Severe Weather Outbreak

June 27th, 1998


On June 27th, 1998 an intense line of thunderstorms tracked from southcentral Minnesota through western Wisconsin, continuing eastward the next day toward the New England states. As this line passed over the upper Mississippi Valley, it produced winds in excess of 90 mph. After storm surveys were done on the damage, winds speeds were estimated to have been over 100 mph in some locations.

In the La Crosse Weather Service's area of responsibility, the strongest and most destructive winds took two different paths. One traveled from just northeast of Wabasha county Minnesota into Juneau county in Wisconsin. The other ran from west of Mower county Minnesota into southcentral Wisconsin. It is important to note, however, that winds were in excess of 60 mph along almost the entire line of storms.

This is an animated loop of the 0.5 degree base reflectivity. The loop starts around 730 pm and ends at 930 pm. This is an animated loop of the 0.5 degree base velocity (same time frame as the reflectivity loop). The winds toward the radar are indicated by the darker colors (blues), while the winds going away from the radar are indicated by the lighter colors (oranges). Notice how the northern bow echo shows up even better here then in the reflectivity.


0.5 degree Reflectivity at 732 pm 0.5 degree Velocity at 732 pm
Two bow echoes can be seen, with the southern one more distinct. This southern bow went through Austin (AUM) MN, where the airport there recorded 92 mph and had extensive damage. The two bow echoes show up very well here, as indicated by the bright blue areas. Those areas correspond to winds of 64 knots (74 mph) or greater. At this time, radar was indicating that somewhere on this velocity product winds were up to 122 knots (140 mph)! It is important to remember here that the highest winds are aloft (approximately 8000 feet), and not at the surface.
0.5 degree Reflectivity at 802 pm 0.5 degree Velocity at 802 pm
Rear inflow notches can be seen on the backside of each bow echo. These "notches" are indicators of strong winds being channeled from aloft to the surface. What happens is that these strong mid-level winds get caught up in the downdraft of the storm, and then forced to the ground. The two bows can't be missed here. The northern one is about to push into Winona (ONA) MN, where the airport there reported 90 mph winds.
0.5 degree Reflectivity at 832 pm 0.5 degree Velocity at 832 pm
The two bowing shapes continue, but the reflectivity seems to have weakened somewhat. That is probably not the case, however. What happends as storms approach the radar is that they usually "appear" to weaken because the storms are too close for the radar to sample the entire storm. The storms then "reintensify" as they move away from the radar, as the sampling improves. In this velocity product, the northern bow continues to go strong, but the southern one seems to have dissappered. This is not the case. The way the radar "sees" the wind is by sampling the wind speeds the come directly into the radar (parallel with the radar beam). So the radar will "see" the best when the winds are headed straight toward the radar, while it will not "see" winds that are perpendicular to the radar beam. In this case, the northern bow still has a component of the wind headed into the radar (it is traveling southeast). The southern bow is headed due east, so when it gets directly south of the radar, the winds are perpendicular to the radar beam.
0.5 degree Reflectivity at 902 pm 0.5 degree Velocity at 902 pm
The bowing structures have now seemed to merge into one large bow. Notice a large rear inflow notch just south of La Crosse. Also, a small inflow notch can be seen to the north. The southern bow has "magically" reappeared (see the explanation of the 832 pm velocity if you haven't already read it). There is also a rather large rotation just north of La Crosse on the northern end of the bow echo. This is referred to as a "bookend" vortex. This is not a tornado (although small tornadoes can be spawned by these), but rather a broad scale rotation that indicates winds that can produce tornadic-like damage. The small inflow notch on the reflectivity product is associated with this rotation.


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