  A hail-producing supercell on 11 May 2017 produced a small (EFU) tornado near
Perkins, Oklahoma (35.97, -97.04) at 2013 UTC. Two infrasound microphones with
a 59-m separation and a regional Doppler radar station were located 18.7 km and
70 km from the tornado, respectively. Elevated infrasound levels were observed
starting 7 minutes before the verified tornado. Infrasound data below ~5 Hz was
contaminated with wind noise, but in the 5-50 Hz band the infrasound was
independent of wind speed with a bearing angle that was consistent with the
movement of the storm core that produced the tornado. During the tornado, a 75
dB peak formed at ~8.3 Hz, which was 18 dB above pre-tornado levels. This
fundamental frequency had overtones (18, 29, 36, and 44 Hz) that were linearly
related to mode number. Analysis of a larger period of time associated with two
infrasound bursts (the tornado occurred during the first event) shows that the
spectral peaks from the tornado were present from 4 minutes before to 40
minutes after tornadogenesis. This suggests that the same geophysical
process(es) was active during this entire window.
