Regions of Iowa

Two Interstates cross here and nothing else interrupts the ground. Des Moines sits in the open middle of the state with no lake, no ridge and no river valley deep enough to change a forecast, so what the Plains send arrives intact — the dryline storms that fire over Nebraska on a May afternoon, the January clipper that puts freezing rain on the I-35 climb to Ames, and the August 2020 derecho, which was still doing 100 mph when it crossed Polk County.

The Corridor took the worst of it. On 10 August 2020 the derecho reached Cedar Rapids with 140-mph winds and held them for forty minutes, destroying two-thirds of the city's tree canopy in an afternoon — the costliest thunderstorm in United States history. The same flat, open ground that let the storm keep its structure also puts the Cedar River through the middle of town, and 2008 and 2016 are the two crests everyone here still measures against.

Davenport is the largest city on the Mississippi with no floodwall, and it has decided that on purpose — the riverfront floods, the barricades go up at Second Street, and 1993 and 2019 are the two crests everyone measures against — 2019 the higher, and the record. Upstream weather is local weather here: a wet April in Minnesota is a Davenport crest in June, and the river runs warm enough into December to put fog on I-74 when the first hard cold arrives.

This is where Iowa starts to get cold. The Cedar Valley sits far enough north and far enough from any moderating water that it holds the state's harshest winter averages outside the Minnesota line, and the same flat tile-drained ground that grows the corn gives an Alberta clipper nothing to slow it down. Summer brings the northern edge of the severe season — the Parkersburg EF5 of 2008 came out of this ground.

The ice sheets went around this corner and left it standing, so northeast Iowa is the one part of the state with real relief — 400-foot bluffs, cold spring-fed trout streams and hollows that hold their own weather. Valley fog forms here on nights the rest of Iowa stays clear, the snow lies longer on the north-facing slopes, and the Mississippi below Dubuque is where the state's ice jams set up.

The last glacier stopped here and left the ground pocked with sloughs and shallow lakes — the Prairie Pothole country, flat enough that the wind has run unobstructed since it was tallgrass. That makes this the coldest, snowiest and windiest tier of the state: the I-35 corridor north of Mason City closes to ground blizzards on days the snow stopped falling hours earlier, and Clear Lake's ice goes on in December and comes off in April.

The driest corner of Iowa and the most exposed. Siouxland sits at the edge of the Plains where the rainfall gradient turns down and the wind turns up, so it takes the state's first blizzard and its worst — and the Iowa Great Lakes at Okoboji and Spirit Lake, glacial and deep, hold the one bit of lake weather Iowa has. The Missouri River bluffs at Sioux City are the north end of the Loess Hills, and the Floyd River flood that came off them in June 1953 killed fourteen people in the city's stockyards and is still the flood of record.

This is the black-dirt middle of the Corn Belt, and the crop is part of the forecast. In late July an evapotranspiring corn canopy puts enough water into the boundary layer to run dewpoints into the upper 70s — the corn sweat that makes an Iowa heat index worse than the air temperature ever suggests — and the same flat, tiled, treeless section-road grid gives a February wind a clean 40-mile run at the drifts.

The Loess Hills are a wind deposit — 200 feet of glacial silt piled along the Missouri and found at this depth in only one other place on earth — and they make the west edge of Iowa the one place the state has a lee side. Council Bluffs shares Omaha's weather and Omaha's severe season, the bottoms below the bluffs are what floods when the Missouri runs high, and 2011 and 2019 both put I-29 under water for months.

Iowa's warm corner, and the one that sees ice instead of snow. Southeast Iowa sits far enough south that a winter storm arriving on a southerly return flow puts the freezing line right across US-34, so the same system that snows on Des Moines glazes Ottumwa. Keokuk sits at the foot of the Des Moines Rapids where the Mississippi drops 22 feet, and the lock and dam there is the river's oldest.

The oldest glacial till in the state, weathered long enough to have been cut into real hills, and the emptiest ground Iowa has — nine counties and 66,000 people. That matters in winter: the I-35 corridor south of Des Moines is where a storm's rain-snow line usually sets up, and the ice that takes the line down here takes it down a long way from the nearest crew. Greenfield's EF4 in May 2024 is the newest reminder that the severe season reaches this far south.