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.