6 p.m. update
We’ve decided to make a first estimate of snowfall totals for the storm system Monday night into Tuesday. It is quite possible we may need to adjust this (upward or downward), but we thought we’d give you a sense of our initial thinking.
Generally, we thinking a coating to two inches of wet snow, mixing with rain at times, is most likely in the immediate metro region. But to the north and west, toward Loudoun, upper Montgomery County, northern Fauquier County, and Frederick County, two to four inch snow amounts are more likely. Even higher amounts are possible into the mountains.
Stay tuned for more coverage on this winter weather event tomorrow.
Original post from 1:30 p.m.
A storm system gathering over the central United States on Sunday will move in our direction early in the week, affecting the D.C. region by Monday afternoon or evening. By Monday night and through Tuesday, a risk of accumulating snow increases considerably for parts of the area, although the chances for a major snowstorm continue to be low.
The best odds for accumulating snow are to the west and north of the immediate D.C. area, but the whole region is in play for this event. Like all late-season snowstorms, accumulation will tend to favor elevation and grassier areas. There’s also the likelihood of some messy mix.
One wild card is arrival time: If rain late Monday flips to snow overnight and falls heavily, March’s increasingly strong sun, of course, won’t be a factor. Given the potential for the heaviest precipitation to coincide with darkness, the low track and a few degrees in temperature will be critical. Anyone who gets into the heaviest activity, should it be all snow, can end up seeing the most impactful winter storm of the season, just hours before the official change to spring.
Temperature forecast from the NAM weather model. The storm is off the Delmarva Peninsula at this point, with precipitation potentially ongoing over the area.
On Friday, we were probably premature in offering scenarios, as the models were showing too many possible outcomes to distill them into a coherent number.
Today, there still are a number of possible scenarios. Though the risk of some snow accumulating has grown, the idea of a heavily disruptive snowstorm has perhaps diminished slightly.
We still don’t know whether the storm is likely to be a mostly rain event with a little wet, inconsequential snow or a storm that produces moderate to heavy snow. We do know that there will be a storm tracking toward Tennessee or Kentucky and that it will re-form to our south across the Carolinas before tracking off the coast.
Most models have the low track far enough east to spare us a major snowstorm. However, there are still are a few model ensemble members that slam us with heavy, wet snow. Such a storm is a long shot, since the upper-level support seems too fragmented to produce one.
Below, we offer our best guess at the most likely scenarios.
Scenario 1 — 50 percent chance: A snowstorm produces moderate accumulations west and north of the city, plus lighter accumulations in and around the city.
In this scenario, precipitation would start as light rain or a mix late Monday afternoon, changing to snow during the night as a band of heavier precipitation moves into the area. Temperatures west of the city would fall to and below freezing, providing the precipitation intensity stayed high. Accumulations would depend on how far north the heavy band of precipitation spreads. Because of the uncertainty of how far north the low-pressure system would track before pushing out to sea, there is no way this early to predict who would get the heaviest snow.
Last night’s European model, with the heaviest precipitation to the south. It was on the cold side, though. (weathermodels.com)
The storm track suggests that our area will be near the northern edge of the heavier band of precipitation. If it ends up staying a little south of the region, the lack of precipitation intensity would keep temperatures warmer, even west of the city. This could keep accumulations down, especially in and around the city. Friday night’s European model had the heaviest snow south and west of the city. Today’s NAM model has it north of us.
Scenario 2 — 40 percent chance: A mostly rain event for the city that could mix with or change to snow toward its end. Little or no accumulation would occur around the city and only minor accumulation north and west of D.C.
Any snow accumulation here would be predicated on a period of heavier snow falling at night. If the precipitation stays light, no accumulation would occur. Rain overspreads the area Monday afternoon and lingers into Tuesday morning. The low tracks to Tennessee or Kentucky and the redevelopment to our south occurs too late to hold in the cold air. Southerly winds around 5,000 feet above the ground warm the atmosphere above freezing. Surface temperatures also nudge up above freezing.
The farther north the low track across Tennessee into Kentucky, the higher the probability that such a scenario will occur. Recent GFS runs have offered an iteration of this scenario.
Scenario 3 — 10 percent chance: A major snowstorm hits much of the area. Folks west of the Beltway are assured their biggest snow event since the January 2016 blizzard.
Moderate to even heavy accumulations could affect the Interstate 95 corridor, as well, but even in this scenario, some mixing with rain and sleet would likely cut down on snow totals compared to colder areas to the west. This scenario remains the least likely of possibilities, as it would require a very specific storm track that would both draw down cold air and unload persistently heavy snow.
Any slackening in the intensity of the snowfall would allow warming, and with temperatures near or even above freezing, the snow could up mixing with or even changing to rain. A minority of ensemble members cling to this solution.
Bottom line
We’re closing in on the storm, and details are beginning to emerge.
Storm system moving into our region Monday night per the high resolution NAM model. (Pivotal Weather)
There is still a way to go, but accumulating snow seems likely to affect parts of our area from this event. The best chance for higher totals is north and west of the city, largely because it is expected to be colder there.
The low track and the precipitation intensity will be critical to who sees what and how much is measured. Because the most active part of the storm may hit the area overnight, there is some chance for much of all of the region to see accumulating snow. If we do not see heavier precipitation move through our area, accumulation will be more difficult and precipitation could even remain more rain than snow.
Keep watching with us, we’ll update as new info arrives.
Capital Weather Gang’s Jason Samenow contributed to this article.