The Storm Prediction Center has issued a Severe Thunderstorm Watch effective until 1am Friday morning for the following counties, Ida, Woodbury, Adair, Adams, Audubon, Black Hawk, Boone, Bremer, Butler, Calhoun, Carroll, Cass, Clarke, Crawford, Dallas, Greene, Grundy, Guthrie, Hamilton, Hardin, Jasper, Madison, Mahaska, Marion, Marshall, Polk, Poweshiek, Sac, Story, Tama, Union, Warren, Webster, Fremont, Mills, Montgomery, Page and Pottawattamie. Thunderstorms have developed across western Iowa. The main threats will be from large hail and damaging straight line winds. Heavy rain is possible as well which may lead to some urban and small stream flooding in areas where numerous thunderstorms train.