OTTUMWA — A severe weather risk is taking shape for later this week in Iowa.
The timing is a bit tricky with this round of storms. The Storm Prediction Center places much of Iowa in the slight risk category. Southeast Iowa is outside the risk area right now, but it’s close enough to bear watching.
Jim Lee, a meteorologist with the National Weather Service Des Moines office, said part of that caution is simply because the storms are days in the future.
“We’re still three, four days out. Things can change,” he said.
Zach Sharpe, one of the Iowa Storm Chasing Network’s forecasters, said Wednesday will have all the elements needed for strong storms across most of the state. But neither Sharpe nor Lee expect the storms to fire up until the evening and overnight hours.
The thing that should delay the storms is what’s called a cap. It’s when air conditions are right for storms but relatively warm air at higher altitudes prevents formation. Storms need rising air to form, and rising air depends on temperature differences. When the air aloft is close to the same temperature as the air trying to form a thunderstorm, not much will happen. Wednesday's highs are expected to be in the upper 80s, well above the average for this time of year.
When the air starts cooling in the evening and overnight, the cap breaks. That’s when storms Wednesday could form.
“Right now, there is nothing that will break that cap, so all storms on Wednesday will hold off after dark for much of Iowa, and these storms will be widespread with hail being the main threat,” Sharpe said.
Thursday is different. The ingredients for storms aren’t quite as good as on Wednesday, but there won’t be a cap to prevent them from forming. The risk Thursday will not be confined to a particular time.