Flood of Hurricanes Could Follow Drought |
By D'Vera Cohn
There's only one quick cure for the drought that's parching the mid-Atlantic: a tropical storm or two, meteorologists say. And predictions are that this year will bring a flood of them to the North Atlantic.
Two separate forecasts -- from the National Hurricane Center and Colorado State University tropical storm expert William Gray -- see a more active than usual hurricane season this year. On average there are 9.3 tropical storms in the typical season from June 1 to Nov. 30.
National Hurricane Center predictions don't include numbers, but Gray's updated forecast, released yesterday, calls for 14 tropical storms during the Atlantic hurricane season. He said nine will swell into hurricanes, including four whose winds will intensify to over 111 miles an hour, putting them into the "severe" category. Usually, fewer than six storms grow into hurricanes.
Whether this tropical moisture will reach the dusty Washington area is beyond such predictions, meteorologists say. This area seldom gets a tropical storm but is drenched by rain from their fringes every few years.
Gray said there is a 54 percent chance that at least one of the most intense hurricanes will hit the East Coast, odds that are greater than the long-term average.
Don't be fooled, he said, by the fact that there's been only one named storm this year -- a relatively wimpy effort called Arlene.
"People should conclude nothing from the fact that we have had only one storm up to this period," Gray said yesterday. "Things are boiling. The conditions are changing. We see [atmospheric] pressures going down. We see sea surface temperatures warming up. A whole lot is going to happen."
The drought has left this region with a rain deficit of more than 14 inches since last summer, drawing down water supplies and parching farms. Maryland imposed the first statewide water-use limits this week in its history, and conditions in the mid-Atlantic and parts of the Northeast are among the driest on record.
Short, intense summer thunderstorms don't help. Often, evaporation cancels them out.
National Weather Service meteorologists say there are two ways to climb out of a drought: Autumn rainfall or summer tropical storms. But fall rainfall can be slow -- three inches a month or so.
A tropical storm, on the other hand, does the job fast, sometimes producing an inch an hour. But it can also bring destructive flooding.
Remember the leftovers from Agnes, which delivered more than seven inches of rain to National Airport in 24 hours in 1972? Or the three inches from the remnants of Hurricane Fran that hit the District in a day in 1996, wiping out the C&O Canal for the second time in a year? That was the last time the region got any significant rainfall from a tropical storm.
Tropical storms are an important source of rainfall here. In the Washington area, more than a quarter of September's rainfall usually comes from tropical storms or their remnants.
But storms also often fizzle before they get here, hitting Cape Hatteras in North Carolina and heading to sea, pulled by mighty winds. The region has never had a truly intense hurricane and has had direct hits from only half a dozen weak ones since record-keeping began in the late 1800s, said National Hurricane Center spokesman Frank Lepore.
On average, he said, a tropical storm passes within 75 nautical miles of Ocean City, Md., once every 3.4 years. It's been 44 years since a hurricane -- Connie -- hit Maryland's Eastern Shore.
Still, he said, "these systems are state-sized. You can miss by a considerable amount and still get a lot of soaking rain."
At the moment, though, meteorological conditions mean it would be impossible for any storm to get close to Washington. A trough of low pressure sits near the Eastern seaboard like a massive bulkhead of swirling air.
Right now, "you can't drive a hurricane into that region," said Max Mayfield, deputy director of the National Hurricane Center. But "I can assure you this pattern will not last through the hurricane season."
Gray's forecast is based on statistical models that combine current weather data with surface and sky-level patterns from past hurricane seasons. The National Oceanic and Atmospheric Administration uses a similar process, but it forecasts only general trends, not a number of storms.
The government forecast issued in June called for "a strong likelihood of an above-average tropical storm and hurricane activity over the North Atlantic Basin." An updated forecast, also calling for a busy storm season, is expected next week.
Scientists base their predictions principally on meteorological forces that are remarkably similar to last year, when there were 14 tropical storms, 10 of which became hurricanes. Among those forces is La Niña, the atmospheric phenomenon that cools the Pacific Ocean and is the flip side of its cousin, El Niño. Gray's research has linked La Niña years to a substantial increase in hurricane activity.
Tropical storms are cyclones that are given names when their sustained winds reach 39 miles per hour. They turn into hurricanes when winds surpass 74 miles per hour. The tropical storm season really gets going in late August -- Gray rings a bell on Aug. 20 as his signal that it's off and running. Storm activity peaks, on average, about Sept. 9 and 10, and remains busy through October.
© 1999 The Washington Post Company