JULY 10-11, 2011 DERECHO
"The Cross Country Derecho of July 2011"

A series of derecho-producing convective systems produced a relatively narrow but lengthy swath of wind damage that extended all the way from the central High Plains to the Mid-Atlantic states on July 10-11, 2011. The event began on the evening of Sunday, July 10, and produced the most concentrated damage and the strongest measured wind gusts over parts of Iowa, northern Illinois, and southern Wisconsin eastward into southern Michigan, northern Indiana, and western Ohio the following morning (Figure 1). The high winds also produced a notable seiche (pronounced "saysh;" a sudden fluctuation of the water level) on Lake Michigan. In the Chicago area, six people were injured in the collapse of a festival tent, and a man was killed by a falling tree near Grand Rapids, Michigan. A million households and businesses were left without electrical power from Des Moines to Detroit as storm winds in some locations gusted to more than 100 mph.

Figure 1. Storm reports and color-coded break-down of the areas affected by the various convective systems associated with the July 10-11, 2012 derecho. Reports are for the 30-hour period from 4:00 p.m. CDT (2100 UTC) Sunday, July 10 to 10:00 p.m. CDT Monday July 11 (0300 UTC Tuesday July 12). Wind damage or wind gusts ≥ 50 kts (58 mph), small blue squares; estimated or measured wind gusts ≥ 65 kts (74 mph), large black squares with yellow centers; hail ≥ 0.75 inches, small dark green squares; tornadoes, small red squares. Isolated severe weather reports not related to the derecho events here discussed are not shown. Blue, green, red, and yellow-shaded regions depict the approximate geographical areas affected by the four main convective systems that were directly or indirectly involved with the main derecho event shown in green (see text for details).

The initial thunderstorms that ultimately grew into what might be termed the first phase of the July 10-11, 2011 derecho (denoted by yellow shading in Figure 1) formed in a very warm, desert-like thermodynamic environment over eastern Colorado on the afternoon of the 10th (Figures 2 and 3). Low-level moisture was relatively limited (surface dewpoints in the upper 40 °F), but sufficient to generate numerous storms given the presence of convergent surface winds and a weak jet stream disturbance aloft. The storms were efficient downdraft-producers; by early evening their outflow had consolidated into a sizable cold pool. This cold pool moved and expanded preferentially northeastward in response to the seasonably strong, 45 mph (40 kt) southwesterly mid-tropospheric winds encountered by the storms.

The leading (northeastern) edge of the elongating cold pool (i.e., the progressive part of the gust front; see Derechos and Flash Floods) served as the primary focus for new storm development as it moved into northwest Kansas and southwest Nebraska. There, it reached a subtle west-east front that separated cooler, more moist air to the north from the desert-like environment over eastern Colorado and southwest Kansas. This boundary --- and the presence of rich moisture to its north (dewpoints near 70°F) --- encouraged stronger and more rapid storm development compared to that which had occurred earlier. At the same time, the direction most preferred for new storm development shifted more eastward along the boundary. The net result was that the developing squall line accelerated east-northeast across central and eastern Nebraska through mid to late evening (Figures 2 and 3). The incipient bow echo surged forward, producing a swath of 70 mph wind gusts that overturned a trailer home in the town of Holdrege (near the Kansas border). Farther north, gusts to 80 mph were reported near Broken Bow, and winds in excess of 70 mph were sustained for one and half minutes near Norfolk.

Figure 2. Forty-hour composite radar base reflectivity loop showing development and evolution of the various derecho-producing convective systems associated with the July 10-11, 2011 derecho event, and that of smaller convective systems that subsequently formed over the middle Ohio Valley. "Ping-pong" animation begins at 3:00 p.m. CDT (2000 UTC) Sunday, July 10, and ends at 7:00 a.m. CDT (1200 UTC) Tuesday July 12.  Reflectivity intensity scale not shown, but stronger returns (higher dBZ values) depicted in orange and red.  Surface data plotted using conventional format, with standard three-letter station identifiers: Temperature / dew point (°F, with dewpoint in green) on left, and pressure / pressure tendency (mb) on right.  White lines / barbs depict wind direction and speed (knots; one full barb = 10 knots = 11.5 mph).

Figure 3. Twenty-seven-hour infrared data satellite loop showing the development, evolution, and demise of the main derecho-producing convective system of July 11. 2011, and of the smaller convective system that subsequently formed in its wake over Indiana, Ohio, and Kentucky. "Ping-pong" animation begins at 4:00 a.m. CDT (0900 UTC) July 11 and ends at 7:00 a.m. CDT (1200 UTC) July 12. Color enhancement scale not shown, but cloud tops colder than -65 C appear black (approximately 14 km or 46,000 ft), -70 C, gold (approximately 15 km or 50,000 ft), and -75 C, aqua (approximately 16 km, or 53,000 ft). Cloud-to-ground lightning strikes (red crosses) and surface data (using conventional station plot format, as in Figure 2) also shown.

The Nebraska squall line moved east into western Iowa after midnight. While it continued to produce marginally severe wind and hail, the north-south band of storms otherwise was not noteworthy. Around this time, however, the squall line encountered a west-east outflow boundary that was drifting southward into central Iowa. The outflow boundary marked the leading edge of cool, outflow air derived from a separate severe weather system that had originated over the Dakotas and southern Minnesota Sunday afternoon (blue shaded region in Figure 1; also see radar and satellite evolution in Figures 2 and 3). The storms in the Dakotas-Minnesota complex weakened considerably upon entering Iowa. But the outflow boundary they left behind fostered rejuvenated storm development at the northern end of the western Iowa squall line as it neared Des Moines, where an area of very warm and moisture-rich air resided at the surface (Figures 2 and 3). These rapidly intensifying storms quickly evolved into a strong bow echo that surged east across the remainder of the state, reaching the Illinois border around 6:00 a.m.  At the northern end of the bow, near its intersection with the outflow boundary, a bookend vortex formed near Marshalltown, about 50 miles northeast of Des Moines. The formation of the bookend vortex marked the beginning of an extended period of more significant wind damage that continued eastward through the Chicago area into northern Indiana and southern Michigan by late morning (depicted by green shading in Figure 1).

The Iowa bow produced a nearly continuous swath of damaging wind that was especially acute over Marshall, Tama, and Benton counties, where crops were flattened on a widespread basis. Winds gusted to more than 100 mph near Marshalltown, and to more than 110 mph in Garrison and Vinton. Structural damage included numerous lost roofs and collapsed walls. Tractor-trailers were overturned on Interstate 380 near Cedar Rapids, and a radio tower was snapped near the town of Toledo. The high winds also bent two steel towers of an electrical transmission line between Garrison and Vinton, causing a regional power outage. Despite the intensity of the pre-dawn event in eastern Iowa, no injuries or deaths were recorded.

Shortly after 6:00 a.m. the bow echo crossed the Mississippi River into Illinois and Wisconsin. Although the storms slightly weakened, their 60 to 80 mph gusts continued to produce widespread crop, tree, power line, and structural damage. Tractor-tailers were overturned near Rockford, Illinois, and 70 mph winds persisted for several minutes in McHenry County (Illinois). The storm complex swept through the Chicago and Milwaukee areas around 8:00 a.m., downing countless trees and utility poles, and causing widespread power disruptions. In Chicago, winds gusted to 63 mph at O'Hare Airport, and to 75 mph at Midway Airport. A trailer was overturned at Midway, and six people were injured when a festival tent collapsed in Palos Heights in southern Cook County. More than 860,000 home and business customers of Consolidated Edison lost power, tying the record for the most ever in the history of the Chicago utility. On Lake Michigan, the winds produced a seiche that caused waters levels to oscillate wildly along the lake shore, including a fall nearly two feet in twelve minutes at Calumet Harbor (near the Illinois-Indiana border). Sizable fluctuations in the lake's level also were observed at Milwaukee, Wisconsin and at Holland, Michigan.

By 9:30 a.m. the bow echo crossed southern Lake Michigan and plowed into southern Lower Michigan and northwest Indiana. Sixty to 70 mph straight-line winds uprooted trees, toppled utility poles, and caused sporadic structural damage. A gust to 85 mph was measured at Michigan City, Indiana. For some parts of lower Michigan, arrival of the Illinois-Wisconsin squall line marked the second bout of severe weather to affect the area since midnight. About two hours earlier, a remnant line of bowing storms associated with the convective system that had crossed Dakotas and southern Minnesota late Sunday reached southwest Michigan, producing 80 to 90 mph winds in Kent and Ottawa counties. A man died in the Cutlerville area (near Grand Rapids) when a falling tree crashed into the garage of his home.

Figure 4. Fourteen-hour visible data satellite loop showing the evolution of the July 11, 2012 derecho-producing convective systems over the Midwest, and that of smaller convective system that subsequently formed over Indiana, Ohio, and Kentucky. "Ping-pong" animation begins at 5:45 a.m. CDT (1045 UTC; sunrise) and ends at 8:45 p.m. CDT (0045 UTC July 12; sunset). Surface data also shown, using conventional format as in Figure 2. Thunderstorms over southwest Michigan at start of loop are remnants of the derecho-producing convective system that moved from southern North Dakota into southern Minnesota and northern Iowa the previous evening.

The storms reached the far western suburbs of Detroit and northeast Indiana around noon local time (middle part of animation in Figure 4). Seventy-one mph winds were measured in Ann Arbor, Michigan, and three-foot diameter trees were uprooted near the Detroit Metro Airport. Two tractor-trailers were blown over on Interstate 69 in De Kalb County, Indiana.  Shortly after noon, storm winds gusted to 71 mph in nearby Paulding County, Ohio. And, about 45 minutes later, measured gusts in excess of 60 mph toppled trees and poles in the Toledo and Lima areas of northwest Ohio.

The thunderstorm complex turned somewhat to the right (southeastward) as it continued east into central and eastern Ohio a bit later in the day. This occurred as the system encountered a weak, north-south warm front near Columbus. The front appears as a band of haze and cloudiness along the Indiana-Ohio border at the start (sunrise) of the visible data satellite loop in Figure 4, and may be tracked as it advances east into central Ohio by early afternoon. The boundary served to change the propagation component of the convective system's overall motion (see "Movement" section in Derecho Forecasting). Once the squall line reached the front, new storm development (i.e. the propagation component of the convective system's motion) occurred more readily near the boundary. Thus, in contrast to earlier in the day when bowing segments within the squall line surged from west to east, those that formed after the boundary had been overtaken moved more southeastward, i.e., toward West Virginia. Toward early evening the direction of preferred storm growth took an even greater right-ward turn, causing the western part of the squall line to curve south and, later still, south-southwest into eastern Kentucky (Figures 2 and 3).

Numerous instances of damaging winds continued as the storm complex moved across southern and eastern Ohio into West Virgina, southwest Pennsylvania and northeast Kentucky. However, the magnitude of the winds gradually diminished and, by evening, the convective complex, while still of fairly broad west-east extent, had became only marginally severe. At the same time, interaction between the outflow boundary of the Ohio Valley squall line and the mountainous terrain of southwest Pennsylvania, western Maryland, and northern West Virginia sparked the development of a new line of storms over the Blue Ridge Mountains (on far right side of Figure 4 animation at end of loop). This squall line produced scattered reports of locally damaging wind east to the Chesapeake Bay through mid-evening (see Figure 1; the area affected by this band of storms appears in red), but it did not attain the strength or duration of the convective systems that occurred west of the Appalachians.

During the early evening, as the main convective system weakened over parts of West Virginia and Kentucky, a new line of storms arose over central Indiana (just before sunset in Figure 4 animation). This activity formed along the stationary part of the elongating cold pool (see Derechos and Flash Floods) left by the afternoon system. A pocket of very warm and moist air, part of the same air mass that had existed over Iowa the previous evening, fueled rapid and intense storm development; by mid-evening, a new squall line with embedded bowing segments extended from north of Indianapolis to north of Dayton, Ohio. This line moved southeast and, later, south, across parts of Indiana, southern Ohio, and eastern Kentucky, producing another round of severe weather. Most of the damage, however, was confined to downed trees. Minor structural damage did, however, accompany the storms in Richmond, Indiana and Wilmington, Ohio. The storms finally diminished early the following morning over southeast Kentucky, southwest Virginia, and eastern Tennessee (see end of animation in Figure 3).

Arguably the most noteworthy aspect of the Cross Country Derecho was its west-to-east extent --- approximately 1300 miles. As has been shown, however, the event was not due to a single convective system but rather to a series of storm complexes that together produced a swath of damage that ranged from intermittent to nearly continuous from eastern Colorado to Maryland. The most "classic" part of the event --- that is, the portion where conditions were most favorable for "textbook" derecho development --- affected the Mid-Mississippi Valley. It was over Iowa, Illinois, and southern Wisconsin that an exceedingly warm and moist low-level environment was surmounted by relatively cool air at mid-levels (i.e., by an elevated mixed layer) in the presence of fast, unidirectional flow. This setup fostered the rapid development of a forward-propagating squall line and, ultimately, a strong, large-scale bow echo. While events similar to the one that occurred over the Mid-Mississippi Valley and the central Great Lakes on the morning of July 11, 2011 appear over that region nearly every summer (e.g., in recent years: June 29, 1998, August 24, 1998, July 3, 2003, August 3, 2004, August 2, 2006, August 23, 2007, August 4, 2008, June 19, 2009, June 18, 2010, and June 23, 2010), those with both the intensity and extent of the July 2011 bow complex are comparatively rare.


Additional information (arranged geographically from west to east; "NWS" = National Weather Service):

North Platte, Nebraska NWS Office - Development of the derecho-producing squall line over southwest Nebraska on July 10, 2011 (evening)

Hastings, Nebraska NWS Office - Continuation of the derecho-produucing squall line into central and eastern Nebraska on July 10, 2011 (late evening)

Iowa Environment Mesonet (IEM) - Slow radar reflectivity animation of the main (Iowa to Michigan-Ohio) July 11, 2011 derecho-producing convective system

Des, Moines, Iowa NWS Office - Early stages of the July 11, 2011 derecho over central Iowa (pre-dawn)

Quad Cities, Iowa-Illinois NWS Office - Maturation of the July 11, 2011 derecho over eastern Iowa and northwest Illinois (early morning)

Milwaukee/Sullivan, Wisconsin NWS Office - Maturation of the July 11, 2011 derecho over southern Wisconsin (early morning)

Chicago, Illinois NWS Office - The July 11, 2011 derecho over northern Illinois and northwest Indiana (early to mid-morning)

Grand Rapids, Michigan NWS Office - The July 11, 2011 derecho over southern lower Michigan (mid-morning)

North Webster, Indiana NWS Office - The July 11, 2011 derecho over northern Indiana, far southern lower Michigan, and northwest Ohio (mid-morning to early afternoon)

Wilmington, Ohio NWS Office - Storm reports with the July 11, 2011 derecho over western and central Ohio (afternoon and evening)

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