OVERVIEW

 

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.

 

The weather 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. 

Storm Prediction Center Surface Weather Analysis at 8:00AM EDT Sunday August 9, 2009
Figure 1:

 

GOES Infra-red satellite image at 8:15AM EDT
Figure 2: 

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.

WSR-88D Composite Reflectivity and cloud-to-ground lightning strikes at 1:46PM EDT Sunday, August 9, 2009 
Figure 3:


WSR-88D Composite Reflectivity animation and cloud-to-ground lightning data from 12:42PM through 3:00PM EDT Sunday, August 9, 2009 
Figure 4:

 

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. 

WSR-88D Base Velocity at 1:46PM EDT 
Figure 5:

 

WSR-88D Base Velocity animation from 12:46PM through 9:00PM EDT Sunday August 9, 2009 
Figure 6:

 

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. 

WSR-88D Storm Relative Motion at 2:05PM EDT  
Figure 7:

 

WSR-88D Storm Relative Motion animation from 1:33PM to 2:28PM EDT Sunday August 9, 2009 
Figure 8:

 

 

tornado path and thunderstorm wind damage map for Allegany County
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.
 
Figure 9:

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