General Observations
A little taste of fall was the common theme across much of the corn belt this past week as a strong cold front worked its way across the nation’s mid-section. High temperatures across Nebraska dropped from the upper 80’s to mid 90’s early in the week to the high 70's to low 80's after the cold front moved east of the state from July 30 - August 4.
As one would expect with the passage of such a strong cold front, localized heavy precipitation was recorded, especially for counties bordering Kansas. The most widespread precipitation event this past week occurred the evening of August 29th through the morning of August 30th with coverage south of I-80 ranging from 0.25 inches up to four inches. One to three inch totals were reported across the southwest and southeastern corners of the state, with 2-4 inches recorded in south central Nebraska.
The remainder of the state received less than normal moisture during this past week as Figure 1 indicates. Surplus moisture was concentrated in pockets of southeast, south central, and southwest Nebraska. Another very small pocket of 1-2 inch totals fell across extreme northeastern Nebraska. It is highly likely that the heavier events across southwest and south central Nebraska will lead to improvements on the U.S. Drought Monitor with this week’s release. Certainly, the below normal temperatures this past week as indicated by Figure 2 helped ease crop water demands and improve the efficiency of precipitation events.
Figure 3 shows this week’s U.S. Drought Monitor Nebraska depiction for Nebraska based upon data through the morning of July 26th. Improvements were limited to southwestern, south central, and east central Nebraska. Portions of Dundy county were reverted to D0 (Abnormally Dry) in the aftermath of 6-12 inches of moisture that fell across the western half of the county. Surplus moisture fell along the Kansas border in south central Nebraska, reducing D1 (Moderate Drought) to D0, while a small sliver on the western fringe of the D0 in east central Nebraska was removed.
Unfortunately, degradation on the U.S. Drought Monitor’s depiction for Nebraska largely focused on areas of the state that have shown continual weekly declines over the past month. A small pocket of the southeastern Panhandle was upgraded to D2 (Severe Drought), as well as an expansion of these same conditions across the northwestern Panhandle region. The area of D1 (Moderate Drought) across south central Nebraska that was improved on its southeastern flank was expanded westward to include the Kearney to Lexington region largely based upon precipitation deficits accumulated over the past 60 days.
Crop Progress
According to the Nebraska Agricultural Statistics Service (NASS) pastures and soil moisture conditions continued to improve slowly, large due to favorable moisture events across southwest and south central Nebraska during the past month. Top soil moisture improved slightly with 37% rated short to very short, a decrease of 4 percentage points from last week. Subsoil moisture improved 2 percentage points from 38% to 36% short to very short.. Pastures were rated 9% very poor to poor, an improvement of 5 percentage points from last week’s release.
As the month turns to August, grain fill becomes the focal point going forward. July is ruled by corn pollination and soybean blooms, but preliminary temperature data from the Nebraska Mesonet suggests that extreme high temperatures during the month were not conducive to widespread pollination issues. In fact, the core area of extreme heat fell across the western 1/3 of the state during the mid-month period, just prior to reaching the silk stage. Even then, high temperatures peaked out in the mid to upper 90’s and only lasted a couple of days before dropping back into the 80’s.
The biggest pollination issue during July was likely the result of the intense wind event the evening of July 7th through the morning of July 8th when straight line thunderstorm winds of 70-95 mph hit a large area of south central and southeast Nebraska. General green snap coverage was reported between 2 and 20%, approaching 85% across portions of Clay county.
According to NASS, 43% of the corn crop had reached the dough stage, compared to 23% last year, and the 5-year average of 31%. Soybeans setting pods reached 64%, compared to 46% last year, and the 5-year average of 53%. As has been the case the entire growing season, crop development for corn and soybeans has outpaced last year and the 5-year average. With normal temperatures going forward, corn maturity would be 10-14 days ahead of last year and 7-10 days ahead of the 5-year average.
Outlook
As we enter the month of August, numerical weather models are not running consistent with their daily outlooks, especially more than a week into the future. During the past week, the GFS has pushed the idea of above normal temperatures, below normal, and back to above normal temperatures the second full week of August. This changeable forecast brings considerable uncertainty into the forecast for the next two weeks.
The current GFS 500 mb run suggests that the upper air trough over the Great Lakes that brought below normal temperatures to the region will weaken and the jet stream will take more of a zonal flow across the central United States. At the same time, another upper air low will move slowly from the Pacific Northwest to south central Canada from August 4-13. Nebraska is currently forecasted to be on the southern flank of these westerlies and on the northern flank of the southern Plains upper air high pressure.
Temperatures are projected to return to the upper 80’s northeast to low 90’s August 6-10, before cooling slightly in response to the upper air low moving across southern Canada on its way to the Great Lakes region. If this system doesn’t pull significant cold air southward into the Dakota’s, then our return to the upper 80’s and beyond will likely continue through the end of the forecast period, which is August 20th. The GFS model currently cools Nebraska back into the low to mid 80’s August 10-13, before trying to build the southern Plains ridge northward into the central Plains August 14-20.
Under the changeable jet stream pattern seen this past week from the GFS model, precipitation events forecasted going forward carry a high degree of uncertainty. Almost every day into forecast period out through August 15th carries a chance for moisture, outside of August 8-9 which is depicted as dry. The most widespread coverage is assigned to the period of August 4-5 across western Nebraska, August 6-7 across eastern Nebraska, and August 10-12 statewide.
If the extended GFS outlook is correct, then the period from August 14-20 will see the southern Plains and Rocky Mountain high pressure aloft expand northeastward into the central corn belt. This air mass would have drier air associated with it, which would translate into reduced convection chances across western Nebraska if the models verify.
The remainder of the month also remains in question as the Climate Prediction Center released its updated August temperature and precipitation outlook. Figures 4 and 5 detail the updated temperature and precipitation forecasts preliminarily issued two weeks prior. Above normal temperatures have been replaced with below normal temperatures, while precipitation has moved toward a wetter forecast.
Since the jet stream pattern when this update was issued favored a northwest flow aloft bringing cooler air and moisture to the central Plains and points east, it is not likely that this pattern will verify. The areas that have consistently received above normal moisture this growing season lie north and east of this updated forecast.
Al Dutcher at the Nebraska State Climate Office