La Nina may persist through next winter

In the recent “ENSO diagnostic discussion“, a weekly must-read for any synoptic meteorologist, the Climate Prediction Center makes the case that La Nina conditions may persist through the fall of 2008 and possibly through winter 2008-09.

The current La Nina event has been classified as “strong” and represents the first strong La Nina since the 1998-2000 event and the second strong La Nina since the 1988-89 event. Like the 1998-2000 La Nina, the current event will probably persist through next winter, although as a weak-moderate La Nina. With my research into ENSO, I have found that multi-year La Ninas are more common than multi-year El Ninos. In fact, there have been three instances of three-year La Ninas (1954-56, 1974-76, 1998-2000) and only one three-year El Nino (1986-1988; although some scholars classify the 1991-92 and 1994-95 events as one weak El Nino from 1991-1995).

An oddity with this particular La Nina event is that thus far this winter there is no dipole in precipitation over the western United States. When a strong La Nina coincides with the cold phase of the PDO, there is usually a sharp precipitation gradient over the western United States where the Pacific Nortwest is very wet (>130% of normal) and the Southwest is very dry (<70% of normal). This winter, the entire western United States has experienced above normal precipitation to the point that for the first time since 1996, there is no place west of the Rockies that is experiencing extreme or exceptional drought (according to the U.S. Drought Monitor categories). In fact, the CPC seasonal drought outlook for Feb-Apr shows very little drought anywhere in the United States this spring. Winters like 2007-08 make seasonal forecasting that much more difficult when such a strong predictor of winter-season precipitation fails to match historical correlations.

If La Nina does continue through the summer and the SE ridge continues to be the dominant synoptic feature over the eastern United States, I would predict that a hot and at times dry summer should be expected over the Mid-South. While the drought over KY was ameliorated by abundant fall and winter rainfall, the drought in the Southeast will continue through the spring, which means that water managers across KY should be concerned about how the weather patterns evolve this spring.

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