Provided by Justin Glisan, Ph.D., State Climatologist, Iowa Department of Agriculture and Land Stewardship
Iowa experienced unseasonably wet conditions as a very active storm track brought several waves of showers and thunderstorms to Iowa. Multiple rounds of severe storms produced large hail, gusty winds and weak tornadoes along with heavy rains. Stations across northern Iowa registered rain totals of two to six inches above average. Temperatures were also unseasonably warm with departures from four to 10 degrees east to west; the statewide average temperature was 66.9 degrees, 7.7 degrees above normal.
Scattered thunderstorms moved across northern and central Iowa on Sunday (7th) afternoon with strong to severe storms firing towards the evening hours. Thunderstorms consolidated into a severe-warned line in eastern Iowa, fueled by highs in the upper 80s and low 90s and ample instability. A secondary line of severe thunderstorms formed in western Iowa and joined a complex in northeastern Iowa that dove through southeastern Iowa into early Monday (8th) morning. Widespread reports of large hail up to 2.75 inches and straight-line winds were found over the southern half of Iowa with a brief EF-1-rated tornado in Muscatine County. Stronger thunderstorms brought heavier rains with over 100 stations measuring an inch or more and a statewide average rainfall of 0.71 inch; 10 stations observed at least 2.00 inches with 2.54 inches reported in Le Claire (Scott County). Tuesday (9th) was a generally pleasant day with easterly winds, mostly clear skies and temperatures in the 70s. Wednesday (10th) saw southeasterly winds build in with partly cloudy skies in western Iowa as afternoon highs made it into the low to mid-80s; upper 70s were observed east. Isolated storms formed in northwestern Iowa and then, more broadly, across the Iowa-Minnesota border after midnight on Thursday (11th). A separate wave of moderate rainfall propagated north over the state through the day as clouds held highs in the mid-70s with low 80s interspersed. Rain totals were above 0.25 inch at most stations with a statewide average of 0.34 inch; early morning thunderstorms trained over Mitchell County producing a 4.83-inch reading at Osage.
Showers lingered in northern Iowa through Friday (12th) morning as temperatures gradually climbed into the upper 70s and low 80s in western Iowa. A potent low-pressure center in Nebraska forced severe thunderstorms that pushed across the western border into the evening hours, leaving behind multiple reports of strong winds and torrential downpours; 10 northwest stations measured more than two inches over several hours. A separate line of thunderstorms brought more rainfall to central Iowa early on Saturday (13th) with morning lows in the 60s. As a warm front lifted north through the day, unseasonable warmth, ample moisture and atmospheric wind shear popped discrete supercells in east-central and northwest Iowa. Several thunderstorms put down brief, multi-vortex tornadoes around Knoxville (Marion County) with more reports of weak tornadoes in Buena Vista, Calhoun and Pocahontas Counties. Weak steering flow impeded normal storm motion with several slow-moving storms producing anomalous rainfall rates; two gauges in Pocahontas County, located in Varina and Havelock, observed 4.62 inches and 4.95 inches, respectively. Statewide totals were highest over Iowa’s northern one-third and a pocket of central Iowa with many stations reporting over 2.00 inches; widespread totals between 0.75 and 1.00 inch were also prevalent. Winds shifted to the east-northeast into Sunday (14th) with rain still falling in northern Iowa as morning lows held in the 60s south to low 50s north.
Weekly precipitation totals ranged from 0.27 inch near Council Bluffs (Pottawattamie County) to 5.24 inches in Osage. The statewide weekly average precipitation was 1.72 inches, more than double the normal of 0.85 inch. Osceola (Clarke County) and Shenandoah (Page County) recorded the week’s high temperature of 91 degrees on the 7th, on average 21 degrees above normal. Rockwell City (Calhoun County) reported the week’s low temperature of 42 degrees on the 8th, three degrees below normal.