Excessive Rain/Flash Flood Event, Monday August 11, 2008
Very cold air aloft (-18° C @ Albany at 12z and 00z at 500mb), associated with the upper air low more than compensated for the cool surface conditions, with temperatures mainly in the 60s and surface dewpoints near 60°, to create a significantly unstable air mass over the Northeast during the morning and the afternoon. A few breaks of sunshine during the morning in the Hudson valley caused a little heating to occur which enhanced the instability in the valley and potentially aided in the formation of a particularly strong zone of torrential t-storms which lead to the majority of the flash flooding that occurred in the region which primarily affected parts of Columbia, western Rensselaer, and northeast Albany counties. Flash flooding also occurred in Montgomery County during the late morning and early afternoon in and around the town of Glen as repeat torrential downpours early in the event caused overflowing water leading to damaged roads and swamped fields. Pockets of flash flooding also occurred in Savoy, MA late in the afternoon and in Middleville in Herkimer County during the evening.
The main zone of heavy rain, spiraled to the north-northwest up the Hudson valley, around the eastern side of the upper low's circulation, during the early to mid afternoon. Locally severe thunderstorms occurred in this narrow axis of excessive rain with large hail and frequent lightning accompanying the torrential rain.
The radar image in Figure #1 shows the most intense thunderstorm at 2:10pm, August 11, as it tracked over Rensselaer on its way to Troy where it produced quarter sized hail. This thunderstorm developed a mid level rotation which sustained its updraft allowing it to produce large hail and the rain that ultimately lead to severe flash flooding in East Greenbush and especially the city of Rensselaer in Rensselaer County. Figure #2 is the Albany Doppler radar storm total rainfall estimate through 10pm on August 11, 2008.
Figure #1: Albany NEXRAD base reflectivity 2:10 pm, Monday August 11, 2008: Severe t-storm with rotating updraft (rotation center marked by the circular shear markers on the north side of the storm) The main hail core is represented by the area depicted in dark purple and black on the radar display. This storm only slowly moved to the NNW at 10-15 mph allowing it to dump upwards of three inches of rain (Doppler estimate) in approximately an hour as well as hail up to an inch in diameter.
Figure #2: Albany Doppler radar estimated storm total rainfall for Monday August 11, 2008 through 10pm. Note the narrow corridor in the Hudson valley where thunderstorms moved over the same communities resulting in the enhanced rainfall amounts ranging from two to three inches. Also note the area of heavier accumulated rainfall in the Mohawk valley, associated with the morning flash flooding in Montgomery County.
This table lists the cumulative rainfall amounts that occurred from midnight through the afternoon and evening on August 11 as measured and reported by WeatherNet 6 spotters. Additional rainfall occurred with the storm system lingering into the pre-dawn hours of Tuesday August 12. The additional rain that fell with the storm is not included in the tabular listing. ( Note: In most cases these rain fall totals represent rain that accumulated in two to four hours.)
FLOODING and SEVERE WEATHER Reports received by the National Weather Service. These reports are gathered for the purpose of verifying the National Weather Service flash flood and severe thunderstorm warnings that were issued and do not represent all of the occurrences of flooding and severe weather that resulted from this event.