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Here’s what made Monday’s snowstorm so severe

Capital Weather Gang’s Jason Samenow talks about what caused Monday’s disruptive snowstorm across the D.C. region and provides a forecast for Thursday. (Video: The Washington Post, Photo: Matt McClain/The Washington Post)
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On Sunday, it was 63 degrees. Many were skeptical that accumulating snow would fall the next day. But by Monday afternoon, five to 10 inches or more of cement-like snow had plastered large parts of the region. More than a half-million customers in Virginia and Maryland lost power because the weight of the snow combined with strong winds brought down tree limbs and power lines, and traffic on Interstate 95 came to a standstill. More than 12 hours later, many motorists are still stranded there.

Both sides of I-95 south of D.C. still blocked after snowstorm; drivers stranded overnight

The storm’s severity can be attributed to 1) its unusual strength and its course, taking an ideal track to wallop the zone from Central Virginia and Southern Maryland to the District with exceptionally heavy snow and strong winds; and 2) the weight of the snow, which snapped thousands of tree limbs across the region.

The snow intensity Monday morning was simply too much for trees and road crews to handle.

In less than 30 minutes, bare, wet pavement turned white as snowfall rates leaped to one to three inches per hour. Visibility was reduced to less than a quarter-mile. In just a few hours, half a foot of snow had piled up in many locations.

A winter storm dumped several inches of snow across the D.C. region on Jan. 3. (Video: Amber Ferguson/The Washington Post)

Because temperatures hovered near freezing as the flakes poured down, they were heavy and wet — perfect for packing snowballs but perilous for tree limbs. As winds gusted over 30 mph, branches snapped under the snow’s heft.

The last time a snowstorm generated outages as numerous as those Monday was the infamous “Commutageddon” snowstorm of Jan. 26, 2011. That storm also stranded vehicles on area roadways for many hours. The two storms had remarkably similar effects on the region. The Commutageddon storm’s wrath was focused just a little farther north and happened in the late afternoon and evening rather than the morning and midday.

Much like Commutageddon, the effect of Monday’s snow on roads was exacerbated as temperatures plummeted amid the intensifying precipitation. While paved surfaces were initially wet from rain, a treacherous bottom layer of ice formed as precipitation transitioned to snow and temperatures sank into the upper 20s. The situation was made worse Monday night when the mercury plunged into the teens, solidifying the slushy conglomeration.

How much snow fell across the region

In the immediate Washington area, within about a county radius of the District, five to 10 inches were most common. But to the south, totals ballooned as high as 12 to 16 inches in the area around Fredericksburg and in parts of Southern Maryland.

The snowfall in the immediate Washington region was notable on several counts:

  • The 6.9 inches observed at Reagan National Airport was the most from a single storm in nearly three years (since Jan. 13, 2019).
  • More snow fell in a single day at National Airport than in the two previous winters combined (5.4 and 0.6 inches).
  • It was the third-biggest first snowfall of a season on record at National Airport, tied with a storm on Nov. 30, 1967. The biggest first snowfall on record was the 1987 Veterans Day storm.
  • It was the most snow on record the day after temperatures reached 63 degrees or higher.
  • The snowfall set Jan. 3 records at all three major airports. In additional to National Airport’s calendar day record of 6.9 inches, Dulles and BWI Marshall airports posted Jan. 3 records of 3.9 and 6.8 inches.
  • Washington has received more snowfall than Denver so far this winter.

Snowfall amounts cut off sharply north of central Montgomery County and west of Fairfax County, where totals of one to four inches were common.

Photography: Snow blankets the Washington D.C. region

Remarkably, Frederick, Hagerstown and Martinsburg, W.Va., received no accumulating snow. These areas typically get the highest snow totals in Mid-Atlantic winter storms while areas around Fredericksburg and Southern Maryland see the least. But, because of this storm’s southern track, the resulting snowfall pattern was the opposite of what the region is accustomed to.

Why so much snow fell

The heavy snowfall was generated by a coastal storm or “nor’easter” that intensified as it moved out of the Carolinas and over the coastal Atlantic. Storms that follow this track are notorious for producing significant snowfall in our area.

The satellite image from 9 a.m. Monday shows a textbook “Hatteras Low” with an extensive band of precipitating cloud north of the warm front (warm front is shown by solid red line).

Within that broad cloud shield was a much narrower corridor of heavy snowfall, seen in the radar image below. The band, which featured very intense radar “reflectivity” values more akin to a warm season band of heavy showers (with embedded thunder), stalled over the same region of north-central Virginia and Southern Maryland for several hours. Snowfall rates exceeding two inches per hour were common within the band.

We look to the snow-to-liquid ratio for why a pasty, slushy snow fell in this region. These ratios indicate whether snow is heavy and wet or dry and fluffy. For a typical snowfall, these ratios are around 10 to 1, indicating 10 inches of snow would fall from the equivalent of an inch of rain. Wet snowfalls feature ratios closer 7 to 1 while dry, fluffy snows have ratios of 15 to 1 or higher.

In north-central Virginia and Southern Maryland, where power outages were most numerous Monday, the ratios were around 7 to 1.

Farther north and west of the District, where colder surface air was emplaced, ratios were closer to 10 to 1, and the snow took on a slightly drier texture, particularly later in the evolution of the coastal storm.

Strong, gusty winds were another aspect that helped knock out power for those half-million customers south and east of D.C. The graphic below shows the coastal storm, in its quick deepening phase, during midmorning. Those concentric lines are isobars (lines of constant pressure), and the big red “L” defines the low-pressure center of the storm.

The pressure gradient describes how closely isobars are compacted together, and you will note how large that gradient is on the north side of the storm. An intense wind from the north was marked by periodic gusts surging above 30 mph. When tree limbs are loaded with pounds of pasty, wet snow, bounding winds are sure to snap those limbs, and even topple trees top-heavy with thousands of pounds of snow clinging to their crowns.

Coastal lows form in the winter months because exceptionally cold air outbreaks juxtaposed with warm oceanic waters create a strong temperature contrast, providing the potential energy source for a developing storm. Throw in an exceptionally vigorous upper-level disturbance riding the jet stream, and you have the makings of a big snow event.

In this instance, a notably intense high-altitude disturbance featured a vortex with a compact region of spin that induced air to rise vigorously ahead of it. This removed air rapidly from the surface storm, dropping its pressure, which accelerated the inflow of subfreezing air from the north.

The forecast was passable, though too conservative and late-coming

This snow event sort of snuck up on forecasters. For days, computer models showed a storm developing to the south but did not simulate much precipitation reaching the D.C. area.

But over the weekend, meteorologists were astonished by how rapidly models went from “zero to sixty” with their prediction of a heavy snow event across the region. The evolution of the intense upper-level disturbance that aided the exceptional snowfall rates was not well captured by the models until late Saturday or early Sunday.

Our forecast posted midday Sunday called for moderate snow in the immediate area (2-6 inches) and heavier snow (4-8 inches) just to the south. By early Monday morning, as the snow was just beginning, we issued our final forecast calling for four to eight inches in the immediate area and five to 10 inches to the south.

If you look at how much snow fell vs. how much we predicted, you see a pretty good match for the northern half of the region. We accurately predicted where heavy snowfall would start and stop.

But we underestimated how much snow would fall in the southern part of the region. Computer models did show the potential for excessive amounts, but we were somewhat skeptical. Considering the ideal storm track for snow and the strength of the storm, maybe we shouldn’t have been.

Still, in our forecasts posted both Sunday afternoon and early Monday, amplified on social media, we emphasized the likelihood of heavy snowfall Monday morning and dangerous travel conditions. We urged people to stay off the roads. We also said we would not be surprised to see double-digit snowfall totals south of the District, as reflected in our “boom” scenario.

However, we did not adequately convey the risk of widespread power outages; that aspect took us somewhat by surprise. This event serves as an important reminder that whenever there is the potential for at least four inches of heavy, wet snow and strong wind gusts, forecasters need to advise residents to be prepared to lose power.

Overall, we — along with our partners at the National Weather Service and broadcast media — adequately warned people that this would be a significant, disruptive storm. But we missed the mark on how severe it would be in many communities and were not able to provide more than one day of lead time.