A weak, relatively stationary, boundary found itself across portions of western and central Iowa during the evening and overnight hours of the 1st into the 2nd.  While surface observations were not tremendous, especially for early September, in the upper 70s to mid 80s, SBCAPE values approached 4000+ J/kg in areas of southwest and south-central Iowa concurrent with dew points into the upper 70s. Support for organized storms was marginal to weakly supportive with bulk effective shear generally less than 40 kts and hail CAPE around 800 J/kg. Heavy rain was also favored with the slow moving boundary, expected 850mb jet, and strong moisture advection into the area.
Resulting weather involved initially discrete cells producing severe hail, quickly morphing into multicellular convection and messier convection as the low level jet began to influence conditions. Early on, severe hail dominated reports, eventually giving way to heavy rainfall and flash flooding. The most impressive reports included up to 2 inch hail on the north side of the Des Moines metro area and heavy rainfall in excess of 6 inches in portions of Poweshiek County.

Flood in STORY, IOWA
From 02-SEP-18 02:47:00 to 02-SEP-18 11:30:00 (CST-6)
Location: NEVADA MUNI ARPT
Flood cause: Heavy Rain
Property damage: 50.00K
Crop damage: 50.00K
Reported by: Law Enforcement

Law enforcement reported several county and rural roads across the county were under water from the heavy rainfall.

Flood in POWESHIEK, IOWA
From 02-SEP-18 02:47:00 to 02-SEP-18 11:30:00 (CST-6)
Location: GRINNELL
Flood cause: Heavy Rain
Property damage: 50.00K
Crop damage: 50.00K
Reported by: Law Enforcement

Law enforcement reported several county and rural roads across the county were under water from the heavy rainfall.

Heavy Rain in POWESHIEK, IOWA
From 01-SEP-18 00:00:00 to 01-SEP-18 22:58:00 (CST-6)
Location: MONTEZUMA
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter reported heavy rainfall of 6.48 inches for the last 24 hour period. Due to limitations in the database entry, beginning time coded as midnight CST.

Heavy Rain in POLK, IOWA
From 01-SEP-18 08:00:00 to 02-SEP-18 08:00:00 (CST-6)
Location: POLK CITY
Property damage: 0.00K
Crop damage: 0.00K
Reported by: CoCoRaHS

CocoRaHS observer recorded heavy rainfall of 3.44 inches over the last 24 hours.

Heavy Rain in JASPER, IOWA
From 01-SEP-18 18:00:00 to 02-SEP-18 01:00:00 (CST-6)
Location: NEWTON
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter reported heavy rainfall of 5.65 inches near Highway 14 and Interstate 80.

Flood in MARSHALL, IOWA
From 02-SEP-18 02:47:00 to 02-SEP-18 11:30:00 (CST-6)
Location: DUNBAR
Flood cause: Heavy Rain
Property damage: 50.00K
Crop damage: 50.00K
Reported by: Law Enforcement

Marshall County 911 reported the shoulder washed out of Hwy 146 south of 290th St.

Hail in POLK, IOWA
From 01-SEP-18 18:05:00 to 01-SEP-18 18:05:00 (CST-6)
Location: GRIMES
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Social Media

Public reported quarter sized hail.

Hail in POLK, IOWA
From 01-SEP-18 18:15:00 to 01-SEP-18 18:15:00 (CST-6)
Location: DODGE FLD ARPT
Magnitude: 1.25 
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Public

Public relayed report of half dollar sized hail.

Hail in POLK, IOWA
From 01-SEP-18 18:11:00 to 01-SEP-18 18:11:00 (CST-6)
Location: DODGE FLD ARPT
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter reported quarter sized hail.

Heavy Rain in JASPER, IOWA
From 01-SEP-18 06:00:00 to 02-SEP-18 06:00:00 (CST-6)
Location: NEWTON MUNI ARPT
Property damage: 0.00K
Crop damage: 0.00K
Reported by: AWOS

Newton Municipal Airport recorded heavy rainfall of 4.14 inches over the last 24 hours.

Flash Flood in POWESHIEK, IOWA
From 01-SEP-18 20:47:00 to 02-SEP-18 02:55:00 (CST-6)
Location: SHERIDAN
Flood cause: Heavy Rain
Property damage: 50.00K
Crop damage: 50.00K
Reported by: 911 Call Center

Local dispatch reported a gravel road washed out along 120th St between 310th and 320th St.

Flash Flood in POLK, IOWA
From 01-SEP-18 21:43:00 to 02-SEP-18 02:15:00 (CST-6)
Location: POLK CITY
Flood cause: Heavy Rain
Property damage: 50.00K
Crop damage: 0.00K
Reported by: Social Media

Public stated that Highway 17 northbound is underwater and closed approximately 1 mile north of Granger.

Heavy Rain in JASPER, IOWA
From 01-SEP-18 06:00:00 to 02-SEP-18 06:00:00 (CST-6)
Location: NEWTON
Property damage: 0.00K
Crop damage: 0.00K
Reported by: COOP Observer

Coop observer (radio station KCOB) recorded heavy rainfall of 2.90 inches over the last 24 hours.

Flash Flood in POWESHIEK, IOWA
From 01-SEP-18 20:47:00 to 02-SEP-18 02:55:00 (CST-6)
Location: SHERIDAN
Flood cause: Heavy Rain
Property damage: 10.00K
Crop damage: 10.00K
Reported by: Law Enforcement

Poweshiek County Sheriff's Office reported a gravel road washed out; 120th Street between 320th and 310th streets.

Hail in POLK, IOWA
From 01-SEP-18 18:30:00 to 01-SEP-18 18:30:00 (CST-6)
Location: DODGE FLD ARPT
Magnitude: 1.25 
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Public

Hail in POLK, IOWA
From 01-SEP-18 18:47:00 to 01-SEP-18 18:47:00 (CST-6)
Location: BONDURANT
Magnitude: 1.75 
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Social Media

Public reported golf ball sized hail.

Hail in POLK, IOWA
From 01-SEP-18 18:38:00 to 01-SEP-18 18:38:00 (CST-6)
Location: CAPITOL HGTS
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Public reported quarter sized hail.

Hail in DALLAS, IOWA
From 01-SEP-18 18:48:00 to 01-SEP-18 18:48:00 (CST-6)
Location: DALLAS CENTER
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Dallas Center Fire Department reported nickel sized hail.

Flash Flood in POLK, IOWA
From 01-SEP-18 22:47:00 to 02-SEP-18 02:15:00 (CST-6)
Location: HERROLD
Flood cause: Heavy Rain
Property damage: 20.00K
Crop damage: 50.00K
Reported by: Department of Highways

Iowa DOT reported that Highway 141 between Grimes and Granger was essentially a sheet of water, with speeds of only 10 mph possible.

Heavy Rain in POWESHIEK, IOWA
From 01-SEP-18 06:00:00 to 02-SEP-18 06:00:00 (CST-6)
Location: MONTEZUMA
Property damage: 0.00K
Crop damage: 0.00K
Reported by: COOP Observer

Coop observer recorded heavy rainfall of 5.16 inches over the last 24 hours.

Heavy Rain in POWESHIEK, IOWA
From 01-SEP-18 06:00:00 to 02-SEP-18 06:00:00 (CST-6)
Location: GRINNELL ARPT
Property damage: 0.00K
Crop damage: 0.00K
Reported by: AWOS

Grinnell Regional Airport recorded heavy rainfall of 3.20 inches over the last 24 hours.

Flash Flood in STORY, IOWA
From 02-SEP-18 00:25:00 to 02-SEP-18 02:55:00 (CST-6)
Location: COLO
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

Iowa DOT reported foot deep water along Highway 30 east of Highway 65. Story County Sheriff also reported Highway 30 on-ramp to Highway 65 under water.

Hail in POLK, IOWA
From 01-SEP-18 18:10:00 to 01-SEP-18 18:10:00 (CST-6)
Location: LOVINGTON
Magnitude: 2.00 
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter reported hail larger than golf balls.

Hail in POLK, IOWA
From 01-SEP-18 18:35:00 to 01-SEP-18 18:35:00 (CST-6)
Location: CAPITOL HGTS
Magnitude: 1.75 
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Delaware Township Fire Department reported golf ball sized hail.

Hail in POLK, IOWA
From 01-SEP-18 18:39:00 to 01-SEP-18 18:39:00 (CST-6)
Location: NORWOODVILLE
Magnitude: 2.00 
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Broadcast Media

WHO relayed reports of 2 inch hail in the Norwoodville Neighborhood. Time estimated from radar.

Flash Flood in MARSHALL, IOWA
From 02-SEP-18 01:42:00 to 02-SEP-18 02:55:00 (CST-6)
Location: DUNBAR
Flood cause: Heavy Rain
Property damage: 20.00K
Crop damage: 0.00K
Reported by: Department of Highways

Marshall County 911 reported the shoulder washed out on Hwy 146 south of 290th St.

Heavy Rain in JASPER, IOWA
From 01-SEP-18 18:00:00 to 01-SEP-18 19:27:00 (CST-6)
Location: COLFAX
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter reported heavy rainfall of 3.5 inches.

Heavy Rain in JASPER, IOWA
From 01-SEP-18 18:00:00 to 01-SEP-18 21:55:00 (CST-6)
Location: NEWTON
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter reported heavy rainfall of 4.25 inches since 7pm.

Heavy Rain in POWESHIEK, IOWA
From 01-SEP-18 06:00:00 to 02-SEP-18 06:00:00 (CST-6)
Location: GRINNELL
Property damage: 0.00K
Crop damage: 0.00K
Reported by: COOP Observer

Coop observer recorded heavy rainfall of 4.54 inches over the last 24 hours.

Flash Flood in STORY, IOWA
From 02-SEP-18 00:25:00 to 02-SEP-18 02:55:00 (CST-6)
Location: COLO
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

Iowa DOT reported Highway 65 at Old Lincoln Highway was completely under water. The local fire department was blocking the road.

Heavy Rain in STORY, IOWA
From 01-SEP-18 06:00:00 to 02-SEP-18 06:00:00 (CST-6)
Location: AMES ARPT
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

Ames Municipal Airport recorded heavy rainfall of 2.14 inches over the last 24 hours.

Hail in POLK, IOWA
From 01-SEP-18 18:48:00 to 01-SEP-18 18:48:00 (CST-6)
Location: BONDURANT
Magnitude: 1.75 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Public

Public reported golf ball sized hail via the mPING app. Time estimated from radar.

Hail in POLK, IOWA
From 01-SEP-18 18:12:00 to 01-SEP-18 18:15:00 (CST-6)
Location: JOHNSTON
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Heavy Rain in STORY, IOWA
From 01-SEP-18 18:30:00 to 02-SEP-18 02:30:00 (CST-6)
Location: AMES
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Social Media

Public reported heavy rainfall of 2.10 inches on the north side of Ames.

Heavy Rain in JASPER, IOWA
From 01-SEP-18 18:00:00 to 02-SEP-18 01:00:00 (CST-6)
Location: KELLOGG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter reported heavy rainfall of 7.5 inches.

Hail in POLK, IOWA
From 01-SEP-18 18:20:00 to 01-SEP-18 18:30:00 (CST-6)
Location: SAYDELL
Magnitude: 1.50 
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter reported ping pong ball sized hail at Saydel High School.

Hail in STORY, IOWA
From 01-SEP-18 22:01:00 to 01-SEP-18 22:01:00 (CST-6)
Location: AMES ARPT
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Social Media

Public reported nickel sized hail.

Flash Flood in DALLAS, IOWA
From 01-SEP-18 19:41:00 to 02-SEP-18 02:10:00 (CST-6)
Location: GRANGER
Flood cause: Heavy Rain
Property damage: 20.00K
Crop damage: 50.00K
Reported by: Fire Department/Rescue

Local fire department reported a motorist rescue along Xavier Ave, where water was up to the hood of the vehicle.