NOAA: U.S. could soon feel El Niño impact

Effects may show in summer; strength remains unknown

Map of 2002-2003 El Nino predictions

CAMP SPRINGS, Maryland (CNN) -- A developing El Niño could show its first impact on worldwide weather by mid-summer, but how extensive that impact will be remains uncertain, NOAA scientists said Thursday.

"This El Niño is still forming, and it's unclear now at what level of intensity it will be once it's fully developed," said NOAA Administrator Vice Adm. Conrad C. Lautenbacher. In past El Niño episodes, April and May were critical months in determining how severe the weather phenomenon's impacts would be, he said.

VIDEO CNN's Ann Kellan reports on the possible weather effects of an upcoming El Nino (March 7) Play video   EXTRA INFORMATION Notable effects attributed to El Nino in 1997-98    RESOURCES Fact Sheet: Charting El Nino

The El Niño phenomenon was named by Peruvian fishermen, who in some years observed changes in their fish catch at Christmastime -- the birth of 'El Niño,' the Christ Child.

El Niños -- which can last 18 months or more -- occur when the waters of the tropical Pacific heat up, triggering a near-global shift in wind and water currents.

A 1997-98 El Niño caused droughts and wildfires from Indonesia to the Brazilian Amazon basin, but brought extra rain and billions of dollars in flood and storm damage to the California coast.

NOAA operates a system of buoy monitors across the Pacific that provides detailed information on water temperature. Created in the 1980s after an El Niño episode took California by surprise, the buoy system gives early warning on emerging El Niños, as well as La Niñas -- a phenomenon in which the Pacific waters cool, and weather impacts are reversed.

La Niña years can frequently, but do not always, follow El Niño conditions.

Possible El Niño effects on the United States include:

The National Oceanic and Atmospheric Administration issues monthly updates on El Niño forecasts. In March it confirmed that an El Niño was forming in the Pacific, but its extent and timetable were uncertain.