By Gary McManus, Associate State Climatologist Oklahoma Climatological Survey
Much as I have feared over the last month, long-range sea surface temperature computer models and atmospheric conditions are pointing more and more towards another possible La Nina event for this winter. Those factors and others have prompted the National Weather Service’s Climate Prediction Center to issue a La Niña watch. Remember, La Niña CAN (not DOES) mean a warmer and drier than normal winter in Oklahoma with a jet stream/storm track pushed farther to the north.
http://ticker.mesonet.org/archive/20110805/typical_la_nina_impacts.jpg A La Niña watch is very much like the tornado watches Oklahomans are so familiar with, except they look forward on a climate time scale. A La Niña watch is issued when conditions are favorable for the development of La Niña conditions within the next six months. Most of the long-range model forecasts expect neutral conditions to continue into fall. After that, however, the news is not as good. Here's a look at some of those model runs. The lines that go above 0.5C indicate El Niño, those between 0.5C and -0.5C depict neutral conditions, and those below -0.5C represent La Niña forecasts. http://ticker.mesonet.org/archive/20110805/models1.png From the CPC's own words: "Beyond the early fall, the forecasts are less certain with half of the models persisting neutral conditions continuously through early 2012. Along with a few other models, the latest runs from the NCEP Climate Forecast System (CFS) models predict La Niña to re-develop during the fall. This forecast is also supported by the ongoing La Niña-like tropical atmosphere, subsurface temperature trends, and the historical tendency for significant wintertime La Niña episodes to be followed by relatively weaker La Niña episodes the following winter. Therefore, neutral conditions are expected to continue into the Northern Hemisphere fall 2011, with neutral or La Niña equally likely thereafter." Here's a look at the CPC/National Centers for Environmental Prediction (NCEP) forecast model runs. Each line is a different model run with the dashed black line the mean of those forecasts. Again, those that dip below -0.5 are the ones of interest. http://ticker.mesonet.org/archive/20110805/CFSv2_models.png This is particularly bad news for the Southern Plains and Oklahoma since another drier-than-normal cool season could result. Last winter's La Niña was a primary factor in the development and persistence of the current drought and was timed perfectly to devastate the 2011 wheat crop. Another such episode would not be welcomed by agricultural producers, to say the least. The resulting U.S. precipitation forecasts give a hint of the possible dryness if La Niña does in fact return. http://ticker.mesonet.org/archive/20110805/US-precip-forecasts.gif The forecast dryness is not extreme during the winter months but then again that is the driest time of the year so there is less of a reduction possible. Notice in particular the intensity of the precipitation deficits forecast in western Oklahoma late next winter. Even if the dryness was not significant, ANY hint of drier than normal weather is terrible news given the state of the current drought. The statewide average rainfall total of 16.57" since October 1, 2010, is nearly 14" below normal and the driest such period on record in Oklahoma since records began in 1895. Let's cut to the chase. A return to La Nina is not certain. If it does happen, it's not a guarantee of a continuation of drought, just an increased likelihood. Other climate factors can have a say. However, it bears watching.
