Nebraska Weather a Little of Everything
March 7, 2008
Looking Back, Looking Forward
Review and Forecast
Temperatures - It was a red flannel winter
The current La Nina event in the equatorial Pacific continues to produce wild weather across much of North America, with expectations that the event will continue through spring and possibly repeat next winter. As winter slowly fades into spring, it is time to review the past three months of weather and what may be in store as the 2008 production season quickly approaches.
If you thought it was colder than usual this winter, you are correct. After an unusually warm period in October and through the Sunday before Thanksgiving, a sharp cold front arrived. During a 24-hour span, highs in the upper 70s across north central Nebraska dropped to lows near 0°F, nearly an 80-degree swing. This was the first signal of what was to come this winter.
For most of the state, monthly average temperatures averaged below normal during each of the winter months, with the most significant departures reported across the Panhandle and eastern fourth of the state. As a whole, winter temperatures ranged from 3-5 degrees below normal across the Panhandle, 2-4 degrees below normal in central Nebraska, and 3-6 degrees below normal in the eastern third of the state.
Even with this below-normal trend, there were several extended periods of warmth, including the first two weeks of January and February. Each warm spell was followed by significant Arctic air masses that dropped temperatures 10-15 degrees below normal for up to two weeks at a time. The end result is that mean winter temperatures averaged between 20°F and 25°F across the state.
Scottsbluff endured its fourth coldest winter in 110 years, while Omaha recorded its ninth coldest winter in 73 years. Other reports include Lincoln, 12th coldest of the last 42 years; Norfolk 12th coldest in the last 60 years; North Platte, 21st coldest in the last 60 years; and Valentine, 45th coldest in 118 years.
The cold temperatures increased energy demand this winter as measured by heating degree day units. In comparison to the 1971-2000 normals, Scottsbluff used 11% more energy; Omaha, 8.7%; Norfolk, 8.1%; Grand Island, 5%; Lincoln, 4.0%; North Platte, 3.6%; and Valentine, 2%.
The most significant month for precipitation this winter was December. Strong storms rolled out of the southwestern U.S. during the first three weekends, bringing rain, ice, and wet snow. A broad swath of central and eastern Nebraska received an average of 1.5 to 4.0 inches of moisture, or 200-500% of normal precipitation. Unfortunately western Nebraska was not as fortunate, with precipitation totals ranging from 0.75 to 1.50 inches, or normal to 200% of normal.
January and February precipitation bucked the December trend. Only three cooperative weather observer sites out of 220 locations reported receiving normal to above normal moisture in January, with most reporting 25-75% of normal moisture. February fared a little better, with 19 of 72 locations reporting above normal moisture. Portions of southeast, northeast, and north central Nebraska reported normal to 125% of normal moisture. The remaining sites were in the 60-90% range, with a few in southwest Nebraska at 30-60% of normal.
High Plains Climate Center soil moisture monitoring sites indicate an abundance of soil moisture for most areas east of a line from Holdrege to Lexington. In addition to the heavy December precipitation on unfrozen ground, much of this region experienced significant moisture last August and September. Available soil moisture in the top 4 feet of the profile is in a general range of 5-7 inches, which is exceptionally good considering average soils in this region will hold 8 inches of available moisture in a four-foot profile. Within this region and extending from Hastings to Grand Island to Central City soils are at 90% of field capacity, leaving little room for additional moisture. Available soil moisture across eastern Nebraska is at levels not normally seen until early to mid-April.
There is a rapid decrease in available soil moisture the further west one moves from the Holdrege/Lexington area. Soil moisture monitoring sites indicate 1-3 inches of available soil moisture in the top 4 feet, with the northern Panhandle in the worst shape. Areas around Chadron have about 1 inch of available moisture while the southern Panhandle has 1-2 inches. Two to three inches of available soil moisture is the general trend for southwest and west central Nebraska. Without good spring moisture these locations will be especially vulnerable to drought reemerging or strengthening as crop water demands increase with warmer temperatures.
Mountain Snow Pack — Reservoir Status
Western snow pack has responded well to the numerous large upper air lows that have traversed the region during the past three months. Most basins have had below normal temperatures for most of the winter. This has kept the snow pack in place rather than being lost to evaporation or runoff. On average, March and April are the two biggest snow months for western mountains, so a reduction in storm activity would be detrimental to a promising snow pack.
In terms of the Platte River basin, there is good and bad news. The good news is that the southern branch of the Platte should see normal to above normal runoff this spring due to a generous snow pack and normal to above normal reservoir levels in the watershed. The upper areas of the northern branch of the Platte are projected to have spring runoff rates that are 100-120% of normal.
The bad news is that the upper two Platte River reservoirs, Seminole and Pathfinder, stood at about 20% of capacity in mid-February. Their capacities are 30-34% of normal for this time of year. If normal precipitation falls through April, inflows to Seminole, but not Pathfinder, will be above normal. Pathfinder is fed by spillover from Seminole and drainage from the Sweetwater basin, which is projected to have 50-70% of normal flows given normal precipitation for the remainder of the snow season.
What does this mean for Lake McConaughy? Expect little if any contribution from Seminole and Pathfinder to Platte River flows. Runoff for Lake McConaughy would need to come from within the basin from just west of Casper to the headwaters of McConaughy. Currently stream flow rates are below normal, but ahead of comparable data from the same time last year. It is going to take a couple significant snow events to increase stream flow rates above the long-term average.
Lake McConaughy is definitely in better shape than at this time last year. Currently McConaughy is at 38.9% of capacity, compared to 33.3% at this time last year, and less than one foot below last yearï¿½s peak elevation set in May. McConaughy could reach 45% of capacity by May.
The strong upper air lows that have consistently rolled across the country during the past couple of months have led to frequent, large-scale severe weather outbreaks. Massive tornado outbreaks have occurred ahead of the lows, while significant snowstorms and an occasional blizzard have been the norm on their western flanks.
These events have stayed south of the Central Plains for much of the winter, but there has been a gradual movement northward. Since there has been no sign that the progression of strong upper air lows will not continue, this could be an exceptionally active severe weather season in April and May.
All of the key ingredients are in place. Temperatures have been consistently below normal across the Canadian prairie provinces. Warmer than normal temperatures have covered the southeastern U.S. most of the winter and strong low pressure systems continue to march across the lower 48 states. Unless significant upper air ridging develops across the inter-mountain west for an extended time (30-60 days) this spring, upper air lows will have the perfect environment to wreak havoc.
With the abundance of soil moisture across eastern Nebraska, any heavy precipitation event is likely to have a significant runoff component. Until Nebraska experiences several consecutive weeks of no precipitation and above normal temperatures, the flood risk will be greater than normal.
I fully expect some planting delays this spring, but it shouldn't be as bad as in the eastern Corn Belt. Much of eastern Iowa, Illinois, Indiana, and Ohio have received more than 6 inches of moisture since the beginning of the year.
Spring and Summer Outlooks
Official forecasts from the Climate Prediction Center indicate that in March through May most of Nebraska will have below normal precipitation and normal to above normal temperatures. In June through August most of the state has equal chances of receiving below normal, normal, or above normal temperatures and precipitation, except for the western fourth of the Panhandle where there is a tendency for above normal temperatures.
Looking at past La Nina events since 1950, my research indicates that March has a tendency for colder than normal temperatures coupled with below normal precipitation across the eastern two-thirds of the state. The western third has a tendency toward above normal moisture and warmer than normal temperatures. In April through May the tendency is for above normal temperatures and precipitation, especially in the western third of the state.
If the current La Nina event continues into the summer, the tendency is for drier than normal conditions and below normal temperatures June through August across the eastern two-thirds of the state. The forecast gets more complicated for the western third of the state. Statistics indicate June tends to be drier and cooler than normal, July is drier and warmer than normal, and August will be wetter and warmer than normal.
These forecasts are dependent on the La Nina event continuing into the summer. As an interesting side note, there is a tendency for drier than normal conditions over a large area of the central and eastern Corn Belt through most of the summer when La Nina conditions continue in the equatorial Pacific. If this occurs, then La Nina will generally restrengthen into the fall, resulting in a tendency for above normal temperatures and below normal precipitation for Nebraska and surrounding states.