EF1 tornado, Nepas to Pella, Iowa, March 6, 2017

1. EF065 to 85 mph

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

2. EF186 to 110 mphthis tornado

   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

EF1

Date

March 6, 2017

Touchdown

7:29 PM CST (01:29 UTC)

Lifted

7:34 PM CST (01:34 UTC)

On the ground

5 minutes

Path length

5.3 miles (8.6 km)

Path width

120 yards (110 m)

Deaths

0

Injuries

0

Property damage

$50,000

County

Marion County

Touchdown point

41.276, -93.291

Lift-off point

41.331, -93.218

Record number

1703061929-01

14 damage points were recorded along this path by NWS DMX, in the Damage Assessment Toolkit the survey teams carry. The strongest indicator was rated EF1. The highest wind the damage implies is 100 mph.

• Snapped pole. Corn stalk debris. Broken branches across road.
• Cinder block foundation. Side wall failure.
• Numerous trees snapped. No structural damage.
• No structural damage. Tree damage. Corn stalk debris.

Survey points are preliminary and are the raw material the published rating was built from. Source: NWS Damage Assessment Toolkit.

A low pressure system developed off the Rockies in the Wyoming, Nebraska, South Dakota region and deepened as it moved eastward through South Dakota through the day on the March 6th. Southerly flow had already established itself and was bringing up Gulf of Mexico moisture into the Plains. A dry line also developed out ahead of the parent cold front, sparking initial storms in eastern Nebraska during the afternoon hours. Storms relatively quickly developed into a linear system, though a few isolated storms developed out ahead of the main line and proceeded to produce severe hail. As the line of storms progressed eastward through Iowa, they produced numerous severe hail and damaging wind reports. Additionally, a few tornadoes developed along the line, producing damage primarily in central and southern Iowa.

NWS storm survey confirmed a tornado touchdown approximately 5 miles west of Knoxville. An outbuilding was heavily damaged in this area and significant roof damage occurred to a residence. This is consistent with winds of 90 to 100 mph - or EF1 damage.

NWS storm survey confirmed another tornado touchdown approximately 4 miles northwest of Knoxville. Roof damage occurred to a few residences with large sections of shingles removed along with tree damage. This is consistent with winds of 85 to 95 mph - or EF1 damage.

NWS storm survey found the last tornado in a series from the same storm in northeast Marion county. The tornado touchdown and moved quickly northeast producing mainly tree damage. However, one barn was shifted off its foundation with several other open air structures being destroyed altogether. Damage was rated high end EF0.

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

8 minutes of warning. NWS DMX issued a tornado warning for this county at 01:21 UTC, 8 minutes before the tornado touched down, in force until 01:45 UTC.

The county was under a tornado watch, issued at 19:55 UTC and running to 02:59 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 0.05 chance of a tornado within 25 miles. The outlook in force was issued at 00:54 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), 61 km away, was scanning. The volume nearest the touchdown began at 01:30 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 2017 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

53 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.