Flash Flood Event in Southern Erie, Northern Chautauqua and Northern Cattaraugus Counties |
OVERVIEW |
| This page documents select meteorological parameters
that came together over Western New York on the evening of Sunday, August 9,
2009 to produce one of the most significant flash flood events to hit the
region in memory. This is not meant to be an in-depth study of the
meteorology behind the flash flood, rather is intended to give the reader an
idea of the evolution of the severe weather on the evening of Sunday,
August 9, 2009.
The area barely had time to recover from a round of severe weather that afternoon, which produced extensive wind damage across several counties including a tornado in western Allegany County. As that round of severe weather moved south of western New York, a second round of severe thunderstorms was evolving out to the west in northern Michigan and Wisconsin. During the evening a cluster of severe thunderstorms dropped southeast across Western New York from Southern Ontario province. As the storms moved onshore across Niagara and Orleans counties, their main impact was damaging winds and near continuous lightning, very similar in fact to the severe weather that occurred earlier that day. However, during the evening the situation evolved from damaging winds to major flash flooding as the storms moved south of Buffalo and approached the Southern Tier. Over the course of a couple of hours late Sunday evening, roughly between 1030PM and 1230AM, some of the highest short-term rainfall totals ever recorded in western New York occurred. Those rainfall totals resulted in the worst flash flooding the area has seen in decades. In addition to preliminary estimates of tens of millions of dollars of property damage, the flooding also resulted in the direct loss of one life and indirect loss of another that night. |
| Figure 1 outlines the area that sustained the most damage associated with flash flooding. The communities of Silver Creek and Gowanda in particular had tremendous damage and much of the area between the two villages was also inundated. Smaller streams that flow through both of those villages became raging torrents with walls of water reported by many individuals. |  |
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| Figure 1: Google Map showing the approximate location of the worst flash flooding that occurred. | ||
METEOROLOGICAL DISCUSSION
| Figure 2 is a GOES IR satellite imagery loop that covers the time frame from 5:15PM Sunday afternoon through 2:15AM Monday morning. In the animation you will see a cluster of storms over northern Georgian Bay, often referred to as a Mesoscale Convective System (MCS) and a second MCS over southern Lake Michigan. As the night progressed, the Georgian Bay storms moved southeast across western New York while the storms from Michigan moved in a more easterly track. The two storm systems eventually combined and reached their peak intensity over western New York before heading southeast to Pennsylvania. At the time of their peak intensity, the cloud tops associated with the storms dropped to a temperature of -71C, which in that evening's atmosphere equates to storms building to a height of approximately 52,000ft. when the torrential rainfall occurred. |  Figure 2: GOES IR satellite loop from 515PM EDT Sunday August 9 through 215AM EDT Monday August 10. |
| The Composite Reflectivity radar image shown in Figure 3 at 8:27PM looked very similar as it entered western New York to the wind-dominated derecho that occurred earlier that day. As indicated above, early in the evening this line of storms was dominated by damaging winds and constant lightning as it moved across the counties north of Buffalo. However, as the line of severe storms tracked toward southern Erie County, a second line of severe storms began to erupt over western Lake Erie and extend east toward Silver Creek as shown in Figure 4 at 9:36PM. |
 Figure 3: WSR-88D Composite Reflectivity at 8:27PM Sunday, August 9 showing the line of storms over the Province of Ontario, Canada. |
 Figure 4: WSR-88D Composite Reflectivity at 9:36PM Sunday August 9, 2009 showing the second line of storms developing over Lake Erie. |
| Over the course of the next two hours that evening, the weather system evolved from a wind damaging line of storms to flash flood producing storms. The torrential rainfall culminated along the lower half of the Cattaraugus Creek Drainage Basin. Through a complex interaction of the two lines of storms, the topography of the area and already saturated ground from earlier storms, the adjacent parts of southern Erie, northern Chautauqua and northwest Cattaraugus counties were in the bulls-eye for catastrophic flash flooding. Figure 5 shows the Composite Reflectivity loop of the evolution of the flash flood from 8:46PM to 12:55AM. It is hypothesized that the first line of storms that were moving southeast intersected the second line of storms heading due east and combined to produce a period of torrential rainfall with several thunderstorm cells crossing the same location between 10:30PM and midnight. The entire thunderstorm complex then continued its track southeast to the Pennsylvania border overnight. |  Figure 5: WSR-88D Composite Reflectivity animation from 846PM EDT Sunday August 09, 2009 through 1255AM EDT Monday August 10, 2009 |
HYDROLOGY / FLASH FLOODING
 Figure 6: Map of the tri-county area of southern Erie, northern Chautauqua and northwest Cattaraugus Counties where the worst flooding occurred. |
The WSR-88D radar has the ability to estimate rainfall from the signal that is returned to the radar from the storms. This is only an estimate of the rainfall amount however and is subject to many factors that can affect the actual values. You can learn more about radar-derived rainfall in this link about WSR-88D Precipitation Images. Over the course of the 24 hours leading up to the flash flooding, there had been two other rounds of rainfall that occurred across the area. The ground was saturated from the rainfall and therefore would not be able to absorb much more rain, causing most of the precipitation to run off into streams and low lying areas. As the two clusters of thunderstorms merged that evening, the rainfall rates increased greatly across the area. Figure 6 shows the area that we will concentrate on for the rainfall analysis. |
| Figure 7 shows the 3-hour radar-derived rainfall between 9:04PM and 12:04AM Sunday evening. The red squares in the image indicate as much as 5 inches of rain. As noted, radar-derived precipitation is only an estimate of the actual amount of rain that may have fallen. The National Weather Service is fortunate to have a Cooperative Weather Observer located in Perrysburg, roughly half way between Gowanda and Silver Creek. In spite of flooding at the observer's residence, she was able to go out and measure rainfall throughout the storm. In the timeframe from 10:30PM and midnight, she recorded an incredible 5.98 inches of rainfall. When compared to the radar-derived rainfall at this location, the ground truth measurement suggests the radar-derived rainfall may have been underestimated somewhat. Based on the added information, it is likely that the entire area outlined in red for the 3-hour radar-derived rainfall would equate to about 6 inches of rain in less than three hours, likely in as a little as an hour and a half. Figure 8 is the same as Figure 7, except that the four waterways that caused the disastrous flash flooding are sketched in. For the village of Silver Creek, it is readily apparent that not only the headwaters, but a significant stretch of both Walnut and Silver Creeks received tremendous amounts of rainfall into their channels. In the village of Gowanda, Thatcher and Grannis Brooks also saw tremendous amounts of rain that turned these normally placid streams into deadly torrents. |
 Figure 7: WSR-88D 3-hour radar-derived rainfall between 9:04PM and 12:04AM Sunday evening. |
 Figure 8: Same as Figure 7 with Silver and Walnut Creeks, Thatcher and Grannis Brooks overlaid. |
| To give a better local perspective of the areas that received the worst flash flooding, we used Google Earth to view the area around the villages of Silver Creek and Gowanda. Figure 9 shows the location of Silver Creek and Walnut Creek which course their way from the highlands of the Chautauqua Ridge through deeper ravines before winding their way through the village of Silver Creek, where they eventually merge and flow out into Lake Erie. Figure 10 zooms in on Silver Creek and in particular, the area around the mobile home community in the village that was severely damaged from the flooding. Damage photos taken by the NWS survey team, who were escorted by law enforcement are included at the end of this report to show the tremendous power and danger of flood waters. |
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| Figure 9: View of the Silver Creek and Walnut Creek Basins as they course through the village of Silver Creek. | Figure 10: Zoomed in view of the Village of Silver Creek and the location of the mobile home community destroyed by flash flooding. | ||
| The next images will focus on Cattaraugus Creek and the tributaries that flow into the creek in the village of Gowanda. In Figure 11 you can see the main branch of Cattaraugus Creek joined by the south branch of the creek a couple of miles upstream of Gowanda. Cattaraugus Creek flows through an area with significant topography on both sides of the waterway and cuts its way through a deep gorge as well. In fact, several campers who had been in the gorge that night had to be rescued by a basket dropped from a Sheriff's helicopter the next morning in what could only be described as a very heroic effort on the part of the emergency response team. In Figure 12 you can see how Cattaraugus Creek cuts through the village of Gowanda. There are a number of tributaries that flow into the creek through the village as well. Here we have pointed out Thatcher Brook which runs behind Tri-County Hospital. The hospital sustained significant damage from flooding likely due to the overflow from Thatcher Brook. The high school football field is also clearly visible in the Google Earth image. The football field was under several feet of water at one point and also sustained significant damage from the mud that was left after the water receded. |
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| Figure 11: Map of the confluence of the south channel and main channel of Cattaraugus Creek just upstream of the village of Gowanda. | Figure 12: Map of the confluence of Thatcher Brook and the main channel of Cattaraugus Creek and Tri-County Hospital in the village of Gowanda. | ||
| Some of the most interesting images of the indirect impacts the flash flooding had on the region were taken a couple of days after the storms when the skies cleared. Figures 13 through 16 show MODIS polar orbiter satellite imagery of the eastern Great Lakes. The polar orbiter spacecraft orbits at a much lower level than the GOES satellites and provides higher resolution imagery. Figure 13 shows the region a few days before the flash flooding. Figures 14 through 16 show the turbidity plume (muddy water) along the east end of Lake Erie as a result of the runoff/mud/debris that came from streams emptying into the lake. Even more fascinating is the turbidity plume that is evident at the mouth of the Niagara River as it empties into Lake Ontario. It is likely that the plume is a result of all of the rainfall that occurred across the river basins that empty into the east end of Lake Erie from the torrential rains of August 9th, 2009. |
 Figure 13: MODIS full color image from 2:49PM EDT August 5, 2009 on a clear day 4 days before the floods. |
 Figure 14: MODIS full color image at 2:55PM EDT August 12, 2009, 2 days after the floods. |
 Figure 15: MODIS full color image at 2:05PM EDT August 13, 2009, 3 days after the floods. |
 Figure 16: MODIS full color image at 10:22AM EDT August 14, 2009, 4 days after the floods |
| Finally, we have included a few photos of damage that occurred as a result of the flash flooding in the villages of Silver Creek and Gowanda during the damage survey conducted by National Weather Service officials in conjunction with the Chautauqua and Cattaraugus County Emergency offices and personnel who staffed the respective Emergency Operations Centers. |
| Silver Creek Damage Photos |
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Gowanda Damage Photos
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