December 21, 2011

As we enter winter, it’s time to analyze whether this fall’s climate trends are likely to continue through winter (Dec-Feb) and what regions of the country are susceptible to production problems in 2012. This is the second consecutive winter with La Nina conditions in the Equatorial Pacific and there appear to be as many dissimilarities as similarities to the same time in 2011.

Review of National Weather Trends

During fall 2011, drought began to rapidly develop across the southern U.S. Texas bore the brunt of drought which developed into one of, if not, the worst agricultural disasters in history. This drought has now expanded northward and includes Oklahoma and the southern two-thirds of Kansas.

In contrast, the eastern Cornbelt was extremely wet from fall 2010 to spring 2011, leading to planting delays and poor root development. The shallow rooting depth left crops vulnerable to dry conditions during the summer, resulting in significant production declines from Illinois east through Ohio. This fall excessively wet conditions redeveloped and are forecast to continue into spring.

The northern and western Cornbelt turned excessively dry during late August through mid-November. This mostly duplicated conditions a year earlier, with one exception. Heavy rainfall that dropped up to 7 inches of moisture across southern Minnesota and northern Iowa in fall 2010 wasn’t repeated this year. A broad area of southern Minnesota, northeast Nebraska, eastern South Dakota, and northwest Iowa has received less than a quarter of its normal precipitation during the past 90 days.

This fall the Dakota’s didn’t receive the abundance of snow they did in early 2011 when record runoff led to  flooding in the Missouri River basin. At this point, many locations across eastern Montana and western North Dakota have not received two inches of snow this winter. By this time last winter, 20+ inches of snow was the norm and the average snow depth ranged from 10 to 20 inches.

With the redevelopment of La Nina conditions this fall, there was considerable banter between climatologists as to whether 2011 winter trends would return in force this winter. So far the answer is no. First and foremost, equatorial sea surface departures are averaging about 1.5° C below normal, while last year  the basin was nearly 3° C below normal. The current La Nina is rated as a weak to moderate event, compared to last year’s rating of exceptionally strong.

Because La Nina was so strong last year, the northern jet stream was especially active and winter storm activity was concentrated across the northern and north central Rockies eastward through the Great Lakes and northeast. In the last 30-45 days the jet stream has tended toward a split flow pattern with a portion of the energy moving into the northern Rockies and northern Plains and the remaining energy diving down into the southwest and the southern Great Basin.

What does this really mean? Essentially, the upper air lows moving across Texas are keeping moisture from moving north into the northern plains. As a result, the upper plains moisture patterns will depend on moisture from the Pacific Ocean rather than the Gulf of Mexico.

What it Means for Nebraska

Nebraska has been caught between these two pieces of energy. If the low in the southern plains begins to lift northeastward before the northern Plains trough arrives, rain and/or snow could develop. If the northern Plains trough moves in first, a dry cold pattern will develop and significant moisture will remain south of Nebraska. The two competing pieces of energy eventually merge east into a strong upper air trough east of Nebraska, likely resulting in more rain/snow for the eastern Cornbelt.

As long as this pattern continues, areas of southwest through east central Nebraska will likely have the best opportunity to receive normal to above normal moisture. Unfortunately, north central and northeast Nebraska will likely be dry and, barring a significant snow storm prior to the ground freezing, moisture infiltration into soils will be limited.

Without significant snowfall across the plains of Montana and the Dakota’s, I would rate the flooding potential for this spring as low. While the northern Rockies are getting snow, it’s not concentrated in a single region as it was last year. There currently is no runoff potential with the existing northern Plains snow pack, but this could certainly change before the spring melt begins.

Three-month Forecasts

The latest Climate Prediction Center (CPC) forecast for January-March indicates above normal temperatures for the southern Plains (Figure 1), extending north to the extreme southeastern Nebraska. The precipitation forecast indicates below normal moisture for the southern plains, extending north to the Kansas-Nebraska border (Figure 2), and moisture for the northern Rockies and eastern Cornbelt.

Nebraska precipitation forecasts for February-April (Figure 3) and March-May (Figure 4) are more worrisome. Below normal moisture should cover the southern and central Plains region, extending north to include much of Nebraska. This scenario would suggest that the northern Plains dryness is not likely to disappear until April or later. Nebraska climate data also suggests drier than normal moisture (less than 50% of normal) in February and March.

It also suggests that the eastern Cornbelt will remain wet and flooding and/or planting delays likely will occur this spring in the Ohio and mid-Mississippi River valleys. Closer to home, the current snow pack is rated normal to below normal in the headwater region of the Platte and Missouri watersheds because most of the recent snows have been across the southern Rockies.

There is enough reservoir storage space in these two watersheds to handle normal to slightly above normal snowfall without major floods; however, if snowstorms pick up, this could change. But for right now, I would rate the flood potential as average to below average.

Al Dutcher
Extension Climatologist