Updated 2 December 1997

We are currently in the midst of a strong El Niño (warm) episode, which is forecast to continue through February-April 1998. During this period the tropical ocean surface temperatures are forecast to remain comparable in magnitude and areal extent to that of 1982-83, which is considered to be the strongest warm episode of this century. In contrast to the 1982-83 El Niño, which caught the country by surprise, the present El Niño was predicted several months in advance. This improvement in climate prediction is the direct result of intensive research efforts by NOAA and its partners during the last 15 years. Part of this research effort, which is still ongoing, has been devoted to determining the effects of El Niño on temperature and precipitation patterns in the U. S. and globally. Some results of this research for your state are discussed below.

The information on state impacts is derived by looking at what has happened in those years during the past century that featured strong El Niño episodes. Four products are included for every period for which effects are highlighted. El Niño impacts vary with season. The first contrasts average conditions during El Nino episodes with what is expected in general. These figures for precipitation also include what took place in 1982/83 as a plausible scenario as to what might happen, because the present El Niño is at least as strong as any on record. These two estimates give a range of what might be expected. However, because of the probabilistic nature of the forecasts, the range could be greater. The second gives the change in probabilities for the respective variable and period. The Climate Prediction Center of NOAA makes seasonal forecasts for temperature (precipitation) probabilities in three categories: the warmest (wettest 1/3); the near normal 1/3; the coldest (driest) 1/3. The probability of getting any one of these three categories would be equally likely in the absence of El Niño. Probabilities of being wetter or drier or warmer or colder than normal can be dramatically changed by El Niño. The third and fourth products for each period are U.S.-wide maps of respectively (1) average temperature or precipitation rankings during El Niño episodes and (2) corresponding probabilities of the three categories. These maps place effects at the state level in a broader context.

Historically, strong El Niño episodes have featured below normal precipitation (about 80% of normal) during January-March. During the 1982-83 episode much drier than normal conditions (up to 6 inches below normal) were observed during January-March. Pre-Winter (November and December) has tended to be warmer than normal for all of Tennessee, with the probability of temperatures for the period ranking among the warmest 1/3 of the 102-year record increasing steadily from west to east across the state.