A snowstorm that could drop 4 to 7 inches of snow on the Seattle area is likely to hit before the morning commute, the National Weather Service said Tuesday night.

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After initial forecasts saying a major snowstorm could drop 5 to 10 inches of snow on Seattle, and up to 4 feet in the Cascades, forecasters late Tuesday revised their predictions downward, saying Seattle likely would receive 4 to 7 inches before the snow lets up late Wednesday.

The white stuff was expected to begin falling in the Puget Sound area early Wednesday morning — in plenty of time for a messy morning commute.

National Weather Service meteorologist Chris Burke late Tuesday was pulling together models that showed the low-pressure area farther south, with the winter storm whacking Southwest Washington from Olympia south, but mostly sparing the Puget Sound region.

“I’m not going to completely rule out an impactful snow event, but it definitely looks like less snow than was earlier predicted,” Burke said. “This is what the models show.”

By about 9:30 p.m., the National Weather Service had updated its forecast, showing an expected snowfall from Tacoma to Everett of 1 to 2 inches overnight and 3 to 5 inches more Wednesday.

Dustin Guy, a NWS meteorologist, said there likely would be a big difference in snowfall between northern and southern areas of the state, and possibly even between North and South Seattle.

While exact snowfall totals were impossible to predict, forecasters said heavy snow was expected through the day in some areas and that some faced the danger of bitter cold — wind chills near zero in Whatcom and San Juan counties.

Temperatures should be warmer Thursday with mixed rain and snow expected, turning to rain Thursday night.

School districts and transit agencies were prepared for serious snow Wednesday.

Some districts, including Seattle, canceled classes for Wednesday, and others will have restricted schedules. The University of Washington late Tuesday canceled classes on its Seattle and Bothell campuses.

King County Metro Transit has canceled up to 30 routes; the Washington State Department of Transportation (WSDOT) had more than 100 snow-response vehicles ready; and utility crews were bracing for power outages as tree limbs sag or snap into power lines under the weight of snow.

But even with state and county crews prepared, some secondary roads could be treacherous if there’s heavy snow, perhaps impassible.

“Bus riders need to have realistic expectations, when the road conditions are bad, traffic is slowed down, buses are not going to be able to keep to their schedule,” Metro spokeswoman Linda Thielke said.

In past storms, a number of buses have been disabled, blocking busy streets. Steve Pratt, Seattle street maintenance director, said 15 bus spinouts were reported during Sunday’s comparatively light snowfall.

WSDOT planned to operate 110 snow-response vehicles on highways from the Canadian border to the King-Pierce county line on Wednesday, not counting its crews on Stevens and Snoqualmie passes.

“Our crews will be out there plowing and treating the roads, but we need drivers to do their part and prepare for really challenging travel conditions,” Washington Transportation Secretary Paula Hammond said Tuesday.

High avalanche danger also could mean periodic closures of major mountain highways.

As temperatures rise to the mid-40s by Friday, relief from the snow carries the hazard of urban flooding as snow and ice rapidly turn to slush. Residents are asked to help clear storm drains near their homes.

Although all parts of Western Washington will receive snow Wednesday, snow depths are likely to vary widely.

South Sound areas may receive 5 to 10 inches, while Northwest Washington, hit hard by snowfall over the past few days, may see only 1 to 6 inches of snow, depending on which forecast you use.

The heaviest snow will be in the mountains, with up to a foot expected in the Olympics, 38 inches in the North Cascades and up to 48 inches in the South Cascades, according to the Weather Service.

Earlier forecasts were for a Wednesday storm expected to deliver a greater single-day punch, but over a much shorter duration, than a snowy stretch of December 2008 in which residents dealt with streets covered with snow and ice for days on end and many commuters were stranded as a large portion of Metro’s fleet was unable to negotiate city streets.

Public discontent with Seattle’s response to the 2008 storms was seen as a factor in Mayor Greg Nickels’ loss in the August 2009 primary in his bid for re-election.

During Wednesday’s storm, Seattle transportation crews planned to position a front-end loader next to the Alaskan Way Viaduct to scoop away snow.

The highway will close for 15 to 20 minutes before the commute, Pratt said. After that, there may be times when the road closes for several minutes if snow falls very quickly, he said.

Pratt said the city is in a better situation than on Nov. 21, 2010, when temperatures dropped into the teens and remained below 32 degrees for more than two days.

Mild weather Tuesday allowed workers to drop salt brine on streets, and focus on particularly difficult areas.

“This time, at least we have a fighting chance,” Pratt said.

Jack Broom: 206-464-2222

or jbroom@seattletimes.com

Seattle Times staff reporter Carol M. Ostrom and news researcher David Turim contributed

to this report.