September 24, 2010

In the following story series Extension State Climatologist Allen Dutcher outlines the weather trends forecast for fall, winter, and spring and how the current La Nina may affect December-January snowstorms in Nebraska and possibly lead to dry conditions in summer 2011.

La Nina Conditions

Pacific Equatorial sea surface temperatures cooled dramatically during the past three months and La Nina conditions are now firmly established. The extent of the cold pool now encompasses the region from South America westward through the dateline with the depth of the cold pool extending over 200 meters in the central Pacific. Numerical models indicate further strengthening of this event through the end of the year.

The significant increase in hurricane activity during September within the Atlantic basin can be directly attributed to this unfolding event as waves exiting western Africa and moving into the eastern Atlantic are encountering reduced wind shear aloft. During the past three weeks, virtually every wave exiting the African coast has formed into a tropical system. Four systems have already obtained major hurricane status and this trend will likely continue through September.

These hurricanes currently developing southwest of the Cape Verde Islands have moved west-northwest, eventually curving northward and following a path between the eastern seaboard and Bermuda. As we progress through September, hurricane formation will likely occur further westward into the central equatorial Atlantic and increase the likelihood that the hurricane track will bring storms into the southeastern U.S. and/or Gulf of Mexico.

Hurricane climatology would suggest that tropical storm and hurricane formation will shift from the eastern Atlantic to the western Atlantic, Caribbean Sea, and Gulf of Mexico. Any systems moving into the eastern Gulf of Mexico will encounter above normal sea surface temperatures and hurricane strengthening could be explosive, similar to what was experienced five years ago when Katrina and Rita obtained major hurricane status.

Drought conditions have gradually increased across portions of the southern and southeastern United States during the past two months, with significant dryness now spreading north into the eastern Corn Belt. It is expected that much of the southern Plains and southeastern U.S. will continue the trend of below normal moisture and above normal temperatures through much of the fall and winter. It is possible that one or more tropical systems could impact the region through early November and bring a temporary respite from the persistently dry weather.

See Fall Forecast

Also

• Winter Forecast
• Spring Forecast