Snowfall forecast for Jan. 29. (Capital Weather Gang)

* Winter weather advisory Tuesday PM entire area | Winter weather advisory early Tuesday AM far north and west | School delays/early dismissals/closings *

9:25 p.m. - A little AM iciness possible north of D.C., PM forecast still on track for rain changing to snow

Light snow with perhaps a bit of sleet and freezing drizzle is possible around 4-8 a.m. mainly north of D.C., which is why the winter weather advisory now starts at 4 a.m. for northwest Montgomery, northwest Howard, and Frederick counties. If there’s any accumulation, it shouldn’t be more than a light coating. But we could see a little iciness on the roads with temperatures in the mid-20s to near 30, so school delays aren’t out of the question for counties north of the District like Montgomery, Frederick and Howard. Check conditions before heading out and use caution on untreated surfaces—both walking and driving.


Forecast radar from the High-Resolution Rapid Refresh (HRRR) model shows light snow (perhaps mixed with a bit of sleet or freezing drizzle) passing mainly north of D.C. early Tuesday morning around 4-8 a.m. (WeatherBell.com)

Looking at the latest model data, our forecast for the main event tomorrow midday into the evening remains on track as described below...

3:15 p.m. - Winter weather advisory issued

The National Weather Service has issued a winter weather advisory for the region Tuesday afternoon and evening. The advisory does differ some depending where you live:


(NWS)
  • Along and east of Interstate 95, the advisory stretches from 3 p.m. (Tuesday) to midnight (Wednesday) and calls for one to two inches of snowfall.
  • West and north of Interstate 95, the advisory stretches from noon to 10 p.m. Tuesday and calls for one to three inches of snowfall.

In both zones, the Weather Service cautions slippery roads could develop during the evening commute and that conditions could turn hazardous as temperatures drop and roads freeze, most likely later in the evening.

Original story from early afternoon

As an arctic cold front sweeps through the Washington region, rain will change to snow Tuesday afternoon and evening as temperatures steadily fall toward the freezing mark.

Snow could reduce visibility and put down a slushy accumulation during the afternoon and evening commute.

Pockets of difficult travel are possible because of localized bursts of heavy snowfall. The National Weather Service issued a special statement early Monday that warned of “rapidly deteriorating driving conditions” Tuesday evening. “Plan ahead by allowing for extra travel time, and consider using public transportation and telework options,” it advised.

We would not be surprised to see some schools cancel after-school activities or even dismiss a little early Tuesday.

While the snow could cause delays and slushy travel, temperatures through 6 or 7 p.m. may remain high enough to prevent widespread icy conditions until later in the evening.

The coldest temperatures, falling into the 20s, are expected to come after the snow ends between 7 and 10 p.m. — which could cause wet and untreated slushy areas to quickly turn icy overnight. Some school delays and cancellations are possible Wednesday morning.


Forecast temperatures between 4 p.m. Tuesday and 7 a.m. Wednesday from high-resolution NAM model. (PivotalWeather.com/PivotalWeather.com)

An added wrinkle is some frozen precipitation (snow and freezing drizzle) may arrive early Tuesday morning — before the main activity in the afternoon and evening — to cause a few slick spots, especially north of Washington. It will probably be light and spotty but could also result in slick spots for the morning commute. It would be wise to check conditions before heading out Tuesday.

In terms of accumulation, a coating to two inches seems likely for most of the region. Models suggest the potential for a little more north and northeast of town, where developing low pressure may draw in additional moisture, allowing for somewhat heavier and longer-lasting snow.

Model snowfall forecasts have generally decreased slightly since Sunday, predicting somewhat lighter snow and somewhat higher temperatures. However, models could still shift over the next 24 hours — and given the late-day commute timing, it is worth monitoring the forecast closely and having flexible plans.

Tuesday event timeline

3 a.m. to 9 a.m.: Chance of light snow and sleet, possibly mixing with and changing to rain from the Beltway south. A coating possible — best chance is north of Beltway. Temperatures 30 to 34, coldest northwest.


HRRR model simulated radar at 5 a.m. Tuesday. (PivotalWeather.com/PivotalWeather.com)

9 a.m. to 12 p.m.: Areas of light rain developing, except light snow and sleet in colder areas north and west of Beltway. Temperatures 35 to 40, coldest northwest.


High-resolution NAM model radar simulation at 10 a.m. Tuesday. (PivotalWeather.com/PivotalWeather.com)

Noon to 3 p.m.: Rain changes to snow northwest to southeast. Temperatures 32 to 38, coldest northwest. Minimal accumulation.


Forecast radar from high-resolution NAM model at 1 p.m. Tuesday. (PivotalWeather.com/PivotalWeather.com)

3 p.m. to 6 p.m.: Snow, with localized heavy bursts. Temperatures 31 to 35, coldest northwest. Slushy accumulation possible.


High-resolution NAM model forecast radar at 4 p.m. Tuesday. (PivotalWeather.com/PivotalWeather.com)

6 p.m. to 9 p.m. Snow ends west to east. Temperatures fall into upper 20s, and wet/slushy areas turn icy.


HRRR model simulated radar at 7 p.m. Tuesday. (PivotalWeather.com/PivotalWeather.com)

Event impact

On our winter storm impact scale, this event rates as a Category 2, or disruptive, event. Even though this snow may not amount to much or last long, its timing during the evening rush increases the potential to cause problems. Also, given the freezing temperatures spilling into the region behind the storm, which is not departing until Saturday, icy areas will develop and stick around if left untreated.


CWG

While delays and areas of difficult travel are likely Tuesday evening, we do not expect this event to cause the widespread gridlock on roads witnessed during the traumatic Jan. 26, 2011, or Jan. 20, 2016, p.m. rush hour snow events — in which some commuters were stranded for many hours. Five to 10 inches fell in the 2011 event — more than twice as much predicted for this one.

In the 2016 event, road and air temperatures in the 20s leading up to the snow and a lack of pre-treatment allowed very icy conditions to develop. Road and air temperatures will be substantially warmer this time around during the commuting period — mostly above freezing — although they will fall quickly as the snow pulls away.

Model snow amounts

Predicted snow amounts from the models are generally in the range of one to two inches in Washington, but some show two to four inches just north and northeast of Washington. But, as temperatures will be above freezing while snow is falling, these amounts may be a bit higher than what actually falls as some flakes melt rather than stick. Here are the projected amounts for Washington:

  • HRRR model: Around an inch.
  • High-resolution NAM: 1.5 inches.
  • NAM model: 1.5 inches.
  • GFS model: Around an inch.
  • European: 1.5 inches.
  • High-resolution Canadian: Coating.
  • Canadian: Around an inch.

The National Weather Service’s forecast of two to three inches right around Washington is a bit higher than the consensus of current models, but we agree these amounts are reasonable to the north and east.


National Weather Service snowfall forecast for Tuesday. (National Weather Service/National Weather Service)