The National Weather Service has issued a winter storm watch because of the potential for significant snow and ice, which is expected to lead to “very difficult” travel conditions Wednesday.

The snow should begin before dawn Wednesday, and we expect the worst conditions will occur in the morning when widespread school closures and air travel delays are likely. Commuters on Wednesday morning are likely to experience hazardous, snow-covered roads and low visibility if they decide to venture out.

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This will probably be the highest impact storm since the January 12 to 13 snowstorm, which dumped about six to 12 inches of snow on the region. But that storm happened over a weekend, and this one will coincide with a busier midweek travel and commuting period.

The storm will have two phases: a slick, snowy phase Wednesday morning and then, along and west of Interstate 95, a sloppy icy phase that follows into the afternoon and evening.

Snowfall is most likely to occur between Wednesday predawn and midday, with ice and rain in the afternoon and evening. In between these two phases, there will be a transition period in which snow changes to sleet and freezing rain from south to north.

The highest snowfall amounts of three to seven inches are expected in our northern areas, which will take longest to transition to ice and which will have the lowest temperatures. In the immediate metro region, two to five inches of snow are most likely.

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However, we cannot rule out localized higher amounts to four to eight inches or so depending on if and where small-scale areas of heavy snow develop. Some models suggest a zone of heavy snow could set up just southwest of Washington.

Southeast of Washington into Southern Maryland, the period of snow will be brief while temperatures near and above freezing will limit accumulation to one to three inches or so.

After the snow ends, sleet and then freezing rain become the dominant precipitation types along and west of Interstate 95 (in Zones 1 and 2). Expect snow and sleet to change to plain rain in our southern and eastern areas near the Chesapeake Bay and into Southern Maryland (Zone 3).

A concern about freezing rain is that it glazes trees and can lead to power outages if its weight starts snapping tree limbs. This is a risk later Wednesday, especially when you factor in the added heft of the earlier snowfall. The threat will be highest in our colder areas west and north of the Beltway (Zone 1). Temperatures in this area may remain at or below freezing through the entirety of the event, which should subside overnight Wednesday.

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We expect residual closings and delays Thursday morning from the precipitation that fell through Wednesday night, especially in Zones 1 and 2.

The above projections and the storm timeline, presented below, are subject to adjustment in future updates as the storm is still over a day away.

Wednesday storm timeline

4 to 8 a.m.: Snow developing southwest to northeast. It could start as sleet or a snow-sleet mix, but it should change over to snow. Temperatures 28 to 34, northwest to southeast. Accumulation of a coating to an inch or so.

8 a.m. to noon: Snow, possibly heavy at times. Could mix with or change to sleet south of Washington in the late morning. Temperatures 27 to 32, northwest to southeast. Accumulation of a few more inches.

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Noon to 4 p.m.: Snow changes to sleet everywhere, except to rain in east and southeast areas. Temperatures 29 to 34 northwest to southeast. Up to an inch or two of additional snow and sleet accumulation, highest amounts north of Washington.

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4 to 8 p.m.: Sleet changes to freezing rain along and west of Interstate 95 (Zones 1 and 2) with cold rain elsewhere. Temperatures 30 to 35 northwest to southeast. Light glaze of ice possible.

8 p.m. to midnight Thursday: Freezing rain in colder areas well north and west of Washington (Zone 1), with rain elsewhere. Temperatures 31 to 35 northwest to southeast. Coating of ice accumulating in colder areas (Zone 1).

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Midnight to 4 a.m. Thursday: Precipitation tapers off. Temperatures 31 to 36 northwest to southeast.

Storm impact

On Capital Weather Gang’s winter storm impact scale, this rates as a high-end Category 2 “disruptive” storm in the immediate metro area (Zone 2) but a solid Category 3 “significant” storm in our colder areas to the north and west.

From a practical standpoint, we don’t expect a big difference in the effects in these two zones during the day Wednesday. Both will deal with widespread accumulating snow and some ice, and slick roads. But in Zone 1, the amounts of frozen precipitation will be somewhat higher, and the slick conditions will last longer into Wednesday evening and night.

In our milder areas east and southeast of town (Zone 3), expect one to three inches of snow or less, this is more of a Category 1 “nuisance” storm, where the effect of snow and ice will be more patchy and short-lived — with probably just a brief period of challenging conditions Wednesday morning.

Why are we predicting less snow than the National Weather Service?

As of Wednesday morning, the National Weather Service was predicting 6 to 8 inches of snow in the Washington region. We think this is too high by about a factor of two on average.

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“I am not a fan of predicting big snow amounts in cases like this that have a low pressure zone tracking to our north and west, drawing north mild air,” says Wes Junker, Capital Weather Gang’s winter weather expert. “Heavy snow forecasts could bust low if the axis of heavy precipitation were to lift north of Washington and the warm layer arrives earlier than forecast. I don’t remember Washington getting more than four or five inches from this type of event.”