General Observations
Another upper air low passed eastward across southern Canada this past week, a trend that has been in place since late June. As the low progressed eastward, pieces of energy rotating around the upper air low brought scattered convection across the state on a daily basis. At least one inch or more of moisture was recorded somewhere in the state 5 of 7 days between August 4th and 11th.
The problem with these heavier moisture events is that they rarely covered more than a couple of counties, with no one region getting fortunate enough to receive multiple rounds of storms. Pockets of 1-2 inches of moisture were reported across the southwest corner, southwestern Panhandle, and northeast Nebraska. A narrow strip of two inch rainfall, a half county wide, fell from the west central Sandhill region southward to the Kansas border. Another small pocket of 2-4 inches fell from Omaha to the Plattsmouth area.
Below normal moisture was observed for the remainder of the state. Precipitation totals for the week were generally in the 0.25-0.75 inch range, except for portions of central Nebraska that received less than a quarter inch of moisture. This area included south central Nebraska northward into the eastern Sandhill region, then eastward to Columbus. Towns within this general area include Columbus, York, Hastings, Grand Island, Kearney, and Central City.
On the University of Nebraska Extension Crops call, there were some reports of corntip back in central Nebraska. Looking at Figure 1, precipitation the past 90 days has been less than 10 inches over northeast and western Nebraska. This matches up well with the current dryness issues depicted by the U.S. Drought Monitor. But western Nebraska is a drier environment, so by using percent of normal moisture, two distinctly different climate regimes can be compared to each other.
Figure 2 shows the percent of normal moisture for the past 90 days, while Figure 3 shows the same metric for the past 60 days. If these two maps were placed as overlays on the U.S. Drought Monitor, the drought regions would match up with the 70% of normal moisture or less for the last 90 days. At the 60 day interval, the worst of the drought conditions are in areas receiving less than 50% of normal.
It is entirely possible, that soils are beginning to be depleted to the point that corn ears are beginning to abort kernels and showing tip back issues on the ear. Areas of less than 70% of normal the last 90 days and 50% of normal the last 60 days would be most likely to have tip back issues. If precipitation continues to be below normal through August, these areas are likely to expand across central Nebraska and tip back will intensify in areas already experiencing stress issues.
Crop Progress
According to the Nebraska Agricultural Statistics Service (NASS) pastures and soil moisture conditions declined this past week after several weeks of improvement. Although temperatures were not extreme, pockets of above normal moisture were limited to 1-2 county areas around Omaha, Norfolk, North Platte, Holdrege, and Hebron. Top soil and sub-soil rating in the very short to short category stood at 40%, an increase of 3 percentage points from the previous week. Pastures In the very poor to poor category increased from 9% last week to 19% this week.
Corn condition ratings improved one point from last week and currently stand at 89% good to excellent, while soybeans increased 2 percentage points to 81% good to excellent. Extension educators statewide indicate that the irrigated corn crop looks good, but dryland corners across the western 1/3 of the state look ugly.
The underlying question for central and eastern Nebraska is how soon will soil moisture losses from below normal precipitation these past 60-90 days begin to impact yields. Some tip back on corn has been noted by Extension Educators in central Nebraska, so it is possible that soil moisture declines are just now beginning to impact yields
Corn is advancing toward eventual maturity and continues to remain ahead of last year and the 5-year average. NASS reports that 67% of the crop has reached the dough stage, compared to 37% last year and the 5-year average of 51%. Soybeans are also ahead of schedule with 81% of the crop setting pods, compared to 62% last year and the 5-year average of 69%.
With a less than optimistic forecast going forward from numerical weather models, an expansion of dryness is probable for the central Platte valley and the western Sandhill region. Improvements across eastern Nebraska will depend on whether moisture develops from systems moving southeastward along the periphery of the upper air ridge. Unfortunately, the current GFS model runs do not show strong precipitation output from these opportunities.
Outlook
The weather outlook as depicted by the morning run of the August GFS model indicates that the upper air pattern will favor a continuation of upper air troughs moving into the northwestern United States then eastward through southern Canada and into the Great Lakes region. The first trough is projected to take this path over the next couple days, ushering in below normal temperatures beginning this weekend and holding through the 21st. The upper air ridge builds back into the region August 21-25th before another upper air low moves toward the northern Plains August 26-27.
Although no widespread precipitation event is forecasted statewide during the next two weeks, there still several opportunities for scattered convection that will be beneficial for those lucky enough to be caught under one or more of these events. The GFS models points toward daily pieces of energy rotating around the upper air low crossing south central Canada from August 11-15.
For Nebraska, the best chances are August 11 across the northern third of the state, August 12 across north central and northeast Nebraska, August 13 across northeast Nebraska, and August 15 across the eastern 1/3 of the state. Dry weather is forecasted from August 16-18, with high temperatures in the 70’s to low 80’s east, low to upper 80’s west.
From August 18-25, the upper air trough across the eastern United States is projected to break down and the upper air ridge over the southern Plains and southern Rockies expands northeastward. Dry conditions are forecasted across the eastern half of the state, with isolated to scattered convection possible across western Nebraska. High temperatures during this period should range from the upper 80’s to mid 90’s west to the mid80’s to low 90’s east.
Better precipitation chances return to eastern Nebraska as an upper air low begins to traverse the northern Plains and push cooler air into the central Plains August 25-26. If the GFS model is correct, temperatures would return back into the 80’s statewide and last through the end of month. Unfortunately, the prospects of a widespread storm system is not supported by the GFS and we will likely have to wait at least until September to experience a widespread drought reducing rain event
Al Dutcher at the Nebraska State Climate Office