September 5th, 2020 by Roy W. Spencer, Ph. D.
The August 10, 2020 derecho event caused an estimated 40 million acres of nearly-mature corn crop to be significantly damaged or destroyed, mainly in Iowa, but also in portions of Nebraska, South Dakota, Illinois, Minnesota, Wisconsin, Indiana, Ohio, and Missouri.
I put together this NASA Terra satellite MODIS imager comparison of the area as imaged on September 2 in both 2014 (a normal crop year) and in 2020, a few weeks after the derecho struck. This date is sufficiently past the event to show areas where the crops are dead and dying. (Click on image if it doesn’t animate.)
The dashed line in Fig. 1 shows the approximate area where crop damage seems most extensive.
What Causes Derechos? How Common are They? Can they Be Predicted?
Derechos are severe thunderstorm “squall line” high wind events that are particularly widespread and long-lived, typically moving rapidly across multiple states. This video taken in Cedar Rapids shows about 25 minutes of very high winds, with occasional gusts taking out trees and tree limbs.
Derechos are particularly difficult to predict. For example, the NWS Storms Prediction Center early morning outlook (issued at 7 a.m.) for severe weather showed little indication of unusual severe storm activity prior to the August 10 event.
Once the derecho formed over eastern South Dakota and Nebraska, though, the forecast advisory was updated to reflect the high probability that it would persist and move east.
Like all severe thunderstorms, we know that derechos require an unstable air mass (usually during summertime), with some wind shear provided by an advancing cool front and upper-level trough to the west. But most of these synoptic situations do not cause derechos to form, and forecasters can’t predict one every time such conditions exist or there will be a lot of false alarms.
The following plot shows an 18 year climatology of derecho events during May-August of 1996 through 2013 from a 2016 study by Gaustini & Bosart.
Note that a farmer in the corn belt will be impacted by maybe one or two derecho events per growing season, depending upon their location, although ones of the severity of the August 10 event are much more rare. Of course there is nothing a farmer can do about such events, even if they were accurately forecast.
Given the central placement of derecho activity in the corn belt, I suspect that these events are made somewhat worse by the huge moisture requirements of corn, which leads to very high dewpoints (oftentimes in the low-80s F) when the corn is actively growing and transpiring water. Any extra water vapor is extra fuel for these storms.
via Watts Up With That?
September 6, 2020 at 12:03PM