NEWS

Most N.C. floods involved decaying weather systems, not intense ones, new research shows

By DAVID WILLIAMSON
UNC News Services

CHAPEL HILL -- Analysis of a century’s worth of weather data across North Carolina and other records of storms and flooding has unearthed facts that may surprise some people.

The most intense hurricanes to strike the state -- such as Hazel in 1954 -- were not associated with the worst flooding, said Dr. Peter J. Robinson, professor of geography at the University of North Carolina at Chapel Hill. And the devastating flooding that followed Dennis and Floyd in 1999 was by no means unprecedented.

“There is growing concern that the southeast United States will experience greater climatic extremes as climate changes, and it is becoming more vulnerable to their impacts,” Robinson said. “For North Carolina, this finding was especially pertinent following the sequence of two hurricanes in 1999. The event was unprecedented in the historical record, some people said, and it was considered a herald of future climatic conditions.”

But that’s likely not true, the meteorologist said. Careful examination of that record showed serious inland flooding was not uncommon in the 20^th Century, and that flooding had different causes, including lingering non-tropical storms and even melting snow.

A report on the study appears in the June issue of Natural Hazards, a professional journal. Robinson based his study on stream flow and weather data from the archives of the State Climate Office of North Carolina at N.C. State University and the National Climatic Data Center in Asheville.

In North Carolina, airflow -- and hence most weather -- comes primarily from the west, and precipitation is evenly distributed through much of the year, Robinson said. All the westernmost river basins, except the Savannah, drain into the Mississippi River, and all others flow into the Atlantic.

He found 31 major inland floods during the past century, including eight directly related to hurricanes. Each of the latter required a precursor storm, often an earlier hurricane, to overcome the dry soils and low stream flows typical of late summer and fall. He found five double-hurricane floods in the last 100 years.

“The frequencies of these floods by decade were poorly correlated with the total number of hurricanes, with no important hurricane floods between 1955 and 1999 despite frequent hurricanes,” Robinson said. “Most floods involved slow-moving, decaying weather systems, not intense ones. An increase in hurricane intensities, often suggested as a consequence of climate change, may actually lead to fewer floods.”

Other floods, he said, were produced by unnamed, non-hurricane storms or squall lines, and precursor storms also were required to cause them.

“These floods were common in the first and last three decades of the 20^th Century and were virtually absent during the middle 40 years,” Robinson said.

As global temperatures rise, as they did in the early 1900s, N.C. cyclonic floods also might increase in the future, but that is by no means certain, he said. Most meteorological analyses employ no more than the past 50 years of observations.

“The present data indicate that use of this period alone would give very misleading results in terms of the frequency, magnitude and cause of floods.”

No one can make regional weather projections for upcoming decades confidently, and so no one should put too much stock in them or worry about them unduly, Robinson said.

“Something that is extremely important, however, is for people never, ever to drive through roads covered with moving flood waters,” he said. “Water is much heavier and moving water is much more powerful than it looks and often deeper, and too many people have died trying to drive through it.”

Robinson also will present his findings Tuesday (May 13) at a meeting of the American Meteorological Society in Raleigh.

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Note: Robinson can be reached via cell phone at (919) 280-9710, 962-3875 (w), 967-6225 (h) or e-mail at pjr@email.unc.edu

Contact: David Williamson, (919) 962-8596