Weather Update
The past week featured roller coaster temperatures and several rounds of severe weather. High temperatures were in the 90’s statewide July 5-6, with upper 90’s reported across much of the Panhandle. The upper ridge responsible for the heat began to retrograde back toward the southern Great Basin July 7 which allowed a short wave trough moving across the northern Plains to move southeastward into the central United States.
A series of short wave troughs moved through the central United States July 7-10 along the periphery of upper air ridge that brought widespread 100-110 F temperatures to the southwestern ¼ of the nation. High temperatures across the eastern 2/3 of the state remained in the 80’s, while the western 1/3 of Nebraska averaged 5-10 F warmer due to its proximity to the upper air ridge southwest of the region. For the July 5-11 period, average temperature were well above normal across the Panhandle, near normal across the central third of Nebraska and below normal for areas east of Grand Island (Figure 1).
Perhaps the biggest feature during this past week was the thunderstorm development associated with the July 6-7 and 9-10 troughs that moved through the northern half of the High Plains and central corn belt (IA, Il, IN). The most concentrated precipitation July 6-7 according to NeRain occurred in the McCook, Maxwell, Lexington to Kearney and Broken Bow to Burwell areas, as storm totals were generally in the 0.50 to 1.00 inch range. Precipitation totals outside of these areas ranged from nothing to less than a half inch of moisture.
<>Widespread severe weather moved through the state July 9-10 and high winds caused significant power outages in Omaha, with smaller outages reported from Hastings through northeast Nebraska. Estimated wind gusts in Omaha exceeded 90 mph and left 190,000 people without power (30,000 plus still remained without power on July 12). Softball size hail devastated crops in the Alliance area, while straight line winds damaged center pivots, grain bins and corn fields approaching pollination.These thunderstorms did produce pockets of heavy rainfall (1-3 inches) in the Broken Bow, Grand Island, Hastings and Aurora areas. There were small pockets of 1-2 inches reported to the north of Omaha and east of Norfolk. The cumulative weekly precipitation totals can be seen in Figure 2, while Figure 3 shows how much these totals deviated from normal. Although Figures 2 and 3 show general coverage patterns, but even in areas receiving more generous precipitation there were large variations over short distances.
Crop Progress
The latest U.S. Drought Monitor depiction for Nebraska based upon conditions through July 6 and issued July 8 can be viewed in Figure 4. This depiction does not include the impacts of the last week’s storm activity the second half of the week. In addition, some of the severe weather impacts on crops may not be entirely included in this week’s crop ratings due to the July 9-10 event falling right before the crop condition report deadline of July 11.
For the first time this crop reporting season crop condition ratings for corn in the good to excellent range dropped below 80%. As of July 11, Nebraska Agricultural Statistics Service (NASS) reports that 77% of the crop was in good to excellent conditions, a drop of 5 percentage points from the July 6 report. Although some of the decline likely was the result of severe thunderstorm activity, short term dryness has accelerated moisture stress across parts of eastern Nebraska, as well as long term dryness issues (> 6 months) across the Panhandle and northeastern Nebraska.
Although corn had the largest weekly decline of the warm season crops using the good to excellent rating scale, soybeans and sorghum also declined compared to last week’s NASS crop progress report. According to NASS, soybeans in the good to excellent category stood at 79%, a drop of 1 percentage point from last week. Sorghum in the good to excellent categories stood at 80%, which is 3 percentage points lower than last week.
Corn is now entering the critical pollination period and NASS reports that 19% of the crop has entered the silk stage. This compares to 17% last year and the 5-year average of 25 percent. Last week only 2% of the corn crop had reached the silk stage. Soybeans in the bloom stage stood a 59% as of July 11, which compares with 55% last year and the 5-year average of 44 percent. NASS indicates that 16% of the soybean crop is setting pods compared to 12% last year and the 5-year average of 4 percent. Sorghum has begun to head according to NASS with 3% of the crop reaching this stage as of July 11. Last year 11% of the crop had reached this stage and the 5-year average is 9 percent.
Topsoil moisture conditions declined last week, due in large part to the lack of widespread above normal precipitation across the southwestern 1/3 of the state. NASS indicates that as of July 11 topsoil conditions were rated 7% very short, 32% short, 57% adequate and 4% surplus. There was a 1 percentage point increase in the very short rating, a 4 percentage point decrease in the short rating, a 1 point decrease in the adequate rating and a 2 point decrease in the surplus rating.
Subsoil moisture conditions were unchanged from last week according to NASS. Subsoil moisture was 8% very short, 38% short, 53% adequate and 1% surplus. In essence, NASS indicates that areas receiving above normal moisture, which would have improved subsoil conditions, were offset by the lack of moisture over the southwestern 1/3 of the state. The wheat harvest picked up steam across the southern Panhandle and southwest Nebraska due to the lack of widespread moisture. NASS indicates that 23% of the crop has been harvested as of July 11 and this compares to 45% last year and the 5-year average of 40 percent.
Weather Outlook
The latest GFS model issued the morning of July 13 indicates that a broad upper atmospheric trough centered from the northern Rockies to the western Great Lakes will be pushed eastward in response to the southwestern U.S. upper air ridge building north and east into the nation’s midsection. As this ridge builds into the region, a progressively warmer and drier pattern should materialize for most of Nebraska as we move through the second half of July.
Before the upper air ridge builds eastward, there are several short waves that are forecast to move along the periphery of this ridge which should assist in the development of thunderstorm activity. The first wave is expected to impact Nebraska the evening of July 13 through the first half of July 14th. Current precipitation forecasts place the northern 1/3 of Nebraska and the southern ½ of South Dakota in the region with the best opportunity for 0.50 to 1.00 inches of moisture. The GFS model is not very aggressive bringing this moisture to the southern half of the state, so precipitation is likely to be more scattered (ie hit and miss activity).
Two additional pieces of energy are projected by the GFS to move along the ridge periphery July 15th and 17th. The two short wave troughs are currently expected to impact the eastern half of the state, although modeled precipitation appears to be on the light side, without widespread rainfall. This could change as we get closer to the dates of impact, but there is no strong currently associated with these systems.
What is more certain is that high temperatures will likely return to the 90’s next week statewide, with mid 90’s to low 100’s possible across the panhandle periodically for the last two weeks of the month. The upper air ridge placement will determine how long the ridge will remain over the central United States as Figure 5 shows that it will extend into south central Canada on July 29. Some of this heat will build into eastern Nebraska, but shouldn’t be as severe as western Nebraska due to elevated dew points.
With drought firmly in place across the northern High Plains, air temperatures are likely return to similar conditions experienced in the middle of June. How far north the ridge builds will be determined by the intensity of the upper air trough that will be building into the western United States. If this growing season is any measure of how long the upper air ridge will remain in place, then the western U.S. upper air trough should begin to move toward the central United States at the beginning of August.
Before the western United States upper air trough begins to move eastward at month’s end, precipitation chances across Nebraska look weak due to no significant trough action to promote lift and convergence around frontal boundaries working through the state. The GFS model hints at a very juicy atmosphere over the eastern half of the state as monsoon moisture from the southwestern U.S. begins to lift east-northeast into the southern half of the Rockies. Any weak wave and/or daytime heating could trigger scattered thunderstorms, but widespread coverage is not showing up currently in the models.
If the GFS is correct in the ridge/trough placement at month’s end, then he monsoon moisture that is current projected to be shifted east of western 1/3 of the state next week will pool across western Kansas and Nebraska as the upper trough along the west coast begins to approach the Rocky mountains by the end of the month. Before this happens, crop water requirements will likely exceed forecast precipitation totals, which would increase irrigation demand and intensiy moisture stress to dryland cropping areas that missed the pockets of above normal moisture these last two weeks.
Al Dutcher, Agricultural Extension Climatologist, Nebraska State Climate Office