Dec 28, 2020 A map showing expected snow and ice accumulation expected on Tuesday across Iowa. Courtesy National Weather Service OSKALOOSA â Mahaska and Marion Counties are included in a winter storm warning issued by the National Weather Service ahead of a Tuesday snow event. Forecasters say a wet, heavy snow is coming for central Iowa and could lead to power outages due to the weight of the snow on trees and power lines. Some freezing rain could fall late Tuesday, making road conditions even more slick. About 5-8 inches of snow are expected in Knoxville, Pella and Oskaloosa, as well as the Des Moines metro. Northern Mahaska County, and most of Jasper and Poweshiek Counties, may receive up to nine inches of snow.