Excessive Rain/Flash Flood Event (Former Hurricane
Following just over a week from a very heavy rain event, loosely related to the remnants of Hurricane Frances, came a second excessive rain event, that originated partially as a result of the remnants of what was once powerful Hurricane Ivan.
The synoptic set-up for this excessive rain event was quite similar to the event that lead to the heavy rains associated with the remnants of Frances. A frontal system, separating a chilly air mass from air of tropical origins, moved across New York and New England during the afternoon of Friday, the 17th. Very warm, humid conditions prevailed across much of the region prior to the frontal passage during the morning of the 17th. The cold front moved through the Albany area between noon and 1pm bringing showers and a slow but steady temperature drop into the upper 50s and the low 60s by 6pm. As the cool, dense air, settled into the region, it set the stage for a significant overrunning event that would lead to between 2"-5" of rain on average and widespread flash flooding in several counties in the Catskills on Saturday the 18th.
The remnant circulation of Ivan never made it much further north than eastern Kentucky through the event. However, the plume of deep tropical moisture associated with the former Hurricane was easily transported northward by the jet stream, directly over Pennsylvania, where significant flooding occurred in the western part of the state, and into New York and western New England. As the moisture was driven north by the jet, it encountered the stalled frontal system lying across northern Pennsylvania which forced the air to rise even more quickly causing a solid area of exceptionally heavy rain to fall extending from the center of Ivan over the Southeast, up through New York, through the morning of Saturday the 18th. Rainfall rates during the height of the event late at night on the 17th and through the early morning of the 18th reached a hefty 1"-3" per hour at times. The result was widespread flash flooding in Delaware , Schoharie, Greene, southern Albany, southern Rensselaer, Columbia, and Ulster counties on Saturday, the 18th.
As is typically the case, the mountainous terrain, this time in the Catskills, received very high amounts of rain due to the effects the mountains have on further lifting the already rising air. This process of orgographic lift further enhances rainfall amounts in warm season storms in the same way that it enhances snowfall amounts during cold season storms. Some of the hardest hit areas from flooding included the Ravena/Selkirk area in Albany county, the Windham, Lexington,and Catskill areas of Greene county, as well as much of southern Schoharie county.
Rainfall ended quickly on Saturday during the late morning and very early afternoon as the jet stream disturbance that transported the tropical moisture into the Northeast rapidly pulled out. By mid afternoon across much of the region, sunshine developed, beginning what would be a significant period of dry weather for the region.
Rainfall totals from Friday, September 17-Saturday, September 18 (Hurricane Ivan Related Event)
The following is a listing of the reported Flash Flood Occurrences on the 18th as a result of the excessive rainfall. (Information obtained from the Albany National Weather Service Local Storm Report)
This series of four photographs, taken by Catskill, NY WeatherNet 6 Weather Watcher Steve Meicht, illustrate some of the flooding on the 18th in the Catskill area of Greene county by the Catskill Creek.