[This post has been updated.]

Don’t let Wednesday’s warm, rainy weather guide your planning for Thursday. Forecasters are giving themselves some wiggle room on when the cold wave will arrive to turn rain to sleet and snow, but they are leaving little doubt that it’s going to happen, and the result will be 4 to 10 inches of snow in the D.C. region, affecting local travel from morning to night.

Metro already has announced it will reduce service on Metrobus and cancel MetroAccess service. The District government announced a Snow Emergency will take effect at 7 a.m. Thursday, so that main routes can be cleared. After 7 a.m., any vehicles parked along snow emergency routes can be ticketed and towed. These routes are marked by red and white signs. See a map of the emergency routes on this pdf.

Here are some tips on what to expect Thursday.

Treating roads before storm: Spreading brine on pavement before a storm arrives can be very effective in preventing snow and ice from bonding with the road surface — unless the storm starts with rain and washes away the treatment. This storm is likely to start with rain.

What gets done first: Local governments have somewhat different policies on road clearing, but the general idea is to give top priority to the roads that carry the most traffic, then work down to the level of neighborhood streets. That means the street in front of your door may not be passable for many hours after the snow stops falling.

What it looks like: It would be highly unusual to see a plow blade on the pavement as soon as the snow starts falling. Most highway departments wait till several inches have accumulated before they drop the blades. At first, the trucks will be dropping salt and sand. So if you’re out in the early part of the storm, you may well be driving on roads that are white, or at best, covered with slush. It will be difficult to see highway lane markers.

Even the main highways, where the departments deploy their biggest equipment to preset locations to await the storm, are likely to be difficult for drivers during the day Thursday. When plowing starts, it takes the trucks a while to come back around for another pass, and during that time, fresh snow will have accumulated.

Whatever the level of traffic, there will be a problem. If Thursday’s commuter traffic is light, the road surfaces won’t get the benefit of all those tires turning snow to slush and enhancing the effect of the plows and salt trucks. If the traffic is heavy, then the plows won’t be moving any faster than the drivers are.

The storm’s timing: The Capital Weather Gang’s latest forecast says “Snow is likely throughout the day Thursday, heavy at times – especially mid-morning to mid-afternoon.” That’s awful, which is good. Governments and school systems have plenty of time to see this one coming. History tells them what the consequences are when the snow is heavy during a weekday. Decisions usually aren’t made till very late on the eve of a storm, but it’s quite likely we’ll have a lot of closings announced for Thursday. And they will be closings, rather than delays.

That will give those weary snow crews a chance to do their job with little impediment from traffic.

Meanwhile, commuters will have a chance to assess the situation. They’ll raise the window shades, turn on TV and radio, fire up their computers. Most will do the right thing and go back to bed. Some who think it’s not so bad at daybreak and head to work will get stuck later in the day as snow accumulates.

Sidewalks: Road crews won’t do them. The local governments make that the responsibility of the property owners. As the street cleanup progresses, the sidewalk conditions can get worse. Plows will push the snow up into ridges, either covering the sidewalks or creating a barrier where you want to cross the street.

[Metro information updated at 2:45 p.m. Wednesday]
Metro: Metrobus will begin Thursday service on a Moderate Snow Plan. Some routes will be suspended while others will detour around hills, narrow streets or other difficult terrain.

Metrobus school trips in the District will operate, unless school is cancelled.

Metrorail plans to open on schedule at 5 a.m. and operate regular weekday service. If the snow should reach about 8 inches, it’s possible that above-ground train service could be suspended. It’s highly unlikely that we’d be in that snowfall range during the morning and early afternoon. Metro officials try to give riders plenty of warning about an impending shutdown in above-ground service, but riders need to be alert for this.

The MetroAccess paratransit service will be suspended all Thursday.

The transit authority is supposed to clear the areas immediately around the rail stations and the parking lots. But it does not clear the thousands of bus stops across the region.