90 MPH: NWS storm survey details the high wind gusts on Saturday

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More: Straight-line winds vs. a tornado: What caused Saturday’s storm damage

Public Information Statement
National Weather Service Huntsville AL
1200 PM CDT Mon May 22 2017

…NWS Damage Report for 5/20/17 Thunderstorm Wind Event…

Peak wind Estimate: 90 MPH
Path length /Statute/: n/a
Path width /Maximum/: Estimated 40 miles (at max width)
Fatalities: 0
Injuries: 0

Survey Summary: During the afternoon and early evening hours of
Saturday, May 20th a northward moving line of severe thunderstorms
produced extensive wind damage across Madison County. The damage
was particularly concentrated and intense along and south of US
Highway 72 and its eastward extension to Interstate 565. These
storms were also responsible for producing damage in several
other counties including Limestone, Cullman, and Jackson counties
in north Alabama and Franklin county Tennessee. All totaled,
the width of the damage was approximately 40 miles when the line of
storms was at its peak intensity.

Several factors led to this intense round of storms including: a
very unstable atmosphere due to daytime heating and high atmospheric
moisture content; an approaching upper level disturbance; and low
level convergence and cell mergers with weaker clusters of storms
from the west. Based on the totality of the reports and the widespread
nature of the damage, the National Weather Service concludes the vast
majority of the events were due to intense straight-line wind damage.

In addition, the pre-storm environment was most conducive to loosely
organized storms favoring bowing structures (favorable for damaging
wind gusts). Given that the event occurred during the afternoon
hours across a highly populated area, the NWS office and its core
partners received numerous photos and videos many of which documented
the storm structure in near real-time. Reviewing these photos revealed
a well-defined and sharp shelf cloud supportive of intense outflow
dominant winds. At least one video along with a brief ground survey
and other documented public reports, indicated the possibility of very
brief gustnadoes, or spinup vortices, along the leading edge of the
storm’s shelf cloud.

The damage associated with these very shallow vortices were overall less
intense and shorter lived than the damage associated with the bowing line
segment itself. And by definition these are not classified as separate
tornadic events. The maximum wind speeds from this event was estimated
from the various photos and documented reports of uprooted hardwood trees
and snapped utility/power poles. That magnitude damage would be consistent
with straight line winds of approximately 80 to 90 mph. Given the totality
of the evidence, this event will be classified as a straight-line/damaging
wind episode.

The information in this statement is preliminary and subject to
change pending final review of the event and publication in NWS
Storm Data.

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