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Home , Anticyclone

The of April 13 th , 2006 in Iowa and Wisconsin

Brian Miretzky

Senior Undergrad at the University of Wisconsin – Madison

Final Case Study AOS 453

May 9 th , 2006

Photo taken by Ryan Pfannkuch and Ben McMillan.

Photo taken by Dr. Pao Wang.

ABSTRACT

The night of April 13 th , 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 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 produced many tornadoes in Iowa and many reports of large in Wisconsin. Two pictures which represent this event are the placed before the abstract. The first picture is of the 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 and the produced. With the help of hand The movement of analyses, surface data, reports, and are never that easy to model products the storms that hit Iowa, predict. Supercells have their own Wisconsin, and Illinois on the night of distinct circulation patterns that lead to April 13 th , 2006 will be diagnosed in their hard to predict movement. order to gain better insights into their Sometimes they follow the synoptic flow peculiar movements. pattern and sometimes they do not. Supercell thunderstorms are most 2. Data prevalent during the and early when the temperatures warm For this case study hand analyses and the synoptic flow is still relatively are used to aid in the discussion. Some strong. There are also many other of these hand drawn diagrams were atmospheric characteristics that are developed with the aid of computer conducive to supercells. Some of the plotting programs. The computer characteristics are shear, instability, software packages used in this case moisture, and low level convergence. All study are General Meteorological four of these characteristics are present Package (GEMPAK) and General on April 13 th , 2006 in the evening hours. Meteorological Package Analysis and The intent of this case study is to Rendering Program (GARP). In addition concentrate on the movement of the to actual observational data, model data supercells from their inception through from the North American Model (NAM, the time they produce their impressive formerly known as the ETA) model on severe weather. It is hypothesized that the 211 and 212 grids was also available this movement was a product of for use. Model data from the Rapid individual circulations and large scale Update Cycle (RUC) model was also circulations. The severe weather they used since in it is run more frequently. produced will be discussed in relation to Also, surface and upper air plots from each cells movement. In each supercell www.weather.unisys.com are used to lies an interesting correlation between view the overall situation. Radar and Satellite images are taken from the along with storm information. The radar images 3. Synoptic Overview come from WSR-88D Doppler radar network, which is run by the National The first way to see the synoptic Weather Service. Lastly, the satellite set up is by looking at a Unisys surface images come from the GOES-8 satellite map from April 14 th , 2006 at 0Z shown run by NASA. in figure 1.

Figure 1. Surface map for 00Z on April 14 th , 2006.

This is a few hours after the beginning of as a stationary front, but it more the convection but gives a good view of probably was just a warm front. This the synoptic situation. The surface map diagnosis is made based on both surface shows a front stretching from Minnesota observations in figure 2 and upper air southeastward through southern observations in figure 3 in addition to Wisconsin and into northeastern Illinois. figure 1. The surface map from Unisys shows this

Figure 2. Streamlines and dewpoints for April 13 th , 2006 at 22Z.

Figure 3. 4 panel plot for April, 14 th , 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 in more westerly and slightly northwesterly southern Manitoba and associated cold flow around the southern edge of the low front. A developed low usually has a pressure and along the axis, as cold and warm frontal feature. In denoted by the yellow dotted line over addition the Unisys upper air plot in Iowa in figure 1, a figure 3 shows warm air advection south was formed in western Iowa. In order to of the supposed stationary front along satisfy the mass continuity equation, this with a temperature gradient, which is convergence must result in upper air emblematic of a baroclinic zone. Also, transport—an important stimuli for the surface observations in figure 2 show convective triggering. A final feature a distinct shift along this front from that was important in the set up was the south and southwest to east and location of the Jet. A jet streak was southeast. There is distinct warm pool of located over the South Dakota eastward air to the southwest of the front and into Wisconsin. This jet was not cooler air northeast of it. important in the initiation of the storm, Now that the warm front has but was important later in the been diagnosed its importance can be development. The jet location to the discussed. The position of the warm north and northeast of the storms means front leads to intense warm air advection the inertial stability was low due to the and heating in western and central Iowa anticyclonic shear. This allowed the during the day of April 13 th . to more easily flow to the north Temperatures reached 90 degrees in and northeast, which created a negative some places while the normal average back pressure thus sucking up the high temperature is in the lower 60’s. updraft. This enhanced the instability Omaha, Nebraska set their record high and allowed for the storms to continue to for April 13 th with a recorded develop and stay vigorous as they moved temperature of 89 and Des Moines, Iowa eastward. tied their record high with the same temperature. April 13 th was the hottest 4. Mesoscale Analysis day in April for most of this region. This advection coupled with solar radiation In order to have severe storms from the lack of cloudiness resulted in certain mesoscale features must be an overall destabilization of the air over present in addition to the right synoptic Iowa. The process, known as differential set up. Two of these features are large advection, was not an instantaneous amounts of small scale instability and event, but a process that developed from wind shear. These features are not persistent heating and transfer from the necessary for the initiation of Great Plains. This instability can be seen convection, but must be present for the in figure 3 with the high lifted indices. enhancement of convection. The small This combined with the moisture laden scale instability can be seen by figures 4 flow from the Gulf of Mexico as seen in and 5. figure 2. When this air collided with the

Figure 4. A cross section taken from W-E along a line from Des Moines, IA to West Bend, WI on April 14 th , 2006 at 0Z. Wind vectors are in black and lines of equivalent potential temperature are in blue.

Figure 5. SPC analysis of CAPE (blues) and CINH (orange, red), 4.13.06, 00Z

Figure 4 shows a moisture tongue Bend the clockwise turning is extending eastward from Des Moines. emblematic of positive helicity is Combine this with figure 5, which shows present. The positive helicities high Convective Available Potential everywhere suggest left moving Energy (CAPE) values over Iowa and supercells are unlikely. Another feature the result is high small scale instability. that is conducive to supercells is the lack Wind shear with height is shown by of much upper level wind shear. If the figure 4. Across the whole cross section upper levels change too much the the winds increase greatly with height up anvil can be torn apart and the updraft until the at 250mb. Also, the destroyed. Figure 4 shows that there is winds veer with height. This means that not much upper level wind shear on the winds turn counterclockwise. This is April 13 th allowing for the supercells to symbolic of warm air advection and form and maintain there updrafts. positive helicities. Helicity is a product Another way to see mesoscale of the vector dotted with the features is by looking at soundings. Two velocity of the wind vector. Higher soundings that are somewhat helpful in helicties increase the likelihood that this case are the 0Z sounding for supercells will form. Usually, helicities Davenport, IA and the 12Z sounding for are the highest right in the vicinity of the Lincoln, IL on April 14 th , 2006. These warm front. Even in the eastern part of soundings are shown in figures 6 and 7. the cross section in figure 4 at West

Figure 6. Sounding taken at 0Z on April 14 th , 2006 at Davenport, Iowa

Figure 7. Sounding taken at 12Z on April 14 th , 2006 at Lincoln, Illinois.

The Davenport sounding is taken about to the west of Davenport. A an hour before the supercells move is possibly due to a rear inflow through the area. The air balloon quit jet from the west where there are lower sending data at 400mb, which is why the dewpoints. Above this a small elevated sounding cuts off at that point. This was mixed layer is present around 600mb. more than likely due to the severe When the convection approaches it will weather entering the area. The break through this mixed layer and use convective available potential energy its plentiful moisture. The sounding from (CAPE) is just over 700 Joules/kilogram. Lincoln is taken after the storms moved There are two possible explanations for through. But it is still useful in this relatively modest assessment of understanding the supercells. The feature energy. The first is simply error. As that is interesting is the strong westerly shown, the sounding stopped at 400mb, wind at approximately 900mb, which is which could affect this value. The larger then both the winds below it and second, and non-mutually exclusive above it. This strong wind is what is left case, could be convective activity the low level jet. The low level jet already began ‘overturning’ the air mass. started out from the south approximately Either way, this sounding shows the 12 hours beforehand and turned destabilization by differentially advected westward with time due to the inertial air. Only a slight inversion exists, which oscillation. This helped to steer the will be eroded even further as the colder supercellular development into Illinois and moister air moves in. This in turn with time as the axis of warm advection will lower the lifted condensation level was shifted eastward. and level of free convection bringing the The best way to see the convection lower to the surface. This is movement of the supercells is using one likely what was happening just to the of the most powerful mesoscale tools, west of Davenport at this time. The which is radar. Figure 8 is radar images lowering of the base makes it taken at different times which show the more likely a tornado could occur, which propagation of the supercells. is what was happening in Iowa City just

Figure 8. NEXRAD Radar images taken from 0Z thru 3Z on April 16 th , 2006. The order of the images is top left, top right, bottom left, bottom right.

There are several interesting features of and weaken slightly, while the southern note from these figures. Initially one cells continue to have more cells develop major supercell seems to dominate in on both the forward and rear flanks. This central Iowa, but as the cell moves east split of the cells in Iowa is very more cells develop to the north and west interesting especially when considering of it. These seem to be separate cells and the factors involved. Normally a split not the original cell splitting. As time would be associated with two different goes on the northern cells continue make cell circulations forming; right and left a slight move to the northeast and then moving storms. This implies storms continue eastward into Wisconsin. The moving to left and right of the mean southern cells meanwhile move flow. But this case is different, which southeastward through western Iowa and can be seen through the 700mb map into Illinois. The northern cells presented in figure 9. eventually move over Lake Michigan

Figure 9. 700mb map from SPC at 0Z on April 14 th , 2006.

This map shows slight divergence in the motion. By the time the northern cells flow over Iowa and western Illinois. It have really developed any also shows the direction of the mean they are to the north of the warm frontal flow from the southwest over Iowa boundary in Wisconsin. This is more meaning the direction of mean conducive to hail formation rather then propagation would be to the northeast. tornadoes. This is because north of the This then implies then the northern warm front the temperatures are colder supercells move with the mean flow and and thus hail will be less likely to melt. are not left movers. It also implies the Also, tornadoes are less likely to form storms that the southern storms move because of the lack of moisture in the against the mean flow and are right lower part of the . Thus the movers. These right movers are due to cells in Iowa produced tornadoes and the intense circulation in these cells. A then wind damage in Illinois, while the possible for reason for this is they are cells in Wisconsin were mostly hail more mature cells and thus the northern producers. Finally, even though not fully cells have small or no mesocyclones discussed a probable reason the present at the time. These mesocyclones Wisconsin storms produced record hail act like large scale baroclinic systems was the small amount of upper level and as the air rises up into them new wind shear and thus strong updrafts in mesocyclones form to the right every the supercells. This most likely helped in time leading to the abnormal storm allowing for the hail to be circulated many times in the cloud before finally supercells are different entities becoming too heavy and falling to the especially from the radar images when ground. they split. But another helpful mesoscale Up until now the overall tool that helps show this is not the case organization of these cells has not been is satellite. Figure 10 shows infrared answered. Because of the different satellite images taken at different times. circulations it might seem as if these

Figure 10. Infrared Satellite images using the GOES-8 satellite taken from April 13 th at 22Z thru April 14 th at 0530Z. The images are sequenced clockwise from top left.

Infrared was used because it can still be have distinct circulations at the surface. viewed during the night and cloud top This would seem to imply that the cells temperatures can be deciphered from it. are organized in a supercell squall line at Initially there are no present over the time, which is a subset of a Iowa near 22Z on the 13 th of April. The mesoscale convective system (MCS). clouds increase in coverage as the Mesoscale convective systems are convection begins with the anvils groupings of thunderstorms and as they spreading out. Just before the storms get bigger they produce system scale split in Iowa the satellite imagery gives motions in addition to the motion of the the impression they are connected at individual cells. MCS implies that there outflow level even though they seem to are one or dynamics driving the system. In this case the organization is likely (MCC). An MCC is just another type of based on the synoptic circulations with MCS which is circular in appearance and possible mesoscale self organization very large. The MCC has many occurring through the interaction of the characteristics that make it slightly individual cells. Eventually the MCS different then other MCS’s. MCC’s must continues to grow during the evening of be large with a warm core vortex at April 13 th and by the time April 14 th rolls middle levels and a cold core meso- around the cloud mass occupies the anticyclone at the surface associated states of Illinois, Indiana, Ohio, and with a density current. An anticyclone is Michigan. The cloud mass is circular in also present aloft feeding into the jet orientation and the outflow extends streak. This outflow aloft creates a outward very far to the north and east. dynamic flywheel that stores energy and The tremendous outflow is persists the system for a long period of representative of the fact that there is time even after the energy supplying the low inertial stability to the north and convection stops. Some of the features east; a result of the jet location and that can not be seen from the satellite are anticyclonic shear below the jet. When presented in figure 11, which is a the outflow has less back pressure on it conceptual model of an MCC (credit Dr. an increased updraft results to replace Greg Tripoli). Model data also the loss of mass at the top. By the early confirmed this theory when looked at hours of April 14 th the MCS has most through the vertical cross section tool in likely evolved from a supercell squall GARP. line into mesoscale convective complex

Figure 11. Conceptual model of a MCC, credit Dr. Greg Tripoli

This model shows all the typical In the case of the supercells that characteristics mentioned before. The developed over Iowa on April 13 th many previous information shows the factors were shown to be involved. The evolution of this storms organization is a location of the right synoptic and key component in the movement of the mesoscale features conducive to supercells. supercells was the first thing discussed. Another important part was the individually cellular circulation. Finally the overall system scale circulation was 5. Conclusion the last key part. When these factors combined it led to the interesting It is still no that well known why movement of the cells and the different supercell thunderstorms move erratically severe weather events produced. The sometimes. Why the cells move types of severe weather can be seen in atypically can be different in each case. the SPC storm reports in figure 12.

Figure 12. SPC storm reports for April 13 th , 2006

One person died, many were injured, and and the night of April 13 th , 2006 be a large amount of damage was caused discussed for a long time. from these supercells. The hail produced was of record proportions in Wisconsin. Because of the unusual evolution of these storms many people were affected 6. References Dr. Greg Tripoli- Professor University of Wisconsin- Madison (provided lecture Holly Hassenzahl- TA University of material) Wisconsin- Madison (provided National Weather Service and National background information and answered Data Center (used their figures questions) and storm information) www.weather.unisys.com (for use of their figures)