Heavy rains don't prove warming, scientists say

Recent extreme weather is traced to other factors

July 14, 2004|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Monday's rains were extraordinary - more than 6 inches fell in North East and more than 11 inches in Smyrna, Del. An astonishing 13 inches fell in Tabernacle, in central New Jersey.

For Marylanders, the deluge seemed all the more amazing, coming just five days after 3 to 4 inches of rain raised the Jones Falls and Gwynns Falls in Baltimore by 6 to 8 feet, stranding motorists and flooding businesses.

What's going on here?

Meteorologists attribute Monday's heavy rains to an unusual combination of factors that converged over a region stretching from northeastern Maryland into northern New Jersey.

The first was an "instability" in upper-level jet stream winds - a typical summertime pattern that causes surface air to rise and form towering thunderstorms.

Added to that was a very large, very wet mass of warm, tropical air. As it rose, it cooled and condensed in torrents of rain.

Finally, a low-pressure center formed along a stationary cold front to our north and west. The circulation fostered slow-moving thunderstorms over a wide area.

"Those three things combined allowed for a large area of 5-inch-plus rain," said Christopher Strong, a meteorologist at the National Weather Service forecast office in Sterling, Va.

That's "pretty uncommon outside a tropical storm system," he said.

The heaviest rains in Maryland - 5 inches or more - were fairly isolated in Harford and Cecil counties. "Most of the rest of the Baltimore-Washington area had more typical summertime accumulations," Strong said.

Baltimore-Washington International Airport saw only a trace of rain Monday. A 6-inch deluge like Cecil's would have been the heaviest one-day rain on record for a July day in Baltimore. The record is 5.85 inches, set July 8, 1952.

The heaviest rainfalls here usually come from hurricanes or tropical storms. But not always.

In August 1955, Hurricanes Diane and Connie helped push rain totals for that month in Baltimore to 18.35 inches.

The remnants of Hurricane Isabel last year dropped 2.2 inches of rain at BWI. But four days later, a non-tropical storm unloaded 2.4 inches. Four days of rain in October left 4.2 inches, and flooding rains in November killed a schoolboy in the city and three construction workers in Woodlawn.

It's tempting to conclude these heavy rains are examples of the extreme weather events that global warming theorists predict will become more frequent as the planet's atmosphere heats up.

But meteorologists urge caution. Global warming, after all, is about climate, not weather.

"You can't link individual weather events to global warming," Strong said. "You have to link long-range patterns and long-range weather trends to global warming."

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