A frontal boundary was located to the northwest of Iowa and remained nearly stationary as a shortwave advanced across the state.  The airmass became very unstable with in the 3000 to 4000 J/kg range and lifted indices in the -6 to -9 C. range.  The atmosphere was weakly sheared with 20 to 30 kts of shear.  A fair amount of downdraft CAPE was available at the onset with values in the 800 to 1000 J/kg range.  CAPE in the -10 to -30 C. layer of the atmosphere was somewhat limited, in the 200 to 300 J/kg range and the freezing level was around 14,400 feet.  Considerable moisture transport took place during the evening with precipitable water values rising to between 1.8 and 2 inches.  Thunderstorms initially formed fairly close to the frontal boundary, then propagated south-southeast across the state.  Initially, the storms produced three quarter to one inch diameter hail, along with occasional wind gusts to around 60 MPH.  As the later part of the afternoon and evening hours wore on, the storms transformed into a heavy rain event.  As the storms moved south across central Iowa, rainfall rates were incredible, with one of the storms producing a rainfall rate of 8.5 inches per hour for about 10 minutes.  The area received nearly four and three quarters of an inch of rain northeast of Ames.  There were numerous reports of rainfall in the 2 to 3 inch range, much of it falling in about an hour.  The ground conditions were relatively dry as little rainfall had occurred during the previous 2 weeks.  The heavy rain did not cause major problems in rural areas.  The Des Moines metropolitan area was hard hit with one of the heavier reports in Windsor Heights where 3.54 inches fell, 2.48 of which fell in a 40 minute time span.  This caused extensive ponding of water around the city, in some places nearly 3 feet deep.  Walnut Creek, in Polk County, rose 10 to 11 feet in just 3 hours and crested just over flood stage.  There were numerous other heavy rain reports, including 3.89 inches in the Webster City area, 3.61 inches in Marshalltown, and 3.38 inches in Johnston.

Flood in POLK, IOWA
From 17-JUL-08 21:27:00 to 17-JUL-08 23:45:00 (CST-6)
Location: DES MOINES
Flood cause: Heavy Rain
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Three to 4 inches of water was reported over the road.

Flood in POLK, IOWA
From 17-JUL-08 22:15:00 to 17-JUL-08 23:45:00 (CST-6)
Location: URBANDALE
Flood cause: Heavy Rain
Property damage: 20.00K
Crop damage: 0.00K
Reported by: NWS Employee

One foot of water was covering the road near 86th and Meredith.

Heavy Rain in POLK, IOWA
From 17-JUL-08 20:50:00 to 17-JUL-08 21:30:00 (CST-6)
Location: WINDSOR HGTS
Property damage: 25.00K
Crop damage: 0.00K
Reported by: NWS Employee

Rainfall recorded in the past 40 minutes totaled 2.48 inches.

Heavy Rain in STORY, IOWA
From 17-JUL-08 15:30:00 to 17-JUL-08 21:19:00 (CST-6)
Location: ROLAND
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Amateur Radio

Storm total for the day was 4.73 inches.

Hail in WEBSTER, IOWA
From 17-JUL-08 18:11:00 to 17-JUL-08 18:11:00 (CST-6)
Location: CLARE
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 10.00K
Reported by: Trained Spotter

Hail in WEBSTER, IOWA
From 17-JUL-08 18:25:00 to 17-JUL-08 18:25:00 (CST-6)
Location: (FOD)FT DODGE MUNI A
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 10.00K
Reported by: Trained Spotter

Hail in WEBSTER, IOWA
From 17-JUL-08 18:27:00 to 17-JUL-08 18:27:00 (CST-6)
Location: FT DODGE
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 10.00K
Reported by: Broadcast Media

Dime to quarter size hail was reported by KCCI Television.

Hail in WEBSTER, IOWA
From 17-JUL-08 18:08:00 to 17-JUL-08 18:08:00 (CST-6)
Location: CLARE
Magnitude: 0.88 
Property damage: 1.00K
Crop damage: 10.00K
Reported by: Trained Spotter

Thunderstorm Wind in WEBSTER, IOWA
From 17-JUL-08 18:20:00 to 17-JUL-08 18:20:00 (CST-6)
Location: FT DODGE
Magnitude: 52.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Street flooding was also reported in the area.

Heavy Rain in HARDIN, IOWA
From 17-JUL-08 14:29:00 to 17-JUL-08 16:00:00 (CST-6)
Location: IOWA FALLS
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A trained spotter reported 2.71 inches of rain and also measured a 57 MPH wind gust in Alden.

Hail in CERRO GORDO, IOWA
From 17-JUL-08 13:22:00 to 17-JUL-08 13:22:00 (CST-6)
Location: EMERY
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 10.00K
Reported by: Law Enforcement

Penny size hail was reported at mile post 183 on the Avenue of the Saints.  Relayed via the Iowa State Patrol.

Hail in CERRO GORDO, IOWA
From 17-JUL-08 13:25:00 to 17-JUL-08 13:25:00 (CST-6)
Location: EMERY
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 10.00K
Reported by: Law Enforcement

The Iowa State Patrol reported that hail had increased to quarter size at mile post 183 of the Avenue of the Saints.

Hail in WEBSTER, IOWA
From 17-JUL-08 18:26:00 to 17-JUL-08 18:26:00 (CST-6)
Location: FT DODGE
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 10.00K
Reported by: Broadcast Media

Report received from KCCI television.

Heavy Rain in BLACK HAWK, IOWA
From 17-JUL-08 14:50:00 to 17-JUL-08 15:40:00 (CST-6)
Location: DEWAR
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Two and one quarter inches of rain fell in less than one hour.

Heavy Rain in BLACK HAWK, IOWA
From 17-JUL-08 15:26:00 to 17-JUL-08 16:26:00 (CST-6)
Location: WATERLOO
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

KWWL reported 2.77 inches of rainfall measured in downtown Waterloo.

Lightning in BLACK HAWK, IOWA
From 17-JUL-08 15:00:00 to 17-JUL-08 15:00:00 (CST-6)
Location: WATERLOO
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Broadcast Media

KWWL reported that lightning struck a home in Waterloo around 1500 CST.  Fire crews were called to the scene.

Hail in CERRO GORDO, IOWA
From 17-JUL-08 13:31:00 to 17-JUL-08 13:31:00 (CST-6)
Location: MASON CITY
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 2.00K
Reported by: Broadcast Media

Hail was reported in the city by KIMT television.