September 21, 2011

September temperatures across the state have averaged 2-4 degrees below normal east of the Panhandle, with isolated pockets of 4-6 degrees below normal reported in central and eastern Nebraska.

Growing degree day (GDD) units for September are running 100-125 units behind normal through Sept. 20, but forecasts for the final 10 days of the month indicate that average temperatures will be 3-5 degrees above normal and cut these deficits by 30-50 units.

The primary factor influencing these colder than normal temperatures is a strong upper air trough that developed over the eastern U.S. Sept. 13-19. Frost and hard freeze conditions hit the eastern Dakotas, Minnesota, northern Iowa, northeast Nebraska, Wisconsin, and Michigan. Major damage to the soybean crop in Minnesota and north central Iowa has been reported, with initial loss estimates ranging from 25-110 million bushels.

Weather models are in strong agreement that an upper air ridge will dominate the central U.S. during the next 14 days and bring dry weather to the region. This should allow crops not recently impacted by freezing temperatures to reach full maturity. If this occurs, I would expect to see harvest activity across southern Nebraska to increase in intensity as the month draws to a close.

With La Nina conditions developing in the Equatorial Pacific region, current models are hinting at a repeat performance of the harvest weather experienced last fall. In other words, above normal temperatures and below normal precipitation are likely across Kansas, Oklahoma, and Texas. If the upper ridge remains into early October, below normal precipitation is likely for Nebraska, while areas across the eastern Corn Belt and Great Lakes could see above normal moisture.

90 Day Forecast: Above Normal Temperatures Likely

The official forecast for the October-December period from the Climate Prediction Center indicates below normal precipitation is likely across the southern Plains into the southwestern corner of Nebraska. A broad area of above normal temperatures is likely for the southern U.S., including the southern and central Plains, as well as areas of the central and eastern Corn Belt (Figures 1 and 2).

Normally in Nebraska 50% of the moisture for this three-month period occurs in October. If October comes in at less than 50% of normal moisture, November and December will need to average nearly double normal precipitation just to bring the three-month total back to normal. October is also a key month for building soil moisture profiles for next years’ growing season. Any moisture shortfalls will need to be made up in later months to reduce the likelihood that the severe drought conditions of the southern Plains don’t expand northward into Nebraska.

January-March Forecast

Long range forecasts for the January – March period indicate equal chances of above or below normal precipitation and temperatures for Nebraska, with below normal temperatures across the northern Plains dipping south to the northern Nebraska border region. To our south, above normal temperatures are projected for drought stressed Texas and Oklahoma. Precipitation is expected to be above normal across the Pacific Northwest and northern Rockies, which includes much of the upper North Platte basin (Figures 3 and 4).

If the Corn Belt does receive above normal moisture, spring planting delays could develop. This is not uncommon during a La Nina spring and will really depend on the length and strength of this event. History indicates that the second year of a La Nina is weaker than the first year and generally dissipates by late spring, but this is not guaranteed.

I expect that current drought areas from Iowa through Ohio will disappear by next April, while drought conditions may improve slightly across the southern Plains. However, it is likely that an extensive area of drought conditions will exist from Arizona east through Georgia by next April. If spring rains fail to materialize for Texas and Oklahoma, then rapid expansion into some portions of the primary Corn Belt would likely occur during the first half of next summer.

Allen Dutcher
Extension State Climatologist