November 21, 2013

 

Figure 1. U.S. Climate Prediction Center temperature forecast for December. 

Figure 2. U.S. Climate Prediction Center precipitation forecast for December.

Figure 3. U.S. Climate Prediction Center temperature forecast for Winter 2013-14.

 

Figure 4. U.S. Climate Prediction Center precipitation forecast for Winter 2013-14.

The December forecast, released Thursday by the U.S. Climate Prediction Center, shows Nebraska with equal chances for above, below, or normal precipitation and temperatures. However, based on recent trends, I wouldn’t be surprised if Nebraska sees below normal temperatures and normal to above normal precipitation in December.

Nationally, the CPC December forecast (Figure 1) is showing below normal temperatures for the northern half of Montana and North Dakota and above normal temperatures for Arizona, New Mexico, Utah, western Texas, western Oklahoma, southwest Kansas, and the southwestern two-thirds of Colorado.

The precipitation forecast (Figure 2) for December indicates below normal precipitation for Missouri, western Arkansas, western Louisiana, southeastern Kansas, Oklahoma, most of Texas, and eastern New Mexico. Above normal moisture is projected for the northern two-thirds of Idaho, northwestern Wyoming, and the western four-fifths of Montana. No precipitation trend was depicted for the remainder of the country, including Nebraska.

I have serious reservations about this December outlook and expect it to radically change when the forecast is updated in late November. Current weather models indicate a significant Arctic surge will occur the first week of December and drive cold air south into Oklahoma. If this occurs, models indicate that Oklahoma and the northern half of Texas could see two to three days of precipitation. These overrunning events occur when warm air south of the front gets lifted over the shallow cold layer near the surface. If this develops most of the area currently depicted as dry for December will likely receive a month’s worth of precipitation with this single event.

Additionally, over the past two months we have seen significant cold outbreaks occurring at regular intervals. Generally, the trend has been 10-14 days of below normal temperatures followed by equal lengths of above normal temperatures; however, the cold outbreaks have been slightly stronger than the warm outbreaks.

Here in the northern Great Plains, the cold air has won out, likely due to improved surface moisture conditions limiting the runaway surface heating that was common the past two winters when soil moisture was limited. I expect the below normal temperature forecast for December is very conservative and is likely to extend much further south and east of current projections. I believe Nebraska is likely to see below normal temperatures and normal to above normal precipitation in December.

The Climate Prediction Center also has released its Winter 2013-14 Outlook (December-February), which forecasts average temperatures and precipitation for Nebraska (Figures 3,4). Nationally, above normal temperatures are forecast from Arizona east through southern Alabama, with the highest probabilities assigned to the southern two-thirds of Texas. An additional pocket of above normal temperatures is projected for the northeastern US. Below normal temperatures are projected for the northeastern half of Montana, northern half of South Dakota, northwestern third of Minnesota, and all of North Dakota.

The winter precipitation forecast projects above normal precipitation for the northern half of Wyoming and all of Montana. Below normal precipitation is projected for Arizona, New Mexico, and western Texas in the southwest and an area south and east of a line from New Orleans, La., to Norfolk, Va.

For the past 18 months the CPC has had a fairly dismal record of accuracy with its three-month forecasts. This is not unexpected as its long-term outlooks are highly dependent on La Nina and El Nino events. When the equatorial region of the Pacific is in neutral conditions, their forecast accuracy is no better than a flip of a coin.

In reality, this winter’s forecast will likely depend on December's weather. If several potent winter storms develop during December and a solid foundation of snow blankets the Corn Belt before Christmas, it is likely to be a cold and stormy winter here in Nebraska.

Al Dutcher
State Climatologist