MIAMI (AP) _ The current El Nino weather pattern - blamed for disastrous Midwest flooding and hurricanes in the Pacific - has become the longest on record and may be strengthening.

And that could upset the world's weather for another year, scientists said.

El Nino, which starts with a temperature increase in the western Pacific, normally occurs twice a decade and lasts 12 to 18 months, University of Miami oceanographer Donald Hansen said Thursday.

The current El Nino began in early 1991 and was forecast to end in July 1992. But it's now almost 3 years old - and Pacific waters are heating up again.

''It's unlike anything we've seen among the El Ninos,'' Hansen said. ''Maybe it's not just an El Nino, but is really signaling that two or three years ago we had a relatively persistent change in the climate.''

Scientists caution that data on El Nino is limited. The phenomenon has been studied scientifically only since World War II, although meteorologists examining historical records have traced it back a century.

The current El Nino is by far the longest on record, said Gerald Bell, a National Oceanic and Atmospheric Administration meteorologist with the Climate Analysis Center in Washington.

''Pacific temperatures are already above normal and they are increasing again,'' Bell said. ''That's why we're putting out the flags now.''

Normal water temperatures near the international date line are 83 degrees. But in the last month, NOAA figures show an increase from 85 degrees to 86 degrees, a sign El Nino is not only continuing, but strengthening.

El Nino could still fizzle. Henry Diaz, a NOAA forecaster in Boulder, Colo., said current long-range forecasts predict a return to a normal winter climate. But those forecasting models erred in predicting El Nino would end in 1992, he cautioned.

''No one is sure just what the world is experiencing,'' Diaz said.

El Nino - Spanish for the Christ child, and so named because it often appears in South America around Christmas - begins with a warming of western Pacific waters believed to be part of a normal global climate cycle.

Prevailing wind blows the warm water east, where it generates wide areas of thunderstorms in the eastern Pacific. Those storms in turn disrupt the jet stream in the Northern Hemisphere, Bell said.

The results can be catastrophic.

''It was definitely a cause of the summer floods in the Midwest,'' Bell said. The 1993 floods did $10 billion in damage and killed 48.

El Nino tends to produce fewer Atlantic hurricanes, although Florida was battered by Hurricane Andrew in August 1992, Hansen noted.

But the phenomenon increases the number of hurricanes in the Pacific and shoves them east - a worry for Hawaiians and other island residents.

The weather pattern also has been blamed for California's rain-drenched winter last year. Generally, the West is drier and warmer in the winter, while the central and eastern United States has colder, wetter weather during El Nino.

Tim Barnett, a research oceanographer at the Scripps Institute in La Jolla, Calif., doubts this year's phenomenon is El Nino at all. He said he has found that oceans change temperatures slightly every few decades and theorizes that it's part of a gradual warming trend in the Pacific and Indian oceans that could last up to 20 years.

But NOAA's Bell said there's no evidence the Earth is witnessing a long- range climate shift.

''It's totally premature to conjecture,'' he said. ''Right now it just looks like a very stubborn El Nino.''