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Earth Matters: Looking back at ’93 storm

March can be a month of wild weather extremes — heavy snow one week and sunny days in the 60s the next. Winter stubbornly holds on as spring begins its northward march.

And when moist tropical air from the Gulf meets frigid air from Canada, big snowstorms are possible.

This past week marked the 25th anni­ver­sary of one such storm, 1993’s “Storm of the Century.”

The storm produced snow from Florida to southern Canada, dropping a rare accumulation of several inches of snow as far south as Birmingham, Alabama.

A few places here in the Northeast got 50 inches.

Though Altoona was among the places paralyzed by the storm, the official accumulation here was less than 15 inches. But some parts of the county had more than 20 inches, and both State College and Pittsburgh got more than two feet.

The March 15 edition of the Altoona Mirror reported that the measurement of the snow was clouded in uncertainty. Snowfall varied due to locally heavier bands, but the persistently high winds caused so much drifting that it was difficult to measure many locations accurately.

Drifts in my own driveway were easily five feet deep.

Like many March snowstorms, the relatively warm temperatures also made for a wet, heavy snow. The six inches we got the final day of the storm had nearly an inch of liquid equivalent, about a third more than an “average snowfall” (which is about 10 to one ratio of snow to liquid).

The weight and subsequent drifting made the cleanup very difficult.

That made a very bad storm seem even worse. The heavy snow caused an unusually high number of roof collapses and power outages. Some people went three weeks without electricity.

Storms in both warm and cold weather become more severe as their air pressure goes lower. Though the physics of such storms is complicated, we could picture the spinning air mass like a drain sucking moisture toward the lowest pressure. The lower the pressure, the more intense the storm becomes. This is why meteorologists so often mention barometric pressure when discussing the severity of hurricanes.

The air pressure of the March 1993 storm went as low as 960 millibars, the equivalent of a Category 3 hurricane in the summer or fall.

Big snows in the eastern United States also need a source of moisture, either from the Western Atlantic or the Gulf of Mexico. Nor’easters usually tap the Atlantic and, consequently, bring much snow to the coast, but not so much inland. (This year’s Nor’easters have brought large snowfall amounts to Philadelphia and none to Pittsburgh or Erie.)

Strong upper winds from the southwest meant that the ’93 storm had a direct pipeline from the Gulf. At one point, the cloud shield from the storm stretched from Honduras to southern Canada and the western edge clipped Chicago.

Thanks to notable developments in mid-range weather forecasting through computer modeling, it marked the first time the National Weather Service had been able to predict such a storm five days before it occurred.

Like Altoona, this was not necessarily the biggest snowfall ever for many places. But the geographic size and total snowfall of the storm ranked it among the most devastating storms since detailed weather statistics began being recorded and compiled.

John Frederick (www.johnjfrederick.com) writes about the environment every other Wednesday.

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