7/18/97 Chase by Jeremy Charles

From: Jeremy Charles (N9VHT) (n9vht@SKYWARN.CS.WISC.EDU):

Subject: Chase summary, 7/18/97 With the area under an SPC-issued slight risk for severe weather, a line of storms began forming in southern Minnesota/far northern Iowa at around 10:15 on 7/18/97. Cap strengths, LIs, CAPEs, and upper level winds looked favorable for the line to fire up to severe levels and rumble toward north-central Illinois, clipping the southwest corner of WI on the way. I was also suspicious that the northern edge of the line would extend farther in to southern Wisconsin based on similar conditions in that area. The Storm Prediction Center issued severe thunderstorm watch 625 effective from 10:30 to 16:00. The watch box included the area I was suspicious of plus a few additional counties a bit farther northward in Wisconsin. Just after 11:00, I decided to see if my training, dry runs and practice chases of rain showers would pay off. The game plan was to head southwest on highway 151 from Madison to intercept the northern portion of the line as it traveled on its projected path across far southwest Wisconsin. I picked up my chase partner, Tom Fleming, N9SZF, on the south side of Madison just before 12:00. As we headed toward the southwest corner of Dane county on US highway 151, we got word of new development in Iowa county heading our way from the west. At 12:30, we departed from the original game plan and perched atop the hill at Brigham County Park, just inside of the Dane/Iowa county border, to check out this new development as it entered Dane county. At 12:43, a cell approached from our west apparently heading east-southeast. The cell structure was heavily obscured by clouds and haze, but a powerful precipitation region was clearly visible through the haze. We could also see a shallow but wide and well-defined wall cloud following the precipitation. We could not tell if there was any rotation due to the haze. We did not report this feature because it dissipated in the six minutes that we waited to see if it would grow or show rotation. This first cell appeared to weaken as it passed to our south, but then grew darker and started producing more precipitation as it started to float away from us to the southeast. At 13:10, we decided to get back on 151 heading east to follow it and see what it would do. We caught up to the cell on its north side after a few minutes and stopped to view it as it had taken on new life. From the intersection of county highway ID and US highway 151, just east of Mt. Horeb, we could clearly see the updraft base to our south. A small amount of scud was present and was being slowly pulled up toward the updraft. CG began to strike from around the updraft base. We moved farther east to the intersection of county highway P and US highway 151, putting the cell due south of us again, and continued to observe. While observing the first cell to our south, we noted the winds picking up slightly out of the west and turned to see the hills off in the distance beginning to disappear. Given the thick haze, I wasn't too impressed by this until wind-whipped rain became *very* clearly visible battering the surface just before the closest hill about three miles to our west. Tom and I quickly figured out that we were becoming the targets of a rain foot from a second cell and started heading for daylight to the north and east on county highway P. Fierce wind-driven horizontal rain bands from the second cell quickly took over the area around us as we continued, at reduced speeds, to the northeast. Before too long, we were just barely caught in the leading edge of the wind and rain. We would break in to clear air only to see the next pocket or rain bands several hundred yards up the road. On a couple of occasions during our escape, we noted small twigs being blown off of trees and scattered on the road. The damage was reported to our base station, but it is unknown if the reports made it to the NWS. At 13:41, the NWS Milwaukee/Sullivan issued a severe thunderstorm warning for Dane county effective until 14:40. A portion of the text read: * AT 140 PM DOPPER RADAR INDICATED A LINE OF SEVERE THUNDERSTORMS NEAR MOUNT HOREB...18 MILES WEST OF MADISON. THE LINE OF STORMS WAS MOVING EAST AT 20 MPH. At one point when we stopped to take observations, we noted the clouds above us churning as the leading edge of the strong winds were causing air to be forced up in to the cloud base. Shortly after this observation, we broke free of the heavy rain and wind for good just west of Middleton. Reports from other chasers in our group started coming in of possible wall clouds and other related events to our east-southeast. Our observations concluded that our escape put us just to the northeast of the second cell (which had essentially followed just to the northwest of our first cell). Since most spotter reports showed cells moving east-southeast, we got on US highway 12 near Middleton and followed it east-southeast around the south side of Madison. Highway speeds allowed us to continue to outrun the leading edge of the second cell. We were again positioned just slightly to the north of our first cell a few miles west of the intersection of US highway 12 and Interstate 90. We investigated a report of possible rotation in the updraft area of the cell. We concluded that this was mostly a repeat of the "winds being shoved up in to the cloud base" event that we had observed earlier, although we did spot an area which appeared to have some weak rotation. This dissipated a few minutes later as we took to US highway 12 again and then headed south on Interstate 90. Shortly after getting on the Interstate, we spotted a wall cloud with a very pronounced beaver tail following it. The wall cloud itself was not very well defined (the leading half of it was not very solid), but the inflow from behind was very pronounced with condensation streaming in from below and behind. We stopped at the intersection of county highway N and Interstate 90 to observe the cell. This put is about 10 miles north of it. As the cell passed to our south, we noted a sudden change in wind direction. The winds had been breezy out of the west but shifted to moderately windy from the north. After we continued south on the Interstate, we got out of this inflow and the trees were demonstrating light winds out of the west again. After another several minutes of chase, the features we had observed dissipated and we called the chase off as the line exited our area. As it turns out, the bulk of the line traveled slightly north of where I had originally expected it to go. It went right through the middle of the SPC's watch box. The cells we had chased were part of the northward extension of the line - a result of the line extending farther north than I had anticipated. Due to hunger and the need to return to work, we did not go back and scout for damage in the area where we had encountered the rain foot. Overall, this turned out to be a very good day for my first real severe storm chase! Total chase distance: 121 miles. -Jeremy C. (N9VHT) South-Central Wisconsin SKYWARN NCS & Training Coordinator Madison Area Repeater Association Vice President & Swapfest '98 Coordinator n9vht@skywarn.cs.wisc.edu Online SKYWARN logging: http://www.cs.wisc.edu/~jeremyc/skylog/

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