Gov. Martin O'Malley said Marylanders will wake up this morning to the most impassable streets since the 27-inch-deep snowstorm of 2003 - and, if they have power, will learn that perhaps 100,000 households do not.

In a late-afternoon news conference at the State Highway Administration operations center in Hanover, the governor said the overnight snowstorm was expected to be much heavier and wetter than the December snowstorm that dumped up to 22 inches of fluffy powder on the region. The heavy snow is likely to bring down many tree branches, cutting power to homes around the state, officials said.

"This one will be a much more stubborn snow," O'Malley said, urging residents to have patience with state and local officials after a snowfall that forecasters said would reach 20 to 30 inches.

Today "will be a day when everyone's digging out - and into Sunday for that matter," O'Malley said. Among those digging out will be workers at Baltimore-Washington International Thurgood Marshall Airport. Most flights were canceled after midafternoon Friday, and the airport is expected to be closed until Sunday.

Officials urged motorists to stay off the roads today unless it is absolutely necessary to go out. They asked drivers who do venture out to take it slow, use special care at highway interchanges and give snowplows a wide berth.

"Never pass a snowplow or a snowplow train," said State Highway Administrator Neil J. Pedersen, using the term for a convoy working to clear several lanes.

Transportation officials, citing reports that traffic was moving reasonably well in light of the storm, were deliberating late into Friday night about possible road closures.

There were few serious incidents to impede traffic Friday. An exception was a collision in which a vehicle carrying two adults and three children hit the back of a salt truck on the shoulder of Interstate 95 at Route 462 in Harford County. Officials said an adult and a child were critically injured.

Earlier Friday, O'Malley declared a state of emergency that cleared the way for the Maryland National Guard to offer assistance - and the use of its Humvees - to local first-responders.

The declaration could clear the way for Maryland to receive federal aid if it meets the threshold of 28 inches of snow. Such aid could help state and local governments cope with the budget-busting effects of the snowiest winter in recent memory - one that was expected to put the state well above the $60 million allocated for snow removal in the current budget year.

Most commuters avoided the worst of the storm by leaving work at midday - encouraged by liberal leave policies adopted by the state and federal governments and many companies. O'Malley said the biggest surge in traffic occurred between noon and 2 p.m., and most roads appeared to be lightly traveled by the time the snow intensified and visibility diminished in late afternoon.

Transit riders joined the early exodus as MARC trains and Maryland Transit Administration commuter buses left early Friday afternoon in an attempt to get commuters home before the storm hit in earnest. State Transportation Secretary Beverley K. Swaim-Staley said both MARC and the commuter buses performed well.

Lindsay Griswold of Hanover caught a 1:20 p.m. Penn Line train out of Washington rather than her usual 5:20 p.m. departure after her employer, a federal contractor, let workers go home early. She said the crush of riders as she passed through Union Station was the worst she had seen.

"Going through the gate was unbelievable. They just packed us in," she said as she waited for her husband to pick her up at the BWI station.

The MTA planned to run local buses as long as possible, but Swaim-Staley said it might discontinue operations sooner than during December's major snowstorm, when some buses got stuck and had to be towed. She said some buses will run on primary routes only.

MTA officials also planned to run unoccupied "snow trains" through the night to keep subway and light rail tracks clear. But the Washington Metro was expected to suspend operations once the snow exceeded 6 to 8 inches.

Private bus companies also were affected by the storm. Megabus canceled service from White Marsh to New York from Friday afternoon through early Sunday. BoltBus halted service between New York and Baltimore as of Friday evening, and Greyhound canceled routes from Baltimore to New York as well as Ocean City.

BWI's biggest airline, Southwest, canceled more than 200 flights Friday and today, forcing ticket-holders to improvise. Southwest, like most other airlines, won't start flying again until Sunday.

Southwest called Rose Maldonado on Thursday to give her the news. She immediately got on the phone to rebook her 2:50 p.m. flight home to San Antonio. After 45 minutes on hold, she secured a 10 a.m. flight. To be sure, she got to the airport four hours early Friday.

"I got stuck here in December," she said. "I had to stay an extra weekend. ... I really didn't want that to happen again."

She was one of the lucky ones. Flights filled up quickly Friday, and those who came early to rebook or fly standby were largely disappointed. By 2 p.m., the Southwest concourse was nearly deserted except for those who work at the airport, and no passengers were being checked in. At 3 p.m., a Maryland Transportation Authority police officer was seen pedaling his bicycle through the terminal at nearly full speed - impossible on a normal afternoon.

One of the few passengers remaining was Josh Watts, 18. He had endured a grueling overnight bus trip from Dubois, Pa., to Pittsburgh to Washington to Baltimore before walking much of the way to BWI - only to find that no flights were going to Lynchburg, Va., where he hoped to join his wife for their 1-year-old son's birthday party. Now, Watts was stranded at the airport with little money and no idea where to stay until Sunday or Monday.

"This would have been the first time I rode a plane," he said.

Dan Smeglin, 23, of Marietta, Pa., said his 4:30 p.m. Southwest flight to Boston was still scheduled when he called the airline early Friday, but upon arriving at the airport about 2 p.m. he learned otherwise. He was left with the unappealing choice of driving home through the snow to Lancaster County or trying to find another flight.

Smeglin's story had a happy ending at AirTran, among the last airlines to halt departures. He got a seat on a 4:30 p.m. flight to Boston - one of the last flights to make it out, according to AirTran.

On the lower level, other thankful passengers were arriving. Eugene Proctor flew in Friday morning from Indianapolis to meet with relatives who were to drive him to North Carolina for a funeral. Initially, he'd been scheduled to take a later flight, but his wife had asked whether he could come a bit earlier. So he had rebooked - before learning that a storm was approaching.

"It worked out," he said. "Getting home will be another story. I need to get home before the [Super Bowl] game Sunday. I've already decided to fly out of Raleigh-Durham instead of Baltimore."

SOME CANCELLATIONS

* The American Visionary Art Museum in Baltimore will be closed today.

* Amtrak Carolinian trains 79 and 80 will operate only between Raleigh, N.C., and Charlotte with no alternate transportation between New York and Raleigh; Palmetto trains 89 and 90 are canceled between New York and Savannah; Northbound Northeast Regionals: Train 82 is canceled between Richmond, Va., and Boston; Train 152 is canceled Washington to New York; Southbound Northeast Regionals: Train 195 is canceled from Washington to Richmond with no alternate transportation; Train 165 is canceled from Boston to Washington; Train 199 is canceled from New York to Washington; Train 159 is canceled from New York to Washington.

* Anne Arundel Community College is rescheduling its Saturday Black History Month presentation, "The Boss: Blue Angels Capt. Donnie Cochran." The free presentation will now take place Saturday, Feb. 13.

* Sunday's Bach Concert Series at Christ Lutheran Church, Inner Harbor, has been canceled. The next concert, Bach's St. John Passion, will be presented March 7.

* The Baltimore Bridal Show scheduled for today and Sunday at the Baltimore Convention Center, has been rescheduled for Feb. 13-14.

* The Baltimore's Child Camp and Summer Programs Fair scheduled today at the Glenwood Community Center in Howard County has been canceled. Another camp fair will be held from noon to 3 p.m. Feb. 28 at the Crowne Plaza Hotel in Timonium.

* The Baltimore Health Department's Field Health Services division will operate on a call-and-confirm basis only for dialysis patients today. Clients must call 410-396-7007 within an hour of their scheduled ride time to confirm they will be using the service.

* The Baltimore Museum of Art will be closed today. The museum's African-American History Month Family Day (originally scheduled for Sunday) has been postponed until Feb. 28.

* The Baltimore Symphony Orchestra performance of "Porgy & Bess" for today is canceled. The BSO is adjusting its normal exchange policy and allowing the following options to be made through Feb. 15: liberal exchange, ticket donation and refund. Go to bsomusic.org for more information.

* Baltimore County's 2010 Black Saga Competition scheduled today has been rescheduled to Feb. 13, from 8 a.m. to 1 p.m. at New Town High School, 4931 New Town Blvd., Owings Mills.

* Center Stage has canceled all performances today of "Cyrano." The theater will announce a decision about the remaining performances by 11 a.m. Sunday. Visit centerstage.org for updates.

* The Enoch Pratt Free Library's Booklovers' Breakfast featuring author and actor Hill Harper scheduled for 8:30 a.m. today has been canceled. A new date will be announced soon.

* Everyman Theatre's "Two Rooms" performance for today is canceled.

* The Franklin High School Athletic Boosters' Basket Bingo event scheduled for today has been postponed. The event will be held Saturday, March 20, at Northminster Presbyterian Church, at 705 Main Street in Reisterstown. Admission is $15 in advance and $18 at the door.

* Harford Community College's You Can Afford College event has been rescheduled to Feb 13.

* LaurelPark and Pimlico Race Course will be closed today. The $70,000 Marshua Stakes, originally scheduled for today, is moved to Monday.

* The following malls are opening at noon today: White Marsh Mall; The Mall in Columbia; Owings Mills Mall; Towson Town Center; Mondawmin Mall; Village of Cross Keys.

Department store and restaurant hours might vary; call restaurants and stores directly to confirm hours. Harborplace & The Gallery will be closed today. Some restaurants might be open.

* Merritt Athletic Club group fitness classes: call for cancellations and modified schedules at each location.

* The National Aquarium in Baltimore will be closed today. All associated programs and events are canceled.

* The Peabody Symphony Orchestra concert scheduled for 8 p.m. today has been canceled. Persons who have purchased tickets to today's concert will receive a full refund or may call the box office at 410-234-4800 to arrange to exchange their tickets.

* The men's basketball game between Towson and James Madison scheduled for today has been postponed until 7 p.m. Monday at the Towson Center.

* The Walters Art Museum's Free Walk-in Tour scheduled for 2 p.m. Sunday has been canceled. ArtKids Preschool: Zomo the Rabbit today is canceled. And the Family Fun morning donor event has been canceled for today and rescheduled for March 6.