Heavy rain falls on ponding water. (Fred King via Flickr)

* Winter storm warning for Frederick County *

* Winter weather advisory for northern Fauquier, Loudoun, Montgomery and Howard counties *

The looming weekend storm is less than 24 hours away. After a day’s worth of persistent precipitation in the form of rain, snow or wintry mix Saturday — depending on your location — gusty winds and Arctic high pressure blast in from the north to deliver the coldest temperatures of the season by Sunday night.

We are happy to report that the forecast has mostly held steady since Thursday. We’ve made only a few tweaks here and there to the timeline. Overall, we want to emphasize that this will be a dangerous winter ice storm for our neighbors in the far north and west areas of the region, mainly north and west of Frederick — where a winter storm warning is in effect.


Winter storm warnings and winter weather advisories in effect this weekend. (National Weather Service)

Saturday during the day: Wintry mix

Precipitation slides in from west to east between Saturday morning and early afternoon. Wet snow could fall initially but then should quickly change to a light wintry mix.

Even though winds from the south will be pulling in warm air at the higher levels of the atmosphere, temperatures at the ground are only going to be in the low to mid-30s when the storm begins. Rain will fall into this cold air and potentially freeze into sleet or freezing rain, which could create slick road conditions Saturday morning and afternoon, especially in our north and west suburbs.

Pavement temperatures may be high enough that mainly untreated roads are a concern within a county radius of Washington.


NAM model simulation of precipitation from Saturday morning to Sunday morning. (PivotalWeather.com/PivotalWeather.com)

The greatest potential for a more significant icy glaze will occur in Frederick, northwest Loudoun, northern Fauquier and points west. Untreated areas of Interstates 70 and 270 around Frederick and Hagerstown could become very slippery Saturday as this storm blows through, especially on bridges, ramps and overpasses.

Farther west into Maryland and the West Virginia panhandle, a full-fledged ice storm could develop, with ice accumulation anywhere from ¼ to ½ inch. That could make travel along the Interstate 81 corridor very difficult, and ice could build up on trees and power lines — resulting in outages.


Ice accumulation forecast for the weekend storm. Red indicates the thickest ice expected between Saturday and Sunday.

Even though it’s not reflected in the graphic above, we can’t rule out a light slushy accumulation of mixed precipitation in the Beltway as the storm begins, mostly on untreated surfaces.

Saturday night: Most places change to rain except far northwest

We expect temperatures will warm up enough so that precipitation changes over to rain by Saturday evening, and the heaviest rain falls overnight. It tapers off Sunday morning and exits before noon, just as temperatures begin to drop. There’s a slight chance for snow as the storm ends, although it shouldn’t be significant.

Total rainfall amounts will come to about one inch area-wide over a short period of time, and it will be falling on top of the leftover snow, so we should be prepared for local flooding, ponding and rising streams. The National Weather Service in Sterling says it expects flooding along minor streams and creeks Saturday night and on main rivers Sunday night.

Howling winds and tumbling temperatures Sunday

The cold front will blow through on the heels of the rain, and the winds will start to gust soon after that. In the Beltway and near suburbs, gusts to 35 mph are possible. In the higher elevations north and west around Leesburg and Frederick, gusts may reach 40 or 45 mph.


Predicted 24-hour temperature change from 1 a.m. Sunday to 1 a.m. Monday. (Washington Post Staff/The Washington Post)

Sunday night is when the cold air arrives. Over the course of 24 hours, the temperature will drop around 40 degrees. Sunrise temperatures Monday will be in the single digits in the suburbs and around 12 degrees in the District. Factoring in the wind, it will feel more like minus-10 to minus-15 area-wide.


Wind chill forecast for Monday morning from NAM model. (PivotalWeather.com/PivotalWeather.com)

High temperatures will be of little relief, reaching only 20 degrees Monday afternoon.

The bitter cold continues through Tuesday, and daytime temperatures don’t climb above freezing again until Wednesday.