La Nina Developing over the Pacific Ocean

La Niña Developing across the Pacific Ocean
The National Oceanic and Atmospheric Administration’s Climate Prediction Center recently announced that weak La Niña conditions have developed in the tropical Pacific Ocean. La Niña is the periodic cooling of ocean waters in the east-central equatorial Pacific, an event which recurs approximately every three to five years and impacts the typical alignment of weather patterns around the globe.  Scientists estimate La Niña conditions will remain in place for at least three to six months, but state it is too early to predict the full impact of this weak event.
Historically, La Niña has been associated with below normal precipitation in the central High Plains, but appears to have little impact on temperature.  However, taking into account the strong trend of above normal temperatures in the past decade associated with large scale climate change distinct from La Niña, temperatures will likely continue to be above normal. As a result, the forecast of a weak to moderate La Niña means an enhanced risk that drought conditions will re-develop in the Tri State area in the coming months. The official Seasonal Drought Outlook issued by Climate Prediction Center on January 19, 2006 placed all of the Tri State area under a “drought likely to develop” classification. This outlook is valid through April 2006. Drought conditions have already developed over large portions of Arizona, New Mexico, southern Colorado, north Texas and Oklahoma, and may expand as far north as Nebraska in the next few months.
It is important to remember that long term weather forecasts are inherently probabilistic. Although below normal precipitation is the most likely outcome in the months ahead, it is still possible that near normal or even above normal precipitation will fall. A review of the historical record in Goodland, Kansas during past moderate to strong La Niña years will serve to illustrate this point. La Niña conditions occurred in 1945, 1950, 1956, 1974 and 1999. Of those years, four were below normal in precipitation and one was above normal. The above normal year occurred with the most recent La Niña of 1999 when Goodland received 20.29 inches of precipitation, nearly an inch above normal.  However, on June 10-11 in that year, Goodland received over four inches of rain from slow moving thunderstorms which produced nearly a quarter of the yearly total.  If not for that one event, 1999 would have ended below normal in precipitation as well.  Typically, the Tri State area receives most of its annual rainfall from thunderstorms during the spring and early summer months.  Thunderstorm coverage and intensity is very difficult to forecast even a few days in advance, and impossible at very long time ranges.  However, if a particular location happens to be under a slow moving and intense storm, such as occurred in Goodland in 1999, it can skew an entire year’s worth of rainfall.  It is because of this difficulty in forecasting thunderstorms that near or above normal precipitation must still be considered a possibility, just not the most likely one in typically dry La Niña years.

These charts show that below normal precipitation becomes more likely in the spring months of a typical La Niña year in western Kansas. Nearly 60% of the periods covered by April-May-June (AMJ above, first box, second row) were below normal during La Niña events. However, by the fall, La Niña influence typically wanes and there is a nearly equal chance of above, normal, or below normal precipitation, as shown in the September-October-November box (SON) in the last row. The data has been adjusted to account for long term climate trends not directly attributable to La Niña.

The above charts show that La Niña has little influence on temperatures in western Kansas . Some months show below normal temperatures as the most likely to occur (FMA, MAM, JAS). Others show above normal as the most likely (OND, NDJ) while still others do not favor any particular category (JFM, MJJ, JJA). If the whole year is averaged, below normal temperatures are slightly favored. The data have been adjusted to account for long term climate trends not directly attributable to La Niña.

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