|
| |
|
November 4, 1999 DES MOINES -- This winter, Iowans can expect some extra work with their snow shovels and snow blowers. Iowa is expected to receive more snow than usual this winter as a result of La Nina, a phenomenon in the Pacific Ocean that affects global weather patterns. Harry Hillaker, state climatologist at the Iowa Department of Agriculture, predicts October through December will be slightly warmer and drier than usual, but January through March will be colder and receive more precipitation than usual. He said previous La Nina winters have exhibited similar trends, and although this winter will follow the typical La Nina trends, every winter has its own unique weather patterns. "It's the best guess," Hillaker said, "but it's not a guarantee of what we're actually going to get." La Nina usually affects each season differently in Iowa, he said. In general, fall and spring weather is slightly warmer than normal with slightly less precipitation than normal. On the other hand, summer and winter weather is more extreme, with colder temperatures and more precipitation than average in the winter, and warmer temperatures and more precipitation than average in the summer, Hillaker said. The amount of snowfall January through March will be affected not only by increased precipitation but by lower temperatures, he said. Rainfall received during warmer winters will fall as snow during colder winters, increasing the snowfall levels in colder winters. Although there will be less snow than average from October through December due to slightly less precipitation and warmer temperatures, the large amounts of snowfall predicted for January through March will probably lead to slightly higher than average snowfall levels for the winter as a whole, Hillaker said. "[La Nina] very strongly supports more snow than usual," he said. La Nina occurs when trade winds in the Pacific Ocean push the warm tropical water from the west coasts of North and South America west to the east coasts of Asia and Australia, Hillaker said. As a result, colder water moves from the ocean bottom and the area surrounding Antarctica to fill the space of the warm water near the west coasts of North and South America. Although La Nina affects the Pacific Ocean most directly, it also affects the movement of weather systems across the world, including Iowa, Hillaker said. "The Pacific Ocean is such a big part of the Earth's surface, and it has a tremendous impact on the weather," he said. Central Iowa received more snowfall than usual last winter Ñ also a La Nina winter Ñ Hillaker said. Des Moines received 43.8 inches of snow last winter, compared to the average of 32 inches. Snowfall is measured from the first snowfall in the fall, which generally occurs around Nov. 1, to the last snowfall of the spring, which usually occurs around April 15, he said. La Nina will probably lead to above average precipitation in the northern third of the nation, Hillaker said. In particular, the pacific northwestern part of the United States will probably see a significant increase in the amount of precipitation because of its proximity to the Pacific Ocean, he said. "In general, with La Nina, the northern third of the United States might be a little wetter and colder, but the southern two-thirds of the United States will probably be warmer and drier," Hillaker said. Hillaker also said it is rare for La Nina to last two winters, but because La Nina usually intensifies in the winter, the current La Nina will probably endure until the end of the winter, if not longer. "It is unusual for them to last as long as this one," he said. La Nina was preceded by El Nino, a weather pattern with climate trends opposite those of La Nina, Hillaker said. During El Nino, there are no trade winds over the Pacific Ocean, and the warm water near the west coasts of North and South America remains in place and grows warmer, he said. As a result, most of the nation experiences weather patterns under El Nino that are stormier than usual, Hillaker said. Although La Nina and El Nino have been present over much of the past few years, there are occasions when neither condition exists, he said. "In reality, a majority of the time, there is neither one," Hillaker said. Because La Nina and El Nino are persistent weather patterns, it is easier to predict weather trends under La Nino or El Nino than under neither pattern, he said. Even under La Nina, however, each season has its own unique characteristics. "[Predicting the amount of snowfall is] never an easy question to answer," Hillaker said. |