Oskaloosa.com

April 17, 2013

UPDATED: Stormy day brings weather risks


CNHI

OTTUMWA — Forecasters say large hail is the biggest threat for southeast Iowa today because of how the storms are forming.

While it may not seem like it, there's a relatively stable mass of air at the surface, according to Jim Lee of the National Weather Service's Des Moines office. Lee said that means storms are forming above that stable air, rather than drawing their strength from ground level.

It's hard for tornadoes and strong winds to penetrate to the ground in that setup. Wind from those phenomena have to push the stable air out of the way; all hail has to do is drop.

“Usually, what we see is hail forming in the thunderstorm above that stable air,” Lee said.

It's important to remember, though, that a lower risk of wind and tornadoes is not the same as no risk. People will need to pay attention to the situation.

There's another threat Wednesday: Flooding. It's a different issue than the hail.

“Heavy rains we don't technically consider a severe weather threat,” Lee said.

Still, flash flooding can be exceptionally dangerous, especially if people decide to drive through flooded areas. A flash flood watch is in effect and there have already been some flood warnings issued for rivers and streams in the area.

The timing for the heaviest rain could compound the issues. Two to three inches of rain are forecast overnight, when darkness makes flooded roadways harder to spot.





 

Original story:

Most of the eastern half of the United States faces at least a chance of severe weather today, with southeast Iowa in the slight risk category.

The Storm Prediction Center says strong winds and hail are the greatest threats from today's storms. While the system does bring a threat of tornadoes, that threat is focused on central Oklahoma and parts of Kansas and Missouri.

That doesn't mean southeast Iowa can ignore the situation. The National Weather Service has issued a severe thunderstorm watch that runs through 5 p.m. The highest risk from these storms is heavy rain, strong winds and the potential for large hail.

The area is under a flash flood watch as heavy rains moved through overnight and more storms arrived in the morning. Heavy rains produce more runoff, leading to the possibility of fast-rising streams. If you come to a roadway that has been flooded, do not drive into the water.

Most of the severe weather is forecast to be out of the area Thursday, but the threat will persist in northern Missouri and extreme eastern Iowa. That's close enough to warrant staying aware of conditions.