The Wisconsin Derecho Event of 31 May 1998 MICHELLE BUERGER University of Wisconsin – Madison, Madison, Wisconsin 13 May 2007 ABSTRACT A derecho, a family of downburst clusters, passed through the upper Midwest on 30 and 31 May 1998. The squall line intensified as it moved through Wisconsin due to the synoptic and mesoscale conditions of the environment in the region. An extratropical cyclone at the surface propagated directly into the state to support the storm. Two upper jet streaks caused divergence aloft to enhance uplift within the severe thunderstorms. The vorticity field at 500 hPa was aligned to advect positive vorticity and its associated upward vertical motion into Wisconsin. Flows at various elevations brought moist air into the boundary layer and drier air into mid-levels, and past derecho analyses have concluded that these conditions are critical to the maintenance of such a windstorm. Finally, speed and directional wind shear in the vertical profile of the atmosphere supported the development of mesocyclones, some of which produced tornadoes on this day. 1. Introduction On 31 May 1998, an intense line of severe thunderstorms roared through the state of Wisconsin at an average speed of 60 miles per hour (27 meters per second). Damaging winds and seven tornadoes were reported in Wisconsin between 0600Z and 0900Z (1:00 AM and 4:00 AM local time) that morning, with a peak wind gust of 128 mph (57 m/s) in Dodge county (NWS Milwaukee). The windstorm was one of the most damaging weather events in U.S. history, ranking only behind nine hurricanes (using data up to 2003; Ashley and Mote 2005). Wisconsin experienced a derecho, which consists of a family of downburst clusters and is often marked by a bow echo signature in radar data. In general, there are two types of derechos: progressive and serial. According to the National Weather Service, a progressive derecho consists of a relatively short line of thunderstorms perpendicular to the mean flow and usually occurs along stationary fronts. Serial derechos are produced by multiple bow echoes within a squall line that extends over hundreds of miles. These windstorms are associated with intense low pressure systems. The derecho on 31 May appeared to be a mixture of these two types. It was similar to a progressive derecho because the squall line was almost perpendicular to the environmental flow. However, progressive derechos are usually smaller in length than observed in this event. The storm also had characteristics of a serial derecho, such as being forced by(a) (b) Fig. 1. 0000Z on 31 May 1998 (a) Composite base radar reflectivity (dBZ) and surface observations, and (b) Infrared satellite imagery (brightness temperature) and surface observations. an extratropical cyclone moving through the upper Midwest and having a longer squall line. Unlike typical serial derechos, it was not associated with a front or a strong shortwave trough at mid-levels. Overall, the derecho that moved through the region took place due to the surface low pressure system propagating into the area, the correct atmospheric conditions of moist low levels and dry mid-levels with conditional instability, and uplift enhanced by upper divergence and vorticity advection. This paper examines the environmental conditions at the synoptic- and mesoscales before the derecho took place (0000Z on 31 May) and as the squall line was propagating through Wisconsin (0600Z). 2. Data All model data was from the Eta model, utilizing the 0 and 6 hours of the 0000Z run on 31 May. The Department of Atmospheric Science at the University of Wyoming created the upper air sounding. WSI NOWrad provided the composite radar data. The National Weather Service office Milwaukee, Wisconsin, provided the single site radar data during the event. Four-kilometer resolution infrared (11(a) (b) (c) (d) Fig. 2. 0000Z on 31 May, (a) Sea level pressure (hPa, green solid lines), wind (kts, blue barbs), and temperature (°C, yellow dashed line); (b) 850 hPa geopotential height (m, green lines), equivalent potential temperature (K, orange lines), and wind (kts, blue barbs); (c) 500 hPa geopotential height (m, solid green lines), absolute vorticity (s-1, orange dashed lines), and wind (kts, blue barbs); and (d) 250 hPa geopotential height (m, solid green lines), wind (kts, isotachs in blue lines and yellow barbs), and divergence (* 105 s-1, orange dashed lines). μm), imagery came from the GOES East satellite. Surface observations were from the National Weather Service METAR stations across the United States. GEMPAK Analysis and Rendering Program (GARP) was used to visualize the data and produce most of the figures in this paper. 3. The Setup: 0000Z a. Synoptic Overview The derecho had not yet formed at 0000Z, but there was light, scattered precipitation over the state. The maximum reflectivity of the rainfall was only about 10 to 15 dBZ (Fig. 1a). Areas of clouds had also formed (Fig. 1b) due to a warm front associated with the surface low pressure system centered near southeastern Nebraska, with a minimum pressure of about 998 hPa (Fig. 2a). The circulation it induced caused light winds with varying directions, from east-northeasterly to south-southeasterly, in Wisconsin. Also affecting wind direction was the frontal boundary that was draped zonally across the middle of Wisconsin. However, the temperature gradient was rather weak in the area, so there was only slight warm air advection into the state. At 850 hPa, the geopotential height minimum was in southwestern Ontario, quite removed from the surface cyclone center (Fig. 2b). This caused a south-southwesterly flow in Wisconsin. Therewere high values of equivalent potential temperature to the west and southwest Fig. 3. Upper air sounding over Green Bay, Wisconsin, at 0000Z on 31 May. of Wisconsin, indicating warm, moist air from southeastern South Dakota to west-central Missouri. The atmospheric flow was bringing this air mass into Wisconsin, thus increasing the moisture at low levels. According to past analyses (Evans and Doswell 2001; Coniglio et al. 2004), this condition is crucial to the intensification of squall lines. The high moisture content in the boundary layer has two main effects on the later development of the windstorm. First, air parcels become more positively buoyant as their temperature and moisture level rise. This enhances convective motions, including strong