Between 0500 UTC and 0900 UTC 31May 1998, a severe and widespread wind event roared across southern Wisconsin. This extreme wind event, known as a "serial derecho" (Johns and Hirt 1983), killed one person, injured 37 others and caused nearly $60 million damage to property and crops (NCDC 1998). This was the worst derecho to strike Wisconsin since the "Big Blowdown" in the North Woods on July 4, 1977 (Fujita 1978). Not only was this event unusual in terms of ferocity, it was also unusual in the time of occurrence for severe weather in Wisconsin (early morning hours instead of the climatologically favored late afternoon and evening). The environment that supported this event required large amounts of CAPE and rather strong low level vertical wind shear (Weisman 1993).
The squall line, racing east at 60 mph, produced surface winds of hurricane force in many locations. Embedded in the squall line were several intense macrobursts and microbursts that resulted in wind speeds of over 100 mph. The maximum measured wind gust was 128 mph just northeast of Watertown, in southern Dodge County (Figure 1). Live broadcasts on NOAA Weather Radio were relayed to the public concerning this dangerous weather situation. These broadcasts, combined with excellent warnings, kept the deaths and injuries to a minimum.
Figure 1. Peak wind gusts and areas of most intense damage (counties are listed).
The following analysis documents an excellent case of a serial derecho and examines the KMKX Doppler Radar data to indicate where the most destructive winds were located. This type of severe weather event will continue to be a challenge to forecasters in the future.
The synoptic pattern at the end of May 1998, consisted of a nearly stationary intense Hudson Bay low and a persistent strong sub-tropical ridge centered over Texas (which contributed to a costly drought in the Southern Plains during the summer of 1998). An intense band of mid and upper level winds then set-up between these two dominant features across the Upper Midwest.
At 1200 UTC 31 May, both Wisconsin and southern Minnesota were located at the nose of a very strong jet streak at 300 mb which put northern Wisconsin and northern Michigan in the left exit region of the jet, and in the right entrance region of a jet streak centered over Ontario, Canada. The ageostrophic circulations associated with these jet streaks may have coupled over the Great Lakes, thus enhancing upward, jet-induced vertical motion. This may help to explain the intensification of a rapidly moving surface cyclone tracking through the Great Lakes region. The associated strong cold front then triggered the necessary thunderstorms to transfer the intense winds aloft down to the surface.
The initial development of this eventual derecho occurred near St. Cloud, Minnesota at around 6:00 p.m.( CDT) on May 30, 1998. About an hour later, a nearly solid squall line was producing a considerable number of large hail reports in Minnesota. Meanwhile, the southern end of the line produced the devastating tornado at Spencer, South Dakota. This line then evolved into a derecho as it blew through the Twin Cities late in the evening, crossed southern Wisconsin in the early morning hours of May 31, 1998, and eventually reached eastern New York State and parts of New England during the evening.
The KMKX VAD wind profile showed a strong low level jet that was transporting moist and unstable air into southern Wisconsin ahead of the squall line. West-southwest winds between 3,000 feet and 10,000 feet were already at 50 to 60 kts. As the squall line approached, the winds increased to 60 to 80 kts, between 5,000 feet and 20,000 feet AGL.
At 0624 UTC, the WSR-88D 0.5° base reflectivity (Figure 2) indicated the squall line (with high reflectivities) extended from central Wisconsin into extreme southwest Wisconsin. Surface reports of damaging winds of 60 to 80 mph were common along this line at this time.
Figure 2. 0.5° base reflectivity at 0624 UTC.
By 0724 UTC, the derecho had raced eastward to an Oshkosh-Madison-Dubuque line. The base velocity product (Figure 3) indicated a fairly pronounced area of 50 to 63 kt inbound velocity which was fairly close to the leading edge of the convection. A small area of >64 kt inbound velocities, with a max of 100 kts, was occurring over northwest Jefferson County associated with the bowing out feature described above. During this same time, an area of lower reflectivity to the west of the high inbound velocity became more evident, indicative of a well-defined rear inflow notch (RIN) (Figure 4). The RIN is common in bow echo storms (Przybylinski 1995). Damaging winds of 80 to 100 mph were reported in association with this particular bowing of the squall line. Just to the north of this bowing segment, there was a hint of another developing bow over southwest Dodge County. This bow was responsible for the highest measured wind gust of the event, 128 mph 1.5 miles northwest of Lebanon in southern Dodge County (about 17 nm north of the RDA). The forecasters' training of these severe weather features enabled them to issue timely warnings, in some cases 30 to 60 minutes in advance, to alert the public to these dangerous winds.
Figure 3. 0.5° base velocity at 0724 UTC.
Figure 4. 0.5° base reflectivity at 0724 UTC.
Reflectivity data (Figure 5) continued to show the well defined squall line at 0743 UTC, with the bowing feature earlier in southwest Dodge County, now approaching southwest Washington County. However, a distinct RIN is not as evident with this feature as earlier indicated. Base velocity indicated significant weakening compared to previous volume scans with only a small area of 50 to 63 kt inbound winds along the leading edge of the convection. However, it was shortly after this time (0750 UTC) that a wind gust of 105 mph resulted in the only direct death of the event, in the town of Erin in southwest Washington County. Even though the velocity data showed a marked decrease in wind speed, intense damaging winds continued to be reported. This diminishing trend was likely due to the fact that the strong winds were nearly perpendicular to the radar beam, crossing the zero isodop, resulting in only a small portion of the total wind vector being measured by the radar. The squall line continued it's rapid movement to the east and by 0823 UTC the line was out over Lake Michigan.
Figure 5. 0.5° base reflectivity at 0743 UTC.
An intensifying storm system, and accompanying intense wind field, as evident by conventional upper air and WSR-88D data, produced a severe derecho across southern Wisconsin on May 31, 1998. This derecho, the strongest in Wisconsin in over 20 years, killed one person and injured 37 others and caused millions of dollars in damage. The derecho, striking in the early morning hours, probably would have caused more death and injury if it would have occurred in the afternoon or evening when more people would have been outdoors. Also, live broadcasts on NOAA Weather Radio gave the public advanced warning of the event, which helped to minimize deaths and injuries.
The author would like to thank John Eise, SOO at the Milwaukee/Sullivan Forecast Office and Wes Browning, Forecaster at the Chicago Forecast Office for their helpful reviews of this paper and assistance in preparing the final manuscript. The author would also like to thank the National Climatic Data Center for their timeliness in sending some of the data used in this paper.
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Przybylinski, R.W., 1995: The Bow Echo: Observations, Numerical Simulations, and Severe Weather Detection Methods. Wea. Forecasting, 10, 203-218.
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