OTTUMWA — Severe thunderstorms in the early morning hours Thursday were just the first round. Forecasters predicted another severe outbreak for Thursday night into Friday and again later Friday.

The National Weather Service said the storms dumped 1.9 inches of rain on Ottumwa and the surrounding area. More than an inch fell between 3 a.m. and 4 a.m. alone.

The storms also produced strong winds. A spotter report from Eldon indicated a 70 mph gust during one storm. Similar gusts were reported in Beacon and Oskaloosa. Another spotter reported road signs blown over in Ottumwa, along with some street flooding.

Both Mahaska and Keokuk counties are under flood warnings. The Mahaska County warning involves the South Skunk River, which forecasters expected to rise above flood stage and remain there through Friday. Flooding on the North Skunk River near Sigourney triggered the Keokuk County warning.

Both warnings anticipate minor flooding, with the rivers falling below flood stage by Saturday.

That could change if storms make another visit to southeast Iowa, as seems likely. The Storm Prediction Center is warning of strong storms Thursday night into Friday and again during the day Friday. Both predictions indicate a “moderate risk.”

State Climatologist Harry Hillaker said the center tends to act conservatively in its predictions. They rarely forecast a high risk of severe weather, he said, but when they do they are worried. Even a prediction of moderate risk means a strong liklihood of storms, which can be dangerous even if they fail to reach severe thresholds.

Hillaker said current models for Friday suggest real risks for all four types of severe weather: tornadoes, hail, high winds and heavy rain.

“Things have changed. We’ve had a very quiet period,” Hillaker said. “Starting [Wednesday] we’re starting to get the very warm, very humid air.”

Warm, humid air is critical to storm formation. Strong thunderstorms form when warm air is able to carry large amounts of moisture to high altitudes. Subtract the humidity or the heat and you don’t get the storm.

Hillaker said the jet stream is strong over the area. That is both positive and negative. The strong jet increases the risk of strong storms, he said, but it keeps them from lingering over an area. That’s why Ottumwa did not see flooding on Thursday despite heavy rains the previous night.

Hillaker said area residents should pay attention to the forecasts and any watches or warnings that may be issued over the next 36 hours. Residents with weather radios are also encouraged to make sure they are working.

Matt Milner can be reached at (641) 683-5359 or via e-mail at mwmilner@mchsi.com