On Sunday August 9th, 2009 a series
of severe weather events unfolded across western New York State. A combination
of severe thunderstorms with damaging straight line winds and a confirmed
tornado pelted the region during the afternoon hours. Later that evening, a
second severe weather event developed and resulted in one of the worst flash
floods to hit the region over the past 30 years. This web page provides a preliminary review of the
meteorological factors that contributed to the severe weather that afternoon and
the catastrophic flash flood from that night, particularly in the
villages of Silver Creek and Gowanda, NY. The day and evening
will be addressed in two parts, the severe weather event from that afternoon
and the flash flood from that night.
pattern on Sunday morning August 9th featured a warm front that
extended southeast across western New York from a Low centered in the Upper
Great Lakes Region (Fig 1). Very warm and moist air was in place across
far western New York. NOAA's Storm Prediction Center had outlined Western
New York in a Slight Risk Area for severe weather that afternoon.
|At 815AM EDT, an Infra-red image of the cloud
tops taken from the GOES Satellite showed a cluster of thunderstorms over the
Upper Great Lakes Region (Fig 2). Over the course of the morning, the NWS
office in Buffalo discussed the potential for severe weather across western New
York with the Storm Prediction Center (SPC) in Norman, OK. It was
agreed that there was a significant threat for severe thunderstorms that
could produce damaging winds that afternoon and a Severe Thunderstorm
Watch was issued for the region.
|As the afternoon progressed an extensive line of severe
thunderstorms developed over the southwest corner of the province of
Ontario, Canada and headed on a southeast track toward western New York.
The Composite Reflectivity radar image taken from the WSR-88D radar at Buffalo, NY at 1:46PM EDT is shown in Figure 3.
The radar image is overlaid with cloud-to-ground lightning strikes
taken over a 5-minute interval. There were nearly 600 strikes
that were tallied over the area outlined by the image during the 5
minute period ending at 1:46PM EDT. The activity evolved into a system that had
characteristics of a parallel derecho.
The line of storms moved rapidly across the region and was accompanied
by a wide swath of damaging winds. Figure 4 is a corresponding
loop of the Composite Reflectivity and lightning data as the storms moved across the region.
|Figure 5 shows a Base
Velocity image taken from the WSR-88D radar at Buffalo, NY as the
line of storms rolled across the Southern Tier. The maximum
velocity sampled from the leading edge of the activity was 65 kts.,
which is approximately 70 mph. The radar beam is at a height
that is roughly 800ft. to 1200ft. above ground level at this point. Rather than a pulse-type severe thunderstorm that
goes through a life-cycle of developing, maturing and weakening over a
period of several minutes, this was a long-lived line of damaging storms
that continued to progress
through Cattaraugus and Allegany counties. Figure 6 is a Base
Velocity loop of the damaging winds associated with the derecho
as it raced across Cattaraugus and Allegany counties. The
leading edge of the gust front is outlined on the image to give you
a history of the track of the damaging winds.
|As the system entered
Allegany county, a rotation developed along the gust front as depicted
in the Storm
Relative Motion field on the doppler radar (Figure 7). This
rotation was later verified to result in an EF-1 tornado that briefly
touched down in a 6 mile long path in the western portion of Allegany
county. Figure 8 shows a loop
of the Storm
Relative Motion field. The yellow circle outlines the
approximate scale of the rotation of the parent mesocyclone that produced
the tornado during the time that it had briefly touched down.
||Figure 9 is a preliminary map of
damage reports that were confirmed in Allegany County associated
with the derecho and embedded tornado. The tornado path
is shown by the yellow line and was confirmed from a combination of
radar signatures and a ground survey by the NWS. The tornado
was categorized as an EF-1 on the Enhanced
Fujita Scale with wind speeds estimated at 100 mph. The
NWS office in Buffalo also summarized the tornado survey in a Public Information Statement. Other markers show
locations where damage was reported from the straight line winds
associated with the larger scale derecho. The winds from
the derecho also were estimated at 80 to 90 mph across a large swath
of eastern Cattaraugus and Allegany counties. The
severe thunderstorm activity that produced widespread wind damage finally
exited western New York state after 4PM that afternoon.