Record highs set this weekend
Several locations in western Nebraska set record highs over the weekend. The 110°F at Ogallala is the warmest temperature recorded since 1954 before widespread irrigation for crops existed. Scottsbluff and Chadron are both experiencing a top 5 warm July and in Chadron, only 1936 was hotter between July 1 and July 26. 1936 is not good company to keep.
Chadron, 108°F (Saturday, 7/25)
McCook, 111°F (Sunday, 7/26)
North Platte, 108°F (Sunday, 7/26)
Ogallala, 110°F (Sunday, 7/26)
Scottsbluff, 108°F (Saturday, 7/25)
Scottsbluff, 106°F (Sunday, 7/26)
The entire state was very hot on Saturday and Sunday with the vast majority of sites hitting 100°F. Only the far southeast corner of the state and areas with significant amounts of irrigated corn were held under 100°F yesterday. Same story for Saturday, though more sites in eastern Nebraska stayed under 100°F. Some sites in the Panhandle and North Central sections of the state were a few degrees cooler yesterday than on Saturday. The award for earliest 100°F goes to Whitney 4 SW in Dawes County for hitting that value at 9:45 MDT on Saturday morning.
Figure 1. High temperatures at Nebraska Mesonet sites on Sunday, July 26th.
Dangerous heat
It also was very humid over the weekend in the states eastern and south central sections where corn and soybean are a dominant feature of the landscape. Dew points cracked 80°F in many places and got as high as 85°F in Rulo on Saturday evening and at Fordyce 4N yesterday. The very high dewpoints combined with temperatures well over 90°F led to heat index values going over 110°F and in some cases exceeding 120°F. Looks like there was a tie for worst heat index as both Axtell 5NE and Holdrege 5 N both managed a heat index of 126°F yesterday. Those likely were influenced by irrigation but maximum heat index values over 120°F were also felt at Plattsmouth, Nebraska City, and Rulo where irrigation is non-existent. It is worth noting that some areas have seen pivots sidelined during daytime hours by local affiliates of NPPD to preserve capacity for other residents on these hot days.
Figure 2. Maximum heat index values on Sunday, July 26th.
Why is it so hot?
A look at the 500-mb map from Saturday evening (00Z Sunday) shows a massive ridge in place over the central U.S., with the center around Dodge City, KS where that 500-mb height was 600 decameters (or 6000 meters). That height is very high for a 500-mb level and is reflective of how warm the atmosphere is. Temperatures in the lower part of the atmosphere were exceptionally warm in the Central/High Plains and this warm air was able to mix down to the surface via adiabatic warming. There was plenty of sunshine over the weekend and thus all of the ingredients were in place to create very hot air temperatures. Combine the heat with low-level moisture coming up from the south and east, crop evapotranspiration adding extra water vapor to the atmosphere, and a more shallow boundary layer and we had the perfect recipe for a dangerous combination of heat and humidity.
Figure 3. Map showing 500-mb level wind speeds, air temperatures, and heights (upper right) in decameters from 00Z Sunday.
Water stress becoming a bigger concern
Last week did deliver some beneficial rains to some but generally was not enough to offset the previous heat spell or to put a surplus of moisture in place before the current heat wave. Root zone soil moisture was short going into the summer for most of the western half of the state and the recent dryness has led to a non-trivial decline in available soil moisture in much of eastern Nebraska into western Iowa. The map below should be taken with some grain of salt but areas shown in yellow and brown are likely starting to see dryland crops experience water stress issues or will soon.
Figure 4. Areas with less than 30 percent of available root zone soil moisture according to SPORTLIS.
Rain chances
The good news is there will be chances of showers and thunderstorms starting tonight for much of western and central (possibly also eastern NE) and later this week for the eastern half of the state. Temperatures will also be cooling back to more seasonal levels in the 80's (central and east) to lower 90's (west) later this week, which would bring some much needed relief. Given the levels of drought in the west and developing (or near-developing) water stress issues in the south cenrtral and eastern sections, this rain could make or break the chances for higher yielding rainfed or irrigated crops limited to lower amounts of water.
Figure 5. ECMWF probability of 1" or more of precipitation.
Thoughts on impacts to yield
The recent heat in the Central Plains and Western Corn Belt has not been well-timed but would have been worse earlier in the month during pollination. It also has not been a 10-day heat wave with no break, so yield potential is still there for fields that are still holding on. Nevertheless, the prospects of achieving last year's higher corn yields in Nebraska are starting to dwindle and will be effectively zero if the rain later this week doesn't materialize. Soybean can still turn in a good year if August is cooler than July and brings good moisture. But there is little margin for error for soybean (as well as crops like sorghum) and rain later this week will be critical to move that pig skin down the field another 10 yards. The opportunity to score the touchdown is still there but the rain will need to be more consistent and more generous in August for much of the area than it has been of late.
Looking at the bigger picture in the Corn Belt, the latest outlook from Patricio Grassini's group shows most places are likely to be around trend with higher chances of above-trend corn in Illinois and western Indiana. Iowa looks to be around trend and Nebraska's yields look to be closer to an at-trend projection if you also consider irrigated crops (see related link). Most of the Corn Belt is in ok shape with moisture but could use cooler overnight temperatures in August and a bit more consistent rain than what July has delivered.
Figure 6. Corn yield projections for rainfed corn
