A complex of thunderstorms developed over portions of Wyoming Sunday afternoon (July 20, 2008). The storms became severe while shifting east through northern Nebraska Sunday evening. The severe thunderstorms moved into northwest Iowa very early Monday morning (July 21, 2008). As the severe thunderstorms continued moving southeast they further intensified and produced widespread and destructive straight-line winds of 60 to 90 mph through east central into southeast Iowa, and northwest into central Illinois. The highest measured wind gust was 94 mph in Moline, IL. Other measured wind gusts included 84 mph in Princeton, IL and 72 mph in Mount Pleasant, IA. These extreme winds produced a large swath of wind damage 20 to 40 miles wide comprised largely of downed trees and power lines. Power was knocked out to over 130,000 residents in the Quad Cities. This type of long lived and destructive wind storm is referred to as a derecho. Click here to learn more about derechoes.
The image above shows the swath of damage reports. Hail is in green (diameter in inches) and wind is in orange (gust in knots).
Thunderstorm complex moving through northern Nebraska Sunday evening.
Main complex of storms moving through northwest Iowa prior to 1 am. Note the additional storms, some severe, developing out ahead through central Iowa.
Destructive, long-lived windstorm (derecho) moving out of central Iowa into eastern Iowa. 0.5 degree base reflectivity image from KDVN at 0926z (426 A.M. CDT).
0.5 degree base reflectivity image from KDVN at 1009z (509 A.M. CDT) showing derecho moving through eastern Iowa. Notice the bowing segment from near Hills southward to Washington... marking where the strongest winds were occurring.
Corresponding 0.5 degree base velocity image from KDVN at 1009z (509 A.M. CDT). The radar was showing an area of 80 to 90 kts between Hills and Riverside at roughly 4300 ft msl (3655 ft agl). Green indicates air moving toward the radar (indicated by dark circle north of Davenport) while red indicates air moving away from the radar.
Line of destructive storms moving into the Quad Cities Metro. Note the bowing segment from southwest of Moline to Burgess denoting where the strongest winds were occurring.
0.5 degree base velocity image from KDVN at 1100z (6 A.M. CDT). Note the pronounced drop-off in velocities. This is due to sampling issues... because the wind is perpendicular to the radar beam. Because the NEXRAD Doppler radar measures only radial wind speeds (wind speeds towards and away from the radar along the radar beam) the component of the wind that is perpendicular to the radar beam cannot be measured at all.
0.5 degree base velocity image from KDVN at 1110z (610 A.M. CDT) near the time that MLI ASOS measured 81 mph... before communication was lost. Backup coastal system at the airport measured 94 mph. Radar is indicating a downburst between I-280 and I-74.
0.5 degree base reflectivity image from KDVN at 1114z (614 A.M. CDT) about the time the Indian Trails Resort was hit.
Corresponding 0.5 degree base velocity image from KDVN at about the time the Indian Trails Resort was hit. Radar is showing winds of 90 to 95 kts at roughly 1620-1700 ft msl (950-1050 ft agl) from near Colona southward to the I-80/I-74 interchange.
0.5 degree base velocity from KDVN at 1140z (640 A.M. CDT) still showing large area of 90-100 kts with max of 109.8 kts at 3501 ft msl (2860 ft agl north of I-80 northwest of Sheffield.