A storm bearing down on New Hampshire today could bring as much as 18 inches of powder to some areas of northern New Hampshire, or as little as three inches of slush to southern New Hampshire cities and the Seacoast.

The biggest urban areas of the state - Manchester and Nashua - could see snow convert to mixed precipitation at the end of today's morning commute. Manchester expects four to eight inches of snow before the changeover, said Manchester Public Works Director Kevin Sheppard, citing the city's weather service provider, HomeTown Forecast Service.

Sleet, freezing rain, rain and freezing drizzle are all possible during the day, Sheppard said. Then tonight, the precipitation could change back to snow and add an inch or two.

"(Thursday) will not be a fun day. This is one of those storms where we will have to prepare for the worst," he said.

Snow is expected to be heavy for most of the state, with somewhat smaller accumulations in Coos County, said Weather Service meteorologist Andy Pohl.

The heaviest snowfalls will be between 3 a.m. and 3 p.m. today.

Accuweather.com predicts three to six inches in Manchester and Portsmouth, with northern communities such as Conway and Berlin getting six to 10 inches. But the National Weather Service said Manchester could receive six to 10 inches, and some areas of northern Carroll County could pile up more than 18 inches of fluffy snow.

"Some models pull in warmer air and have a changeover to rain," said National Weather Service meteorologist Chris Kimble. "But Laconia, Conway, we're not even considering a changeover. There's a chance in Concord, but we're not thinking there will be."

The cities of Manchester and Concord issued a parking ban on city streets. The Manchester ban went into effect at 10 p.m. Wednesday; violators will have to pay $110 to retrieve their car from the impound lot next to Derryfield Park.

Power companies put crews on standby and said they had arranged for help from third parties if necessary. The Seacoast could face the worst in power disruptions. Winds are expected to be strongest there, with gusts up to 40 mph, and the snow is expected to be wetter.

"Wetter, stickier snow can have a greater impact on the system compared to the drier, fluffier stuff," Unitil Media Relations Manager Alec O'Meara said. His company, which counts 45,000 customers in the Seacoast region, said Wednesday it had secured agreements with third-party crews to help restore power.

The National Weather Service said its winter storm warning started at 10 p.m. Wednesday and will extend to 4 a.m. Friday.

At Manchester-Boston Regional Airport, many flights from the Midwest and Mid-Atlantic were delayed and canceled as of Wednesday night.

U.S. Airways announced Tuesday that it was waiving its $150 ticket change fee Wednesday and today for travelers flying into or out of many airports, including Manchester and Portland.

Manchester's Sheppard said every snowplow route involves both main arteries and side streets. The arteries are a priority, but drivers are told to keep all streets passable.

"We will keep the streets open as much as possible," Sheppard said.

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Mark Hayward may be reached at mhayward@unionleader.com.