area.

Far north and west areas rating

In northern Fauquier, Loudoun, Frederick, and northern Montgomery County, where 6-12 inches of snow are expected, this storm rates as a Level 4 “major” storm. In these areas, snow will be heavy for an extended period of time leading to snow-covered roads and difficult travel. Widespread school closures are extremely likely in this area.

Answers to frequent questions

When will conditions be worst? The window from roughly 11 p.m. to 8 a.m. will be most challenging from the Interstate 95 corridor to the north and west. This is when temperatures will be near and below freezing. Along and close to Interstate 95, snow and mixed precipitation, heavy at times, will make for sloppy and slick road conditions. Farther to the northwest, heavy snow may make travel difficult if not impossible.

Where will conditions be worst? This is an easy one: Essentially the areas from northern Fauquier County to central Loudoun County to northern Montgomery County and to the north and west. This is where precipitation should remain mostly snow with heavy amounts in excess of eight inches a good possibility. The Weather Service calls this the “sweet spot or Goldilocks zone” for heavy snowfall in this storm.

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When will conditions improve? After 8 or 9 a.m., as the heaviest bands of precipitation move off to the northeast, skies brighten and temperatures nudge up.

Will the snow stick? Yes, easily in our colder areas. Temperatures will drop to near 30, and the heavy snow intensity will allow the snow to pile up.

In immediate metro area and Interstate 95 corridor, assuming some heavier snow materializes before the switch-over to a wintry mix, temperatures should fall enough (to near freezing) for the snow to stick some, especially since the snow will be falling at night. Also, some accumulating sleet is possible. But there may not be a lot of snow buildup, especially in our normally milder areas such as downtown and near Reagan National Airport.

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Will schools cancel Tuesday? We think school systems west of Interstate 95 have a good chance to close Tuesday.

Will flights be disrupted? The worst of the storm will occur overnight when air traffic is light. But there are likely to be some delays late tonight and Tuesday morning — not just because of conditions in Washington but due to heavy snow and wind that could shut down airports from Philadelphia to Boston.

Could this storm be more severe than forecast? Yes. If the storm shifts slightly eastward so that it can draw down more cold air, snowfall amounts would increase substantially in the immediate metro region and reach our “boom scenario.”

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Could this storm be less disruptive than forecast? Yes. If the storm edges a little closer to the coast drawing in some milder air and temperatures tick up just slightly above what models are indicating, mostly sleet and rain would fall in Washington, with little or no accumulating snow. It is not out of the question for some areas to awaken to bare ground Tuesday morning, especially milder areas inside the Beltway and to the south and east. This is our bust scenario.

Why have your snowfall predictions in the immediate metro area fallen some? Since Saturday, model projections for the storm track have shifted ever so slightly to the west, closer to the coast. For a snowstorm, we’d want to see the storm forming over or just east of Cape Hatteras and tracking just east of Ocean City, Md. Instead, the models show it developing just inland of the Cape and then tracking right over Ocean City. This has pushed the likely position of the rain-snow line from east of Washington to over Washington.