The Washington PostDemocracy Dies in Darkness

A messy mix of snow, sleet and rain likely on Thursday. Accumulation and slick travel possible in colder locations.

High resolution NAM model shows snow and sleet over the Washington region on Thursday morning around 10 a.m. (PivotalWeather.com)

This story was updated to add mention of the winter storm watch issued for parts of western Maryland, the eastern panhandle of West Virginia, Shenandoah Valley, and Blue Ridge mountains.

The first winter storm of the season will charge up the East Coast on Thursday, unloading a nasty mix of heavy snow, sleet and rain — weeks before winter officially begins. The Washington region is likely to deal with a messy conglomeration of precipitation, possibly starting as snow and sleet, before changing to rain.

Because frozen precipitation is expected to arrive around dawn or a little before, it could lead to slick conditions and cause delays and cancellations, especially in colder locations north and west of the Beltway.

D.C. winter outlook 2018-2019: We predict snowier than normal conditions for the first time in three years

A changeover from frozen precipitation to rain is expected as Thursday wears on, happening first in our warmer locations to the east and last in our colder locations to the west. Accumulation of snow and sleet is possible, with the greatest amounts in our colder locations.

Treated roads and sidewalks should be manageable, but untreated pavement could accumulate a slippery coating, especially north and west of the Beltway where ground temperatures will be a little colder.

When the precipitation moves in, it is likely to quickly become moderate to heavy at times, which increases the potential for areas of travel disruption. Most of the precipitation should fall between about 7 a.m. and 5 p.m. with the equivalent of an inch of rain or so falling in this period (from both rain and melted snow and sleet).

Another round of rain showers, possibly mixed with snow and sleet in our colder locations, could move through Thursday night as a trailing disturbance swings by.

The total amount of snow and sleet accumulation is a big wild card at this point and will depend on how fast the cold air over the region erodes. Some models predict that precipitation will change from snow and sleet to rain quickly (meaning little or no accumulation around D.C.), while others will take longer.

Our initial thought is that we could see an inch or so of snow/sleet inside the Beltway and to the south and east, with one to three inches possible north and west, before slightly higher temperatures and more rain mixing in limit additional accumulation and reduce road impacts during the afternoon into evening. However, these amounts could be lower or higher, depending on the timing of the changeover to rain.

Southern Maryland is expected to get mostly rain from this event.

The timing of the precipitation is an important variable in this storm. The earlier the precipitation moves into the area, the greater the probability of meaningful frozen precipitation because temperatures will be lower. Later timing would give the atmosphere more time to warm up.

If Washington manages at least an inch of snow, it will be the first instance in November since 1989 when 3.5 inches fell. We think there’s about a 30 to 40 percent chance of this happening.

It could snow in Washington on Thursday. How often does that happen in November?

A more significant winter storm is possible toward north-central and northwest Maryland (near and west of Hagerstown), where at least three inches of snow is possible before a wintry mix, and toward the Shenandoah Valley and west-central Virginia, where a significant freezing rain/ice storm is quite possible. In this zone, a winter storm watch has been issued.

Storm discussion and scenarios

The setup for this event involves a cold zone of high pressure located over New England in a position favorable to feed cold air into our region. This is a phenomenon known as cold air damming and is expected to keep our daytime temperatures in the 30s and even near to below freezing in the usual colder locations well west and north of the city. At the same time, an area of low pressure is expected to organize along the Southeast coast and track northward along or near the coast spreading precipitation into the area.

All the models are advertising that some winter weather will affect the region starting around rush hour Thursday. However, they differ on how much of an effect the storm could have as they differ on how quickly the precipitation transitions from frozen to rain around the city and our western suburbs. During these damming events, the wise path is usually to lean toward the models with the highest vertical resolution and the coldest low-level temperatures as the cold air can be tenacious.

Below we present two scenarios for the Washington region: a cold scenario, in which we see more frozen precipitation; and a somewhat less possible, milder scenario in which snow and sleet change to rain more quickly.

Scenario 1: Colder, snow/sleet scenario (60 percent chance)

The cold scenario begins with the precipitation arriving around just before dawn in the District. The precipitation could start as a brief period of rain and sleet before changing to sleet and eventually a period of snow.

Across the District, the snow might last only an hour or so but could produce accumulations of about an inch. Up to a few inches could quickly come down in our colder areas north and west of the city where snow may last a bit longer.

In the city and the close-in suburbs, temperatures are forecast to remain above freezing so accumulations probably would be limited to grassy surfaces and trees, but if the snow falls heavily, sloppy minor accumulations might be possible on untreated roads.

After this possible burst of snow, precipitation would change back to sleet and then rain as temperatures rise above freezing, before tapering off in the midafternoon. The exception to this might be in areas from Leesburg to Frederick and to the north and west, where temperatures may remain near freezing for the duration of the event, and snow would change to a mix of sleet and freezing rain before tapering off.

The high-resolution NAM and latest run of the European model present this colder, snowier scenario. (A caution is needed about the European model forecast snow maps that may be floating around. The model’s algorithm doesn’t adequately separate snow from sleet or freezing rain, so don’t trust the amounts presented, as they probably are overdone.)

Scenario 2: Milder, sleet/rain scenario (40 percent chance)

The warm scenario offers the immediate D.C. area a brief period of mixed snow and sleet with little or no accumulation and then heavy rainfall. It would be a really wet and raw day across the area with temperatures staying in the 30s but just warm enough for most of the area to avoid much disruption from snow or sleet.

But even in this warmer scenario, areas well west of the city toward Frederick and Leesburg might pick up a quick inch or two of snow but would then change to mostly sleet and rain. Some roads could become slick through late morning, but conditions would improve in the afternoon as temperatures rise above freezing.

Cold pockets could linger near the Interstate 81 corridor and west into the late evening hours, extending the period of possible slick conditions — mainly on untreated surfaces.

The American GFS model generally supports this scenario.

The bottom line is that this is our first legitimate winter weather threat of the season. The models are in relatively good agreement on its evolution but differ on some details. The biggest difference is that the higher resolution models have somewhat lower temperature throughout the atmosphere. If this were late December, the colder scenario would be heavily favored. This time of year, it’s more of a modest lean toward the colder solution.

Loading...