Ag Weather Forum

Possible Fourth of July Heat

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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A dome of upper-atmosphere high pressure brings on hot conditions for the central U.S. for the onset of corn pollination in the next two weeks. (NOAA graphic)

The Pacific Ocean may be trending toward El Nino, but, ahead of that development, there's still time for a heat wave to form over the central U.S., and threaten to stress corn going into the pollination phase. This setup is the case as we look to the final days of June into early July.

Corn is just barely getting into pollination over the Midwest. As of June 24, 5% of the U.S. corn crop was in the silking stage. In the central part of the country, Kansas, at 13%; Kentucky, 16%; and Missouri, 11%, showed the greatest progress in this stage. Illinois, Indiana and Nebraska all had 2% of their corn in the silking stage.

And, with the large majority of the U.S. corn crop still set to go through pollination, upper-air forecast maps for the 500-millibar constant barometric pressure value feature the building of a large dome of high pressure, stretching from the Delta west to the Four Corners, during the July 4-6 timeframe. The dynamics of this lead to hot and dry conditions.

The height of the 500-millibar constant pressure level can be used to estimate the surface temperatures, since the height of a pressure surface is related to the temperature of the air beneath it. For July 4-6, that height of the 500-millibar constant pressure is around 594 decameters, or 5,940 meters in height. In the spring and fall seasons, this height corresponds to surface temperatures of around 90 degrees Fahrenheit. However, in midsummer, that surface temperature needs to be modified upward by 10 degrees F. In other words, 10 degrees Fahrenheit need to be added to the surface temperature. And, that leads to a surface temperature over the south-central U.S. approaching 100 degrees Fahrenheit. Certainly, the mid-90s F range is not out of the question. That is stressful to corn, with possible damage to both the pollen from the tassel, as well as to the silk.

In addition, overnight low temperatures are likely to be very warm as well, with many readings in the low to mid 70s F. There has been a great deal of research and analysis done on the effect of above-normal nighttime temperatures on the total health and productivity of corn -- it's not helpful -- and this gets into the mix as well when we think of the prospect of the upper-air ridge formation and its possible impact on pollination.

There, of course, is substantial soil moisture over much of the central U.S. -- in some areas, too much. That moisture offers a possible buffer against extensive heat damage right away. Forecast maps do indicate the high pressure dome sliding more toward the southwestern U.S. in the 12- to 16-day timeframe, for the July 8-12 period. If that indeed takes place, the excessive heat threats to corn would be limited.

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