October snow would be a historical rarity

Those long-range seasonal outlooks were hinting at an early start to wintry weather this year. But not this early.

Chances are excellent that the region will experience a meteorological rarity Saturday: October snow, and, in some places not very far north and west of Philadelphia, quite a lot of it.

"I know it's only October," said Paul Walker, a meteorologist with AccuWeather Inc. But the computer models and the people who consult them were confident Friday that the improbable was about to become a reality.

The National Weather Service posted winter-storm warnings for Bucks, Chester, and Montgomery Counties for two to six inches of snow. A winter-weather advisory was in effect for the city, adjacent South Jersey, and Delaware County for one to two inches.

Rain was due to overspread the area Saturday morning, then mix with and change to snow generally north to south during the afternoon.

Also, significant coastal flooding is possible, with the biggest threat at the Shore during the late-morning high tide Saturday, said Stewart Farrell, director of the Coastal Research Center at Richard Stockton College of New Jersey.

Given how warm it has been, snow would have a hard time sticking to the streets during the day Saturday, but it could pose a threat to leaf-filled branches and power lines north and west of the city. Peco Energy said it had "supplemental staff" on standby.

Should enough snow fall in the city to qualify as measurable - that is, a tenth of an inch or more - it would qualify as a twice-in-a-lifetime event.

In records dating to 1884, only three times has measurable snow fallen in October in Philadelphia, or once every 40 years or so. It hasn't happened since 1979, and with good reason.

The region's major snows usually come from coastal storms that tap moisture-laden air from the Atlantic Ocean. In late October, ocean temperatures on average are in the 50s all the way up to the coast of Maine.

Plus, the upper atmosphere is still recovering from the leftovers of summer.

In this case, however, the coastal storm is expected to take a path up the Eastern Seaboard that would drive cold winds from the north into the region. And it would be full of moisture.

By the time the storm winds down Saturday night, 2011 could set a record for the most precipitation in any calendar year.

In the 138-year period of record, 2011 already is No. 2 for precipitation at 54.62, less than two inches away from the reigning champ, 1996, at 56.45.