1 AOS 453 Case Study Volume 1 Severe Weather on 11 June, 2008 Jacob Beitlich Department of Atmospheric and Oceanic Sciences, University of Wisconsin Madison Abstract On the 11th of June, 2008 there were at total of 341 severe weather reports across the western edge of the corn belt, starting in central Kansas, and extending northeast into central Minnesota. This paper examines the causes for why the severe weather outbreak occurred, both from a synoptic and mesoscale perspective. There was abundant low level moisture from both nocturnal thunderstorms the night before, as well as the strong advection of Gulf moisture into the region. An area of convergence associated with a surface front provided a lifting mechanism at the surface. There was diffluent flow above 500mb, which aided in the upward vertical motion at lower levels. There was also a dry elevated mixed layer above 500mb which led to large CAPE values. Southerly winds at the surface, and westerly winds aloft meant that there was veering with height, creating an environment favorable for tornadoes. Special attention was given to an F3 tornado, and it was determined that the tornado was most intense while the storm made its right turns. Finally, another possible contribution strong southerly winds, by means of the Sawer-Eliasson Circulation in the jet exit region, was analyzed and left up for debate. ____________________ 1. Introduction Every year dry air from the Great Basin gets ejected out over the High Plains, meanwhile low level moisture from the Gulf of Mexico is advected northward into the Midwest. During the spring and summer months, the combination of the two sets the stage for severe weather. The dry elevated mixed layer can have steep lapse rates (on the order of 8-9oC), and together with the warm moist air at lower levels, leads to a very potentially unstable air mass. However, unless there is a forcing mechanism to release the convective available potential energy (CAPE), the atmosphere will remain just that, potentially unstable. June 11th, 2008 had both of the two ingredients mentioned earlier, as well as several others, which caused that day to be favorable for severe weather to develop. As will be shown shortly, both upper level dynamics and veering winds with height, contributed to the development of supercell thunderstorms. The byproducts of these storms were over a hundred hail and wind reports, as well as several tornadoes (Fig 1).Figure 1: Storm reports from June 11th, 2008 courtesy of the Storm Prediction Center (SPC). The blue, green, and red marks represent wind, hail, and tornadoes. The black triangles indicate reports of hail larger than 1.5 inches, while the black boxes are reports of wind greater than 65kts. Before sunrise, during the early morning hours of June 11th, a complex of thunderstorms moved across virtually this same area of the upper Midwest. There were some minor storm reports, but mostly just heavy rain. The reason for pointing that out is because now there was abundant surface moisture for the June 11th storms to work with, as will be shown later in more detail. Also, as shown in Fig 2, the environmental air around a thunderstorm sinks and slightly warms. The atmosphere Figure 2: Conceptual Model of how during the mature stage of a thunderstorm, the environmental air around the storm is gradually sinking, thus becoming more stable. (Tripoli) wants stay in equilibrium, so a small area of upward vertical motion, such as in a storm updraft, is offset by a large broad area of subsidence. This acted to stabilize the air on the following day, which was responsible for clear skies on June 11th (Fig 3a). Since this severe storm event Figure 3: Satellite images using the .65 and 11µm wavelength show that throughout the afternoon, (a) the sky was clear over much of western Iowa, eastern Nebraska, and northern Kansas. The white clouds over southwestern Wisconsin are remnants of the overnight convection. The yellow clouds indicate warmer temperatures, thus lower cloud tops. These clouds were most likely caused by the surface heating and evaporation of the available water from the previous night’s rain. Notice the cold front made visible by the arc of clouds over southeastern South Dakota, extending down into north central Kansas. (b) During the evening hours there is an explosion of thunderstorms along and ahead of the cold front, as indicated by the bright white clouds. It is even possible to distinguish the over-shooting tops. The subsidence modeled in Fig 2 is very clear both east and west of the severe storm. This image was taken just five minutes after an F3 tornado ripped through Iowa. (Pavolonis) Fig 3.a 19 UTC Fig 3.b 2340 UTCoccurred in mid-June, the sun’s direct rays were almost at their highest latitude in the northern hemisphere. Therefore, clear sky allowed the incoming solar radiation to both evaporate the rain water that had recently fallen, and warm up the temperatures at the surface. However, as the same time, cool and dry air at upper levels was advected over this region. Therefore, by the late afternoon, the combination of this elevated mixed layer together with the warm moist surface air created a potentially unstable environment. A surface cold front (Fig 3a) was enough to lift this unstable surface air up to the level of free convection, and the diffluent flow at upper levels created favorable conditions for the anvil outflow. Rapid thunderstorm development quickly followed (Fig 3b). This set up could be generically summed up as a Type D/E synoptic pattern. As Fig 4 shows, there was a 500mb upper level diffluent jet (brown) associated with an upper level low (purple arrows) spinning over Wyoming. There was also a strong cold front (blue), associated with a surface cyclone (L), which is oriented north/south across eastern Kansas and Nebraska. There was considerable surface convergence due to the wind shift along the cold front. A tongue of warm