EF0 tornado, Tripoli, Iowa, May 22, 2011

1. EF065 to 85 mphthis tornado

   Light damage. Surface peeled off some roofs, gutters and siding damaged, branches broken, shallow-rooted trees pushed over.

2. EF186 to 110 mph

   Moderate damage. Roofs badly stripped, mobile homes overturned or badly damaged, exterior doors lost, windows broken.

3. EF2111 to 135 mph

   Considerable damage. Roofs torn off well-built houses, frame homes shifted on their foundations, mobile homes destroyed, large trees snapped or uprooted, cars lifted off the ground.

4. EF3136 to 165 mph

   Severe damage. Whole stories of well-built houses destroyed, large buildings badly damaged, trains overturned, trees debarked, heavy cars lifted and thrown.

5. EF4166 to 200 mph

   Devastating damage. Well-built houses leveled, structures with weak foundations moved some distance, cars thrown and large missiles generated.

6. EF5over 200 mph

   Incredible damage. Strong frame houses swept off their foundations, car-sized missiles thrown more than 100 meters, steel-reinforced concrete badly damaged, high-rise buildings deformed.

7. Unrated

   The survey recorded a tornado but could not put a number on it, most often because it crossed open country and damaged nothing a rating can be read from.

Rating

EF0

Date

May 22, 2011

Touchdown

3:13 PM CDT (20:13 UTC)

Path length

0.1 miles (0.2 km)

Path width

30 yards (27 m)

Deaths

0

Injuries

0

County

Bremer County

Touchdown point

42.790, -92.230

Lift-off point

42.790, -92.230

Record number

1105221413-01

The upper level low pressure that affected the state the previous day continued to move closer to the state. A complicated setup took place as it approached. Initially, a pseudo warm front lifted north across the state. A strong push of moisture sent dewpoint temperatures into the mid 60s to around 70. Meanwhile, a cold front/dryline pushed into western Iowa during the day with dewpoints in its wake in the low to mid 50s. The atmosphere became unstable as the day progressed. With the cold core low moving toward Iowa, the freezing level was quite low, around 10,500 feet. CAPE in the -10 to -30 layer of the atmosphere was between 600 and 800 J/kg. The downdraft CAPE was between 800 and 1100 J/kg. By mid afternoon, CAPE increased to 2500 to 4000 J/kg and lifted indices fell to -10 to -13 C. Shear increased across the state with 50 to 55 kts of shear available at the time of initiation. The LCL was relatively low, in the 750 to 1000 meters. The 0-1 km SRH values were around 150. There was more moisture available than was in place the previous day with PWAT values in the 1 to 1.25 inch range. Thunderstorms formed in two areas. The first was along the dryline over southeast Iowa. The storms became severe quickly and produced severe hail. The largest was golf ball size with the storms as they moved through Mahaska and Poweshiek Counties. Over northern Iowa, a line of thunderstorms formed on the edge of the cold pool as it interacted with the deepening moisture. These storms produced hail and several funnel clouds. Hail from this line was around an inch or less in diameter. One tornado touched down briefly southeast of Tripoli.

Funnel cloud reached three quarters of the way to the ground.

From NOAA NCEI's Storm Events Database, the narrative the local forecast office wrote for Storm Data.

31 minutes of warning. NWS DMX issued a tornado warning for this county at 19:42 UTC, 31 minutes before the tornado touched down, in force until 20:15 UTC.

The county was under a tornado watch, issued at 17:56 UTC and running to 20:42 UTC.

Every warning and watch covering the touchdown point that day, from the Iowa Environmental Mesonet's archive of National Weather Service products. Lead time is measured from the issue of the warning in force to the touchdown time in the database, both of which were recorded to the minute by different people.

The Storm Prediction Center had this point in a Moderate Risk, with a 0.15 chance of a tornado within 25 miles, in the hatched area where significant tornadoes were expected. The outlook in force was issued at 20:00 UTC.

SPC's Day 1 convective outlook as it stood when the tornado began, read at the touchdown point. The same archive is on the radar archive.

The radiosonde launches on either side of the tornado, from the nearest station that was launching then, drawn and analyzed the way Storm Lab does: the same diagram, the same parcel and shear definitions SPC's mesoanalysis uses.

The La Crosse radar (KARX), 142 km away, was scanning. The volume nearest the touchdown began at 20:11 UTC.

Open the radar archive at this moment, over the track

Reflectivity and velocity, decoded from the Level II archive in the browser. Volumes take a few seconds to open the first time.

Local storm reports within 60 km of the path, in the three hours either side.

From the Iowa Environmental Mesonet's archive of National Weather Service local storm reports.

The live radar over the trackEvery 2011 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

47 more were confirmed in the United States that day.

Ratings are the survey's, read off the damage rather than measured; the rating scale is above. Tornado counts rise through the record as reporting improved, so a year in the 1950s is not comparable with one today except for the strongest tornadoes, which were always noticed.

NOAA Storm Prediction Center tornado database, 1950-2025, the official record kept by NOAA's Storm Prediction Center.