Flash Flood Event in
Southern Erie, Northern Chautauqua and Northern Cattaraugus Counties
|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.
Google Map showing the approximate
location of the worst flash flooding that occurred.
|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
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
|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.
|Figure 9: View of the Silver Creek and Walnut
Creek Basins as they course through the village of
||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
|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
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
Gowanda Damage Photos