Observations and Radar Analysis of a Warm Season Derecho Over Iowa on 29 June 1998

Observations and Radar Analysis of a Warm Season Derecho over

Iowa on 29 June 1998

by Tara Cooper and Jim Lee

Advisor: Dr. Fred Carr Introduction

On 29 June 1998, a Mesoscale Convective System (MCS) intensified into a particularly powerful, well-defined derecho as it moved into central Iowa. Winds in excess of 120 kt were estimated by the Des Moines (DMX) and Davenport (DVN) WSR-88D’s as the storm moved eastward across Iowa and into Illinois. Well over $20 million in structural damage occurred, thousands of trees were destroyed, and several hundred thousand

Iowans were left without electricity for days (Jungbluth 1998). The unusually severe nature of this storm greatly enhanced its observable characteristics. Because it exhibited

“textbook” features, this case is invaluable to a physical and dynamical understanding of derechos.

There are approximately 17 derechos per warm season in the United States (Johns 1987).

Storms of this nature produce tornadic-strength winds and cause significant damage over an extensive area. They are difficult to forecast because the synoptic features associated with them are usually weak, and their dynamics are not fully understood. In particular, it is nearly impossible to forecast the initiation of a derecho system. The rapid development and progression of these storms increase the complexities of predicting their evolution and intensity. Further understanding of derechos will lead to improved numerical prediction and warnings for the general public.

This paper will define the 29 June derecho according to established classification criteria.

Synoptic, mesoscale and storm-scale features of the storm will be compared to those of typical derechos. Data from this event will then be used to illustrate some of the physical processes occurring in the initial and mature stages of the storm.

Definitions and Classification

Johns and Hirt (1987) defined a derecho as “any family of downburst clusters produced by an extratropical mesoscale convective weather system.” "Derecho" is a Spanish word meaning “straight ahead,” and is intended to emphasize the straight-line winds characteristic of this type of storm (Hinrichs, 1888). The following criteria, based on research by Fujita and Wakimoto in 1981, are used to identify a derecho.

1. There must be an area of concentrated reports consisting of convectively

induced wind damage and/or convective gusts greater than 26 m/s (50 kt).

This area must have a major axis length of at least 400 km (250 nm).

2. The reports within this area must also exhibit a nonrandom pattern of

occurrence. That is, the reports must show a pattern of chronological

progression, either as a singular swath (progressive) or as a series of swaths

(serial).

3. Within the area there must be at least three reports, separated by 64 km (40

nm) or more, of F1 damage (Fujita 1971) and/or convective gusts of 33 m/s

(65 kt) or greater.

4. No more than 3 hours can elapse between successive wind damage events.

Derechos can be classified according to environmental conditions and storm development. Dynamic derechos develop along an extensive squall line associated with a migrating low pressure system, whereas warm season derechos typically initiate near a quasi-stationary thermal boundary between May and August (Johns 1993). Warm season events generally exhibit a “progressive” pattern in which a short squall line is oriented nearly perpendicularly to the mean wind direction. In contrast, dynamic systems generally exhibit a “serial” pattern in which individual swaths within an extensive squall line propagate nearly parallel to the mean wind direction (Johns 1993). Derechos can be further classified into one of four radar echo patterns established by Przybylinski and

DeCaire (1985). Distinctions between radar types arise from the number of bowing line segments, the number of rear inflow notches (RIN’s), the magnitude of the reflectivity gradient, and the presence of additional convective cells near the bow echo (Przybylinski,

1995). Fig. 1 illustrates the four different reflectivity signatures associated with derechos.

The 29 June derecho qualifies as a warm season, progressive derecho of radar type two.

Synoptic features expected for this kind of storm include westerly or northwesterly flow at upper levels, an east-west oriented thermal boundary, low-level warm advection, weak convergence, moisture pooling, and strong convective instability (Johns and Hirt, 1987).

Updrafts along the leading edge of the bow echo produce strong reflectivity gradients while downdrafts are chiefly responsible for RIN’s in the trailing edge of the storm.

Synoptic Discussion

The synoptic environment associated with the warm season, progressive derecho of 29

June closely resembled the ideal pattern described by Johns and Hirt in 1987 and by

Johns in 1993. There was an upper-level trough over the midwestern United States and westerly winds at 500mb over the region of initiation. A 500 mb low north of the

Minnesota/Canada border and a 500 mb high centered over Oklahoma combined to produce a velocity maximum over Nebraska, Iowa and Illinois by 1200 UTC (Fig 2 and

3). Warm advection occurred below 700 mb. 1200 UTC surface analysis reveals a weak, quasi-stationary front stretched across eastern Nebraska and Iowa and along the

Illinois/Wisconsin border (Fig 4). Weak convergence and warm advection in the lower levels resulted in temperatures in the mid-80’s in the frontal zone by 1700 UTC.

Transpiration from vegetation (i.e. trees, corn, and soybeans) and advection of warm, moist air from the south developed a moisture pool along and to the south of the frontal boundary, as evidenced by dewpoints in the mid-70’s by this time (Fig 5). These processes generated an extremely unstable air mass over central and southern Iowa, with estimated CAPE values in excess of 4000 J/kg. Convective instability was a significant factor in the initiation and development of the derecho system.

Radar Analysis

Early in the morning on 29 June, an MCS moved into northwest Iowa. The MCS approached Des Moines (DMX) as a high-precipitation, outflow-dominated storm.

Between 1800 and 1830 UTC, two low-level mesocyclones developed northwest of

DMX. At this time, storm movement was estimated at 43 kt from 310 degrees. The storm began to exhibit many of the classical derecho characteristics including high translational speeds and violent straight-line winds. At 1832 UTC the DMX WSR-88D indicated a maximum inbound velocity of 122 kt just west of Granger (Jungbluth 1998).

Over the next ten minutes the leading edge of the storm swept through Des Moines at an estimated speed of 65 kt. An inbound velocity of 111 kt was indicated 1.2 miles west of the radar (Jungbluth 1998). At approximately 1840 UTC the radome door of the WSR-

88D blew open and data transmission was lost. The problem was corrected, but the radar was struck by lightning shortly thereafter and incapacitated for several days. Loss of radar data after 1900 UTC limits analysis in the area east of DMX.

As the derecho entered the effective range of the DVN WSR-88D it exhibited a classic bow echo appearance (Fig 6). At 2007 UTC the derecho was in its most mature stage and displayed well-defined, “textbook” features. Reflectivity data indicates that strong convection occurred along the entire leading edge of the storm, due to convergence and rapid rising motion (Fig 7). Significant convection was also indicated along a line extending downwind (eastward) from the northern portion of the bow echo. This line of convection is known as the warm advection wing (Smith 1990) and is associated with large-scale advection of warm, unstable air over the stationary front. Reflectivity data also shows the location of several RIN’s along the trailing edge of the storm. These notches are the result of descending air (associated with stratiform precipitation) and the injection of drier, cooler air into the back of the storm by the rear-inflow jet (RIJ).

Velocity data indicates four distinct mesoscale vortices along the northern half of the bow echo at 2007 UTC (Fig 8). The northernmost vortex (located over Johnson County) was the most severe and was associated with ground-relative winds in excess of 120 kt. The southernmost vortex (located over Washington County) passed over Washington, Iowa, where an unofficial wind gust of 107 kt was measured. This is the highest wind speed ever recorded in the state of Iowa.

A vertical cross-section through the northern vortex reveals the structure of the derecho

(Fig 9). The most dominant feature was the RIJ, which descended from the mid- troposphere and flowed into the rear of the storm. The downward transport of mid- tropospheric air inhibited development of precipitation, resulting in a distinct minimum in reflectivity (Fig 7). The RIJ advanced rapidly, and was chiefly responsible for the translational speed of the derecho greatly exceeding the environmental wind speed.

Intrusion of the RIJ into the unstable environment resulted in intense vertical motion.

Along the leading edge of the storm, convergence and rising motion caused convective precipitation to develop. This process was responsible for the sharp reflectivity gradient observed in Figure 7.

Summary

The Iowa/Illinois derecho of 29 June initiated just to the north of a quasi-stationary summertime front and rapidly intensified into a devastating windstorm. Surface convergence, warm advection and high dewpoint values in the frontal zone contributed to the development and severity of the system. Radar analysis of the system shows a descending RIJ behind the storm and a strong cyclonic mesovortex at the north end of the bow echo. These two features enhanced each other, creating a positive feedback mechanism by which wind speeds were greatly increased. As a result, extensive damage occurred along the entire path of the storm. At this time, numerical models cannot predict the initiation of a derecho system.

Although many of the physical mechanisms involved can be described qualitatively, current understanding of the associated dynamics is insufficient. As a result, inadequate warnings are often issued to the public.

The derecho of 29 June 1998 was particularly severe in nature and exhibited ideal features. Further study of this case would prove valuable in understanding the nature of warm season, progressive derechos.

Acknowledgments

The authors would like to thank Dr. Fred Carr, advisor for this project, and Mike

Coniglio for their insight, encouragement, and research suggestions. Much gratitude is extended to David Demko for donating his extensive computer assistance, especially in retrieving radar data and preparing the project presentation. The authors acknowledge

Tom Condo for providing archived meteorological data as well as technical support.

Finally, the authors wish to thank Kyle Weisser and Ray Wolf from the DVN National

Weather Service office for providing radar data.

References

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Hinrichs, G., 1888: Tornadoes and derechos. American Metorological Journal, 5, 306- 349.

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Figure 1: Reflectivity signatures of derecho types 1-4 (Adapted from Przybylinski and DeCaire 1985).

Figure 2: 1200 UTC 500 mb analysis on 29 June 1998 (Courtesy of Oklahoma Climatological Survey).

Figure 3: 1200 UTC sounding from Davenport, IA (Courtesy of DVN National Weather Service Office).

Figure 4: 1200 UTC surface analysis of pressure (solid lines) and dewpoint in Fahrenheit (dashed lines).

Figure 5: 1700 UTC surface analysis of pressure (solid) and dewpoint (dashed).

Figure 6: 2007 UTC base reflectivity from DVN at 0.5O elevation. Figure 7: 2007 UTC base reflectivity vertical cross section, taken along the bold white line near the center of Figure 6. Left to right in Figure 7 corresponds to West to East on the line. The dBZ scale is the same as in Figure 6.

Figure 8: 2007 UTC storm-relative velocity from DVN at 0.5O elevation.

Figure 9: 2007 UTC storm-relative velocity vertical cross section taken along the bold white line near the center of Figure 8. Left to right in Figure 9 corresponds to West to East on the line. The velocity scale is the same as in Figure 8.