Tornadoes · United States · Iowa · 1999

F0 tornado, Auburn, Iowa, May 27, 1999

This F0 tornado ran 0.1 miles across Sac County on May 27, 1999, touching down at 7:39 PM CDT and reaching 20 yards wide. It killed nobody.

The rating scale

Tracks are drawn in the color of the rating. The Fujita scale rates the damage rather than measuring the wind: the speeds are what that damage is taken to imply.

  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.

The record

Rating
F0
Date
May 27, 1999
Touchdown
7:39 PM CDT (00:39 UTC)
Path length
0.1 miles (0.2 km)
Path width
20 yards (18 m)
Deaths
0
Injuries
0
County
Sac County
Touchdown point
42.280, -94.880
Lift-off point
42.280, -94.880
Record number
311

What the Weather Service recorded

A weak flow pattern was in place over Iowa with a broad upper level ridge of high pressure over the state. A very weak cold front sagged south into Iowa during the afternoon. Thunderstorms developed along the front, however the dynamics were quite weak and moisture was quite limited. Along the line, winds of 40 to 50 MPH were common, along with numerous reports of pea to marble size hail. A report of hail around three quarters inch in diameter was received during the early afternoon over Emmet County. There wasn't't any other severe weather until the late afternoon hours. A brief spin-up tornado developed in Sac County north of Auburn as two outflow boundaries collided and a strong thunderstorm developed over the area. The tornado was very brief and no damage was reported as it touched down in open country. As the storms moved through Winnebago County lightning struck the Forest City School's wind turbine. Damage was done to the electrical equipment and the turbine was down for a few days.

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

The warnings

No tornado warning for this county appears in the archive as in force at the moment the tornado touched down.

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 forecast that day

The Storm Prediction Center had this point in a General Thunderstorms Risk. The outlook in force was issued at 19:51 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 air that afternoon

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.

Omaha, NE (KOAX), 00:00Z

0.7 hours before the tornado, 163 km from the path. May 28, 1999.

Skew-T log-pSkew-T log-p diagram, Omaha, NE at 00:00Z on 1999-05-28Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel lifted from the surface. Mixed-layer CAPE 0 joules per kilogram, CIN -84127, lifted condensation level 2605 meters above ground. Wind barbs in knots down the right edge.LCL10008507005003002001001 km2 km3 km6 km9 km12 km15 km-30°-20°-10°0°10°20°30°40°
Temperature in red, dew point in green, the mixed-layer parcel dashed. The warm shading is its CAPE, the cool shading the inhibition it has to break through.
HodographHodograph, Omaha, NE at 00:00Z on 1999-05-28The wind through the lowest 10 kilometers as a curve, rings every 10 meters per second labeled in knots. Zero to 1 km storm-relative helicity 103 square meters per square second, 0 to 6 km bulk shear 16 knots.1939Sfc1 km3 km6 km9 kmRMLM
The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km. RM and LM are the right and left moving storms Bunkers' method gives; the shaded area is the 0 to 1 km storm-relative helicity.
Mixed-layer CAPE
0 J/kg
Mixed-layer CIN
-84127 J/kg
Surface CAPE
172 J/kg
Mixed-layer LCL
2605 m above ground
0-6 km bulk shear
16 kt
0-1 km bulk shear
13 kt
0-1 km storm-relative helicity
103 m²/s²
0-3 km storm-relative helicity
147 m²/s²
Significant tornado parameter
0.0
Supercell composite
0.0
0-3 km lapse rate
8.6 K/km

Open this sounding in Storm Lab

Omaha, NE (KOAX), 12:00Z

11.3 hours after it, 163 km from the path. May 28, 1999.

Skew-T log-pSkew-T log-p diagram, Omaha, NE at 12:00Z on 1999-05-28Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel lifted from the surface. Mixed-layer CAPE 0 joules per kilogram, CIN -77486, lifted condensation level 1603 meters above ground. Wind barbs in knots down the right edge.LCL10008507005003002001001 km2 km3 km6 km9 km12 km15 km-30°-20°-10°0°10°20°30°40°
Temperature in red, dew point in green, the mixed-layer parcel dashed. The warm shading is its CAPE, the cool shading the inhibition it has to break through.
HodographHodograph, Omaha, NE at 12:00Z on 1999-05-28The wind through the lowest 10 kilometers as a curve, rings every 10 meters per second labeled in knots. Zero to 1 km storm-relative helicity 23 square meters per square second, 0 to 6 km bulk shear 10 knots.1939Sfc1 km3 km6 km9 kmRMLM
The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km. RM and LM are the right and left moving storms Bunkers' method gives; the shaded area is the 0 to 1 km storm-relative helicity.
Mixed-layer CAPE
0 J/kg
Mixed-layer CIN
-77486 J/kg
Surface CAPE
0 J/kg
Mixed-layer LCL
1603 m above ground
0-6 km bulk shear
10 kt
0-1 km bulk shear
4 kt
0-1 km storm-relative helicity
23 m²/s²
0-3 km storm-relative helicity
-10 m²/s²
Significant tornado parameter
0.0
Supercell composite
0.0
0-3 km lapse rate
4.5 K/km

Open this sounding in Storm Lab

The surface observations

Carroll (CIN), 27 km from the path, in the two hours either side.

Time (UTC)Temp (°F)Dew point (°F)WindGust (kt)Pressure (mb)Weather
22:45 UTC89.655.4250° 12 kt1015.6
23:05 UTC89.655.4220° 15 kt181015.2
23:25 UTC87.853.6230° 14 kt191015.2
23:45 UTC8653.6210° 12 kt161015.2
00:05 UTC84.253.6220° 12 kt161014.9
00:25 UTC82.453.6210° 11 kt1015.2
00:45 UTC82.453.6210° 9 kt1015.2
01:05 UTC80.653.6220° 8 kt1015.9
01:25 UTC75.25950° 4 kt251017.9
01:45 UTC73.45960° 9 kt1018.3
02:05 UTC71.660.820° 9 kt1018.3
02:25 UTC69.862.670° 8 kt1018.3

From the Iowa Environmental Mesonet's archive of airport observations.

The radar

The Des Moines radar (KDMX), 113 km away, was scanning. The volume nearest the touchdown began at 00:37 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.

This ground now

The live radar over the trackEvery 1999 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

The other tornadoes of May 27, 1999

2 more were confirmed in the United States that day.

DateTimeRatingWherePath (mi)Width (yd)DeathsInjuriesDamage
May 273:45 PM CDTF0Lafourche Parish, LA12.050$50,000
May 276:15 PM CDTF0Jasper, TX1.010$10,000

What this means

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.

Sources

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