On April 9, 2011, supercell thunderstorms produced several tornadoes across the northwestern portions of the U.S. state of Iowa, causing extensive damage across the region. The tornadoes were part of an outbreak that occurred from April 9–11, which affected large parts of the central and eastern United States, and occurred as part of a record-breaking monthly season. These tornadoes contributed significantly to Iowa's above-average, but non-lethal season in 2011, according to an experienced climatologist. According to a report published by the National Weather Service in Des Moines, April 9, 2011 was tied with May 22, 2004 as the fifth most active day for tornadoes in Iowa ever since.
Within the state, one storm put down a family that consisted of at least 10 twisters, several of them were strong to violent (EF2–EF4) tornadoes that impacted communities and farmsteads from Monona County, to Kossuth County. One large and damaging EF3 tornado struck the city of Mapleton, causing over a dozen injuries and widespread damage. After dusk, another impactful EF2 tornado hit the city of Early and compromised buildings in town. During the later evening hours, an intense, long-lived and massive EF3 tornado tracked across Sac, Buena Vista and Pocahontas Counties, causing severe damage to several farmhouses and eventually the small communities of Varina and Ware. That tornado spawned approximately five satellite tornadoes, including one brief but powerful EF4 tornado that completely destroyed a farmstead west of Pocahontas (City).
Meteorological synopsis
Episode narrative
One day prior At 1:00 AM CDT (06:00 UTC) on April 8, 2011, the NOAA Storm Prediction Center (SPC) outlined a slight risk for severe weather across portions of the Great Plains and Midwest, from northern Oklahoma to central Wisconsin, and including nearly the entire state of Iowa. Accompanying the slight risk was a 30% hatched probability over northwestern Iowa, eastern Nebraska, southeastern Minnesota, and southwestern Wisconsin. An upper-level trough was forecasted to move over the Intermountain West and into the Four Corners region, with a low-level jet expected to strengthen over an environment with strong and favorable wind shear and sufficient atmospheric instability in the south-central United States. At 12:30 PM CDT (17:30 UTC), the NOAA SPC updated the outlook to move the 30% probability further into Wisconsin and Minnesota, with the 30% hatched probability and a 15% hatched probability encompassing the majority of central and northwestern Iowa.
Day of the event
At 1:00 AM CDT (06:00 UTC) on April 9, the NOAA SPC upgraded to a moderate risk for severe weather centered over northern Iowa and southern Minnesota, with a 10% hatched (EF2EF5) probability of tornadoes over northwestern portions of Iowa as the major western trough was expected to track eastward alongside a powerful jet stream moving northeastward across the High Plains towards the Mississippi Valley. A surface low-pressure area was expected to deepen across northeast Nebraska by that evening, alongside strong southernly low-level winds bringing moist and potentially very unstable air from the Gulf of Mexico into the region, with expected Mixed Layer Convective Available Potential Energy (MLCAPE) values of 2000-3000 J/KG. At 8:00 AM CDT (13:00 UTC), the SPC removed the hatched area from northwestern Iowa, while maintaining the 10% probability of tornadoes. A surface low was deepening over Colorado during the morning hours, and was expected to track east-northeastward into South Dakota and Nebraska by the evening. A strong capping inversion was likely to be present, limiting the number of storms that could form. It was predicted that any storms that formed along the warm front had the greatest chance of producing tornadoes during the late afternoon and evening hours. At 11:30 PM CDT (16:30 UTC), the Storm Prediction Center extended the moderate risk for severe weather southward, covering the majority of northwestern Iowa as models were in agreement of storms developing along the eastern Nebraska/Iowa border by 7:00 PM CDT (00:00 UTC). The low-level jet was expected to strengthen and bring additional warm and moist air into the region as the night progressed, resulting in an expansion of the area where severe storms could develop. Alongside the maintained 10% probability for tornadoes within of a point, a 15% probability of damaging thunderstorm winds over Iowa, Wisconsin, and southern Minnesota, with a 45% hatched probability of hail over covering northwestern Iowa and southern Minnesota.
Event narrative At 5:50 PM CDT (22:50 UTC) on April 9, a tornado watch was issued for portions of northwestern Iowa, northeastern Nebraska, and the southeastern corner of South Dakota, with a high chance of tornadoes but a low chance that those tornadoes are of EF2+ strength. As the day progressed, the low-level jet steadily strengthened, reaching by 12:00 AM CDT (05:00 UTC). Ample moisture advection led to dew points rising up to around by the evening hours, and a powerful warm front moved into northern Iowa that afternoon, extending from western-central Iowa to eastern and southeastern portions of the state. An elevated mixed layer delayed storm development until the evening, allowing for atmospheric instability to build during the day, with elevated CAPE values reaching 30005000 J/kg. Strong shear of was also present, along with downdraft CAPE values of 1000 J/kg and helicity values in the lower levels of the atmosphere reaching 800 J/kg. Several discrete storms formed along the warm front during the late afternoon and evening hours, producing various tornadoes in western and northwestern Iowa, with three being intense (EF3+) and one being violent (EF4+).
Tornado summaries
Mapleton, Iowa
The first tornado Prior to the first tornado of the evening in Iowa on April 9, a tornado warning was issued by the National Weather Service in Omaha, Nebraska at 7:08 PM CDT (12:08 UTC) for northeastern Monona County. Approximately 12 minutes later, a tornado touched down southwest of Mapleton, Iowa.
This large, powerful and dusty tornado began at EF0 intensity, near IA 175. The tornado then steadily increased to EF1 intensity as it moved along the highway, just a few minutes before impacting the southwest side of Mapleton. Trees along this stretch of the highway were snapped or uprooted as the tornado encroached on the Maple River.
Direct hit and widespread damage
After causing another instance of EF0 tree damage along 1st Street, the tornado intensified further and entered the city, causing EF2 damage to multiple metal buildings, with many of them buckling and collapsing as the winds passed through. Along South 1st Street, the tornado caused one localized spot of EF3 damage, as a metal auto service was completely collapsed and destroyed, with the steel beams blown over to the next city block. Moving up through much of the center of town, a brick building in the main part of Mapleton, right on the northwestern side of the intersection of Courtright and South 4th Streets was collapsed at EF2 intensity, as the large and damaging wedge tornado went through downtown before weakening.
Endpoint after Mapleton The tornado would exit the through the north side of the city, having only done EF1 damage to several structures and trees, before soon dissipating north of Mapleton.
The intense EF3 tornado that struck Mapleton, while despite not the strongest of the evening, was the most notable event of April 9. Widespread damage occurred by this large wedge tornado, which destroyed 100 homes. Residents before the tornado struck received early warning of, and thus has this resulted in no deaths and 14 insignificant injuries, though one person suffered from a broken leg. In all, this tornado tracked for and was up to wide at peak width. Much of the early warning was helped by storm chasers and spotters, giving many residents around 15 minutes of lead time to take cover before the tornado struck Mapleton.
Early, Iowa
Activity in Sac County While a large and mile-wide (1.6 km) wedge EF3 tornado was ongoing in western Sac County after dusk settled, the parent storm which previously produced that tornado and the earlier Mapleton tornado, dropped another a strong tornado just north of Odebolt at 8:29 PM CDT (01:09 UTC).
EF2 tornado and satellite timeline
Approximately six minutes after issued tornado warning by the National Weather Service office in Des Moines, Iowa for the area, and after the initial EF3 tornado near Schaller formed at 8:23 PM CDT (01:23 UTC), a second large and significant tornado formed on its eastern flank. The tornado started out just to the east of Fox Avenue in the central-northwest section of Sac County, and headed northeast. Along the path of the tornado, the tornado impacted one farm that was located to the far west of IA 471, heavily damaging one outbuilding, which was impacted at EF0 intensity. After causing this instance of insignificant damage, the tornado persisted to the northeast and began to head towards Early. At around 8:38 PM CDT (01:38 UTC) a satellite tornado developed to the south-southwest of Early, south of the four-way intersection connecting both 270th Street and Hope Avenue, and northeast of a farm near the much larger tornado. This secondary, EF0-rated tornado, primarily stayed across rural farmland and inflicted no notable damage. The EF0 satellite moved north-northwest before it conducted a full loop pattern to the east, and crossed its own initial trajectory before returning back on its primarily northward track. The satellite's path then suddenly jogged to the northeast, before heading due north and encroached on 270th Avenue. After crossing the road, the satellite then moved more northwest again, before dissipating just prior to hitting another farm approximately 2 minutes after its formation, at 8:40 PM CDT (01:40 UTC).
Damage in Early and demise Meanwhile, the main tornado impacted the city of Early, causing significant amounts of damage throughout the 605-people community. Numerous trees and buildings were damaged by the tornado, which at this point reached EF2 intensity, as pieces of branches and debris from structures were blown onto lawns to the west of a restaurant. To the west along Second Street, around 75% of a brick building belonging to a trucking company was compromised and spilled into two lots. At the intersection of Main Street, the worst of the impact was seen, as a Payless Cashways building was completely destroyed. To the northwest at Second and Main Streets, the roof off a repair business and was found propped up against a nearby house. Four nearby trees were strewn with building insulation, including one that had was adorned by a sheet metal piece. After causing much destruction in town, the tornado exited out of Early through the northern side, subsequently weakening as it encroached over US 20. Thereafter, at 8:54 PM CDT (01:54 UTC) the tornado dissipated on the northern side of Early, at the northern intersection connecting US 20 with IA 471.
This large and damaging EF2 tornado with estimated winds, was on the ground along a path long and wide. The tornado caused no reported fatalities or injuries at all during its 25-minute lifespan across rural areas around Early and within the city. Approximately $1 million USD in damages were inflicted by this tornado alone. The brief and erratic EF0 satellite tornado that was accompanied its EF2 parent tornado, lasted for 2 minutes throughout its short track of long, and wide. Estimated winds for this tornado were at . An estimated $1,000 USD in damage was inflicted by this satellite.
Nemaha–Ware, Iowa
Parent EF3 tornado formation