The Supercells of April 13th, 2006 in Iowa and Wisconsin Brian Miretzky Senior Undergrad at the University of Wisconsin – Madison Final Case Study AOS 453 May 9th, 2006 Photo taken by Ryan Pfannkuch and Ben McMillan. Photo taken by Dr. Pao Wang.ABSTRACT The night of April 13th, 2006 was a very interesting night for many citizens of a certain section of the Midwest. The states of Iowa, Wisconsin, and Illinois were all affected by a severe weather event. This event started as convection in west-central Iowa and developed into supercells over eastern Iowa. The supercells then went separate directions into Wisconsin and Iowa. This was a result of mesoscale interaction between the cells and the overall synoptic circulation. The mesoscale interaction is very confusing and will be a subject of debate for a long time into the future. Overall, the storms produced many tornadoes in Iowa and many reports of large hail in Wisconsin. Two pictures which represent this event are the placed before the abstract. The first picture is of the tornado to hit Iowa City, IA area and the second picture is of the hail to hit the Madison, WI area. Using different tools both the synoptic and mesoscale conditions will be discussed to understand the origins and movement of the supercells. 1. Introduction The movement of supercell thunderstorms are never that easy to predict. Supercells have their own distinct circulation patterns that lead to their hard to predict movement. Sometimes they follow the synoptic flow pattern and sometimes they do not. Supercell thunderstorms are most prevalent during the spring and early summer when the temperatures warm and the synoptic flow is still relatively strong. There are also many other atmospheric characteristics that are conducive to supercells. Some of the characteristics are shear, instability, moisture, and low level convergence. All four of these characteristics are present on April 13th, 2006 in the evening hours. The intent of this case study is to concentrate on the movement of the supercells from their inception through the time they produce their impressive severe weather. It is hypothesized that this movement was a product of individual circulations and large scale circulations. The severe weather they produced will be discussed in relation to each cells movement. In each supercell lies an interesting correlation between the movement and the severe weather produced. With the help of hand analyses, surface data, storm reports, and model products the storms that hit Iowa, Wisconsin, and Illinois on the night of April 13th, 2006 will be diagnosed in order to gain better insights into their peculiar movements. 2. Data For this case study hand analyses are used to aid in the discussion. Some of these hand drawn diagrams were developed with the aid of computer plotting programs. The computer software packages used in this case study are General Meteorological Package (GEMPAK) and General Meteorological Package Analysis and Rendering Program (GARP). In addition to actual observational data, model data from the North American Model (NAM, formerly known as the ETA) model on the 211 and 212 grids was also available for use. Model data from the Rapid Update Cycle (RUC) model was also used since in it is run more frequently. Also, surface and upper air plots from www.weather.unisys.com are used to view the overall situation. Radar andSatellite images are taken from the National Weather Service along with storm information. The radar images come from WSR-88D Doppler radar network, which is run by the National Weather Service. Lastly, the satellite images come from the GOES-8 satellite run by NASA. 3. Synoptic Overview The first way to see the synoptic set up is by looking at a Unisys surface map from April 14th, 2006 at 0Z shown in figure 1. Figure 1. Surface map for 00Z on April 14th, 2006. This is a few hours after the beginning of the convection but gives a good view of the synoptic situation. The surface map shows a front stretching from Minnesota southeastward through southern Wisconsin and into northeastern Illinois. The surface map from Unisys shows this as a stationary front, but it more probably was just a warm front. This diagnosis is made based on both surface observations in figure 2 and upper air observations in figure 3 in addition to figure 1.Figure 2. Streamlines and dewpoints for April 13th, 2006 at 22Z. Figure 3. 4 panel plot for April, 14th, 2006 at 0Z using ETA analysis of in clockwise order from top left the (a) 850mb, (b)300mb, (c)850-500mb, (d)surface levels.Figure 1 shows a low pressure system in southern Manitoba and associated cold front. A developed low usually has a cold and warm frontal feature. In addition the Unisys upper air plot in figure 3 shows warm air advection south of the supposed stationary front along with a temperature gradient, which is emblematic of a baroclinic zone. Also, the surface observations in figure 2 show a distinct wind shift along this front from south and southwest to east and southeast. There is distinct warm pool of air to the southwest of the front and cooler air northeast of it. Now that the warm front has been diagnosed its importance can be discussed. The position of the warm front leads to intense warm air advection and heating in western and central Iowa during the day of April 13th. Temperatures reached 90 degrees in some places while the normal average high temperature is in the lower 60’s. Omaha, Nebraska set their record high for April 13th with a recorded temperature of 89 and Des Moines, Iowa tied their record high with the same temperature. April 13th was the hottest day in April for most of this region. This advection coupled with solar radiation from the lack of cloudiness resulted in an overall destabilization of the air over Iowa. The process, known as differential advection, was not an instantaneous event, but a process that developed from persistent heating and transfer from the Great Plains. This instability can be seen in figure 3 with the high lifted indices. This combined with the moisture laden flow from the Gulf of Mexico as seen in figure 2. When this air collided with the more westerly and slightly northwesterly flow around the southern edge of the low pressure and along the trough axis, as denoted by the yellow dotted line over Iowa in figure 1, a convergence zone was formed in western Iowa. In order to satisfy the mass continuity equation,