An unstable airmass was in place over northern Iowa with an outflow boundary draped across the north central counties into southwest Minnesota.  This boundary served as a focusing mechanism for thunderstorm development.  The freezing level was relatively low at around 11,500 feet.  CAPE available in the -10 to -30 layer of the atmosphere was between 200 and 400 J/kg.  The atmosphere became unstable with afternoon highs in the upper 70s to low 80s and dewpoint temperatures in the mid 60s.  CAPE was around 2000 J/kg with a lifted index of -4 C.  The shear was quite weak with only 25 kts of shear available.  Precipitable water increased to 1.4 inches.  The LCL had lowered to 1000 meters by later afternoon.  Isolated thunderstorms formed along the old outflow boundary.  They moved very slowly and dumped copious amounts of rain.  A report of 5 inches in one hour was received from north of Emmetsburg in Palo Alto County resulting in flash flooding in the city.  There were numerous reports of half inch to three quarter inch diameter hail and 1 inch diameter hail falling west of Emmetsburg.  As the thunderstorm continued to the southwest, hail of 1 to 1.75 inch diameter fell with the largest being golf ball size hail that fell west-southwest of Curlew.

Heavy Rain in PALO ALTO, IOWA
From 17-MAY-13 17:15:00 to 17-MAY-13 17:15:00 (CST-6)
Location: EMMETSBURG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Rainfall in the past hour of 1.25 inches.

Heavy Rain in PALO ALTO, IOWA
From 17-MAY-13 17:24:00 to 17-MAY-13 17:24:00 (CST-6)
Location: EMMETSBURG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Rainfall in the past hour of 5.00 inches.  Relayed by KICD radio.

Hail in PALO ALTO, IOWA
From 17-MAY-13 16:40:00 to 17-MAY-13 16:40:00 (CST-6)
Location: EMMETSBURG
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Hail in PALO ALTO, IOWA
From 17-MAY-13 18:00:00 to 17-MAY-13 18:00:00 (CST-6)
Location: CURLEW
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Flash Flood in PALO ALTO, IOWA
From 17-MAY-13 16:48:00 to 17-MAY-13 19:00:00 (CST-6)
Location: EMMETSBURG
Flood cause: Heavy Rain
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Several inches of water flowing over the road in Emmetsburg at Lawler and Main.

Hail in PALO ALTO, IOWA
From 17-MAY-13 17:15:00 to 17-MAY-13 17:15:00 (CST-6)
Location: EMMETSBURG
Magnitude: 1.00 
Property damage: 1.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Hail in PALO ALTO, IOWA
From 17-MAY-13 17:00:00 to 17-MAY-13 17:00:00 (CST-6)
Location: EMMETSBURG
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Hail in PALO ALTO, IOWA
From 17-MAY-13 17:45:00 to 17-MAY-13 17:45:00 (CST-6)
Location: CURLEW
Magnitude: 1.75 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Hail in PALO ALTO, IOWA
From 17-MAY-13 17:55:00 to 17-MAY-13 17:55:00 (CST-6)
Location: CURLEW
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Hail in PALO ALTO, IOWA
From 17-MAY-13 16:27:00 to 17-MAY-13 16:27:00 (CST-6)
Location: EMMETSBURG
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Public

Public reported heavy rainfall along with nickel to quarter size hailstones.