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Active 1999 Tropical Weather
Season Predicted Due to
La Ni�a
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The winter of 1998-1999 has been strongly influenced by the event known as La Ni�a. Its counterpart, El Ni�o, caused a strong and persistent west to east movement of weather systems across North America from 1997 to the first part of 1998. From late summer of 1998 to the present, La Ni�a has been allowing a more north to south, or south to north movement of weather systems across this continent. The result has been a repeating cycle of warmer than normal weather in many parts of the U.S., followed by occurrences of colder than normal weather. Associated with the alternating surges of the differing air masses has been abundant rainfall in many locales, as well as abundant snowfall across the central and northern Plains and the Midwest.

a4cc24i3.GIF (30565 bytes)The cooler than normal sea surface temperatures in the equatorial Pacific, otherwise known as La Ni�a, are expected to continue through this summer and into this fall. La Ni�a is expected to begin weakening during the autumn season, with an active 1999 tropical weather season predicted due to La Ni�a, with the equatorial sea surface temperatures returning to near normal levels by early 2000. Also significant are the warmer than normal sea surface temperatures in the western North Atlantic Ocean, which includes a much warmer than normal Gulf Stream ocean current off the east coast of the U.S. This has so far been associated with stronger than normal high pressure over the North Atlantic Ocean. Persistent high pressure to the east of the U.S. is also expected to continue through this summer and possibly into this fall.

A La Ni�a pattern during the Northern Hemisphere summer typically produces drier than normal weather in the plains states, a few, brief surges of cooler temperatures into the southern states, and increased tropical activity in the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. The expected increase in tropical activity includes tropical waves, tropical depressions, tropical storms and hurricanes. These disturbances typically move east to west along the southern edge of the North Atlantic subtropical areas of high pressure and then northward around the western edge. As a result, their exact movement this summer will be determined by the positions of these areas of high pressure, which are also known as anti-cyclones.

The present indicators tend to show an even distribution of tropical systems along the East and Gulf Coasts of the U.S. for landfall of any disturbance moving out of the tropics of the
Atlantic Ocean during this coming summer. However, not all tropical disturbances will affect the U.S. Many will continue moving westward into Central America and southeast Mexico and some will turn northward while still well east of the U.S. East Coast.

The forecast for the 1999 hurricane season calls for 12 to 15 named storms in the North Atlantic Ocean basin. Of these, seven to nine are expected to become hurricanes. A few of the tropical storms occurring this season are likely to be enhanced by non-tropical weather features. This weather scenario is most likely to occur early in the tropical season and again late in the season, or prior to
the summer solstice and after the autumn equinox.


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