F0 tornado, Audubon, Iowa, April 15, 2006

1. F040 to 72 mphthis tornado

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

2. F173 to 112 mph

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

3. F2113 to 157 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. F3158 to 206 mph

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

5. F4207 to 260 mph

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

6. F5261 to 318 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

F0

Date

April 15, 2006

Touchdown

8:43 PM CDT (01:43 UTC)

Path length

0.2 miles (0.3 km)

Path width

30 yards (27 m)

Deaths

0

Injuries

0

County

Audubon County

Touchdown point

41.700, -94.970

Lift-off point

41.700, -94.970

Record number

387

The intense storm mentioned above progressed eastward slowly during the evening. Moisture was somewhat limited with a narrow band of one to one and one quarter inch precipitable water values lifting northeast into the state. Temperatures warmed into the low 80s during the afternoon, but convection was hindered in most areas with dew points in the 30s to low 40s. A narrow band of dew points in the upper 50s to low 60s pushed into southwest Iowa by late afternoon. At the same time, a pronounced dryline pushed across Kansas. An arc of thunderstorms formed in response to the dryline and moved into western Iowa during the evening. Initially the airmass was quite unstable when the storms formed Lifted indices were around -8 C. with CAPE values in the 1500 to 2000 J/kg range. There were some reports of hail, but that was limited in spite of the 11,500 foot freezing level. The hail reports that were received were in the three quarter to one inch diameter range. During the early evening the LCL fell to around 600 meters. There were two waves of thunderstorms. The initial set moved through during the period of high winds over the southwest and central counties. These storms were very elevated and clearly in an inverted "V" sounding environment. The downdrafts from the storms enhanced the winds as they passed. One produced downburst winds around 60 MPH in Grimes and tore a trailer door off. One tornado touched down in Audubon County southwest of Audubon. The tornado touched down in open fields and did not cause any damage. There were a few reports of high winds due to the storms. A combination of two things caused the high winds. One was the storms themselves initially as the flow in the low levels, the lowest 500 meters, was in the 50 to 60 kt range. The other occurred as the storms started to weaken. Soundings indicated that during the dissipation stage, the storms moved into a near storm environment that exhibited an inverted "V" sounding. This was the case in Taylor County at Lenox, where high winds tor the roof off of a hardware store and ripped the roof off of a hog confinement building. Several hogs were killed at the confinement farm. In Adams County, high winds resulted in the collapse of strong thunderstorms over the northern part of the county. The outflow rush produced winds estimated around 75 MPH south of Prescott. Numerous machine sheds and barns were damaged in the area. Eight inch diameter trees were downed as well as a few TV and Radio towers. Other reports of winds in the 60 to 65 MPH range were received, but little damage was reported.

Tornado touched down in open fields southwest of Audubon. Little to no damage occurred.

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

No tornado warning for this county appears in the archive as in force at the moment the tornado touched down. One was issued for the county at another hour that day.

The county was under a tornado watch, issued at 22:28 UTC and running to 02:44 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 Slight Risk, with a 0.10 chance of a tornado within 25 miles. The outlook in force was issued at 01:04 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 Des Moines radar (KDMX), 104 km away, was scanning. The volume nearest the touchdown began at 01:43 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 2006 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

13 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.