The snow may come in waves, one late tonight into very early Tuesday morning, and a second midday Tuesday into the evening commute.

Models tend to agree that the heaviest snow, coldest temperatures and thus the greatest accumulation potential is in northern and northeastern Maryland, where a winter storm warning is in effect. Amounts of at least a few inches are likely there, and amounts over eight inches cannot be ruled out in a worst-case scenario.

In the immediate metro area, we sit right on the fence between a nuisance snow event in which light snow does not accumulate much to a more disruptive event with several inches of snow, widespread travel issues and school delays and cancellations. We gently lean toward more of a nuisance event, although confidence is not high. Commutes both Tuesday morning and evening could be affected, especially in our colder suburbs.

We must stress that this forecast is exceptionally difficult because conditions will change very quickly over small distances and models do not agree on where and when the more significant snow will start and stop. They more or less agree the heaviest snow should occur north of the District. A few models have shown the potential for three to six inches in the District itself, while some others suggest just a dusting.

Timeline

4-10 p.m. today: Mixed precipitation develops, except more snow north and west areas. Little or no accumulation. Temps: 37-43.

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10 p.m. today to 4 a.m. Tuesday: Mixed precipitation changes to snow, steadiest north of the District. Light accumulations possible, especially north of town. Temps: 31-36.

4 a.m. to 10 a.m. Tuesday: Intermittent light snow in the District and south. Steadier snow north — more accumulation possible. Temps: 30-35.

10 a.m. to 4 p.m Tuesday: Intermittent snow showers through midday, then a chance of steadier snow developing. Temps: 32-37.

4 p.m. to 10 p.m. Tuesday: Areas of snow or snow showers, tapering after 7 p.m. west to east, turning colder. Temps: 26-32.

(Throughout this timeline, coldest temperatures are north and northwest of the District.)

Model discussion by Wes Junker

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The models continue to fluctuate in their handling of how much precipitation will fall across the region from run to run.

Today’s GFS forecasts an area of moderate-to-heavy precipitation developing from the District north and eastward across Baltimore into the Pennsylvania overnight. Consequently, the model advertises the potential for a significant snowstorm across northern Maryland. It spares the District heavy accumulations only by keeping the surface temperature a couple of degrees above freezing during the bulk of the storm, but late tomorrow afternoon it predicts temperatures to drop below freezing while it is still snowing, arguing that we would probably see a couple of inches of snow even in the city.

Today’s NAM model keeps light precipitation going for an extended period and tries to develop a heavier band but never quite gets its act together over our region. The combination of only having light-precipitation intensities and above-freezing temperatures keeps snow from accumulating across most of the region. However, the NAM does drop four to six inches of snow over southeastern Pennsylvania. Shift that bull’s-eye south and west and drop the temperatures a couple of degrees and that snow max could shift closer to Washington.

Last night’s European model predicted an extended period of light snow across the area with temperatures above freezing with its heaviest precipitation falling during the afternoon when above-freezing temperatures would make it difficult for snow to stick except on mulch, grass and the tops of trash cans and cars close to Washington.

The Canadian model — both its regional and global simulations — suggest several inches of snow in the D.C. area, and six inches or more around Baltimore.

With so much model ambiguity, we’ve opted to basically hold onto our general forecast ideas from Sunday.

The complexity of the forecast is illustrated by last night’s Short Range Ensemble Forecast model plume diagrams for Reagan National Airport (see below) of snow accumulation and temperature.

Each line on the top figure is a different model simulation of snowfall.

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The first thing to note is that the snowfall forecast ranged from zero to around five inches assuming a 10-1 liquid-to-snow ratio. That assumption is not a good one if the temperatures stay above freezing. The simulations don’t agree on the amounts or on when the heaviest precipitation will fall. This morning’s version of the model (not shown) has an even bigger spread with forecasts varying from zero to 12 inches using the same dubious assumption regarding snow-to-liquid ratio.

The snow is falling heaviest when the lines sharply rise from the x-axis.

Now look at the temperatures from the same guidance. When the snow lines on the top panel are rising, almost all of the simulations are predicting above-freezing temperatures. Only three simulations drop the temperatures to below freezing. Those three would offer accumulations on roads while the bulk of the forecasts would argue that any accumulations would be mostly on grass, cars and mulch.

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A large part of the forecast problem remains that a number of model solutions still are advertising the development of a smaller scale band of moderate precipitation somewhere between northern Virginia and southeastern Pennsylvania. Where that heavier precipitation is falling, temperatures are liable to drop to near or below freezing especially if it develops over the northern and northeastern suburbs of the Baltimore-Washington area. Anywhere that happens, we could end up with a boom snow scenario. However, without heavier precipitation, we might only see very light accumulations on grass.