The cold front that had moved southeast across the state the previous night became stationary across southern Iowa by the morning of the 17th.  A sharp contrast in dew point temperatures was found across the front.  Dew points were in the upper 50s to mid 60s south of the front, with dew points in the 30s north of it.  During the late afternoon the front began to return north as a warm front.  By evening a strengthening southerly jet in the low levels impinged on the front with wind speeds around 50 kts.  The overall low levels of the atmosphere were relatively dry and the airmass was capped initially.  The LCL height was around 750 mb.  There atmosphere was quite unstable with Lifted Indices in the -6 to -10 C range.  CAPE values were in the 2500 to 3500 J/kg range.  Thunderstorms erupted over northern Iowa shortly after sunset.  The storms formed into discrete supercells.  A tornado touched down east of Mason City from one of the stronger cells.  It was in open area and did not cause any damage during its brief touchdown.  The dominant severe weather mode for the event was hail.  Baseball size hail fell in Palo Alto County in Mallard.  There were numerous reports of hail three quarters of an inch to one and one quarter of an inch in diameter.  The largest hail was around two inches in diameter in Palo Alto County.  Hail reportedly covered the ground as the storms passed.  During the late stages of the event, a thunderstorms produced high winds as it swept across Interstate 35 west of Northwood in Worth County.  Two vehicles and one semi-tractortrailer truck were blown off of the interstate near mile post 214.  In addition, one telephone was blown down by the high wind.

Tornado in CERRO GORDO, IOWA
From 17-APR-04 20:40:00 to 17-APR-04 20:40:00 (CST)
Location: MASON CITY
Tornado F-scale: F0
Reported by: BROADCAST MEDIA

Brief touchdown in open area east of Mason City.

Hail in KOSSUTH, IOWA
From 17-APR-04 19:04:00 to 17-APR-04 19:04:00 (CST)
Location: LAKOTA
Magnitude: 0.75 
Reported by: TRAINED SPOTTER

Hail in WORTH, IOWA
From 17-APR-04 22:05:00 to 17-APR-04 22:20:00 (CST)
Location: NORTHWOOD
Magnitude: 1.50 
Property damage: 10K
Reported by: GENERAL PUBLIC

Hail in WORTH, IOWA
From 17-APR-04 20:50:00 to 17-APR-04 20:50:00 (CST)
Location: KENSETT
Magnitude: 0.88 
Property damage: 1K
Reported by: AMATEUR RADIO

Hail in PALO ALTO, IOWA
From 17-APR-04 21:15:00 to 17-APR-04 21:15:00 (CST)
Location: MALLARD
Magnitude: 2.75 
Property damage: 25K
Reported by: TRAINED SPOTTER

Hail in WORTH, IOWA
From 17-APR-04 20:09:00 to 17-APR-04 20:09:00 (CST)
Location: JOICE
Magnitude: 1.00 
Property damage: 2K
Reported by: TRAINED SPOTTER

Hail in WINNEBAGO, IOWA
From 17-APR-04 21:28:00 to 17-APR-04 21:28:00 (CST)
Location: THOMPSON
Magnitude: 1.75 
Property damage: 5K
Reported by: TRAINED SPOTTER

Hail in WORTH, IOWA
From 17-APR-04 20:23:00 to 17-APR-04 20:23:00 (CST)
Location: KENSETT
Magnitude: 1.00 
Property damage: 2K
Reported by: TRAINED SPOTTER

Hail in POCAHONTAS, IOWA
From 17-APR-04 21:01:00 to 17-APR-04 21:01:00 (CST)
Location: PLOVER
Magnitude: 1.00 
Property damage: 1K
Reported by: TRAINED SPOTTER

Hail in WINNEBAGO, IOWA
From 17-APR-04 20:06:00 to 17-APR-04 20:06:00 (CST)
Location: LAKE MILLS
Magnitude: 0.88 
Property damage: 2K
Reported by: TRAINED SPOTTER

Hail in KOSSUTH, IOWA
From 17-APR-04 20:58:00 to 17-APR-04 20:58:00 (CST)
Location: ALGONA
Magnitude: 0.88 
Reported by: TRAINED SPOTTER

Hail in POCAHONTAS, IOWA
From 17-APR-04 20:56:00 to 17-APR-04 20:56:00 (CST)
Location: PLOVER
Magnitude: 0.75 
Reported by: TRAINED SPOTTER

Hail in WORTH, IOWA
From 17-APR-04 20:31:00 to 17-APR-04 20:31:00 (CST)
Location: NORTHWOOD
Magnitude: 1.25 
Property damage: 5K
Reported by: TRAINED SPOTTER

Hail in CERRO GORDO, IOWA
From 17-APR-04 21:32:00 to 17-APR-04 21:32:00 (CST)
Location: ROCK FALLS
Magnitude: 0.75 
Reported by: TRAINED SPOTTER

Hail in KOSSUTH, IOWA
From 17-APR-04 21:14:00 to 17-APR-04 21:14:00 (CST)
Location: TITONKA
Magnitude: 1.00 
Property damage: 5K
Reported by: OFFICIAL NWS OBS.

Hail in WINNEBAGO, IOWA
From 17-APR-04 19:37:00 to 17-APR-04 19:37:00 (CST)
Location: THOMPSON
Magnitude: 1.25 
Property damage: 5K
Reported by: TRAINED SPOTTER

Hail in PALO ALTO, IOWA
From 17-APR-04 21:15:00 to 17-APR-04 21:15:00 (CST)
Location: WEST BEND
Magnitude: 1.75 
Property damage: 10K
Reported by: EMERGENCY MANAGER

Hail in KOSSUTH, IOWA
From 17-APR-04 21:01:00 to 17-APR-04 21:01:00 (CST)
Location: WHITTEMORE
Magnitude: 0.88 
Property damage: 1K
Reported by: LAW ENFORCEMENT

Hail in WORTH, IOWA
From 17-APR-04 20:35:00 to 17-APR-04 20:35:00 (CST)
Location: MANLY
Magnitude: 0.88 
Property damage: 1K
Reported by: LAW ENFORCEMENT

Thunderstorm Wind in WORTH, IOWA
From 17-APR-04 21:43:00 to 17-APR-04 21:43:00 (CST)
Location: NORTHWOOD
Magnitude: 70 EG
Property damage: 15K
Reported by: LAW ENFORCEMENT

Hail in PALO ALTO, IOWA
From 17-APR-04 21:00:00 to 17-APR-04 21:00:00 (CST)
Location: WEST BEND
Magnitude: 2.00 
Property damage: 15K
Reported by: TRAINED SPOTTER

Thunderstorm Wind in WINNEBAGO, IOWA
From 17-APR-04 21:30:00 to 17-APR-04 21:30:00 (CST)
Location: SCARVILLE
Magnitude: 65 EG
Property damage: 40K
Reported by: EMERGENCY MANAGER

Hail in POCAHONTAS, IOWA
From 17-APR-04 20:50:00 to 17-APR-04 20:50:00 (CST)
Location: HAVELOCK
Magnitude: 0.75 
Reported by: TRAINED SPOTTER

Thunderstorm Wind in WINNEBAGO, IOWA
From 17-APR-04 21:20:00 to 17-APR-04 21:20:00 (CST)
Location: SCARVILLE
Magnitude: 65 EG
Property damage: 40K
Reported by: EMERGENCY MANAGER

Hail in WORTH, IOWA
From 17-APR-04 20:31:00 to 17-APR-04 20:31:00 (CST)
Location: KENSETT
Magnitude: 0.75 
Reported by: TRAINED SPOTTER

Hail in PALO ALTO, IOWA
From 17-APR-04 21:00:00 to 17-APR-04 21:00:00 (CST)
Location: WEST BEND
Magnitude: 0.88 
Reported by: EMERGENCY MANAGER

Hail in CERRO GORDO, IOWA
From 17-APR-04 20:54:00 to 17-APR-04 20:55:00 (CST)
Location: ROCK FALLS
Magnitude: 1.25 
Property damage: 5K
Reported by: TRAINED SPOTTER

Hail in PALO ALTO, IOWA
From 17-APR-04 21:06:00 to 17-APR-04 21:06:00 (CST)
Location: MALLARD
Magnitude: 1.00 
Property damage: 2K
Reported by: TRAINED SPOTTER

Hail in PALO ALTO, IOWA
From 17-APR-04 21:10:00 to 17-APR-04 21:10:00 (CST)
Location: WEST BEND
Magnitude: 2.00 
Property damage: 10K
Reported by: TRAINED SPOTTER