Weekly Iowa Weather Update, July 24th, 2018

A series of fast-moving cold fronts moved across the state on Monday and Tuesday, bringing measurable rainfall to eastern and southern Iowa. Fairfield (Jefferson County) reported 0.59 inches on the 16th . Lee County observed accumulations between 0.01 and 0.05 inch on Tuesday.

Cooler air moved in behind the fronts, bringing below-normal to normal temperatures for the rest of the week. Average highs were in the low- to mid-80s for much of Iowa, with Oskaloosa reporting the week’s high temperature of 94 degrees on the 16th.

On Wednesday, a thunderstorm complex moved into the state during the evening hours. Pottawattamie, Fremont and Taylor Counties all reported rainfalls of over an inch. Thursday was an active day as a strong low pressure system over Minnesota and an attendant warm front draped over central Iowa fired up discrete severe supercell thunderstorms.

There were over 27 preliminary reports of funnel clouds and tornadoes, with Bondurant, Marshalltown and Pella taking direct hits; these locations reported catastrophic damage and some injuries. The Pella and Marshalltown tornadoes were rated at EF-3, with estimated peak winds at 144 mph; the Bondurant tornado was rated at EF-2, with estimated winds of 115 mph. The storms sped through Oskaloosa and Ottumwa into Van Buren and Lee Counties, where heavy rain and hail were reported. Widespread measurable rain was also observed in Iowa’s northeast quadrant, with Waukon (Allamakee County) recording 3.03 inches of rain, the week’s highest total.

Conditions calmed down on Friday, as the low pressure moved into Wisconsin through early Saturday morning. Lingering isolated thundershowers brought rainfall to Clayton, Dubuque, and Jackson Counties; Guttenberg reported nearly an inch of rain. Sunday was cooler and mostly dry. Average temperatures were variable across the state, with eastern Iowa up to eight degrees below normal.

Source:  Dr. Justin Glisan, State Climatologist, Iowa Department of Agriculture and Land Stewardship