Heat Waves, Hurricanes Predicted for Summer in U.S.

By John Roach
for National Geographic News

May 22, 2003

Summary: Summer weather is notoriously difficult to forecast because there is no jet stream over the United States to stabilize atmospheric conditions. Nonetheless, meteorologists believe there are indications that this year it may be hotter than the recorded average for the southern part of the country while the Gulf and Atlantic Coasts of the U.S. may be battered by more hurricanes than normal. A lot depends on La Niña, the name scientists give to a sustained cooling of surface temperatures of the central and eastern tropical Pacific.

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As the season of long days and short nights, barbecues and cold drinks, and lakeshore and beachfront retreats is kicked off this Memorial Day weekend, forecasters train their gaze on charts and graphs as they attempt to predict the fickle summer weather.

"In summer we are not dealing primarily with a jet stream," said Mike Halpert, a meteorologist with the National Oceanic and Atmospheric Administration Climate Prediction Center in Camp Springs, Maryland. "It typically shifts northward into Canada."

The lack of a main jet stream over the United States makes the prediction work of Halpert and his colleagues difficult. In the winter months, the jet stream tracks storms across the country in predictable patterns that are influenced by long-term ocean currents.

"For summertime rainfall there are not any long-term trends to point us one way or another," said Halpert. Instead, unsettled summer weather tends to be dominated by isolated thunderstorms spawned by localized humidity and heat.

The only statement Halpert and his colleagues at the Climate Prediction Center are making about the general summer weather is that it will be hotter than normal from the Southwest eastward across central and southern Texas across the Southeast to Florida.

The reason cited is a "trend tool" that indicates temperatures in the South have been above normal during much of the past ten years, said Halpert. The center does not ascribe the warm trend to global warming, just a long-term trend that they expect to continue.

As for the Northeastern United States, Halpert said that he and his colleagues "don't have a good handle" on the weather forecast. All winter they were calling for warmer weather, but it ended up colder than normal. For this summer they are keeping their mouths shut.

Hurricanes

The one area where climate forecasters do have traction this time of year is hurricane season, which officially begins on June 1. The season does not usually rev up until the late summer and early fall. It officially ends November 30.

For the 2003 season, both the National Oceanic and Atmospheric Administration and the renowned forecast team at Colorado State University in Fort Collins led by atmospheric scientist William Gray are calling for "above normal" levels of activity.

Get ready for Ana, Bill, and Claudette. Watch out for Danny, Erika, and Fabian. Be braced for Grace, Henri, and Isabel. They, along with Juan, Kate, Larry, and perhaps a few other named storms may form in the Atlantic basin.

The NOAA forecast calls for a potential of 11 to 15 named tropical storms and the Colorado State team calls for 12 named storms. Of those, NOAA predicts that 6 to 9 will form into hurricanes, of which 2 to 4 will be classified as major.

The Colorado State team says 8 will become hurricanes, 3 of them intense with sustained winds of 111 miles (179 kilometers) per hour or greater. The long-term average is 9.6 named storms, 5.6 hurricanes, and 2.3 intense hurricanes per year.

The 2002 hurricane season saw 12 named tropical storms, but only 4 hurricanes, and 2 intense hurricanes. The 2003 season promises to be stronger, said Phil Klotzbach, a member of the forecast team at Colorado State University.

"Most importantly, the El Niño that put a damper on last season is gone now," he said. "Right now conditions are about neutral between El Niño and La Niña and it looks like conditions will go towards La Niña this summer, though it is difficult to say."

Additionally, the North Atlantic continues to be warmer than normal as part of a multi-decadal pattern. Klotzbach said that when this pattern existed in the 1950s and 60s, every year except for El Niño-influenced years were more active than normal.

According to the forecast by Klotzbach and his colleagues, there is a 68 percent chance that one of the major hurricanes will make landfall somewhere along the U.S. Atlantic or Gulf coasts.

Halpert, with the Climate Prediction Center, said "we expect to see more storms develop in the Gulf of Mexico and the Atlantic Ocean, but there is no way to say where those storms will go at this stage."

As a result of the more active hurricane season, Halpert says that the Atlantic and Gulf Coast regions should expect a wetter than normal later summer and fall.

La Niña Wildcard

The wildcard for hurricane forecasters is whether or not La Niña forms over the course of the summer. La Niña is characterized by unusually cold temperatures in the equatorial Pacific Ocean as opposed to El Niño's unusually warm temperatures.

"If you have La Niña, hurricane activity will be enhanced even further," said Klotzbach. The weather phenomenon reduces vertical wind shear in the heart of the hurricane development region.

NOAA says there is a 70 percent chance of La Niña forming. If it does, the influence will be most strongly felt next winter when it should result in drier and warmer conditions in the Southeast.

"In winter, patterns are somewhat more predictable. It looks like we will see a La Niña developing over the coming months, so we may be expecting a La Niña-influenced winter," said Halpert.