(CWG)

Confidence is growing that the Washington region will face a messy winter storm on Wednesday with a slippery and disruptive mix of precipitation.

As usual, the storm is likely to have the biggest impact in colder areas that are north and west of Washington, but the entire region is likely to deal with at least a period of frozen precipitation on Wednesday morning through around midday. There is a strong chance of school delays and closures and slick travel Wednesday morning unless there is a significant change in the forecast.

Based on the latest model projections, which could change, snow is likely to develop over the region between the predawn and early-morning hours Wednesday. Then snow could fall fast for a time, but before too much can pile up, it is likely to transition to sleet and freezing rain.


The GFS model shows snow moving over the Washington region at 7 a.m. Wednesday.

The transition to sleet will occur first in southern areas and last to the north, but it should happen pretty quickly — with snow ending between late morning and mid-afternoon. The timing of the change to sleet is tricky, and models differ some on this.


The NAM model shows snow transitioning to sleet by early afternoon around Washington on Wednesday.

Snowfall amounts are uncertain and likely to change, but we think they will be moderate (two to five inches) in our northern and northwestern areas (Zone 1 in top image), light to moderate (one to four inches) in the immediate area (Zone 2), and minimal (a coating to an inch or so) in our far-southeastern areas.

After the snow, we’ll have to worry about accumulation of sleet and freezing rain, mainly in Zones 1 and 2. Temperatures, especially the farther north and west you go, could linger near freezing into or even through Wednesday night, extending icy precipitation over a long duration. Untreated sidewalks, parking lots, driveways and neighborhood roads could remain slick and icy.


The NAM model shows freezing rain lingering in colder areas north and west of Washington at 7 p.m. on Wednesday. (PivotalWeather.com/PivotalWeather.com)

An additional concern is ice buildup on trees, especially in Zone 1. The weight of freezing rain could cause limbs to break and power outages. Freezing-rain amounts could exceed 0.25 inches, creating ice storm conditions.

From downtown Washington and to the south and east, temperatures should rise to freezing or a little above, which may allow conditions to improve some Wednesday afternoon and into the evening. Precipitation should change to plain rain here, but specific temperatures are uncertain, and we’ll need to watch the forecast.

So what’s behind this mess?


A European model simulation of the weather pattern on Wednesday morning, with high pressure north of Washington feeding cold air south while low pressure ejects from the Gulf of Mexico toward the Tennessee Valley. (PivotalWeather.com/PivotalWeather.com)

A cold high-pressure system to our north leading up to the event will supply cold air east of the mountains, while a low-pressure zone tracking from the Gulf of Mexico toward the Tennessee Valley will draw plenty of moisture northward. But the flow of air from the south also will help warm the mid-levels of the atmosphere. This means the front-end thump of snow is likely to be followed by sleet, freezing rain and, finally, rain before the precipitation ends sometime early Thursday.

The snowfall potential depends on how quickly the heavier precipitation arrives before the warm layer edges above freezing. The models differ some on that point.

The North American Mesoscale and global (GFS) models hammer us with a period of heavy snow before any change Wednesday afternoon. The average snowfall simulation from the GFS modeling system predicts 3.4 inches but ranges from zero to over seven inches.

The European modeling system, which is based on 50 simulations, is quite bullish, forecasting an 80 percent chance of at least three inches in Washington. An important caveat about its snow product is that it tends to count sleet as snow and assumes all the snow will stick. In this storm, sleet should reduce snowfall accumulations, and some of the initial snow may melt, especially from the District to the southeast, where temperatures may start off a little above freezing.


Likelihood of at least three inches of snow through Wednesday from the European modeling system. (WeatherBell.com)

Still, all the models are suggesting the potential for at least a couple of inches of snow before an icy mix. Rarely in such a weather pattern, with low pressure passing to our west, do storms produce much more than four or five inches. That is probably the top end of what this type of event can produce.