A potentially major snowstorm was expected to hit the Philadelphia region from Monday night through Tuesday. Preparations were in high gear Monday night and the impact of the severe weather was already being felt at airports. Here's what you need to know in advance of the upcoming snow.

Forecast

Forecasters are warning of a "nasty" nor'easter, with periods of dangerously high snowfall rates and whipping winds. The storm was expected to move in late Monday and could continue through Tuesday night, when the precipitation should taper off. The National Weather Service says to expect eight to 12 inches of snow in Philadelphia, with higher amounts in the Pennsylvania suburbs and lower amounts in South Jersey.

The final National Weather Service map published Monday projected 12 inches of snow in the city.

winter storm warning for the area was put in place from 8 p.m. Monday through 6 p.m. Tuesday.

Along the Jersey Shore, Ocean, Monmouth, and Middlesex Counties were under a blizzard warning due to the whiteout conditions expected there.

In the Philly suburbs, Montgomery, Chester, and Bucks Counties were also under a blizzard warning.

The heaviest snow and strongest winds are expected overnight and into the day Tuesday, setting the stage for a particularly hazardous Tuesday morning commute.

The weather service warned that people in places northwest of I-95 "should be thinking about sheltering in place" from 2 a.m. to 2 p.m. Tuesday. In parts of eastern Pennsylvania and North Jersey, snow could fall at a rate of two to four inches per hour.

Some rain and sleet could mix in with the snow at times, and throughout Monday forecasters cautioned that there was a range of potential snow totals. Early weather service maps on Monday forecast seven inches as likely in Philadelphia, with the potential for more than 20.

These early National Weather Service maps showed the minimum (left) and maximum snowfalls originally forecast in the Philadelphia region for March 13 and 14.

This storm is also expected to be a windy one, with northeast winds of 15 to 25 mph and gusts of 35 mph near Philadelphia and over 50 mph closer to the coast.

The late-season storm is forecast to slam the bulk of the mid-Atlantic and Northeast.

Tuesday's high temperature was expected to be in the low 30s, with similar highs forecast for Wednesday and Thursday.  So the snow is unlikely to melt quickly.

While the snow is expected to far exceed anything recorded in Philadelphia so far this winter, it would take the worst-case scenario for this storm to rank among the city's biggest snowfalls.

Schools

Philadelphia public and parochial schools will be closed Tuesday.

Temple University said it would be closed for the day.

Transportation

Roads

PennDot is advising drivers to stay off the roads during the storm, but as of Monday afternoon no highway closures were planned, officials said. As of 10 p.m. Monday, Interstates 70, 76, 78, 80, 81, 83, 84, 283, 176, 180, 476, 380, and all expressways had a 45 mph speed limit and a ban on tandem truck trailers, empty trailers, towed trailers, buses, recreational vehicles, and motorcycles to facilitate plowing, PennDot reported. The same restrictions will be in place for the Pennsylvania Turnpike from the Bedford interchange east to New Jersey and on the Northeast Extension.

Gov. Christie has declared a state of emergency for New Jersey ahead of the storm. State offices will be closed Tuesday for nonessential employees. Road conditions can be checked at http://www.511nj.org/.

New Jersey's Department of Transportation had not issued any restrictions or closures as of Monday afternoon. About 2,500 plows were available to deploy across the state, and brining preparations on major roads began over the weekend. In New Jersey, too, officials recommended  that drivers stay off the roads.

The Philadelphia Police Department was resurrecting its #NoSavesies social media campaign, reminding residents not to use objects to save shoveled-out parking spots.

In Delaware, the governor issued a Level 1 Driving Warning for New Castle County Tuesday, and authorized the Delaware National Guard to assist state and local officials.  Roads will not be closed, but nonessential employees are urged not to drive unless they have to.

Airport

Southwest Airlines canceled all Tuesday flights at Philadelphia International Airport and American Airlines  canceled its flights through 5 p.m. Tuesday, said airport spokeswoman Mary Flannery.

Many carriers, including American, the dominant airline in Philadelphia, were allowing passengers to change flights without paying a change fee.

Flights in Philadelphia were impacted by the snow that earlier hit the Midwest. Hundreds of flights at Chicago's airports were canceled, causing a ripple of travel disruptions nationwide.

Long lines were reported at PHL as passengers sought to get away in advance of the storm.

Mass Transit

SEPTA: The Market-Frankford and Broad Street Lines will run 24/7, with no Nite Owl bus service. Regional Rail will operate on a severe storm plan, meaning that most trains run on Saturday schedules, the Wilmington/Newark Line on an enhanced Saturday schedule, and no Cynwyd Line service. Starting at 4 a.m. Tuesday, service will be cut back or suspended on bus Routes 7, 9, 14, 18, 19, 20, 22, 24, 26, 27, 28, 31, 32, 38, 44, 46, 48, 50, 52, 53, 55, 57, 58, 65,70, 73, 77, 84, 88, 92, 94, 95, 97, 99, 103, 107, 108, 112, 114, 117, 119, 120, 124, 126, G, H, XH, J, K and L. Routes 35, 61 and 62 will be suspended. CCT service will run on a limited schedule. The agency warned that service on the Norristown High-Speed Line, trolleys and other bus routes could be curtailed as conditions warrant. More information is at septa.org.

PATCO will operate on a snow schedule.

NJ Transit will shut down all bus and Access Link service at 12:01 a.m. Tuesday, and expects to resume Wednesday. The Atlantic City Rail Line will operate on a regular weekday schedule, but all other rail lines will operate on a weekend schedule Tuesday.  That means no service west of Raritan on the Raritan Valley Line, west of Dover on the M&E Line, or west of Bay Street on the Montclair-Boonton Line.

Hudson-Bergen Light Rail will operate on a weekend schedule with additional trains every 20 minutes between Tonnelle Avenue and Hoboken Terminal. Newark Light Rail will operate on a Saturday schedule. RiverLINE service will operate on a Sunday schedule.

Amtrak  will run a modified schedule throughout the Northeast Corridor. There will be no Acela Express service between Boston and New York, and limited service between New York and Washington. In addition, Northeast Regional and Keystone Service trains will operate on modified schedules.

Utilities

Forecasters warn that the combination of wet, heavy snow and strong winds is likely to knock down trees and power lines, leading to outages. Utility companies say they are preparing to have extra personnel on hand to respond to outages.

Government

Philadelphia and numerous other municipalities have announced snow emergencies.

Philadelphia's declaration means that vehicles cannot be parked on snow emergency routes. The snow emergency goes into effect at 9 p.m. Monday, with ticketing and towing to start at midnight.

Trash and recycling collection is canceled for Tuesday in Philadelphia. Residents with Tuesday pickup should hold their materials until next week.

In Washington, the House canceled votes that had been scheduled for Tuesday.

Closings

Many attractions were likely to be closed due the storm; check with a site before going. (The Philadelphia Flower Show, however, will remain open.)