Weather Review
Drought conditions continue to expand across Nebraska according to the latest Drought Monitor depiction (Figure 1) issued August 18th for conditions through 0700 CDT August 16th. The heat and dryness led to drought intensification, especially over the southern half of the state. All was not lost as extreme northeast Nebraska (northeast Knox, Cedar, Dixon and Dakota counties) NERain observers reported 1.80 -2.70 inches of moisture August 16th. This led to a reduction of D3 (extreme drought) to D2 (severe drought).
The cumulative precipitation totals reported from August 15th through the 22nd can be viewed in Figure 1. Daily precipitation totals exceed one inch on August 16th across the northwestern Panhandle and extreme northeast Nebraska. Smaller pockets occurred on August 17th and were confined to parts of northeast, east central and southwest Nebraska. It wasn’t until August 20th before the next one inch plus precipitation total was reported at Brownsville 0.2 SSW with 1.53 inches.
Although these precipitation totals sound impressive, they were a minority in an overall dry pattern last week. Less than 0.30 inches of moisture was reported last week for the western 1/3 of the state, excluding the northern 1/3 and extreme southwestern corner of the Panhandle. A similar area was located over central and south central Nebraska. These locations are the most likely candidates for drought expansion/intensification.
It should be noted that pockets of significant severe weather developed along a cold front extending from south central through northeast Nebraska on August 18th. This led to the issuance of 24 storm reports for hail one inch or greater. The worst hit area was Nance County where three storm reports indicated hail reached 2.75 inches in diameter (Genoa, Genoa 2N, Saint. Edwards 6 E). Until this storm event, there had only been 4 hail reports of one inch or greater hail and 4 wind gust reports of 57 mph or greater since the first of the month.
The daily maximum precipitation values reported from NERain observers are as follows: 8/15 (2.45 inches: Marsland 3 SE), 8/16 (3.70 inches: Waco 4.3 SE), 8/17 (0.94 inches: Fort Calhoun 2.9 SSW), 8/18 (0.57 inches: Marsland 3 SE), 8/19 (0.83 inches: Rockville 2.2 NW), 8/20 (1.53 inches: Brownsville 0.2 SW), 8/21 (0.00 inches: all stations).
Although weekly precipitation was disappointing once again for much of the state, at least Nebraskan’s got to enjoy a brief taste of early fall air temperatures. Average temperature anomalies for August 15-22 can be seen in Figure 3. Average temperature anomalies ranged from normal to 3 F below normal across the northern 2/3 of the Panhandle and northeast Nebraska. Across the southern Panhandle, average temperature anomalies ranged from 1 F below normal to 2 F above normal. The remainder of the state reported average temperature anomalies of 2-5 F below normal, with the strongest anomalies (3-5 F below normal) occurring across central, east central, north central and southeast Nebraska.
The daily maximum temperature extremes for airport location provided by the National Weather Service are as follows: 8/15 (92 F: Imperial), 8/16 (89 F: Chadron), 8/17 (92 F: Chadron), 8/18 (95 F: McCook, Ord), 8/19 (91 F: McCook), 8/20 (88 F: McCook), 8/21 (93 F: Chadron, Scottsbluff).
The daily minimum temperature extremes are as follows: 8/15 (63 F: Alliance, Kimball, Wayne), 8/16 (58 F: Kimball), 8/17 (52 F: O’Neill), 8/18 (49 F: Sidney), 8/19 (50 F: Alliance, Kimball), 8/20 (42 F: Alliance), 8/21 (50 F: Ord).
Crop Progress
Last week’s cooler temperatures and small areas where weekly precipitation exceeded 0.50 inches were insufficient to stem further degradation to Nebraska’s warm season crops and rangelands. Air temperatures were cool enough across the eastern half of the state that air cooling units could be turned off and consumers could enjoy nature’s air conditioner. Without the welcome break from above normal temperatures, it is likely that crop conditions would have deteriorated at a greater rate than reported in this week’s Nebraska Agricultural Statistics Service (NASS) crop report.
The Nebraska corn, soybean and sorghum crops reported another week of crop condition declines according to NASS. As of August 21st, 47% of the corn crop was in good to excellent condition, which represents a decline of 5 percentage points from August 14th. Soybeans rated good to excellent were 48%, while sorghum was 20 percent. Soybean rated good to excellent dropped 2 percentage points from the previous week, while sorghum dropped one percentage point.
In response to a steady decline in the good to excellent rating for these 3 crops over the last 6 weeks, there has been a strong increase in the percentage of each crop rated poor to very poor. NASS notes that 30 percent of the corn crop, 20% of the soybean crop and 46% of the sorghum crop are rated poor to very poor as of August 21st. Compared to August 14th, there was a 4 percentage point increase in corn rated poor or worse, a 2 percentage point increase for the soybean crop and a 6 percentage point increase for the sorghum crop.
Warm season crops are quickly approaching physiological maturity and harvest activity will ramp up within the next 30 days across the state. As of August 21st, NASS reports 39% of the corn was in the dent stage and 3% had reached maturity. Soybeans setting pods was estimated to be 93% and leaf drop was at 3 percent. The sorghum crop was 75% headed, while coloring was at 17 percent. Corn dent and soybean leaf drop were one percentage point ahead of the 5-year average. Sorghum maturity is at least one week behind the 5-year average according to NASS. Sorghum in the heading stage was estimated to be 75% compared to the 5-year average of 94%, while head coloring was 17% compared to the 5-year average of 32 percent.
Rangeland and pastures also declined due to a substantial majority of western Nebraska failing to receive more than 0.25 inches of moisture for the week ending August 21st. NASS reports that pastures and rangeland are currently rated 53% very poor, 28% poor, 15% fair, 4% good and 0% excellent. There was an increase of 9 percentage points in the very poor category from the prior week. The percentage of rangeland rated poor to very poor (81%) indicates that prospects for fall regrowth are dismal unless a very wet pattern becomes established over the next two months.
Topsoil and subsoil moisture ratings have declined so strongly over the last two months that weekly changes are primarily confined between the very short and short categories until widespread rainfall events materialize. NASS reports that topsoil moisture as of August 21st was rated 43% very short, 36% short and 21% adequate. Subsoil moisture was rated 40% very short, 39% short and 21% adequate. Compared to August 14th, topsoil moisture rated very short increased 4 percentage points and subsoil moisture gained 3 percentage points.
Weather Outlook
There is considerable uncertainty in regards to the weather forecast over the next 16 days, especially during the August 27 – September 4 period. The latest GFS model run issued the morning of August 23 indicates that the upper air ridge currently bring dry weather to the state will begin to weaken as we approach this coming weekend. Two weak troughs are forecast to eject eastward out of the northern Rockies August 26th and 28th.
The first wave is forecast to bring a small chance of isolated showers to extreme northern Nebraska the evening of August 26th. The second wave is forecast to move into the western Dakota’s, cutoff, then drift southwest toward east central Colorado. The GFS model develops precipitation across eastern Nebraska the afternoon of the 28th through the 29th. This precipitation shield is shifted toward western Nebraska August 30-31 before the upper low dissipates on September 1. High temperatures are forecast to reach the upper 80’s northeast to middle 90’s southwest.
Because the GFS is developing a cutoff upper air low, forecast confidence is extremely low. Although cutoff lows are fairly common, numerical weather models over forecast their development. In addition, it is very difficult for models to handle the movement of cutoff lows, let alone their longevity. If the GFS model is correct, then welcome rainfall is likely statewide. If the cutoff upper air low fails to develop, then precipitation chances will be limited to the upper air trough passage on the 28th.
Assuming the GFS model is correct with the placement of the cutoff low, it then begins to move an upper air ridge into the central and southern Plains beginning September 1 keeping it in place through the end of the model run period, which is September 8th. During this period, the GFS model attempts to stream a ribbon of low and mid layer moisture northward from the western Gulf of Mexico. This moisture feed is forecast to pool on the southwest flank of the weak upper air trough positioned over the western Great Lakes and lead to daily chances of thunderstorm development over eastern Nebraska and western Iowa September 2-6.
I suspect that much of the precipitation currently forecast for eastern Nebraska is overdone and will not come to fruition since it just showed up in the models over the last 36 hours. I am more inclined to lean toward warm and dry like western Nebraska simply because the models were consistently pointing in this direction since the middle of last week. There continues to be no sign of a strong cold air push from Canada, thus high temperatures September 1-8 are forecast to be in the 80’s east to low 90’s west. A few isolated 95-100 F reading are possible September 6-8 across southwest Nebraska. If rain does materialize across eastern sections of the state, cloud cover my lower high temperature 5-10 F.
Al Dutcher, Agricultural Extension Climatologist, Nebraska State Climate Office