On 10 August 2020, a strong “derecho” developed over Iowa as a result of a strengthening pre-existing MCS (Mesoscale Convective System) combined with an unstable atmosphere ripe for vigorous convective activity. The derecho led to numerous severe wind reports primarily over eastern Iowa, northern Illinois and southern Wisconsin, with a handful of wind gust reports exceeding 100 mph (Fig. 1 top-panel). Extensive straight-line wind damage was observed, including destruction of farm buildings and equipment (Fig. 1 bottom-panel), crops, trees and weak building structures.
Fig. 1: (top-panel) SPC filtered storm reports for 10 August 2020 (source: https://www.spc.noaa.gov/exper/archive/event.php?date=20200810), and (bottom-panel) heavily damaged grain belt in Marion, IA as a result of straight-line winds from the derecho event (source: https://www.weather.gov/dvn/summary_081020)
Fig. 2 shows a conceptual model of a derecho. A derecho is an MCS (Mesoscale Convective System) in which the leading edge of the storm evolves into an intense “bow echo.” Derechos initially form as a single or multicellular thunderstorm that develops within an environment with modest to high vertical wind shear, instability (i.e., sufficient CAPE, or Convective Available Potential Energy), moisture and some sort of lifting mechanism (typically a frontal boundary or mid-to-upper level “wave;” Fig. 2 panel 1). As the thunderstorm complex matures, cold air sinking within the thunderstorm downdraft diverges at the surface and propagates radially outward. The leading edge of the cold pool moving in the same direction as the mean wind forces warm, moist air downstream of the advancing cold air to lift, and it will create the “bowing” effect of precipitation on radar reflectivity imagery, as this portion of the cold pool propagates faster than the cold pool edges not aligned with the mean wind (Fig. 2 panel 2). The lift ahead of the cold pool leads to further thunderstorm development, and, if the complex continues to strengthen, will develop a stronger downdraft and faster propagating cold pool (Fig. 2 panel 3). The winds associated with the fast-moving cold pool can exceed hurricane force (i.e., greater than 74 mph, or 64 knots), and, within strong derechos, 100 mph (i.e., 86.9 knots)! Furthermore, a “bookend vortex” can form on the northern edge of the bow echo, which gives the derecho a “rotation” appearance on radar.
Fig. 2: Conceptual model of a derecho (illustrations by Dennis Cain, and images from https://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm#types). See text for details.
At 1200 UTC 10 August 2020, remnants of an MCS over southeastern South Dakota moved eastward into Iowa over the next several hours. During that timeframe, the synoptic-scale environment exhibited an upper-level trough over the Saskatchewan province associated with a strong surface extratropical cyclone over the region. The cyclone was associated with a cold front extending south and then southwestward through northwest Wisconsin, southeast Minnesota, central Iowa and southeastern Nebraska moved slowly eastward during the day (Fig. 3). This front, maintained within a deformation zone driven by anticyclonic flow over the northern U.S. Rockies and southeastern U.S., along with the upper tropospheric trough/ridge and jet streak dynamics, provided synoptic-scale lift to aid the MCS during the day.
Fig. 3: WPC surface map analyzed at 1200 UTC 10 August 2020. Weather station data shown using traditional conventions and units, isobars (mb) are in solid brown, and frontal boundaries and high and low pressure centers shown using traditional conventions. Source: https://www.wpc.ncep.noaa.gov/archives/web_pages/sfc/sfc_archive_maps.php?arcdate=08/10/2020&selmap=2020081012&maptype=namussfc
Along with the above, surface-based CAPE values in some portions of eastern Iowa and central Illinois exceeded 4000 J/kg, with modest directional shear in place due to the presence of an upper level trough positioned over the North Dakota/Minnesota/Saskatchewan border (not shown). For example, the KDVN sounding at 1700 UTC 10 August 2020 (with the balloon launch occurring at a non-traditional time of day) shows observed CAPE values over 4500 J/kg (Fig. 4)! These type of conditions were optimal for strengthening of the pre-existing MCS as well as its evolution into a derecho.
Fig. 4: KDVN sounding at 1700 UTC 10 August 2020 (source: https://www.spc.noaa.gov/exper/archive/event.php?date=20200810). Note the large surface-based CAPE value as well as modest speed shear prior to derecho arrival.
As a result of the above (plus some other factors not focused on in this post), the derecho that evolved had a “textbook” appearance on radar, as shown in Fig. 5 below.
Fig. 5: Radar reflectivity factor (color scale based on conventional color scale used for radar reflectivity) .gif of the derecho event (from the Quad Cities, IA radar site) on 10 August 2020. Note the bow echo that develops over time. Note the bookend vortex on the northern edge of the bow echo.