El Nino to Play Role in Summer Weather

Meteorologists report that indications exist for a shift from a condition called La Nina to an El Nino, which will affect conditions during this summer season. La Nina is a situation that occurs in the eastern tropical Pacific Ocean, offshore Peru and Ecuador, when the surface temperature reads cooler than climatological norms. El Nino is just the opposite: the sea surface temperature in the same region warms up above what is considered normal.

El Nino events of the past few decades have varied in strength from strong to weak. An example of a strong El Nino event occurred from 1982 to 1983. As a result, very few tropical storms and hurricanes were noted in the summer of 1983 (see story on the 1997 hurricane season). The most significant hurricane occurring that summer was Alicia, which formed off the Louisiana coast, moved west and struck Galveston and Houston. The summer of 1983 brought above normal rainfall and slightly cooler than normal temperatures to the southern United States. It also produced near to slightly below normal rainfall and above normal temperatures over the northern United States.

We anticipate that the El Nino event expected for this summer will be much weaker than the 1982-1983 event, thereby having less influence on our weather. In addition, we foresee summertime high pressure to be a frequent occurrence in the eastern United States with low pressure occurring more often in the Central Plains and in the Desert Southwest. Further, we predict an upper air pressure trough to occur on a regular basis in the Rocky Mountains and in the Central and Northern Plains. This last feature is likely to force a pressure ridge or high-pressure area aloft to frequent the Western states this summer.

Near normal temperatures from Arizona to Georgia may also result this summer. High pressure aloft over the Appalachians and East Coast states will have a tendency to support above normal temperatures in the Eastern and Northeastern states. An overall tendency of near to below normal temperatures may occur in the Plains and Rocky Mountain states, mainly north of a line from Dallas to Albuquerque. Finally, the states out west can expect a warmer than normal summer.

Precipitation trends expected this summer include near normal rainfall in most parts of the United States. Above normal precipitation is likely from the Southern Plains through the Tennessee River Valley to the Appalachian Mountains. The summer monsoon in the Desert Southwest has the potential of producing above normal rainfall across this region as well.