Using a Rotational Shear Nomogram to Classify Ambiguous Tornadoes Observed in Central California

There have been many nomograms developed, based upon research, to aid forecasters with the
detection of mesocyclones, tornadoes and supercells. One of the more popular nomograms which
is based on rotational velocity developed by Andra (1997). However, these usually accepted
methods of detection typically do not work well for the Central Valley of California. Circulations
in radar velocity are tough to pinpoint across the topographically challenged NWS Hanford
County Warning Area (CWA) for two reasons: many tornadoes are only on the ground for a
short period of time and do not typically produce the gate-to-gate shear found in supercells.
Research by Falk and Parker (1998) from NWS Shreveport, has focused on velocity signatures
for “mini-supercells”, which they determined to be less than 2 nautical miles in diameter. Also in
contrast to the rest of the country, previous research on California tornadoes from Monteverdi
and Doswell has identified that several tornadoes in the Central Valley are from mini-supercells,
whose most dominant feature is 0-1 km low-level shear. Familiarization of the aforementioned
characteristics present unique challenges for radar operators at NWS Hanford, and was the
motivation for our reanalysis.

The research done by Falk and Parker was anecdotally found to be more successful in the
classification of Central Valley tornadoes with regards to rotational shear rather than rotational
velocity observed during the 2018-19 wet season. In order to modify the Shreveport nomogram
for use by the Hanford office, the Tornado Climatology from (Sanger and Andersen, 2014) was
further examined.

The 2018-19 wet year was an above average year for observed tornadoes in the Central Valley.
This further necessitated research to find better ways to identify rotation and possible tornadoes
using velocity data. The goals of our research was to help improve lead time, detection, and the
overall confidence of our forecasters when issuing tornado warnings. To achieve this, we looked
back at confirmed tornadoes from November 1996 through June 2019 and calculated the
rotational shear with the method defined by Falk and Parker. Our customized nomogram with
renewed classifications for minimal mesocyclone, tornado possible, tornado probable and
tornado likely, for the NWS Hanford CWA, will be exhibited along with our data.