The 1997 El Niño: Potential effects in Montana - November 1997 to April 1998
Climate Prediction Center (CPC), NWS/NOAA
Updated 17 November 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 during moderate to strong El Niño episodes. Three types of products are included.
The first consists of U.S.-wide maps of average precipitation rankings during El Niño episodes for
the November-December and January-March periods. El Niño impacts vary with season. The second
product gives the change in precipitation probabilities by season. The Climate Prediction Center of
NOAA makes seasonal forecasts for precipitation probabilities in three categories: the wettest 1/3;
the near normal 1/3; the 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 than normal can be
dramatically changed by El Niño. The third set of figures converts the average rankings shown
earlier to per cent of normal precipitation and precipitation totals. These figures include what took
place in 1982-83 as a plausible scenario as to what might happen since the present El Niño is not an
average event. These two estimates give a range of precipitation totals that might be expected.
However, because of the probabilistic nature of the forecasts, the range could be greater.
Historically, moderate to strong El Niño episodes have featured drier than normal conditions (70%
to 90% of normal) over the entire state during November-March. During the 1982-83 episode similar
features were observed.