In the last week of July in 2009, Southern Wisconsin experienced several rounds of severe weather. Both rounds included some elements of damaging winds, large hail, weak tornadoes, flash flooding, and even a phenomenon known as a wake low. The first round occurred on July 24th, and was restricted to far Southwest Wisconsin. Lafayette County was the only county that was impacted in the Milwaukee/Sullivan county warning area. The second round occurred on July 27th. This round of severe weather impacted a broader area over South-Central Wisconsin.
Radar images were created using Gibson Ridge Level 2 Analyst Edition software.
|July 24, 2009 Severe Weather|
Two supercell thunderstorms tracked southeast across parts of Lafayette County. The first storm basically crossed Lafayette County between 5:00pm and 5:45pm along a line from just southwest of Belmont to near Apple River, Illinois. As the thunderstorm entered northwest Lafayette County, it showed a remarkable hail signature on radar.
Below and on the left is a 4-panel image of 0.5° Refl (upper left), 1.3° Refl (upper right), 1.9° Refl (lower left), 2.4° Refl (lower right) at 5:03pm CDT from the Sullivan, WI radar. A similar 4-panel image, just at 5:31pm CDT is shown below and on the right, when the storm had progressed into southern Lafayette County.
At that range from the radar, the lower right hand panel would indicate the reflectivity at about 23,000 to 25,000 feet above the ground level. The radar also exhibited a "three body scatter spike" signature (click here for an image) as the storm moved across Lafayette County. The three body scatter spike is caused by the radar beam hitting large, wet hail and scattering to the ground, scattering back up into the storm, and then being scattered one final time by the hail aloft again. The second storm clipped the far southwestern part of the county between 6:00pm and 6:50pm CDT, and showed a similar hail signature on radar (click for a 4-panel radar image like the ones above, at 6:35pm CDT).
When the National Weather Service damage survey team was in Lafayette County, they noted areas with especially severe crop and vegetation damage. This, combined with radar hail size algorithms, is conveyed in the map below, on the left (note that smoothing and estimation were used):
Pictures of the hail damage found on the survey is shown in the pictures above (last three images). The second image in the row above shows corn stalks stripped bare and sheared off. These corn stalks were, at highest, a foot and a half. That picture was taken south of Belmont on Highway 126. The third image shows soybeans that were badly damaged or destroyed by the hail. These fields are normally a deep green. That picture was taken near Highway U and Jackson Road, to the north of Shullsburg. The fourth picture shows corn that was stripped by large, wind-driven hail. That picture was taken just northeast of Shullsburg on Highway U.
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What was perhaps most amazing about the hail damage was that most vegetation sustained damage. Corn and soybean crops were the most noticeable, but even weeds and grass by the side of the roadways were snapped or shredded as well. The two supercell's paths tended to merge towards the southern part of Lafayette County. Where the hail swaths merged, they created about a 9-mile wide and 3-mile deep (north to south) swath of severe hail damage. This area roughly paralleled County Highway W and stretched from near County Highway O east to Walker and Cub Hollow Roads. We also received numerous reports of damage to vehicles and structures from wind-driven hail from all over western Lafayette County.
In addition to the extensive hail damage, both supercells produced a weak tornado. Based on surveyed damage, the first supercell likely began producing a rear flank downdraft (RFD) that caused a surge of severe straight-line winds as it crossed from Grant County into Lafayette County. The area of RFD wind damage extended from near Highway 81 and the county border southeast to a rural area a few miles north of Lead Mine (map pictured below, on left; click for larger image). A concentrated swath of wind damage developed from just southeast of the intersection of County Highway X and Highway 81 and progressed to near the intersection of County Highway H and Red School Road. This concentrated swath likely developed around 5:10pm CDT and was the result of a significant downburst.
The most intense wind damage was found near Coulthard Lane. Several corn fields were flattened, a shed was destroyed (pictured above, on right), and there was a considerable amount of tree damage, including some trees being snapped. One snapped tree was about 2-3 feet in diameter. Straight-line wind gusts in this area were likely around 90 mph.
As the downburst event occurred, it likely helped intensify the circulation in the parent thunderstorm, and a few minutes later a tornado developed. The tornado began at about 5:12pm CDT about 4.2 miles NW of Shullsburg in a field northeast of the intersection of Prairie Road and County Highway Q. It initially moved south-southeast. As it crossed Highway Q, a fire fighter reported seeing swirling debris cross the road. As the tornado passed by Dry Bone Road, it began to turn on a more easterly course and the path widened. An image of the tornado path is pictured below. Click on the left thumbnail for the full-resolution version. Radar images are available as two-panels with storm relative velocity on the left and reflectivity on the right, from the La Crosse, WI radar: 0.9° at 5:14pm CDT, 0.5° at 5:18pm CDT, and 0.9° at 5:18pm CDT.
The more significant damage began on a farm just west of the intersection of Jackson Road and County Highway O. On this farm, a barn was badly damaged as the walls collapsed and the roof was blown into the structure (2nd picture from left, above). A large tree had some large limbs and branches snapped off (3rd picture from left, above). Evidence of rotational winds included all vegetation on the north face of the house laid down facing west, and a window on the north side of the house blown in, along with eyewitness accounts. The tornado essentially moved along Jackson road, first snapping a large tree, and then stripping some branches off of a line of trees about half way down the road. The damage path ended around where County Highway U intersects Jackson Road. A shed here was destroyed (4th picture from left, above), with some metal fragments being tossed 100 yards downstream. One metal fragment was wrapped around a stop sign. Corn was flattened on the opposite side of County Highway U in a convergent pattern. The tornado was rated an EF-1 on the Enhanced Fujita Scale with maximum wind gusts estimated to be in the 100-105mph range, and it ended around 5:17pm CDT.
About 20 minutes after the first supercell completely exited Lafayette County, another supercell began to rapidly intensify and show signs of strong rotation in Grant County. Click here for a 4-panel radar image from the Davenport, Iowa radar (upper left 0.5° Refl, upper right 0.5° SRM, lower left 1.4° Refl, lower right 1.4° SRM) at 6:11pm CDT when the storm was in Grant County.
The storm eventually pushed into southwest Lafayette County, maintaining its rotation signature. Straight-line wind damage associated with the rear flank downdraft was much more sporadic, and weaker, with this storm indicating maximum wind gusts approximately in the 60 to 65 mph range. Some branches and limbs were blown down in the far southwest part of Lafayette County. The tornado associated with this supercell thunderstorm began at about 6:32pm CDT 1.8 miles northeast of New Diggings. Near the beginning of the path, a newer metal shed was completely destroyed (2nd picture from left, above) with some surrounding tree damage. Sporadic tree damage was observed along the remainder of the track until the tornado reached a house along Black Hawk Road in the area south of Shullsburg. A portion of the roof, and part of a wall, were ripped off by the tornado, and windows were blown in on the west, south and east sides of the house (3rd picture from left, above). The tornado was rated an EF-1 on the Enhanced Fujita Scale with maximum wind gusts estimated to be around 90mph, and it ended around 6:40pm CDT. Click here for a 4-panel radar image from the Davenport, Iowa radar (upper left 0.5° Refl, upper right 0.5° SRM, lower left 1.4° Refl, lower right 1.4° SRM) at 6:11pm CDT when the storm was in Grant County.
|July 27, 2009 Severe Weather|
A supercell thunderstorm tracked southeast across Marquette County and into southwest Green Lake County in the late afternoon hours on July 27th. Along its track, the supercell produced some straight-line wind damage, along with some severe hail and hail damage to crops. The hail damage here was not as significant as what was observed in Lafayette County from the July 24th storms. However, some hail up to 1 inch in diameter was found about a mile north of the tornado track 18 hours after it fell (when the damage survey was conducted)! Shading from the corn stalks probably contributed to the hail avoiding melting completely, and the hail likely washed into a large pile which kept hailstones near the ground sheltered from the warmer air. A few pictures are included below.
The tornado associated with this thunderstorm began at approximately 5:48pm CDT 2.9 miles southwest of Montello in Marquette County and tracked southeastward to a point about 1.5 miles northwest of Dalton in Green Lake County. The tornado likely ended around 6:16pm CDT. The tornado path is depicted in the graphic below, on the left. Damage with the tornado included almost exclusively damage to trees and crops. Some trees were uprooted. There was some minor roof damage to a house along the tornado path as well. Some of the damage associated with the tornado is shown in pictures below. The tornado was rated an EF-1 on the Enhanced Fujita Scale with maximum wind gusts estimated to be around 95mph
Below are some pictures of the supercell thunderstorm as it moved through southeastern Marquette County. They were taken by Doug Raflik who was in the Dalton area looking northwest. The main features of the supercell are annotated on the image. Doug and other spotters indicated that inflow into the rotating wall cloud was impressive - from the east and from the south.
We also have included several radar images. The first is a 4-panel image (click here) from the Sullivan, WI radar with the upper left 0.5° Refl, upper right 0.5° SRM, lower left 0.9° Refl, and lower right 0.9° SRM at 5:49pm CDT. The second is a 4-panel image (click here) from the Sullivan, WI radar with the upper left 0.5° Refl, upper right 0.5° SRM, lower left 1.3° Refl, and lower right 1.3° SRM at 6:03pm CDT.
A thunderstorm pushing across Lafayette County around 8pm produced some wind damage along with a very brief tornado. The wind damage likely occurred between 7:40pm and 7:50pm CDT along Highway 23 from 4 miles north of Darlington to Darlington. This swath of wind damage included snapped and uprooted trees, flattened corn, and some minor structural damage. A couple of pictures of the straight-line wind damage is included below.
The brief tornado then likely occurred at 8:10pm CDT about 2.6 miles northeast of Gratiot. It had a very short path length and was likely on the ground for less than a minute. Click here for a radar 4-panel image at 8:08pm CDT (upper left 0.5° Refl, upper right 0.5° SRM, lower left 0.9° Refl, and lower right 0.9° SRM). Tree limbs about a foot in diameter were blown down. Some trees were also blown down. There was also minor roof damage. The tornado was rated an EF-0 on the Enhanced Fujita Scale with maximum wind gusts estimated to be around 80mph. A map of the tornado location is shown below, on the left. The remainder of the pictures are of the tornado-related damage.