A blockbuster East Coast storm brushed the D.C. area Sunday into early Monday, dumping heavy, wet snow in some neighborhoods while leaving others with little more than slush. Gusty winds toppled branches and cut power to thousands.
Totals across the region ranged from just a coating to about 6 inches, with 1 to 3 inches most common. The heaviest amounts — generally 3 to 6 inches — fell north and just west of the Capital Beltway and into Southern Maryland. There, the weight of the snow on trees and wires, combined with wind gusts over 30 mph, knocked out power for more than 20,000 customers. Enough snow fell to close most school systems in the area.
The region sat along the storm’s southern and western edge, where snowfall amounts shifted dramatically over short distances.
Forecasting proved challenging as model guidance swung back and forth, even within hours of the storm’s onset.
Our own forecast was a mixed bag. We correctly highlighted sharp gradients in totals and were on target in many areas. But in some spots, we missed the mark.
How much fell
An inch of snow fell at Reagan National Airport, D.C.’s official observing site.
To the west, 2 inches accumulated at Dulles International Airport. In higher-elevation Damascus in northern Montgomery County, 7.1 inches was observed.
A sample of reports is below:
- 5.8 inches in Gaithersburg, Maryland
- 5.3 inches in Laytonsville, Maryland
- 4.6 inches in Vienna, Virginia
- 3.5 inches in Takoma Park, Maryland
- 3.5 inches in East Falls Church, Virginia
- 3.4 inches in Herndon, Virginia
- 3 inches in the Chevy Chase neighborhood of D.C.
- 2.5 inches in Bowie, Maryland
- 2.5 inches in Upper Marlboro, Maryland
- 2.5 inches in the American University Park neighborhood of D.C.
- 1.9 inches at Baltimore-Washington International Marshall Airport
National is up to 9.6 inches on the season, which is right about average to date. While the airport averages 13.7 inches per winter, about 85 percent of it comes before the end of February. March typically sees about 2 inches of snow, while the final handful of days in February add about 1 inch.
At this time last winter, National was sitting at 14.9 inches, which was also the final total for the full season.
Dulles’s seasonal total is now up to 12.6 inches. Damascus’s seasonal total is up to 23.4 inches. These locations are also near or a bit above average to date.
How the storm unfolded and why snow amounts varied so much
Over the weekend, two high-altitude weather disturbances — from the north and the south — merged east of the Mississippi River, carving out an intense dip in the jet stream. The upward motion within that jet-stream dip gave rise to a coastal storm that developed off Cape Hatteras, North Carolina, and tracked toward the northeast.
Such coastal storms, or nor’easters, have been infrequent this winter. But the intensity of this rapidly intensifying storm made up for it. On Sunday, the storm pummeled the Delmarva coast with wind gusts over 50 mph, big waves and about a foot of heavy snow. Blizzard warnings spanned from the southern tip of the Delmarva Peninsula to Massachusetts.
On Sunday afternoon, the intensifying coastal storm pulled down freezing air from the north over the D.C. area. That caused steady rain to transition to wet snow between midafternoon and early evening. The region caught the back fringe of the retreating band of heavy snow (see image below). This snow band, which formed 100 to 150 miles north and west of the storm center, is a common feature of winter storms and delivers the truly blockbuster accumulations.
But this storm had a peculiar twist. In 1993, two meteorologists wrote a scientific paper describing a curious feature of some coastal storms, which they termed a Norlun trough. This forms in the lower atmosphere as a narrow zone of low pressure forms well inland from the storm center (as annotated in the image below).
It develops as winds from northeast and northwest cause low-level air to converge and rise. This creates a very narrow but intense snow band, just a few tens of miles wide, that can remain parked over the same location for several hours, creating large changes in snow amounts over a small distance.
Although the coastal storm’s main snow band quickly departed our region, the Norlun trough became entrenched and fairly robust over Montgomery and Fairfax counties through Sunday evening. It really drove up the snow totals over Fairfax County. One of our forecast models with the highest spatial resolution, or “granularity,” correctly predicted the Norlun snow band just a few hours before it developed over these counties. However, the location was not exact, and its intensity was generally overestimated.
The result was a complicated accumulation map, with some areas cashing in and others falling short across very slim distances.
Reviewing the forecast
This was among the most challenging storms to forecast in two decades. Three factors made it especially difficult:
- Temperatures were marginal — a difference of 1 or 2 degrees had major implications for accumulation.
- The D.C. area sat near the edge of the heaviest precipitation.
- Small-scale features, such as the Norlun trough described above, could dramatically alter totals over just a few miles.
Computer models struggled with these complexities. Three days out, simulations ranged from no snow to 3 feet. Even hours before onset, projections varied from less than an inch to 10 inches.
We leaned on experience to craft what we believed was a reasonable forecast. It wasn’t perfect. Some areas were a bust; others matched our projections closely. But we correctly conveyed that totals would vary sharply and accurately communicated timing and the rain-to-snow transition.
Most important, we emphasized uncertainty throughout. As we wrote Saturday: “It’s entirely possible that snowfall amounts exceed or lag expectations in some areas.”
Where we missed — west and south of Fairfax County and near downtown D.C. and east — the reason was straightforward but unpredictable. The storm intensified more slowly than expected and tracked 20 to 30 miles farther east, shifting heavier snow bands and widening a dry slot.
In a storm defined by tight gradients and volatile model guidance, we worked to provide the clearest information possible. While imperfect, we believe the forecast rose to the challenge.
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