March 27, 2009

The most current Pacific Equatorial seas surface temperatures point to the rapid demise of a weak La Nina event, which is the abnormal cooling of the sea surface between Peru and the international date line.  This represents the second consecutive winter of La Nina conditions.  This event is expected to end in April, although North America may see lingering impacts into early summer.

There have been 10 La Nina episodes since 1950, with seven lasting two years, and four lasting three years.  Current model projections indicate that La Nina conditions will give way to a possible El Nino (abnormal warming) as early as this fall.  During La Nina conditions the northern jet is typically stronger during the winter, while El Nino conditions strengthen the southern (tropical) jet.

La Nina Pattern and Spring Forecasts

Typical winter tendencies for La Nina periods include dryness across the southern third of the U.S., increased moisture in the Pacific Northwest, and more frequent Arctic intrusions into the northern Plains.  During El Nino events, the southern third of the U.S. usually experiences above normal moisture, while the Pacific Northwest is drier than normal and temperatures across the northern and central Plains are warmer than normal.

Statistical analysis of monthly temperature and precipitation tendencies following second year La Nina events indicate a tendency toward above normal temperatures and below normal precipitation from February through August over the western half of Nebraska.  The strongest precipitation tendencies will be in March and July, with August showing the strongest temperature tendency.

Eastern Nebraska has a weak tendency for below normal temperatures from February through April, with above normal temperatures from May through August.  Precipitation tendencies are for below normal moisture during March and from May through August, with April having a weak tendency for above normal moisture.  Strongest temperature tendencies occur during June and August, while precipitation tendencies are greatest in March and August

Official forecasts issued by the Climate Prediction Center indicate no temperature trend for April or April through June periods; however, below normal precipitation is projected for April through June for the western quarter of the Panhandle.  The remainder of the state has equal chances for below normal, normal, or above normal precipitation.  The forecasts project above normal temperatures for the western fourth of Nebraska for June through August. 

Drought Potential Still Looms

Using the outlooks in combination with La Nina statistics, there is an elevated risk for strengthening drought conditions across the western third of Nebraska.  At present, soil moisture values are sufficient to offset drought risk for the remainder of the state through mid-June.  If temperatures average above normal and moisture is below normal, drought could emerge later in the summer.

Al Dutcher
Extension State Climatologist