August 24, 2007

For Nebraska La Nina Conditions

Our Nebraska State Climate Office recently conducted a study of fall, winter and spring climate patterns that occur across Nebraska during La Nina. The state was divided by thirds into three zones — western, central and eastern. We found that past conditions have been significantly different from official National Weather Service Climate Prediction Center forecasts for the nine months of fall through spring.

There have been 10 La Nina events since 1950, five lasting less than one year, two lasting less than two years, and three lasting just short of three years. Of those 10 events, seven occurred in years where we transitioned from an El Nino to a La Nina event. This year will be the eighth such event. Since it is uncommon for climatic conditions to remain stable for two or three years, our examination concentrated on the first year of a La Nina that immediately followed an El Nino event.

Western Nebraska

During the fall, five of the seven previous events brought below normal temperatures to the region. Precipitation statistics were a little less favorable with only three of the seven events showing above normal moisture. Scottsbluff normally receives 2.73 inches during the September-November period. The two wettest events occurred in 1973 and 1998, with 6.32 and 4.37 inches, respectively. On the dry side, the 1964 and 1988 events came in at 0.18 and 1.54 inches.

If past La Nina winters are any indication for this year, wet and cold will be the norm. Five of the seven past La Nina events had below normal temperatures and five had above normal moisture. Scottsbluff normally averages 1.36 inches of moisture during the December-February period. The two wettest La Nina events were 1973-74 and 1964-65 with 1.91 and 1.55 inches, respectively. The two driest events were 1970-71 and 1998-99 with 1.06 and 1.15 inches. Even during the dry years, precipitation was less than 25% below normal.

What may be the biggest surprise in our analysis were the spring months of March to May. Only one out of the past seven La Nina events had above normal temperatures. In addition, four of the seven experienced above normal moisture. Scottsbluff normally averages 5.29 inches in spring. The two wettest La Nina events being 1962 and 1971 with 6.91 and 7.75 inches while the two driest events were 1984 and 1988 with 2.46 and 3.31 inches.

The wet/cool bias in spring is even stronger west and north of the Panhandle. The Panhandle region is on the eastern edge of a broad area of the Pacific Northwest that has a high statistical likelihood of experiencing cool/wet conditions during the winter through spring period during an ongoing La Nina pattern. If past trends are indicative of this winter, significant snowpack accumulations could be in store for the upper reaches of the Platte River system.

Eastern Nebraska

Weather conditions across western Nebraska during La Nina events following an El Nino event are not consistent with eastern Nebraska trends. Six out of the last seven La Nina events brought above normal temperatures to the eastern third of the state, with four of the seven coming in drier than normal.

What our statistics did show was that the three wet years were exceptionally wet. Omaha expects to receive 8.59 inches during the September-November period and the three wettest La Nina years averaged 10.80 inches. During the winter, only two of the seven La Nina events had above normal temperatures. In fact, exceptionally cold temperatures occurred in at least one of the three months during five of the La Nina years. In Omaha, one February and one December had temperatures for the month averaging less than 20°F, while four January's managed the same feat. Precipitation trended toward the dry side with four of the seven events recording below normal moisture.

During March to May, temperatures trended toward the cool side with four of the seven events showing below normal conditions. There was a slight wet bias with four of the seven events having above normal moisture. For Omaha, three of the four events recorded at least 12 inches of moisture, compared to the normal value of 9.51 inches. Only two events were significantly dry, 1974 and 1988 with 5.17 and 3.03 inches, respectively. The remaining event was less than an inch below normal.

Central Nebraska

Statistical analysis of central Nebraska climate records indicated a mixed signal in each of the three seasons. Temperature data indicated warm bias for fall temperatures, with a cold bias in winter and spring. The only precipitation bias was in fall when below normal moisture prevailed.

We believe the lack of precipitation bias in winter and spring is probably due to this region experiencing western Nebraska conditions during some La Nina events and eastern Nebraska conditions during the remaining events. Sometimes winter storms across the northern Rockies spill moisture far enough eastward to include central Nebraska, while typical winter storm tracks that impact eastern Nebraska prevail in other years.

Conclusions

It is evident that our analysis of past El Nino to La Nina transition events produced some remarkable results that are at odds with the official CPC forecast for the next nine months. Their expectations are for above normal temperatures throughout the period, while our analysis suggests that only the fall period shows a warm bias across the state. Analysis of past events indicates a cool bias in the winter and spring, with a pronounced cold bias in eastern Nebraska during the winter and in western Nebraska during the spring.

Our data also indicates a dry bias during the fall for the entire state, although when it is wet, it is exceptionally wet. This year's CPC fall forecast indicates above normal moisture across the Panhandle, with no apparent trend for the remainder of the state. According to our analysis, western Nebraska typically experiences above normal moisture during the winter and spring, most likely in the spring. For eastern Nebraska, there is a slight tendency toward drier conditions during in winter and spring.

If the upcoming nine months precisely follows past La Nina tendencies, decent soil moisture recharge may occur across the western third of the state. Because present soil moisture conditions are unusually wet across portions of central Nebraska, above normal moisture during the fall and winter would lead to a significant spring flooding risk. There is ample room to store additional moisture in eastern Nebraska soils. Even if a dry fall does occur, wet conditions during the late winter and early spring can easily erase sub-par fall moisture concerns.

Finally, take past climate data with a grain of salt. It can provide useful information for future cropping decisions, but is not set in stone. Only time will tell whether past trends will verify as we head through the next nine months.

Al Dutcher
Extension State Climatologist, Lincoln