Enjoy the sunshine today, because you're not going to like tomorrow. The National Weather Service has posted a "Winter Storm Watch" for all of Maryland west of the Bay, effective from this evening through late Friday night.

UPDATE: The storm "watch" has been upgraded to a Winter Storm Warning, effective from 10 p.m. tonight through 10 p.m. Friday, from the Bay to Allegany County. The NWS forecasts light snow overnight, followed by freezing rain on and off into the evening on Friday. Expect a quarter-inch of ice and "substantial travel problems and power outages."  Sounds like a snow day to me, but stay in touch with your school - Sign up for The Sun's school closings alert service. See the end of this post for details.

Earlier post: This storm will likely start tonight as snow, but the thinking in Sterling appears to be that it will be largely an ice storm, with the potential for a quarter-inch of ice forming as the snow changes to sleet and freezing rain.  

We'll see clouds moving in from the southwest by this afternoon, the harbingers of the low-pressure system spinning up from the Texas Gulf Coast. Here's the radar loop.  And here's what the water vapor loop looks like.

If the forecast holds up, it will start as snow, and could accumulate an inch or two before the changeover begins.

Here's the setup: All this cold, arctic air that moved in with yesterday's Alberta Clipper is continuing to build. It's only 25 degrees here at Calvert & Centre streets as I write this. That sets up what meteorologists call a "cold air damming" scenario. This cold air is dense and heavy, and is not easily dislodged by the warm air advancing with the Texas storm.

So, we get warm, moist air aloft, dropping precipitation through the cold air layer at the surface. If the cold air layer is thick enough, we get snow. But as the warm air slowly erodes the cold layer, the cold gets thinner. The precipitation begins to fall as rain right through the thinning cold layer. It lands on sidewalks and tree limbs - where the cold air persists - and freezes in place. That's freezing rain. We get an ice buildup and - while it can be beautiful - all sorts of bad things happen.

Forecasters are having a hard time sorting all this out, and figuring out when the snow will change to sleet and freezing rain, and how much of each we'll get. But we should expect a couple of inches of snow and sleet by daybreak, followed by an extended period of freezing rain. Southern counties could see a change to all-rain, but the ice/rain line remains unknown.

The morning discussion seems to suggest the cold air will hang on longer than some of the computer models might suggest. We could get more snow than they're expecting: "If future (computer) runs come in colder again, or if observations come in colder than (computer) guidance, then (predicted) snow totals will need to be bumped up and ice accumulations toned down. Regardless, there's enough concern to justify continuance of the winter storm watch..."

At the tail end of tomorrow's storm, cold air moves back in, and any lingering precipitation on Saturday could be snow.

Here's AccuWeather.com's Henry Margusity on the forecast. And here's his precipitation map.