(2:00PM - 8/16/20) An in depth recap on the significant severe t'storm event that occurred across the region on Monday, August 10th. This event featured a derecho that moved across Illinois, as well as other states in the region, bringing widespread, significant, and destructive damaging winds.
This event was not very well forecasted as little as 12-18 hours out. As of Sunday evening, 99% of model guidance did not have even near a scenario as to what actually played out. Most guidance supported the idea that overnight severe t'storm activity ongoing across portions of the Midwest would continue deep into Sunday night. Some of this activity would have the potential to affect Monday's setup for some areas, while other areas would have a better threat, with a disturbance moving through the region supporting the overall idea for a severe t'storm threat. Overall, a general minimal to limited (level 1 to 2) worthy threat was likely to be expected for Monday, as of Sunday evening. Our forecast from 9PM Sunday evening only featured a minimal (level 1) severe t'storm threat for the state and surrounding areas, with a mention that it was likely an upgrade to a limited (level 2) severe t'storm threat would be needed for some areas, pending how Sunday night activity progressed by Monday morning. With that said, there was one...single...sole outlier model that showed a significant severe t'storm threat across portions of the state and region for Monday, and showed a potential derecho affecting the region. That model was the HRRR (High Resolution Rapid Refresh model). The HRRR is a short term model, which can be useful at times, but will also spit out far left field/wacky solutions many times. So Sunday evening when it was showing that significant scenario for Monday, with no support from any other model guidance and previous history in these situations suggesting not to jump onboard with it, no thought was given to it by us or pretty much anyone else. However, as Sunday night progressed into the wee hours of the morning, and the evolution of the weather pattern across the region was a carbon copy of the the HRRR model was showing and the environment in place was very supportive of the scenario it had been showing. At that time, attention started to rapidly grow and the likelihood for an upgrade in severe t'storm probabilities for our morning update was likely. By early Monday morning before 6AM, a complex of severe t'storms had taken shape along the South Dakota/Nebraska border,with everything having gone exactly as shown up until that point by the HRRR model. By this time other model guidance had started to jump onboard as well, in addition to the continued very supportive environment in place across the region. We issued a quick forecast update blurb during the 6AM hour Monday morning, hi-lighting the need for a significant increase in severe t'storm probabilities across portions of the state, with the increasing potential for a derecho to impact portions of the state. At 11:30AM Monday morning with issued our full morning update, with an upgrade to upwards of major (level 5) severe t'storm probabilities across Northern Illinois, which is the highest severe t'storm risk on our scale. We also drove home the point on how significant the potential was for widespread, significant and destructive damaging winds across portions of the state. This high level of severe t'storm risk and significant wording was much needed in this scenario. The short term forecast upgrade to upwards of major (level 5) severe t'storm risk verified very well across Northern Illinois. Much of Central and Southern Illinois also saw a large jump in severe t'storm probabilities between the Sunday evening and Monday morning forecast, which also verified well.
A very favorable environment for severe t'storms was in place across the region from very late Sunday night/very early Monday morning on into Monday evening, one in which has been seen in other derecho events. A large reservoir of high instability (3000-6500J/KG), deep moisture (70°+ dew points and 1.50"+ PWATS), high lapse rates (8-9.5C ML and 6-7.5C LL) and increasing shear (40-50KT+) was in place. This type of environment is generally very favorable for severe t'storms, and is also an environment that has been seen during other derecho events over the years in the region. A trigger, the main forcing mechanism for the development of activity, was a strong wave/disturbance pushing east-southeast out of Southern Canada and the Northern Rockies, down into the Northern Plans and Midwest.
The derecho moved across portions of Nebraska, Iowa, Wisconsin, Illinois, Indiana, Michigan, and Ohio... Bringing damaging winds upwards of 80-100MPH+, and significant/widespread damage. This derecho originated as scattered showers and t'storms across the South Dakota/Nebraska border very late Sunday night into very early Monday morning. This activity quickly grew upscale into a large complex, which quickly gained organization and severity as it moved across Northeast Nebraska, Southeast South Dakota and Western Iowa early to mid Monday morning. Continuing eastward, this complex fully matured and became most significant across Central to Eastern Iowa mid Monday morning into early Monday afternoon, from the Des Moines area on east into the Quad Cities area. During this time what is known and a bookend vortex was present within the complex, along with a strong rear inflow jet. The combination of these two features, along with a great environment and other factors, lead to a swath of 80-120MPH winds and widespread/severe damage across a several county wide corridor from near Des Moines eastward into the Quad Cities area. The extremely high wind gusts in excess of 80-120MPH over such a large area, along with the extent and severity of damage, is extremely rare and almost unheard of in this part of the country outside of a tornado. Damage in the Cedar Rapids mimics that of a fairly strong hurricane land-falling in the tropics. As this complex continued east into Illinois Monday afternoon, it quickly changed characteristics...With the main bookend vortex weakening, and the complex being more dominated by numerous meso-vorticies, which tend to enhance wind damage in smaller yet numerous corridors. This ended up being the case across much of Northern Illinois, where numerous swaths of 70-100MPH damaging winds occurred, along with at least 15 tornadoes. While moving through Illinois, the main derecho merged with another severe t'storm complex exiting Missouri into Western Illinois, which ended up impacting much of Central and Southern Illinois with damaging winds of 60-70MPH and some hail. Meanwhile, further north the main derecho continued on across Northern and Central Indiana, Southwest Michigan and Northwest Ohio Monday evening, with damaging winds of 60-80MPH. The derecho finally rapidly weakened and diminished early Monday night, deeper into Michigan, Ohio and Kentucky.
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