A violent tornado destroyed most of Spencer SD on 30 May 1998. According
to DOW data, hurricane force winds (> about 35 m/s) first hit the town
at approximately 8:38pm CDT. The center of the tornado was over Spencer
at 8:39pm (W edge of town at 8:38:30, E edge of town at 8:39:30), winds
of 106-110 m/s (237-246 mph) occured to the south of the tornado at that
time. Winds dropped below hurricane force by 8:40pm. The storm
was moving at approximately 15 m/s (32 mph) to the ESE. Winds would
have increased from hurricane force (35 m/s) to devestating force (~100
m/s or 200 mph) in about 20 seconds.
A roughly 200 m swath of F3/F4 damage (unofficial)
was observed in Spencer by DOW crews. The NWS has categorized the
damage as up to F4. The worst observed damage was S of the DOW center
track, as expected.
Only DOW3 was functional so only single-Doppler data is available.
Peak tornado relative winds were approximately 85
m/s (190 mph)
DOW3 took data in the forming tornado from S of Fulton SD starting at approximately
8:03 (1:03 UTC) until ~8:22 (1:22 UTC) with a 6-7 minute gap. DOW3
redeployed to 2.0 miles (3.5 km) WSW of Spencer by 8:33. The tornado
center was 1.1 miles (1.7 km) to the NW of the DOW permitting extremely
high resolution high prf observations as it moved towards Spencer, hitting
the town at 8:37 (1:37 UTC).
Data is just being translated, but early indications
show Doppler velocities of 100 m/s, indicating ground relative winds
of about 110 m/s (246 mph) to the S of the tornado. (This
is based on a difference of 38 degrees between the observed track and the
radial velocity direction and a 16 m/s velocity. Only obvious dealiasing
has been performed on folds at 32 m/s and 96 m/s. Sub-tornado high
shear/rotation zones have not been dealiased and might hold much higher
velocities)
Image #14: 980531-013659: Tornado
about to hit Spencer. Grids 500 m, N is up, body of supercell is
off top of page. Shear couplet is +98/-64. Tornado is moving roughly
E (left to right). Spencer is to E of high reflectivity disk at (0.8-1.5,3.8-4.5).
Note strong shear zone to W of tornado. 75 m/s (Doppler) = 83 m/s
(ground) zone is ~250 m which corresponds to region of total destruction
in town. Reflectivity values are uncalibrated and saturated.
