OMAHA (DTN) -- The winter 2016-17 forecast issued by the NOAA Climate Prediction Center (CPC) this week took note of the projected influence of La Nina in the Pacific Ocean, stating that if we see a La Nina, it will be a weak one.
La Nina is the term used to identify the occurrence of widespread cooler-than-normal temperatures in the equatorial Pacific, along with jet stream patterns which feature a magnified northern jet stream and limited subtropical jet energy over the continental U.S.
The CPC forecast cites La Nina's influence in calling for above-normal precipitation in the northern tier of the U.S., as well as generally dry conditions in the Southern Plains through the Gulf Coast, along with most of the Far West. But, the Pacific conditions have not been clear-cut. The CPC issued a La Nina Watch back in May, but removed it from ocean-related commentary in September, only to reinstate the La Nina Watch earlier this month.
"If La Nina does develop, it looks weak," said CPC Deputy Director Mike Halpert in a conference call. "In some respects, we have actually gone against the model depictions based on La Nina."
The biggest change in the CPC forecast is in temperature outlooks. The western and southern two-thirds of the continental U.S. have above-normal temperatures predicted, with other areas showing "equal chances" of above normal, normal, or below-normal temperatures. But, especially in the northern states, that's a major difference from La Nina form. "La Nina would favor colder than normal in the northern tier," Halpert said.
Regarding the back-and-forth of issuing or removing a La Nina Watch, Halpert said the weak cooling tendency of the Pacific is responsible. "We had the watch in effect, and then by September the Pacific had relaxed back to normal. The dynamical (forecast) models had everything going flat," he said. "So, the watch was dropped. Then, the ocean changed and the models responded. This highlights the difficulty of predicting a weak event."
There is some question as to whether even a weak La Nina may develop. Eastern Pacific temperatures logged by DTN Senior Ag Meteorologist Mike Palmerino have actually been above normal for some time. "My September temperature reading was plus-0.3 (Celsius), and it's hung there through the first half of October," Palmerino said. "I don't think that you can ignore that warmth." Palmerino's region of emphasis is in the eastern equatorial Pacific; the CPC closely tracks the central Pacific equator region for its analysis of either El Nino or La Nina conditions.
The impact of a possibly weak or short-term La Nina is especially noteworthy for the recently dry Southern Plains. Forecast models are hinting at higher chances for precipitation in the Southern Plains during the early-November time frame. "That would actually be more of an El Nino-ish happening," Palmerino said.
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