The radome of the WHO radar that was blown off the tower along with the radar dish and destroyed near Alleman -Photo by NWS Des Moines
An NWS employee next to a field of flattened corn near Huxley -Photo by NWS Des Moines
An unfinished industrial Building was leveled by the winds near Huxley -Photo by NWS Des Moines
Close-up of an unfinished industrial building that was leveled by the winds near Huxley -Photo by NWS Des Moines
A tractor trailer was also flipped near the industrial building near Huxley -Photo by NWS Des Moines
Many trees were damaged and some uprooted in the town of Maxwell -Photo by NWS Des Moines
A grain bin punched in by the wind southwest of Marshalltown -Photo by NWS Des Moines
Trees uprooted in Union Grove State Park -Photo by NWS Des Moines
Large shed blown over near Union Grove State Park -Photo by NWS Des Moines
Roof blown off a house and garage failed west of Maxwell -Photo by NWS Des Moines
CO-OP bins destroyed in Garwin - Photo by NWS Des Moines
Roof of the Garwin legion hall in a neighboring backyard -Photo by NWS Des Moines
Large metal framed building partially collapsed south of Traer -Photo by NWS Des Moines
Ten power poles snapped off southeast of Traer -Photo by NWS Des Moines
Power pole snapped off southeast of Traer -Photo by NWS Des Moines
3:23am: The storm as it was beginning to intensify over far northern Polk and southern Story Counties. Radar estimates indicated winds of 85 mph only 300 feet above ground.
3:28am: The storm continued to intensify as it tracked eastward between Sheldahl and Alleman. While winds were likely above 70 mph, the radar was indicating winds of only 40 to 45 mph near the surface. This is due to a limitation of Doppler Radar. When measuring the velocity of a storm, the radar can only sample the component of the storm's motion that is parallel to the radar beam. At this point, the storm was due north of the radar and traveling east, thus the motion was almost completely perpendicular to the radar beam. This resulted in a significant underestimate of the storm's winds.
3:37am: As the storm moved further eastward, the storm's motion became more parallel to the radar beam and the radar was able to measure the winds with greater accuracy. At this point the storm was entering Cambridge with the radar estimating the winds at 90 mph (750 ft above ground).
3:46am: The storm began developing a circulation over southeastern Story County known as a bookend vortex, which aided in the strengthening of the surface winds. This vortex is visible on the image as the area of inbound velocity values between Huxley and Nevada (green colors) next the region of outbound velocity values to the south. Radar velocity values were between 90 and 100 mph at 1100 feet over Maxwell.
4:09am: The strongest winds are now located between Marshalltown, State Center, and Haverhill. Velocity estimates were between 100 and 115 mph at 2500 ft.
4:18am: The storm continues plowing ENE at 45 mph, with the strongest winds now between Marshalltown and Le Grand. Velocity estimates were between 110 and 125 mph at 3500 ft.
4:23am: The storm strikes Garwin and Toledo. Velocity estimates were around 125 mph at 4100 ft just SW of Garwin.
4:36am: The bookend vortex is clearly evident on the north side of the storm, with gate to gate shear of 115 mph NE of Garwin. The most destructive winds occurred just south of this vortex.
4:50am: The storm races through Dysart and enters the Quad Cities CWA.
Tops of grain bins destroyed in Green Mountain -Photo by Kip Ladage, Used with Permission
Grain bins destroyed in Garwin -Photo by Kip Ladage, Used with Permission
The roof of a house lifted off and destroyed and the garage collapsed in Garwin -Photo by Kip Ladage, Used with Permission
Ground view of the house with the missing roof and collapsed garage in Garwin -Photo by Kip Ladage, Used with Permission
Pieces of board embedded into a building in Garwin -Photo by Kip Ladage, Used with Permission
Home and attached garage destroyed just SW of Garwin -Photo by Kip Ladage, Used with Permission
A section of a grain elevator collapsed onto the ground below in Dysart -Photo by Kip Ladage, Used with Permission
The roof of a grain bin was lifted off and tossed onto the street below in Dysart -Photo by Kip Ladage, Used with Permission
A hog confinement building was destroyed three miles south of Dysart. Note the length of the debris trail, all blown to the east -Photo by Kip Ladage, Used with Permission
Close up of the hog confinement building destroyed three miles south of Dysart -Photo by Kip Ladage, Used with Permission
Close up of the hog confinement building destroyed three miles south of Dysart -Photo by Kip Ladage, Used with Permission
A section of power lines downed three miles east of Traer -Photo by Kip Ladage, Used with Permission
Debris trail from a garage in the town of Clutier -Photo by Kip Ladage, Used with Permission
Fields of corn flattened throughout Tama County -Photo by Kip Ladage, Used with Permission
A farm outbuilding destroyed in rural Tama County -Photo by Kip Ladage, Used with Permission
Farm outbuildings destroyed in rural Tama County. Note what appears to be a large portion of one of the outbuildings that rolled from the farm in the bottom right of the photo to the top left of the photo, a distance of roughly 1000 yards -Photo by Kip Ladage, Used with Permission
A barn and other outbuildings destroyed on farm in rural Tama County. Note the long debris trail pointing east -Photo by Kip Ladage, Used with Permission
A barn and other outbuildings destroyed on farm in rural Tama County -Photo by Kip Ladage, Used with Permission


 

Event Summary
Synoptic Conditions
Radar Data
Damage Path
Photos
Other Information

 

Event Summary

On the morning of July 11, 2011, a powerful long-lasting straight-line windstorm, known as a derecho, developed over central Iowa and carved a path of extensive damage across east central Iowa. The storm first took shape as a cluster of low end severe storms over southern Nebraska during the late afternoon of July 10. The system continued northeastward and entered western Iowa at 1:00am, still only as a marginal line of severe storms. As the system passed through the northern Des Moines metro area at 3:30am, it interacted with an outflow boundary from the storms to the north and rapidly intensified as it accelerated eastward. Over the next hour and a half the storm plowed eastward through Story, Marshall, and Tama Counties, blasting the area with winds of up to 105 mph, the equivalent of an EF1 tornado. The storm continued to track eastward, plowing through eastern Iowa and the southern Great Lakes region before dissipating in West Virginia in the mid-afternoon. Thousands of trees were downed in eastern Iowa alone and numerous structures were damaged or destroyed.


Synoptic Conditions

Derechos are able to last for several hours, and their gust fronts produce high end damage. In order for these storms to last so long, then need both instability and wind shear. Instability refers to the storm's ability to tap into the warm moist air near the surface as it develops. Meteorologists use the term CAPE (Convective Available Potential Energy) as a way to measure the amount of instability. Wind shear refers to how the wind changes at different heights above ground. Generally, severe weather is likely when the atmosphere has a combination of both high instability and high shear values. In this case, there was plenty of warm moist air at the surface. Temperatures as of 2:00am on July 11 were near 80F across Central Iowa, and dew points were in the mid 70s. This contributed to high CAPE values of 3000-4000 J/kg. At the same time, the wind shear in the lowest 3000 ft of the atmosphere was near 30 kts, which is considerably high.

Synoptic Synoptic Synoptic
Surface Obs and Radar over Iowa at 2:00am July 11 MLCAPE values (Red Lines) over Iowa at 2:00am July 11 1km Shear Values over Iowa at 2:00am July 11 

One quick and easy way to get an idea of the potential for a derecho event is by looking at the "Derecho Parameter". This parameter is calculated from both instability and wind shear, and it highlights the regions where they overlap. The derecho parameter was quite high that morning and was maximized just ahead of the advancing derecho as it moved from Iowa to Lake Michigan.
The Derecho Parameter and Radar Data from 1:00am to 7:00am July 11 


Radar Data

Click an image below to view loops of the storm as it tracked through the Des Moines County Warning Area.

Radar Loop Radar Loop
 Base Reflectivity Radar Loop from 3:09am to 4:55am  Base Velocity  Radar Loop from 3:09am to 4:55am

 

Base Velocity Radar Time Series

Velocity images from the Des Moines radar were particularly impressive during the course of this event.

Radar Data Radar Data Radar Data
Radar Data Radar Data Radar Data
Radar Data Radar Data Radar Data

 

Storm Structure

Outflow Boundary Interaction

Radar Data Radar Data Radar Data
An outflow boundary is a term used to describe the leading edge of the rain cooled air generated by a thunderstorm downdraft. Outflow boundaries can travel many miles out from a storm and persist long after a storm has dissipated. During the late evening hours of July 10, a storm over southern Minnesota developed an outflow boundary that propagated southward into Iowa. This outflow boundary began interacting with the northern end of what was a low end severe thunderstorm just north of Des Moines and likely aided in the system's rapid intensification and generation of the bookend vortex on the northern end of the storm. The white arrows in the images above denote the leading edge of the outflow boundary as it tracked south. The radar images were taken at 2:42am, 2:56am, and 3:18am.

Bookend Vortex

Radar Data Radar Data Radar Data
After the system passed through Marshalltown it became more organized and developed a circulation on the north end of the squall line known as a bookend vortex. The development of the bookend vortex coincided very well with the onset of higher end severe winds (100 mph and greater) as noted on the damage survey. The storm relative velocity image on the left was taken just after the development of the vortex and shows 91 kts (105 mph) of gate to gate shear in the yellow circle. The circulation extended throughout the depth of the storm, as illustrated by the four panel storm relative velocity image from the 0.5 to 1.8 degree scans (circulation shown by black circle). As the vortex tightened up it began to appear on the reflectivity imagery with the higher reflectivities being drawn into the circulation (right image, arrows denoting the path of the winds). The bookend vortex is also visible on the base reflectivity loop.

Rear Inflow Notch/Jet

Radar Data Radar Data Radar Data
As the storm tracked eastward, it developed a classic signature on reflectivity data known as a rear inflow notch. This notch, shown in the left image above (white arrow), appears as a weakening in the reflectivity field directly behind the bowing segment of the storm and is the result of another phenomena called a rear inflow jet. A rear inflow jet a mesoscale jet that transports air from the mid levels of the atmosphere to the front of the storm. This dry mid-level air evaporates the precipitation on the backside of the storm, creating the rear inflow notch. The middle image above is a four panel velocity image at the 0.5, 1.4, 3.2 and 5.1 degree radar scans. Note how the strongest winds in the derecho shift from the leading edge of the bow echo at 0.5 degrees near Garwin (beam height of 4300 ft AGL) to about 30 miles behind the storm east of Maxwell (beam height of 17,000 ft AGL). This is the rear inflow jet descending through the storm. The cross section on the right using storm relative velocity data also illustrates the rear inflow jet, with the black arrows tracing the path of the winds through the storm.

Damage Path

Below is a map of the areas affected by this storm in the Des Moines National Weather Service County Warning Area, with the different color contours denoting the strength of the winds. This map was developed using information from local storm reports received by the NWS, radar data, as well as the storm survey conducted after the event. The derecho continued into the Quad Cities' County Warning Area, hence the cutoff in the damage track on the east side of the map. Click on a marker on the map to view photos or notes from the NWS damage survey.

Please note that this map is only a preliminary estimate of the location and severity of the damage from this event, and is subject to change as new information is received.

Contour Legend Black:  General Severe Winds (58 mph+) Yellow: Winds of 70mph + Red: Winds of 100 mph +

 

Below is a map of the local storm reports for this event within the Des Moines NWS County Warning Area. Click on a marker for more information.

Nationwide Storm Reports (through 7:00am 07/11/2011)  Nationwide Storm Reports (after 7:00am 07/11/2011)

Complete Listing of Local Storm Reports (.pdf)

Photos

Photos taken during the NWS damage survey following the storm.

Storm Survey Storm Survey Storm Survey
Storm Survey Storm Survey Storm Survey
Storm Survey Storm Survey Storm Survey
Storm Survey Storm Survey Storm Survey
Storm Survey Storm Survey Storm Survey

Aerial and ground photos taken by Kip Ladage throughout Tama County.

Aerial Photos Aerial Photos Aerial Photos
Aerial Photos Aerial Photos Aerial Photos
Aerial Photos Aerial Photos Aerial Photos
Aerial Photos Aerial Photos Aerial Photos
Aerial Photos Aerial Photos Aerial Photos
Aerial Photos Aerial Photos Aerial Photos


Other Information

Links to additional news articles about this derecho published by other National Weather Service Offices impacted by this storm.

Quad Cities, IA Milwaukee, WI Chicago, Il Grand Rapids, MI Detroit, MI Northern Indiana Wilmington, OH

Public Information Statement issued by the NWS in Des Moines

...PRELIMINARY DAMAGE SURVEY RESULTS FOR STORY...MARSHALL AND TAMA
COUNTIES...

THIS STORM SURVEY INVESTIGATION IS STILL ONGOING AND SUBJECT TO
FURTHER COORDINATION WITH LOCAL EMERGENCY MANAGEMENT AGENCIES.

...WIDESPREAD AND EXTREME STRAIGHT LINE WIND EVENT...

THE FOLLOWING IS A PRELIMINARY ASSESSMENT FOR THE DAMAGE THAT
OCCURRED OVER STORY...MARSHALL AND TAMA COUNTIES...OR FROM CENTRAL
INTO EAST CENTRAL IOWA...EARLY IN THE MORNING ON MONDAY...JULY
11...2011.

* EVENT DATE: 07/11/2011.

* ESTIMATED START TIME: APPROXIMATELY 320 AM CDT.

* EVENT TYPE: DERECHO. NO TORNADOES WERE INDICATED DURING THIS
  EVENT. THIS PARTICULAR DERECHO BEGAN IN CENTRAL IOWA AND
  TRAVELED ACROSS CENTRAL AND EASTERN IOWA...AS WELL AS ACROSS
  ILLINOIS...LAKE MICHIGAN...MICHIGAN...AND ENDED IN ONTARIO
  CANADA. THIS DERECHO TRAVELED FROM CENTRAL IOWA TO DETROIT
  MICHIGAN...OR A DISTANCE OF APPROXIMATELY 550 MILES...OVER THE
  TIME SPAN OF NINE HOURS. A DERECHO IS A LONG LIVED...
  WIDESPREAD...DAMAGING WIND STORM WHICH IS ASSOCIATED WITH
  RAPIDLY MOVING SHOWERS AND THUNDERSTORMS. DERECHOES ARE OFTEN
  CURVED OR BOWED IN THEIR SHAPE. WINDS IN DERECHOES CAN EXCEED
  100 MPH. THE WINDS ASSOCIATED WITH DERECHOES ARE NOT CONSTANT
  AND MAY VARY CONSIDERABLY ALONG THEIR PATHS.

* EVENT LOCATION: BEGAN JUST WEST OF INTERSTATE 35 IN SOUTHERN
  STORY AND NORTHERN POLK COUNTIES. CONTINUED EASTWARD INTO
  MARSHALL AND TAMA COUNTIES.

* PEAK WIND: THE HIGHEST WINDS WERE AT LEAST 80 TO 105 MPH.

* PATH WIDTH: THE PATH WIDTH OF THE STRONGEST...HIGH-END WINDS
  WAS IN TAMA COUNTY AND WAS 5 MILES WIDE. IN ADDITION...THERE WAS
  A 10 MILE WIDE PATH OF WIND SPEEDS OF 70 TO 80 MPH. THERE WAS
  ALSO A 15 TO 20 MILE WIDE PATH OF WIND SPEEDS OF 60 MPH OR
  GREATER. IT SHOULD BE NOTED THAT THE DAMAGE SWATHS FROM THESE
  WINDS WERE NEARLY CONTINUOUS...AND EXTENDED FROM SOUTHERN STORY
  AND NORTHERN POLK COUNTIES...ACROSS MARSHALL AND TAMA COUNTIES.

* PATH LENGTH: THE DAMAGE SWATHS FROM THESE WINDS WERE NEARLY
  CONTINUOUS...AND EXTENDED FROM SOUTHERN STORY AND NORTHERN POLK
  COUNTIES...ACROSS MARSHALL AND TAMA COUNTIES.

* INJURIES: NONE HAVE BEEN REPORTED TO THE NATIONAL WEATHER
  SERVICE IN DES MOINES THUS FAR.

* FATALITIES: NONE HAVE BEEN REPORTED TO THE NATIONAL WEATHER
  SERVICE IN DES MOINES THUS FAR.

* DISCUSSION/DAMAGE: THIS WIND EVENT HAS BEEN THE MOST WIDESPREAD
  AND DAMAGING ONE TO AFFECT CENTRAL AND EAST CENTRAL IOWA SINCE
  1998. AGAIN...NO TORNADOES WERE INDICATED. THE DAMAGE WAS DUE TO
  STRAIGHT LINE WINDS FROM A DERECHO. HAVING SAID THAT...THE
  STRONGEST WINDS DURING THIS EVENT WERE SIMILAR TO THOSE FOUND IN
  AN EF1 TORNADO. THE WIND DAMAGE FROM THIS EVENT WAS MORE
  WIDESPREAD THAN THE DAMAGE A TORNADO HOWEVER. EXAMPLES OF THE
  NOTABLE WIND SPEEDS FOLLOW AND HAVE BEEN ESTIMATED FROM OBSERVED
  DAMAGE. NEAR HUXLEY...96 MPH. MAXWELL...94 MPH. NEAR
  MARSHALLTOWN...70 TO 90 MPH. JUST EAST OF GARWIN...105 MPH.
  BETWEEN GARWIN AND UNION GROVE STATE PARK...85 TO 97 MPH. JUST
  SOUTH OF TRAER...100 MPH. JUST SOUTH AND SOUTHWEST OF
  DYSART...105 TO 110 MPH. SOUTH OF DYSART...80 TO 100 MPH.


More information on derechos can be found on the Storm Prediction Center's website.

 


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