Enhancement of the Downdraft Wisconsin Derecho, May 30th-31st, 1998 Jason Harder Atmospheric and Oceanic Sciences, UW-Madison, Madison, Wisconsin ABSTRACT Southern Wisconsin was pounded by a long lasting derecho on May 31st, 1998. The synoptic conditions were favorable for the development of severe storms. The set up was conducive to the formation of a bow echo and the derecho that would follow. Ingredients came together to enhance the downdraft of the derecho over south-central Wisconsin and produce wind gusts of up to 128 mph. 1. Introduction In the early morning hours of May 31st, 1998, southern Wisconsin was subjected to a widespread, severe wind event, better known as a derecho. The night before, there was an outbreak of supercells in South Dakota, which through the night developed into a line of storms. Conditions were favorable for this line to develop into a bow echo, however once formed, the magnitude of this system grew and became a derecho that would travel roughly one thousand miles and lasted close to 15 hours. This storm became one of the most devastating derecho events in recorded history. Hurricane force winds in excess of 120 mph sped through southern Wisconsin during the morning of May 31st. A line of storms developed over the Minnesota, South Dakota border earlier in the evening, with this line evolving into a long lasting derecho. The derecho entered Wisconsin just before 5z on the 31st and exited the state at about 9z. It sped through Wisconsin with a translation speed close to 60 mph. Property damages were estimated up to $60 million and another $1.82 million in crop damage. One person was killed and thirty seven were injured from this derecho. A peak wind gust of 128 mph was recorded in Watertown, Wisconsin around 2:30 am, CDT on May 31, 1998 (FIGURE 1). The combination of the rear inflow jet, up-down draft and evaporative cooling enhanced the downdraft, achieving such intense surface wind speeds. 2. Data The conceptual model, Mesoscale-analysis, and cross section were all made by hand and finalized in the Paint program for Windows computers. The four-panel plot of the jet stream, lifted index/relative humidity, surface pressure and 850mb temperatures are courtesy of Unisys Weather. Radar reflectivity and velocity figures are courtesy of the SSEC/CIMSS2 Summer Workshop Case Study Page. Progression of the derecho over the Upper Midwest and also the wind reports courtesy of the Storm Prediction Center. 3. Synoptic Overview Synoptic conditions were favorable for the development of this derecho. There was a low pressure system located over the eastern Dakotas and western Minnesota (FIGURE 2, top left). Also present was a thermal gradient (FIGURE 2, bottom left) that the derecho followed during its journey through the Upper Midwest. With both the low pressure system and the thermal structure, this derecho fits into the serial and progressive derecho set up. Throughout the lifetime of the derecho it followed the thermal gradient, along with having the low pressure system right behind it the whole way through the Upper Midwest. The top right analysis in FIGURE 2 shows the wind pattern and the associated jet streaks during the beginning hours of the derecho. With there being a small jet streak over the Heartland and also one just North of the Great Lakes region, there is an optimal set up for additional vertical motion. The area which the derecho formed in was simultaneously in a left exit region and right entrance region of the jet streaks. The bottom right-hand plot in FIGURE 2 shows the low lifted indices over the derecho’s path, which helped to produce the strong wind event on May 31st, 1998. Both of these created more vertical motion than already seen in the atmosphere, aiding in the development and evolution of the derecho. All of these conditions came together at the right time and right magnitude to produce and sustain such an intense derecho. 4. Mesoscale Analysis There are two types of derechos: Serial and Progressive. A serial derecho is when there multiple bow echoes embedded in a relatively long squall line. They are associated with migrating low pressure systems and generally move in a northeasterly direction. The progressive derecho is normally linked with a shorter squall line, which will sometimes take the shape of a single bow echo. This type tends to form on the pole ward side of a stationary front and moves in southeasterly direction, migrating towards the warmer air south of the stationary front. The Wisconsin derecho, however, is a mix between these two, often called a “hybrid” derecho. There is both a low pressure system and a thermal gradient associated with this derecho. It involves only one large bow echo, and moves southeast to begin with, but then turns and starts to3 move northeastward, reaching into Canada and parts of New York. FIGURE 3 shows the progression of the derecho through the Upper Midwest. As the derecho moved across Wisconsin a series of events came together to produce a small protruding extension of the bow echo where the gust of 128 mph was located. There is a rear inflow jet associated with most derechos. With most storms this is enough to cause extensive damage to property. FIGURE 4 shows a conceptual model of this derecho where there is the typical updraft pulling in warm, moist air from in front of the storm and lifting it up and over the cold pool and associated downdraft. FIGURE 5 shows a vertical cross section through the derecho. The moisture plum is represented by Theta-e, contoured in green, and shows the tilted updraft and moisture flow towards the rear of the storm. Theta, contoured in red, shows the warm air rising in the updraft, and with the cold pool just behind the updraft, there is a sharp contrast in theta between the updraft and downdraft areas. The