75% of the town of Mapleton, Iowa was destroyed yesterday in an EF3 tornado that struck shortly after 700PM Central Time. The 3/4 of a mile wide tornado tracked along a 3.5 mile long path and caused substantial damage to homes and businesses across the town, with the heaviest EF3 rated damage occurring on the southwestern edge of Mapleton.
Statement released from NWS Omaha:
PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
308 PM CDT SUN APR 10 2011
...TORNADO DAMAGE IN MONONA COUNTY IOWA INCLUDING MAPLETON...
THIS IS A PRELIMINARY MESSAGE. ADDITIONAL DETAILS MAY BE PROVIDED
LATER.
A SURVEY TEAM DID A DAMAGE ASSESSMENT OF THE TORNADO THAT MOVED ACROSS PARTS OF MONONA COUNTY IOWA SATURDAY EVENING APRIL 9 2011. THIS INCLUDED THE TOWN OF MAPLETON.
SO FAR...A PATH LENGTH OF AROUND 3.5 MILES HAS BEEN DETERMINED. THE MAXIMUM WIDTH OF THE TORNADO WAS 0.75 MILES. THE WORST DAMAGE WAS NOTED TO BE IN THE LOWER END OF THE EF3 CATEGORY.
MONONA COUNTY EMERGENCY MANAGEMENT ESTIMATED 12 TO 14 INJURIES...THE WORST OF WHICH WAS A BROKEN LEG.
EF3 WIND DAMAGE IS CAUSED BY WINDS ESTIMATED AT 136 TO 165 MPH.
Here is a map released by NWS Omaha of the damage the survey crews observed. Dark blue triangles denote EF0 damage, green indicates EF1 damage, yellow indicates EF2 damage, and the single orange triangle denotes the EF3 damage. Click the image to see it in its original size on the Weather Service's site.
This is a radar image from the Des Moines station a few minutes before the tornadic supercell arrived in Mapleton, IA. You can see that the storm is trying the best it can to develop a hook echo on its lower-left side. When a storm develops a hook on radar, you know it's rotating and there's a good chance of a tornado.
This is the base velocity image from the same Des Moines station taken around the same time as the one above. The base velocity on the radar shows the strength of the winds moving towards or away from the radar site. On this image, green indicates winds moving towards the radar, and red indicates winds moving away from the radar.
When the reds and greens start hooking and swirling like they are in the image above, you can be sure that there's a sight to be seen on the ground. I edited the above radar to show you how the winds were rotating, and approximately where a funnel cloud would have formed with this storm. This rotation got stronger as it moved off to the east-northeast (denoted by the black arrow), and had finally formed a tornado by the time it reached Mapleton (in the black box).
The Sioux City Journal reports that while parts of Mapleton are pretty well destroyed, its citizens lucked out big time.
MAPLETON, Iowa - Monona County Sheriff Jeff Pratt didn't mince words Sunday morning after surveying the damage wrought by a ¾-mile wide tornado that destroyed or damaged more than 600 homes in the small town of Mapleton late Saturday night.
Pratt credited advanced warning systems that gave residents a 15-minute head start to prepare, but that alone may not be enough to explain how the town of 1,294 was spared any fatalities or even serious injury.
[...]
City Clerk Karla Uhl said the storm destroyed roughly 3/4 of the city's buildings, and more than 120 homes were completely destroyed.
Here are a few pictures of the damage, taken by Iowa Governor Terry Branstad as he toured Mapleton this afternoon.
Picture by Gov. Terry Branstad
Picture by Gov. Terry Branstad
Picture by Gov. Terry Branstad
Reposting this from the diary I wrote last night about this tornado, a video from storm chasers showing the tornado as it approached Mapleton.