*  Winter weather advisory for D.C.’s far north and northwest suburbs  6 a.m. to 6 p.m. Tuesday | Winter storm watch late Tuesday night through Wednesday evening entire region except far south *

10:35 p.m. update

We’re sticking with our forecast map below for snowfall Tuesday night into Wednesday. We may consider increasing amounts in the morning, but for now there’s too much conflicting model information to do so. As for tonight into tomorrow, we continue to expect the precipitation to start as rain around 2-4 a.m., before transitioning to sleet from northwest to southeast around 6-9 a.m., although areas south and east of the Beltway may stay all rain for the duration. The sleet could be moderate to heavy for a several hour period north and west of the Beltway, and may mix with a bit of snow and freezing rain, with sleet/snow accumulations up to an inch or so possible before this first wave of the storm tapers mid-afternoon. Slick spots on roads and sidewalks are likely north and west of the Beltway, and possible inside the Beltway, as temperatures drop to right around freezing during the day tomorrow.

6:20 p.m. update

Afternoon model runs (with the exception of the NAM) have us leaning toward upping our snowfall totals for the second wave of this winter weather event starting during the predawn hours Wednesday (see forecast map below or in above video) and continuing through Wednesday afternoon. But we want to review one more set of model runs before doing so. We’ll be back between 10 and 10:30 p.m. with an update.

3:50 p.m. update

The National Weather Service has issued two new alerts:

  • A winter weather advisory for the first phase of the storm Tuesday, from 6 a.m. to 6 p.m., for our northern and northwestern areas, including northern Montgomery, northwest Howard, Loudoun, Frederick, and Carroll counties. The advisory notes the potential for up to one inch of sleet and snow accumulation as well as a light glaze of ice.
  • A winter storm watch for the second phase of the storm late Tuesday night through Wednesday evening, for the entire region, except southern Fauquier and Stafford counties (and locations farther south). This watch indicates the * potential * for 5 or more inches of snow, it is not an actual forecast for that amount.

2:25 p.m. update

The European forecast model, often the most accurate, is just in and supports moderate snow accumulations in the region for the second part of the storm Wednesday. It might actually suggest our snowfall forecast (shown below) is too conservative (i.e. we’re not forecasting enough), but some of the snow it is forecasting would likely melt because it would be falling during the day in March with temperatures a hint above freezing.

This is a delicate forecast (in which small changes can make a big difference) and additional adjustments to predicted snowfall totals are likely.

Original post from 1 p.m.

Just as spring officially begins, a complicated and drawn-out winter storm is set to march through the region Tuesday and Wednesday. It will disperse the full gamut of precipitation types over the D.C. area, starting first as rain and sleet early Tuesday. But precipitation should turn to more snow from northwest to southeast by the time the second phase of the storm initiates late Tuesday night or early Wednesday.

Even as temperatures fall to around freezing during the first portion of the event, Tuesday morning into the early afternoon, road temperatures probably remain warm enough for mostly wet travel conditions. However, some heavier bursts of sleet, which are tiny pellets of ice, are possible and could cause slick spots.

During the second phase of the storm, Tuesday night and Wednesday, there is potential to bring widespread accumulating snow if it comes together.

Exactly how much snow is the big question, but one to three inches is our first estimate around the District, with the possibility of somewhat more in our colder areas to the north and west. New computer model information is showing a strong signal for a big snowstorm for the northern Delmarva through Philadelphia and central New Jersey. As we’re not far from these locations, we’ll watch this carefully and will probably need to fine-tune snowfall estimates over the next 24 to 36 hours.

In terms of travel and commuting disruptions, the periods of greatest concern would be: 1) the late morning through midafternoon period Tuesday, especially north and west of the city, and 2) during Wednesday morning’s commute throughout the region.

Throughout this event, both Tuesday and Wednesday, forecast surprises are possible because of its complexity. “The evolution of the features driving our weather over the next several days is a convoluted mess,” says Wes Junker, Capital Weather Gang’s winter weather expert.

The first wave Tuesday morning: Mixed precipitation, heavy at times

Over the past two days, model forecasts have shifted the track of the first wave far enough to the north, to pretty much take significant snow off the table Tuesday morning.

But a nasty mix of rain and sleet are likely, that could mix with snow, especially in our colder areas to the north and west.

Temperatures are forecast to slowly fall predawn Tuesday through midday, from the mid-to-upper 30s to near freezing, as precipitation picks up in intensity. As temperatures cool, rain may increasingly mix with and even change to sleet.

High resolution NAM model shows falling temperatures Tuesday.

There is even the chance of heavy sleet between midmorning and early afternoon Tuesday. This could cause slick spots on roads, especially in our colder areas. Capital Weather Gang’s winter weather expert Wes Junker said that because of unstable air at high altitudes, we can’t even rule out thundersleet.

When precipitation falls as rain rather than sleet in our colder areas, the rain may freeze on tree limbs resulting in a glaze (but probably not enough to cause power concerns). We think roads and sidewalk temperatures should be high enough that the water runs off rather than freezes during the day Tuesday. But again, if and when precipitation turns to sleet, some slipperiness cannot be ruled out.

NAM model forecast radar on Tuesday.

Precipitation should gradually diminish Tuesday afternoon, with just lingering pockets of light rain, drizzle and/or light sleet into the evening. Road conditions during the evening commute should be okay, but slick spots on sidewalks and untreated roads could become an issue after dark in our colder areas.

The second wave Tuesday night and Wednesday: Mostly snow

The period to watch for the possibility of steady precipitation redeveloping is between late Tuesday night (after the commute) and dawn Wednesday. The atmosphere is likely to have cooled enough for precipitation to shift more from a rain-sleet mix to more snow, mixed with sleet at times. Temperatures should be near freezing throughout the region.

But the intensity of this precipitation is a wild card.

“The big question mark is whether this second wave, forecast to translate into a coastal storm, can pull moderate to heavy precipitation back across the area Wednesday to produce meaningful accumulations,” Junker said. “The heaviest precipitation could stay far enough northeast to only produce light accumulations. Right now, that’s our bet, but in such a complicated set up, we still can’t completely rule out a period of heavy snow developing Wednesday morning.”

GFS model simulation of storm from Tuesday evening to Wednesday night.

The period of greatest concern for traveling would be early Wednesday morning, when temperatures are likely to be coldest and when snow and sleet may fall at the steadiest clip.

As Wednesday wears on, the intensity of snowfall will probably wane except in areas well to the northeast of Washington, and temperatures should sneak above freezing. Unless snow falls heavily, it is hard for snow to stick much in March during the day because of the high sun angle. It’s not out of the question snow continues through Wednesday afternoon, but we think accumulation prospects greatly diminish after about mid-to-late morning.

Next updates

Because of the complexity of this forecast, it’s one you’ll want to keep monitoring. We will be updating this story through this afternoon as new information comes in.

Then stay tuned to our PM Update which will be posted around 5 p.m. We’ll update this, as needed through this evening. We may also post a SchoolCast late this afternoon.

Then, on Tuesday, we’ll have updates beginning early in the morning on the storm as it unfolds, and a new, detailed snow outlook for Tuesday night and Wednesday by midday.