General Observations
Fall crop harvest is underway across the state, as dry weather and warmer temperatures aided harvest activity. Well above normal temperatures developed statewide from September 24-26, before cool and damp conditions developed on September 27 due to the passage of a cold front.
The passage of the cold front on Sunday led to very light rainfall across the northwestern ¾ of the state. Precipitation totals ranged from zero to a tenth of an inch. Thunderstorms developed along and south of the I-80 corridor from York to Omaha, with storm totals ranging from a tenth to a third of an inch of moisture. Although temperatures averaged well above normal these past 7 days (Figure 1), smoke from western wildfires was cited as a possible source of slower than expected grain dry down during the first half of last week.
The continuation of drier than normal conditions led to further expansion of drought coverage across western Nebraska. There was a slight expansion of D1 (moderate) drought and DO (abnormally dry) conditions on the western, southern and eastern fringes of the Sandhill region due to below normal moisture on time scales of 30 to 60 days (see Figure 2).
More importantly, D3 (extreme drought) was introduced into the western North Platte river valley between Scottsbluff and Lake McConaughy. With no significant moisture events projected during the next two weeks, further intensification of drought conditions are expected across the southern and northern 1/3 of the Panhandle.
Frost and scattered hard freeze conditions were reported the morning of September 28th across the northwestern 1/3 of the state, which also brought the second freeze of the season across the northern Panhandle. If forecasts verify, areas north of I-80 that haven’t experienced frost conditions have an above average likelihood of experiencing these condition the morning of October 2.
Crop Progress
Conditions this past week were nearly ideal for widespread harvest activity, especially September 24-26. Nebraska Agricultural Statistics Service (NASS) reports 14% of the corn was harvested as of September 27, a gain of 4 percentage points from last week. Soybeans harvested stood at 29%, compared to 10% last week. Extension reports on the university crops call September 29 indicated that there was heavy soybean harvest activity September 25-26. The sorghum harvest stood at 7%, compared to 1 percent last week.
The dry weather was favorable for wheat planting and 60% of the crop was estimated to be in the ground by NASS, slightly behind last year’s pace of 65% and the 5-year average of 66%. Wheat emergence stood at 15%, compared to 16% last year and the 5-year average of 27%. Pastures continued to decline due to the dry weather. Pastures in the good to excellent category stood at 21%, a drop of 20 percentage points from last week. Pastures in poor to very poor condition stabilized at 34%, an increase of one percentage point from last week.
With the continuation of dry conditions, soil moisture ratings continued to decline according to NASS. Topsoil in the short to very short classification stood at 60%, an increase of 5 percentage points from last week. Subsoil moisture also declined 5 percentage points and NASS reports that 58% of the soils across the state are rated short to very short. The continuation of dry weather for the next two weeks will increase concerns about inadequate fall soil moisture recharge impacts on wheat emergence and stand development, as well as further deterioration of pastures statewide.
Outlook
The most current run of the GFS model used for this forecast was issued the morning of September 29 and indicates precipitation events will be far and few between during the September 29 – October 15 period. In fact, the biggest forecast challenges will be to identify when cloud cover may be sufficient to impede harvest activity by increasing surface relative humidity levels.
The upper air trough the brought an end to last week’s mini heat wave is currently situated over the upper Great Lakes region and will influence weather conditions across the central Plains through October 8th according to the GFS model. Several impulses will slide down the backside of this trough, which will bring reinforcing shots of cool Canadian air with little to no precipitation (see Figure 3).
The first impulse sliding down the backside of the upper air trough is forecast to begin infiltrating cool air into Nebraska beginning September 30th, with the coldest air forecast to arrive October 1. The GFS model indicates widespread spotty frost is likely across the southern ½ of the state, with widespread frost and isolated hard freeze conditions for the northern ½ of Nebraska the morning of October 2. High temperatures will drop from the upper 70’s to low 80’s September 29, into the lower 70’s September 30th, and into the lower 60’s October 1. No precipitation is forecast with this impulse, but cloud cover may impede drying the first day of October.
Another impulse is projected to slide down the backside of the Great Lakes upper air trough on October 2, which will induce the formation of a surface low across eastern North Dakota. This surface system is projected to move east-southeast and increase cloud cover statewide during the overnight hours and into October 3. This represents the best precipitation chances in the entire 16 day forecast period. Currently the GFS model keeps precipitation on the Iowa side of the eastern Nebraska border region, but the European model indicates scattered showers and thunderstorms may develop across the eastern 1/3 of the state. High temperatures may top out in the upper 60’s to low 70’s across the southern 1/3 of the state.
The Great Lakes upper air trough is projected to lift northeast and weaken from October 4-7 which will allow milder air from the southern High Plains to build northward. High temperatures should warm from low 60’s October 4 to the upper 70’s by October 7. High temperatures could run 5 F warmer for southwestern Nebraska into the southern Panhandle region.
The GFS model hints that a final impulse will slide down the backside of the weakened upper air trough over the northern Great Lakes, which is projected to give a glancing blow of cooler Canadian air to Nebraska on October 8. At this point, the GFS model shows no precipitation, but does point to mostly cloudy conditions for most of the day with high temperatures dropping back into the 60’s.
From October 9-15, the GFS model indicates that the western U.S. upper air ridge will push into the central Plains as the Great Lakes upper trough lifts northeast and an upper air tough begins to develop across the Pacific Northwest. Dry and sunny conditions are projected by the GFS model, with highs warming from the mid 70’s October 9 into the mid 80’s by October 14. Cloud cover is projected to increase across the western half of Nebraska October 15, as the western U.S upper air trough moves into the central and northern Rockies (See Figures 4-5).
If the GFS model is correct in its depiction at the end of its 16-day forecast period, a significant storm system will likely develop over the central and northern Rockies October 14-15. The GFS indicates that enough cold air is projected to lay over the Rockies to produce high elevation snowfall. If this trough is able to pull this cold air eastward, the western Dakota’s and eastern Montana could also experience accumulating snowfall, with widespread rainfall possible across Nebraska.
Al Dutcher, Agricultural Extension Climatologist with the Nebraska State Climate Office