Warning Decision Process
KRAX 4 panel imagery from 1920Z (320 PM EDT) on Wednesday, March 28, 2007.
The images
are of Composite Reflectivity in the upper left, Echo Top in the upper right,
0.5 degree reflectivity in the lower left, and VIL in the lower right.
Once convection initiated, warning forecasters made use of a variety of products to
assist in the warning process. Volume scan products such as Composite Reflectivity,
VIL and Echo Tops were used to help prioritize which storms needed further investigation.
The All-Tilts radar product was especially useful in storm interrogation because it
allows forecasters to see every radar elevation angle as it comes in. All-tilts allows
forecasters to easily monitor volume-scan to volume-scan convective trends throughout
the depth of the storm, such as the strength and vertical motion of reflectivity
cores, mid-level convergence and storm top divergence.
A recent study (Gerard 1998) of 39 convective events (64 storms total) in WFO's
Cleveland, OH and Jackson, MS showed that
convective cells have a high probability of being severe (96%) when 65+ dBZ
reflectivity echoes are located at or above the freezing level. This signature
worked quite well with storms during the afternoon of March 28, 2006. For example,
at 1922Z (322 PM) a severe thunderstorm warning was issued for Granville County
when a convective cell near Oxford attained a 63
dBZ core above the freezing level (at roughly 12,000 ft). The signature was seen
on the 1.8 degree elevation slice via All-Tilts at 1920Z (320 PM). Several minutes
later, at 1924Z (324 PM), the convective cell attained a 65
dBZ core at 24,000 ft, over 10,000 ft above the freezing level! This and subsequent
signatures led to very high confidence that the storm in question was severe. Reports of
penny and quarter sized hail accumulating on the ground were received at 1947Z (347 PM)
in Oxford with this storm. As the afternoon progressed, warnings for subsequent storms
were based on signatures similar to the storm in Granville County where very high (~65+ dBZ)
reflectivity echoes were being sampled at or above the freezing level.
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