F2 Tornado — Humboldt, Iowa

2004-05-21 · near Bradgate, Humboldt, Iowa

Event narrative

Tornado intensified as it approached Bradgate. Seventy five percent of the town was damaged or destroyed by the tornado which was one half mile wide as it moved through.

Wider weather episode

A very unstable airmass was over Iowa and helped kick off the seasons first severe weather outbreak. At the surface a warm frontal boundary extended nearly east to west across the state and provided the focus for thunderstorm development. By the late afternoon, lifted indices were approaching -10 C. with CAPE values around 5000 J/kg over western Iowa. Surface temperatures warmed into the upper 80s with dew points in the low to mid 70s. A southwest surface wind of 15 to 25 kts pushed over the front. Northeast of the frontal boundary winds were easterly around 10 to 15 kts. Actually, the surface boundary was further enhanced by outflow from convective complexes during the day, further sharpening the contrast. Thunderstorms continued to fire along and north of the boundary through the afternoon and into the evening. This resulted in widespread flash flooding as very heavy rains fell on areas that had had significant rainfall the previous night. There were reports in north central into northeast Iowa of 2 to 4 inches of rainfall in a little more than an hours time. Major flooding took place in the Mason City area where evacuations were taking place. The thunderstorms became most intense during the afternoon into the early evening. There were several tornadoes across northeast into north central Iowa. Most were relatively brief touchdowns and were in open areas. Reports of multi-vortex tornadoes were received from Grundy County. The days most significant tornado touched down in Pocahontas County east of Rolfe and tracked into Humboldt County through the Bradgate area. The tornado damaged or destroyed over 75% of the town of Bradgate (pop 120). Tornado damage in Bradgate was very extensive impacting most of the town. Hardly a building was not impacted. Outbuildings, light structures and garages were heavily damaged or destroyed. Two homes were destroyed. Several homes were damaged ranging from minor to extensive. The Survey Team found that the majority of the damage was F1, with a few cases of F2 damage. The F2 damage was to the snapping of very large trees just west of town and the structural damage to two buildings. Nearly all of the 53 homes there reported damage with several totally destroyed. Significant widespread property damage was reported in the tornadoes path. There were several injuries in the town, the most significant being a broken leg. A police officer was reportedly hit by debris from the tornado and suffered a broken leg. According to law officials, the damage path of the tornado was 2 miles wide and about 8 miles long. Storm chasers on the scene reported the tornado itself to be one half mile wide at one point. Reports of hail were numerous through the afternoon and evening. Most of the hail was in the three quarter inch to one inch diameter range. There were a few reports of golf ball size hail from the stronger storms. Winds were not a major problem with the activity. There were spotty reports of winds to around 60 MPH, but most reports were in the 40 to 50 MPH range. One of the stronger wind reports was a 64 MPH wind gust in Tripoli in Bremer County. This gust was measured by a mesonet station there. During the mid evening hours, thunderstorms rumbled across Grundy County. Lightning struck the Sheriff's office there and destroyed a significant amount of office equipment. Damage was reported around $100,000.

View location on OpenStreetMap → (42.9833, -93.4833)

Source: NOAA Storm Events Database, event_id 5394317. Narrative written by the NWS forecast office that issued the report.