9 p.m. update

Afternoon model predictions didn’t reveal anything to alter the forecast beyond what’s said below. It is becoming more likely that our far north and western areas have a good chance for a significant snowfall, especially toward the Interstate 81 corridor. But closer to Interstate 95 or anywhere within a one county radius of the District, the prospects of substantial snow (of more than a couple inches) become iffy – but merit close watching.

Probability of at least 4 inches of snow through Wednesday.

Unless there are any major developments with new computer model information this evening or any watches are issued, this will be the last update.

Original post from 2 p.m.

Model depiction of rain, snow and a wintry mix across the Washington area on Tuesday afternoon. (WeatherBell.com)

We’ve been tracking the potential for two waves of wintry weather in the Washington area between Monday night and Wednesday. Parts of the forecast have come into better focus, while significant questions still remain.

Wave No. 1 (Monday night into Tuesday)

We’re fairly confident that the chance of rain showers will start to increase Monday evening as low pressure approaches from the west-southwest, with a steadier rain becoming likely around or after midnight. Most of the area should see just rain through around 8 a.m. as overnight temperatures drop into the 30s. But areas farther north and west such as Loudoun, Frederick and northern Montgomery counties could change to snow or a wintry mix by 5 a.m. or so, with grassy accumulations of about an inch or less possible by approximately 8 a.m., with perhaps a bit of slush on the roads.

We’re less certain about what happens during the day on Tuesday. We lean toward model information that shows periods of rain or wintry mix into the afternoon, with temperatures warm enough for little to no accumulation on road surfaces inside the Beltway and to the south and east, but a continued chance of some slush on the roads north and west of the Beltway. In this scenario, total snow accumulations would most probably range from a trace to two inches (from southeast to northwest) across the metro area, with higher amounts far to the northwest, as shown in our map below.

There is a smaller chance, but not impossible, that a heavier burst of snow or wintry mix between late morning and late afternoon on Tuesday cools temperatures enough for everyone to get a period of more significant accumulation both on grass and roads, especially north and west of the Beltway, but even some areas inside the Beltway and to the south and east. In this scenario, total snow accumulations could reach into the “boom” categories seen in the map above.

Whatever precipitation is falling should taper by Tuesday evening.

Wave No. 2 (Tuesday night into Wednesday)

Forecast confidence drops even lower for the second wave, mainly because it’s farther away. Precipitation may pick up again around 4 to 8 a.m. Wednesday and, if so, would probably be mainly snow with temperatures in the low 30s. The big question here is whether we’re looking at light enough precipitation with only the potential for a dusting to an inch or so, or whether we’re looking at a juicier system capable of producing a few inches. Whatever we get has a decent chance of sticking area-wide and could disrupt the Wednesday morning commute, before tapering off by midday Wednesday.

Note that the snow map above is only for the first wave. We’re not ready to issue a snow map yet for this second wave.

Here’s how our winter weather expert Wes Junker sees things …

This two-wave storm system has evolved into yet another difficult forecast for the area, with the chance of significant snow in the immediate metro area from wave No. 1 on the ropes, but not out. Wave No. 2 is even more uncertain, but could still deliver a bit of snow.

The chance of significant accumulating snow in and around the city has been slowly frittered away by low pressure tracking a tad too far north as it approaches from the west. This spreads southerly winds at around 5,000 feet across the area ahead of the storm, providing just enough warming aloft to push the rain-snow line to our north for much of the storm. The fact that precipitation is unlikely to change to snow until during the day on Tuesday is a downer for snow lovers — this late in March, your best bet for accumulating snow is overnight into the early morning. The rain is likely to change to snow earliest Tuesday morning north and west of the city, where minor accumulations are likely, probably on the order of a dusting to 2 inches (except where higher terrain might enhance accumulations a bit). This morning’s GFS and last night’s European models support such a forecast.

Today’s NAM model, and to some degree this morning’s European model, offers the one way that areas closer to the city could see more significant accumulating snow: If a burst of very intense precipitation snowfall were to develop Tuesday morning, the strong vertical motion and dynamics might lower temperatures enough for snow to stick for light accumulations inside the Beltway, and possibly several inches of snow to the north and west, with even heavier accumulations over higher terrain. The NAM’s high bias for precipitation cautions not to buy into this possibility too quickly without support from other operational models.

However, there are a couple of other complicating factors that argue against completely dismissing our snow prospects. There are two upper-level disturbances at play. The first is forecast to weaken rapidly as it approaches the region. It produces our initial storm and is the impetus for the models pulling the low out to sea. If the first upper-level impulse were to weaken quickly enough, then the first low might stall and wait for the second, much stronger upper-level disturbance that is forecast to amplify and strengthen as it moves east. It offers the potential to initiate a second surface low near the North Carolina coast.  If that second low developed quickly enough and tracked far enough north, we could see light to moderate snow on Wednesday.

Jason Samenow contributed to this post.