During the late afternoon of Sunday August 21st, 2011 severe weather developed rapidly across the Finger Lakes Region of New York State in advance of a cold front crossing the Lower Great Lakes Region.   A severe thunderstorm produced extensive damage across portions of Wayne and Cayuga counties.  A subsequent damage survey conducted by the National Weather Service in coordination with Emergency Management offices from Wayne and Cayuga counties confirmed that an EF-2 tornado touched down and was responsible for the bulk of the damage.   This web page provides a brief review of the meteorological factors that contributed to the severe weather that afternoon.  The photographs of damage at the bottom of this report were taken by the WFO Buffalo Storm Survey Team.


The weather pattern on Sunday afternoon August 21st featured a cold front that extended from a Low in Quebec to another Low over western Lake Erie (Fig 1).  Strong winds aloft and an atmospheric thermodynamic profile that featured a very unstable environment combined to elevate the threat for severe weather during the late afternoon hours.  NOAA's Storm Prediction Center had outlined an area just to the east of the affected counties to be under a Slight Risk Area for severe weather that afternoon and night. 

Figure 1: NCEP Surface Analysis Sunday August 21st at 18z. 


Figure 2:
 12hr forecast valid 00z Mon August 22, 2011 of the NAM 200mb Height, Divergence contours and image of Wind Speed.
Severe thunderstorms are often fueled by strong winds aloft and are more likely in particular quadrants of the Upper level Jet.  In this case, the Finger Lakes Region was under the Left Exit Region of the Upper Level Jet as shown in Figure 2.  This region is often associated with increased upward motion and when combined with other factors, an increased threat for severe thunderstorm development.  The image show the 200 mb height field in thicker green lines, the divergence field in light brown and the image is wind speed which outlines the 200mb jet.     


As the afternoon progressed a cluster of thunderstorms moved across WNY.  As you will see in the animation below, one particular storm among a cluster of showers and embedded thunderstorms becomes stronger and actually turns slightly to the right of the prevailing movement as it crosses eastern Wayne and Cayuga counties.  Right turning storms often produce severe damage. 

Figure 3: WSR-88D 0.5 degree reflectivity loop from 1713z - 1950z
 Sunday August 11, 2011.  Note the cell that intensifies,
 bows out and turns to the right in eastern Wayne county.


Figure 4 shows a Storm Relative Velocity (SRM) image taken about the time of the damage.  The drawing within the image attempts to explain how the radar detects a circulation from the SRM data.  The radar can only detect motion that comes toward or away from it.  Therefore, a circulation will show a specific signature, in this case, red colors show motion away from the radar and green show motion toward the radar.  If adjacent radar pulses shows strong reds next to strong greens, then there is some evidence that a rotation exists in that spot.  The forecaster working the severe weather desk will use this information along with several other indicators to assess the potential for rotation and subsequently a tornado.  Figure 5 is an animation which shows the development of the rotation, a tightening of the rotation during the time of the tornado damage as it moves across eastern Wayne county and eventually it weakens as it heads east of Cayuga county.

Figure 4:  WSR-88D 0.5 degree SRM August 11, 2011 at 1922z.  Note the strong opposing velocity signatures adjacent to each other along the Wayne-Cayuga county border.   


Figure 5: WSR-88D 0.5 degree SRM August 11, 2011 loop from 1859z-1945z shows the tightening of the circulation as it moves across eastern Wayne and western Cayuga counties.


Figure 6 shows the damage path that resulted from the EF-2 tornado on the Enhanced Fujita Scale .  A Survey Team from the National Weather Service met with Emergency Managers from Wayne and Cayuga counties who escorted them to sites where damage was reported.  Based on examination of the evidence and comparison with the WSR-88D radar data, the Team determined that an EF-2 Tornado with wind speeds of approximately 115-120mph cut a swath about 200 yds. wide along a path of 11 miles.  Each of the points along the damage path in the figure represent locations where damage was examined.  


Figure 6: Damage path determined from the
survey conducted by NWS and Emergency
 management officials.

Damage Photos Taken by NWS Survey Team

The four photos above are of a 2-story frame farmhouse in Savannah, NY.  The roof on an addition was torn off, a barn was destroyed across the street and an aluminum boat with motor was carried several hundred yards into a field. 
Damage to homes on Drable St. in Savannah.  The first two photos show a manufactured home whose roof was partially removed.  The third photo is another manufactured home that was moved off its foundation and sustained significant damage.
This home on Taylor St. had its roof completely torn off, the pool deck was tossed on top of the garage roof and an outbuilding was tipped over.
The first photo shows an entire stand of pine trees snapped off about halfway up their trunks.  The second photo shows the tangled mess of trees and a flatbed trailer.  Finally a stately farmhouse in Conquest had its entire roof torn off.

The damage path ended in Cato just east of Rte 34 where the a beautiful backdrop of fair weather cumulus clouds outline a barn whose roof was torn off just the day before.


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