August 20, 2010

Forecasts for this upcoming fall and winter are changing rapidly due to the building La Nina event in the equatorial Pacific. Based on current sea surface temperatures and model projections, this event is expected to be one of the strongest events measured since the 1950s. Current models indicate much of the Pacific Northwest, northern Rockies, and northern Plains should experience below normal temperatures and above normal moisture.

Closer to home, Nebraska is projected to lie on the southern fringe of the colder and wetter than normal area. It also is projected to lie on the northern fringe of the above normal temperature and below normal precipitation area indicated for the southern U.S. If this verifies, we will lie in the battle zone of air masses and temperature swings over short time frames (one week or less) can be expected, with an occasional ice and/or snow storm.

In the near term, current 30-day forecasts (Figures 1-2) for September do not indicate a defined temperature or precipitation trend for Nebraska. That is, there are equal chances of below normal, normal, or above normal temperatures and precipitation. The 90-day forecast for September through November does point toward above normal temperatures and above normal moisture for most of the state (Figures 3-4).

In my opinion, the 30-day and 90-day precipitation forecasts are out of sync with each other. September moisture normally accounts for almost 50% of the September-November precipitation total in Nebraska. Subsequent 90-day precipitation forecasts by the Climate Prediction Center (CPC) indicate equal chances for above normal, normal, or below normal moisture in the October-December and November-January periods.

Since CPC is indicating no defined precipitation trend in September, the only way to verify a wet 90-day forecast is to have October and/or November weighted toward the wet side. If that were the case, October-December and/or November-January should have also indicated a wet trend across the state.

Looking at the past few La Nina events, precipitation has generally been above normal during September, with much of the moisture occurring during the first half of the month. This was followed by below normal moisture from October through mid-November. Temperatures consistently averaged above normal.

The current pattern of upper air troughs beginning to build in the Pacific Northwest and high pressure over the southern and southeastern U.S. blocking the eastward movement of these troughs is beginning to resemble more recent La Nina events. If this trend develops, I expect good field drying weather in late September which should provide much better harvest weather than last year.

Al Dutcher
State Climatologist, Lincoln