PENNSYLVANIA Weather cycles from drought to drenching

A warmer-than-expected El Ni & ntilde;o made for a wet fall for Pennsylvania.
HARRISBURG (AP) -- About this time last year, the administration of Gov. Mark S. Schweiker was warning Pennsylvanians to conserve water and hinting that, without significant precipitation soon, summer could bring dead lawns and record low reservoirs.
It seems so long ago.
Those predictions turned out to be fairly accurate. But forecasts that autumn would bring only a little relief have fallen flat.
Instead, a very wet fall has filled up reservoirs and streams.
Source of precipitation
Much of that wet weather this fall has to do with the temperature of surface waters in the Pacific Ocean, meteorologists say.
The specific spot of the Pacific that correlates to the weather over the United States is about 160 degrees west on the equator, just east of the International Date Line. The spot has been warmer than usual over the past few months, said David Miskus, a meteorologist at the National Weather Service's Climate Prediction Center.
The warm water evaporates, releasing warm air into the atmosphere. The warm air interacts with colder air from the north and creates energy that results in stronger storm systems, Miskus said.
The temperature swing from a La Ni & ntilde;a event -- colder-than-average water -- to an El Ni & ntilde;o event -- warmer-than-average water -- typically does not range more than 12 degrees Fahrenheit, according to the center.
But the temperature has warmed more than predicted by the Climate Prediction Center and resulted in a stronger El Ni & ntilde;o event than expected, Miskus said.
Usually, weaker El Ni & ntilde;o storm systems blow horizontally across the southwest to the southeast and out over the Atlantic Ocean.
But because this is what forecasters call a "moderate" El Ni & ntilde;o event, the stronger weather systems have gathered up moisture in the Gulf of Mexico and then headed up the East Coast and into a gap that, in past El Ni & ntilde;o years, has usually been filled with a pocket of warm air, Miskus said.
That warm-air pocket, which generally covers the Northeast and Great Lakes, is smaller this year, allowing colder Canadian air down into the Northeast states to meet with the storm systems traveling up the East Coast, Miskus said.
Several big storms have pounded the Northeast, Pennsylvania included, and a number of other slow-moving storms have dripped moisture down over a period of several days.
The first sign that a wet fall might be in store was late September's Tropical Storm Isidore, which soaked eastern states from the Mississippi Delta to New England, Miskus said.
Normally, El Ni & ntilde;o suppresses the colder air to the north, resulting in a milder winter.
Not this year. Instead, Pennsylvanians saw a rare white Christmas.
Length of cycles
Although the El Ni & ntilde;o and La Ni & ntilde;a events typically last no more than a year, the most recent La Ni & ntilde;a cycle lasted much longer than that, perhaps up to several years, said Steve DiRienzo, a National Weather Service meteorologist in State College.
The sustained La Ni & ntilde;a, which tends to produce drier, colder weather, left Pennsylvania parched.
What causes the give-and-take between La Ni & ntilde;a and El Ni & ntilde;o? Meteorologists say it is unclear.
"No one's quite sure exactly whether the atmosphere drives the ocean or the ocean drives the atmosphere," DiRienzo said. "It's like the chicken and the egg."
To predict the La Ni & ntilde;a and El Ni & ntilde;o cycles, meteorologists at the Climate Prediction Center analyze historical data to spot trends and use computer models to show the movement in water temperatures documented continually by buoys, ships and satellites, Miskus said.
The data have resulted in predictions of warmer-than-usual weather across Pennsylvania in the coming months. In parts of western Pennsylvania, dryness is expected to set in, while the rest of the state should see enough continued wetness to help make the drought just a memory.