Water in the tropical Pacific is cooling off, and scientists at the national Climate Prediction Center say that's a big factor in their forecast of a cooler winter than Baltimore had last year.
As cold winds and possibly the first Garrett County snows bore down on the state yesterday, Ed O'Lenic, head of operations at the center in Camp Springs, ventured his agency's prediction. He said the Northeast and mid-Atlantic states should average a few degrees above normal, but still cooler than last winter's record balminess. Precipitation should be close to normal.
If you're talking snowfall, "normal" in Baltimore would mean about 22 inches, or nearly three times last winter's paltry 8.
Mr. O'Lenic is braced to take the heat if he's wrong. "It's a dangerous profession at times, because you're accountable for these things," Mr. O'Lenic said.
But his level of confidence is higher than usual this year because his forecast computer models, for the first complete winter season, include data on the behavior of El Nino, a complex pattern of changes in air and ocean temperatures in the tropical Pacific.
Scientists have been studying the warming and cooling of these waters and experimenting with the data for more than a decade. They have gradually come to recognize the fluctuations can have profound and fairly predictable effects on weather in the United States and around the world.
It was the Spanish who first noticed changes in Pacific Ocean temperatures and fisheries off South America in certain years. The warm, fish-poor waters seemed to arrive at Christmas time, and they named them for the Christ child, or "El Nino."
Scientists today refer to the warm phase as El Nino, and to the subsequent cool phase as the opposite gender child, "La Nina."
"When the ocean temperatures are warmer than normal, it causes a stronger-than-normal sub-tropical jet stream, and that causes increased storminess across the southern part of the country," Mr. O'Lenic said. That also means heavy precipitation across the South and warmer-than-normal conditions over the northern part of the United States.
The "moderately strong" warming of the tropical Pacific a year ago was held partly responsible for the record warm winter in the Northeast. All but an inch of last winter's scanty snow fell in a single February storm. There were highs in the 60s and 70s in December and January.
When the tropical Pacific cools, winter weather in the Northeast is often cold and stormy, although other factors can and do intervene to complicate any direct cause-and-effect relationship.
This year, Mr. O'Lenic said, the tropical Pacific is "slightly cooler than normal. It's a very, very weak anomaly however, and the atmosphere above the anomaly is not really responding the way it would during a strong La Nina."
That scenario, he said, translates into "less warmth" than last year in the Northeast and mid-Atlantic, but still a few degrees above normal. For precipitation, the forecast means "relatively dry conditions over parts of the northern U.S. but no indications one way or the other" for the mid-Atlantic.
That means about normal precipitation. It "doesn't preclude the occurrence of real winter-type weather," Mr. O'Lenic said. It just means that when you average out all the good and bad weather at the end of the season, it comes out "normal."