Nor'easters: 'Tis the season

It doesn't have a starting or ending date, nor does it have the same celebrity power or get nearly the attention of the hurricane season, but nor'easters are far likelier to have an impact on the Philadelphia region than tropical storms.

On average, the East Coast experiences 12 nor'easters a year, according to the Northeast Regional Climate Center, and the unofficial 2010-11 nor'easter season is about to get under way.

A storm is to form off the Maryland coast and bomb out later near New England. It is likely to affect the region through Saturday, and its powerful backlash may be a major player in the Phillies-Giants series opener. Fortunately, its worst impacts will be well to our north.

For us, it looks like heavy rain starting in mid-afternoon, and with so many storm drains clogged with leaf-fall, expect road ponding. No major flooding is expected.

The winds aren't going to be big deal until tomorrow, when they swing around to a more northwesterly direction. That may be a stroke of luck.

The lighter winds today will be from the east and northeast, what PECO calls a "counter wind." Prevailing winds around here are from the west, so when the winds come from the other direction, trees and their branches become vulnerable, especially if the ground is saturated. Still, gusts from the northwest over 40 m.p.h. could cause some power outages tomorrow and Saturday given that branches are still weighted with leaves.

If the winds will be from the northwest, why is this thing called a "nor'easter?"

Nor'easters do take their names from the strong winds from the northeast that they generate. The ones that have the greatest impact on our area tend to form off the coast of Cape Hatteras, N.C. Winds circulate counterclockwise around centers of low pressure, so an area to the northwest of the center would experience northeast winds. Typically, the precipitation and winds occur hundreds of miles from the center

This one is to form farther north than the region's more-disruptive nor'easters, such as the great Ash Wednesday storm of 1962, the consensus reigning champ. By the time this one matures off the New England coast, the counterclockwise circulation will mean a strong westward turn to the winds.

Fans in Philadelphia, Boston and New York may think that the baseball postseason causes nor'easters, the way mobile homes lure hurricanes or tornados. But it so happens that baseball has pushed its playoff schedule into the nor'easter timetable. This is the time of year when the maturing temperature contrasts across the northern Hemisphere favor well-organized storms over the showery rains of the warm season.

The nor'easter season peaks in winter, but the "perfect storm" of 1991 occurred in October. The climate center study looked at 47 years of coastal storms. In October, it documented 47 storms.