10:30 p.m. update: Radar shows light snow starting in D.C.’s west and southwest suburbs and streaking northeastward. Flakes should begin inside the Beltway between 11 p.m. and midnight. Snow will vary in intensity overnight with a possible break in the action or lull towards and during the early morning, but should pick-up again by mid-morning to mid-afternoon, when conditions will likely be worst.

7:15 p.m. update: Based on this morning’s and afternoon’s model data, we have decided to increase snow amounts for much of the immediate metro region to 3-5 inches before any changeover to ice and ultimately rain. Temperatures will be very cold when the snow starts overnight and through Monday morning allowing rapid accumulation and models have increased the amount of precipitation they’re generating. Expect snow covered roads and VERY slick conditions Monday morning. Travel will be difficult

Having said that, we think snow may change to sleet a little earlier than we had before – as soon as early to mid-afternoon, which we think will limit snow totals from going even higher than in the above map.

While we increased snow totals, we have reduced ice forecast totals as models pretty uniformly reduce the intensity of precipitation after snow transitions to sleet and freezing rain. They bring in a dry slot. We think the chances of a serious ice storm have dropped, even in areas that take a long time to warm above freezing, well north and west of town.

For a good part of Monday night, precipitation may be quite light in the form of light drizzle and light freezing drizzle (coupled with some fog), which may leave a glaze, but unlikely heavy ice build-up.

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By Tuesday, a second wave of heavy precipitation comes through but by then everyone will be above freezing.

Original post from 1:05 p.m.

A winter storm will begin to affect the Washington region Monday morning, starting as snow and then changing over to freezing rain and rain that night. Although forecast models suggest that freezing rain will taper off before the Tuesday morning commute, we cannot rule out an icy rush hour in some of our cooler locations.

Snow spreads into the region overnight tonight (after midnight) from southwest to northeast.

Temperatures will be very cold, so snow will immediately stick and cause icy roads.

Snow changes to sleet and then freezing rain Monday evening from southeast to northwest.

Freezing rain changes to rain Monday night into Tuesday morning, but icy conditions could last into the Tuesday morning commute in the District and areas north and west.

Heavy rain is likely Tuesday afternoon, possibly causing some local flooding.

With extremely cold air in place ahead of this storm, snow will immediately stick to the roads when it starts to fall Monday, making travel difficult and in some places very dangerous. Snow totals could reach anywhere from one to four inches across the immediate D.C. metro area, with the lower accumulations falling to the southeast, and the higher accumulations falling to the northwest.

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Snow then changes over to sleet and freezing rain Monday evening.

Continue reading for the ice forecast …

There is still some uncertainty about how long the freezing rain will last, especially in the valleys west of the city, where cold air could last much longer than other parts of the region. It’s likely that some icing will continue in those colder locations late into Monday night or early Tuesday morning, and the adverse effect on the Tuesday morning commute cannot be ruled out.

Southeast of the city, the icy conditions should give way to rain in plenty of time to allow melting for the Tuesday morning commute, and accumulations are not expected to be heavy enough to cause power outages.

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In the immediate metro area, freezing rain will probably cause slippery road conditions overnight. While models have been suggesting that temperatures will rise enough so that precipitation will change over to rain before the Tuesday morning commute, there is a chance that freezing rain could linger into the early-morning hours Tuesday. And we can’t rule out an icy rush hour, especially for our north and northwest suburbs.

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Where the ice accumulation approaches 0.25 inches in the northwestern areas, limbs could come down and scattered power outages would be possible. Travel could remain treacherous into the morning hours. The far northwestern areas, in the darker shade of brown in the map below, could end up with a serious ice storm with considerable tree damage and power outages if the colder model solutions pan out.

[7:15 p.m. update: We now think the tree damage and outage risk is not as high thanks to the likelihood of a dry slot pushing through the area overnight Monday, reducing the amount of icy precipitation.]

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Technical forecast

The models are in agreement that the storm will begin as snow, change to freezing rain and then transition to rain, but they differ on the details.

Today’s NAM model shows a pretty good front-end thump of snow, which would give the District and areas to the east two to four inches of snow and would deliver six inches or so to the west of the city. It then forecasts a lull in the precipitation as it changes to freezing rain, which lingers in the city until about 4 a.m. The icing hangs on in a few spots well west of the city through 7 a.m. — much longer than either the GFS model or last night’s European model.

The NAM model’s colder temperatures at the surface are worrisome because a mesoscale model should have some advantages with low-level Arctic air, assuming the surface pattern is well forecast. Plus the models sometimes have trouble scouring out Arctic air. But even the colder NAM brings heavy rain across the area during the day on Tuesday as temperatures warm and as it tracks a strengthening low almost right over us.

By contrast, today’s GFS is much warmer. It places the freezing line already well west of the city by 1 a.m. Tuesday morning (see below). The freezing line then zips northward to the far northwest corner of Pennsylvania by 7 a.m. as the surface low tracks west of the city.

Its snow forecast is tamer than the NAM’s, with two to four inches from the city’s west and one to two inch from the Chesapeake Bay to the city. With the westerly storm track and strong high off the coast, the more-conservative snow forecast seems more likely.