March-April Showers Welcome, Not Likely to Eliminate Deficit - UNL CropWatch, March 15, 2013
March 15, 2013
Spring is fast approaching and it won’t be long before producers begin planting. While the devastating drought of 2012 is still fresh on everyone’s mind, there are glimmers of hope that the atmosphere soon may be generating robust precipitation events across the western High Plains region.
Extreme and exceptional drought conditions continue to plague Nebraska and surrounding states, but intense low pressure systems ejecting out of the western U.S. have provided welcome relief to significant portions of Texas, Oklahoma, and Kansas during the past six weeks. Some of this moisture extended into Nebraska during the past three weeks, but many more events are needed before the end of April to put a significant dent in our drought signature.
Figure 1. March 2012 departure from normal temperatures. The rapid warm-up brought average temperatures that were often 9 to more than 15 degrees above normal. (Source: High Plains Regional Climate Center)
Figure 2. Departure from normal temperatures March 1-12, 2013. The start of this March brought temperatures that were 2-6 degrees below normal for much of the state. (Source: High Plains Regional Climate Center)
At this time last March, temperatures were rapidly warming and native vegetation was beginning to break dormancy (Figure 1). After March 12, 2012, Lincoln failed to see its low temperature hit freezing for the remainder of the month and its high temperature never dipped below 50°F. Average temperatures ranged from 12°F to 15°F above normal. During the first half of March 2013, average temperatures were 2-6 degrees below normal east of the Panhandle and 1-2 degrees above normal across the Panhandle (Figure 2).
Expectations are that the cooler than normal temperature trend will continue through March. A very significant snowpack exists across southern Canada with a broad foundation of 20 plus inches from the Rockies east to the Atlantic Ocean. Last year, the only region of comparable snow depth was an area from the Hudson Bay region east toward Labrador. Air being drawn southward on the backside of the low pressure systems crossing the Rockies has tapped into the cold air over this deep snowpack, delaying the rapid warm-up experienced last March.
The temperature difference between southern Canada and the southern U.S. has been significant enough to energize low pressure systems emerging out of the central and southern Great Basin. As long as the snowpack remains substantial, I expect this trend to continue into early April, perhaps longer. Even though Nebraska was on the northern periphery of two intense snowstorms that hit Kansas and Oklahoma, the moisture that fell should enhance any additional systems that eject out of the Rockies during the next few weeks.
Weather models indicate that a very active pattern will continue through the end of the month. I expect this pattern will continue into April, but our weather models only go out to 16 days. The upper air pattern indicates an endless string of strong upper air lows flowing from the Gulf of Alaska southeastward into the central Plains. Nebraska continues to be in a favorable position to see several widespread, strong moisture events, each separated by a week without moisture.
We are going to need every single one of these projected systems to provide widespread moisture. It should be no surprise to anyone that the top 5-6 feet of the soil profile virtually had no available moisture at the end of the 2012 production season. Unfortunately, moisture was limited in October and November, leading to a poor start to the soil moisture recharge period.
Soil Moisture Profile Improving
The impact of recent storms is beginning to show in the High Plains Regional Climate Centers soil moisture monitoring network. Last weekend’s storm resulted in the top foot of the profile finally reaching field capacity in areas east of Grand Island and south of the Platte River. Most locations are beginning to see a response at the two-foot level; this improvement should continue as the remaining snowpack melts and infiltrates into the profile. Available soil moisture at field capacity for these locations would be in the range of two inches per foot.
Soil Moisture Levels
The best recharge is in an area from Nebraska City south to Falls City where available soil moisture ranges from 3.0 to 5.0 inches. Two additional areas have reached field capacity at a 12-inch depth: south central Nebraska around the Holdrege area and the northern Sandhills from Gordon southeast to Ainsworth. Almost every other site across the state has failed to reach field capacity at the 1-foot mark. It should be noted that all of these sites are under grass and could be under estimating cropland conditions by as much as one to two inches.
The rest of the state shows available soil moisture of 1.0 to 1.5 inches. All of this moisture is in the top of the profile, with all stations not located in wet meadows showing no available moisture at the 3- and 4-foot level. What does this mean? It is going to take incredibly wet conditions the next six weeks to get the top 3-4 feet of the profile to field capacity.
From now through the end of April we normally receive 4-5 inches of moisture across the eastern third of the state, 3-4 inches across the central third, and 2.5-3.0 inches across the western third. Receiving an inch of moisture a week through the end of April with an average infiltration rate of 70% would put 4-5 additional inches of moisture into the profile across eastern ebraska. That would return us to normal soil moisture recharge. This wouldn’t even address the cumulative hydrological impacts from the past 18 months.
What does all of this mean? In order to escape the crop damage experienced last year, timely rainfall will be required from now through June for dryland corn to hit normal yields. We need to build up soil moisture reserves to support several weeks of dry conditions during the peak heat of the summer. In a typical year, we would have enough stored moisture to support 20-24 days of peak corn ET (0.30 inches/day). Right now, moisture reserves at most locations, except for extreme southeast Nebraska, would support only 4 and 10 days of peak ET.
Precipitation deficits from last October to now are 2-3 inches across the northeast and eastern Sandhills, 1.5-2.5 inches across the central Platte River valley, and 1-2 inches across the southwest , western Sandhills, and the southern three-fourths of the Panhandle. For areas west of York, climate statistics from stations with more than 100 years of data indicate there is less than a 20% chance of eliminating accumulated precipitation deficits of 2.5 inches or more by the end of April. Not impossible, but not likely.
Extension State Climatologist