After several hot and mostly dry days across Iowa, the weather pattern is about to change in a big way. Multiple rounds of showers and thunderstorms are expected to develop Thursday afternoon and continue through Friday, bringing the potential for locally heavy rainfall, flash flooding, and a few strong to severe storms.
While a few storms Thursday afternoon could produce damaging wind gusts, the greatest concern across much of Iowa is increasingly becoming excessive rainfall and flooding.
Flooding Threat Increasing Across Iowa
The National Weather Service’s Weather Prediction Center (WPC) has placed much of Iowa within a Slight Risk (Level 2 of 4) for Excessive Rainfall Thursday into Friday, highlighting the potential for flash flooding if thunderstorms repeatedly move over the same locations.
Unlike a standard rainfall forecast, the WPC’s Excessive Rainfall Outlook focuses specifically on the potential for rainfall rates and totals that could exceed the ground’s ability to absorb water, leading to rapid runoff and flash flooding.
Several ingredients are coming together that favor efficient, heavy rainfall:
- Tropical-level moisture with atmospheric moisture values exceeding 2 inches
- Multiple disturbances moving across the Plains
- Slow-moving thunderstorms
- The potential for storms to repeatedly track over the same areas (known as training)
When thunderstorms train over the same location, rainfall totals can quickly add up in just a few hours.
Thursday: Storms Develop in Western Iowa
Thursday will begin warm and increasingly humid before thunderstorms begin developing across western Iowa during the afternoon.
A few storms may become strong to severe, especially across western Iowa where the Storm Prediction Center has placed portions of the state under a Marginal Risk (Level 1 of 5) for severe weather.
The primary severe threat will be:
- Damaging wind gusts
- Frequent lightning
- Torrential rainfall
As the evening progresses, thunderstorms are expected to spread east across central Iowa.
Thursday Night Into Friday: Heavy Rain Becomes the Main Story
The period from Thursday night through Friday morning appears to carry the highest flooding risk.
Forecast models continue to indicate several rounds of thunderstorms moving through Iowa as a slow-moving upper-level system crosses the region. Because storm movement may be relatively slow, individual communities could receive multiple rounds of heavy rain.
Atmospheric moisture values are forecast between 2.0 and 2.5 inches, which is exceptionally high for late July in Iowa and supports extremely efficient rainfall production.
Rainfall Totals Still Uncertain
One of the biggest forecast challenges is determining exactly where the corridor of heaviest rain will develop.
Some forecast models focus the highest rainfall across northern Iowa, while others shift the axis farther south into central Iowa.
Regardless of the exact placement, much of the state has the potential to receive beneficial rainfall, with localized areas seeing considerably higher amounts where thunderstorms repeatedly develop.
Residents should remember that even if widespread flooding does not occur, localized flash flooding can develop quickly wherever storms repeatedly move over the same area.
Why Flash Flooding Can Develop So Quickly
Flash flooding often occurs before rivers have time to rise.
Urban areas, low-lying roads, creeks, and locations with poor drainage are especially vulnerable when rainfall rates exceed one to two inches per hour.
Never attempt to drive across a flooded roadway. It takes only a small amount of moving water to carry away a vehicle, and the road beneath the water may already be washed out.
Cooler Weather Arrives This Weekend
Once the storm system exits Friday, much cooler and more comfortable air will move into Iowa.
High temperatures this weekend are expected to remain in the low to mid-80s, nearly 10 degrees cooler than the heat experienced earlier this week. Humidity will also decrease, making for a much more pleasant start to August.
What You Should Do Now
With flooding becoming the primary concern, now is a good time to prepare before storms arrive.
- Stay updated with the latest forecasts Thursday and Friday.
- Be ready for rapidly changing conditions if thunderstorms repeatedly affect your area.
- Never drive through flooded roadways.
- If you live in a flood-prone location, have a plan in case water begins rising quickly.
Forecast confidence continues to increase that much of Iowa will receive widespread rainfall, but confidence remains lower regarding exactly where the heaviest rainfall corridor will set up. As newer forecast data becomes available over the next 24 hours, the expected axis of greatest flooding potential will become clearer.
The Iowa Storm Chasing Network and IowaWeather.com will continue providing updates as confidence increases on where the heaviest rain and greatest flooding threat will develop.