Radar Tutorial
Chapter 3, Page 3: Derecho: A Thousand Mile Super Storm
May 15th 1998
This is a velocity radar loop of the same storm as it passed into Ramsey County into Anoka. Notice the wind speeds show
a full scale from 50MPH away from the radar to 50MPH towards the radar. Winds were clocked at 99MPH about this time in
northern Ramsey County.
Notice a couple of velocity couplets (dark red and green or blue close together) develop a few miles to the west of the
center of intensity of straight lines winds. What appears to be a couple micro bursts occurred wests of the central
gust front. An intense velocity couplet represents a strong rotation close to the downburst spawns a tornado. This is
not the traditional tornado that developed from a mesocycle in a supercell thunderstorm. Rather this is a lower level
rotation similar to tornadoes you'd find in squall lines, of short duration, but sufficiently intense to do considerable
damage up to F2 in scale.
What appears to be a microburst develops over central Hennepin County. At this time, the major thrust of wind was over
Scott and Dakota Counties and just entering Ramsey County.
Within ten minutes the another possible microburst or possibly the continuation of the first moves into SE Sherburne
County and it is joined by a second in Western Anoka County. But more importantly, notice the leading edge of the
straight line winds have surged into SE Anoka Coutny and NW Washington County.
Just to the SW a strong rotation has spawned a tornado. The tornado tracked 12 miles nearly straight north, likely in
under ten minutes, moving as fast as 70MPH from Roseville to Shoreview to Lino Lakes and into Eastern Blaine. Damage
was rated as F2.
Just a few minutes later, the tornado has apparently dissipated in Blaine. Notice another rotation has developed
near the center of the gust front over north central Washington County, but there were no reported touchdowns.
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