Long Duration Excessive Rain/Flood Event
A complex and particularly slow moving weather pattern involving the passage of a strong cold front and an excessive feed of tropical moisture, including the remnant moisture from the former Tropical Storm Tammy, came together over the Northeast late Friday October 7 through early Sunday morning, October 9, to produce extremely heavy rainfall.
Very dry, warm, summer-like weather prevailed in the week leading up to this storm that brought a dramatic change over the Northeast plunging the region into a much cooler, and extremely wet pattern, that would all but wipe out the above normal temperature and below normal rainfall anomaly that characterized the first seven days of October 2005.
A broad southwest flow straight out of the Gulf of Mexico and the Atlantic had pumped the temperatures up to near 80° with dewpoints approaching 70° on Friday, October 7. A strong, but very slow moving cold front, gradually moved into western New York by the end of the day, keeping eastern New York and western New England in the flow of warm tropical air through much of the night with a showery rain breaking out between 2:00 and 4:00pm across much of the region. Rainfall became moderate in intensity in spots through the evening, but in general was not significant.
The real mechanism for producing the extreme rain event that was to develop on Saturday was the combination of the frontal passage, which occurred around 5am on Saturday, October 8, dropping the temperatures from the upper 60s into the mid and upper 40s, and the presence of an air mass with an unusually high moisture content, being supercharged with the remnants of tropical storm Tammy, that moved into the Southeast during the middle of the week. The axis of heaviest rain developed along and north of the slow moving front early Saturday morning, where the lift in the atmosphere, aided by the passage of a strong upper level jet, and very strong and deep feed of tropical moisture, was maximized. The very chilly air near the surface behind the front acted as a strong lifting mechanism as the very warm tropical air was forced to ascend over the shallow cold dome. The strong southwest to northeast jet stream flow, running parallel to the frontal system, meant an extremely slow progression of the system to the east into New England allowing several waves of low pressure to develop and ride up along the front into the predawn hours on Sunday, October 9. The end result was an extended period of continuous torrential wind blown rains, that lasted almost twelve hours at any one location, with rainfall totals ranging from 8"-12", on average, in a narrow axis across parts of the mid Hudson valley, Bennington, Berkshire, and Litchfield counties in western New England.
The graphic is the Albany National Weather Service Doppler radar storm total rainfall estimate. The zone on the graphic shaded in purple, extending from Ulster and northern Dutchess counties in NY northeast through Columbia, Bennington, Berkshire, and Litchfield counties is the axis of heaviest rainfall where totals approached a foot in a few spots by early Sunday morning October 9. Rainfall totals outside of the purple strip, ranged from 1.5" to 5", including most of the Catskills, Capital Region, eastern Mohawk valley, north to Saratoga Springs and Lake George.
Graphic: Albany Doppler Radar Storm Total Rainfall estimate, ending at Noon Sunday, October 9, 2005.
Extremely dry conditions prevailed throughout the Northeast in the many weeks leading up to this major rain event. Therefore, reservoir, river, and stream levels prior to the rain were quite low. The pre-existing low water levels is the only reason major regional flooding did not occur. Localized moderate to major flooding did occur mainly in western New England through parts of Bennington, Berkshire, and Litchfield counties where the rain was heavier and lasted longer. Flooding was observed on the Hoosic and Housatonic rivers as well as the Wallomsac river at Bennington, VT, and the Batten Kill at Arlington, VT. There was also considerable nuisance type small stream, creek, urban, and poor drainage flooding mainly from late Saturday continuing into the day on Sunday throughout the remainder of eastern New York and western New England, especially in the eastern Catskills and the mid Hudson valley, as rainfall rates exceeded an inch per hour at times. This was the same weather system to produce the devastating flooding that occurred around the Keene area of southwest New Hampshire.
The table below contains a listing of the flooding that was reported to the Albany National Weather Service office, and an estimate of the time the flooding occurred. This is not meant as a complete list of the flooding that actually occurred, only what was reported as a means for the Albany National Weather Service to verify the flood warnings that were issued.
This table below contains a listing of the observed rainfall totals as recorded by WRGB's exclusive WeatherNet 6 spotter network, as well as the Albany National Weather Service Cooperative observers and Skywarn spotters for the October 7-October 9, excessive rain event.