The National Weather Service is clear: A blizzard is on its way to the Northeast this weekend.

But how much that storm will slam the Philadelphia region remains less predictable.

For now, the weather service is forecasting snow in Philadelphia from late Friday into Saturday morning, with a 90-percent chance of precipitation.

The weather service now calls for two to four inches to fall in the region. The service upped its projections for the area Thursday afternoon, and the forecast could change again as the storm nears. (The Inquirer's Weather or Not blog explains why this storm is tough to predict.)

Philadelphia and South Jersey can expect rain for much of Friday. That precipitation should mix with snow in the evening, and turn to all snow after 1 a.m. Saturday, the weather service says. A 30-percent chance of snow continues through 7 a.m. Saturday.

North Jersey and the Poconos, though, could see significant snowfall. The weather service is predicting four to 10 inches in those areas.

Snow there should develop Friday morning, then change to sleet and rain. Then, a heavier snowfall should begin later in Friday afternoon and continue through early Saturday morning, according to the weather service.

In the northernmost parts of New Jersey, "this looks to become a major snowstorm" with "near blizzard conditions," the weather service says. One to two inches of snow could fall per hour, with winds of 30 to 35 mph.

And farther north, the outlook is even more severe.

"A potential historic winter storm and blizzard is expected to drop one to two feet of snow," a blizzard watch from the weather service's Taunton, Mass., office says.

Much of New England is expected to be hit hard by the storm, with the heaviest snow coming Friday night into early Saturday.

The storm could cause havoc for travelers along the East Coast, and airlines are letting customers flying to, from and through certain cities to change their flights without paying fees. Most airlines weren't waiving change fees for Philadelphia International Airport flyers as of Thursday morning, but Delta travelers flying through Philadelphia on Friday or Saturday can change flights without penalty.

Utility companies are also gearing up for the storm. Jersey Central Power and Light said more than 100 workers from other regions are arriving in New Jersey today to prepare for the storm.

Those crews should be "in position ahead of the snow to minimize the impact the severe weather could have on our customers," president Don Lynch said in a statement.