Excessive Rain/T-Storm/Flood Event
The unusually cool and wet pattern which gripped the Northeast in June continued in July with the prevailing polar branch of the jet stream displaced far to the south across the northern third of the United States. A semi permanent area of low pressure continued over the Hudson Bay region of Canada. The Canadian upper low, more common in the winter and early spring months, than during the summer, spun down an unusually strong and southward tracking upper level storm offshoot which when it interacted with the normally high summer moisture levels in the atmosphere, produced an excessive rain event for eastern New York and western New England during the July 14-16th period. This was the second warm season excessive rain event of the year following the anomalous early June Nor'easter.
Friday, July 14
The initial phase of the storm began as the intensifying early spring type upper low pushed south out of Canada on a track for western New York. Typical warm and humid weather prevailed over eastern New York and western New England with temperatures in the low 80's and dewpoints in the mid 60's. Scattered clusters of T-storms developed as a result of the upper atmosphere cooling, in direct response to the approach of the upper low. Daytime heating of elevated terrain, was sufficient to start thermals which became heavy rain making t-storms in Ulster county and in Fulton, Montgomery, Herkimer and southern Hamilton counties. The steering flow on the 14th was quite weak, common in July, so once the t-storms formed they remained nearly stationary over generally the same isolated areas. Rainfall amounts estimated by Doppler radar in parts of central Ulster county in and near the Ashokan reservoir approached a whopping ten inches, with a WeatherNet 6 measurement of 6.68" in West Shokan. Similarly, rainfall amounts between two and seven inches occurred in small parts of Herkimer, Fulton, and Montgomery counties. Localized flooding occurred as a result of the heavy rain. This heavy rain, however, was very isolated in nature, when compared to the remaining area, which had little or no rain.
Saturday: July 15
The widespread rain event developed during the early morning on Saturday, July 14, when the upper low, which had now become quite strong containing very cold air aloft, eased south into western New York and then stalled. The upper storm initiated a weak and strung out area of low pressure at the surface from near Buffalo, NY to Wilmington, NC. Simultaneously, high pressure, on the 15th, was located over the north Atlantic. The combined flow, both at the surface and aloft, between the weak surface lows and the strong high to the east was from the southeast off the moisture rich Atlantic ocean. The flow provided an almost unlimited supply of moist air, which when forced to rise, due to atmospheric instability caused by the cold air aloft and diffluent flow, produced record rains in the Capital Region. 3.23" of rain fell on the 15th at Albany, setting a twenty four hour rainfall record. The fact that the parent upper low stalled out much of Saturday and Saturday night meant that no weather features moved for almost 16 hours resulting in the continuous, semi-stationary feed of moisture. The band of steady moderate to heavy rain was relatively narrow extending from south of Long Island north to the Capital Region and then arced back into the Mohawk valley and the Adirondacks. Steady rain fell in this zone from the early morning until close to midnight. Rain became extremely heavy, from 6pm to midnight on the 15th. In that six hour time frame two to four inches of rain fell, combined with the one to two inches of rain that had fallen in the previous six hours, to produce widespread urban, creek, small river, and stream flooding over a wide area from the eastern Catskills to the mid Hudson valley, Berkshire and Litchfield counties in MA and CT respectively, the Capital District and the Mohawk valley. The following is a listing of some, but not all, of the reported flooding in the area.
Sunday, July 16
The main area of upper level low pressure through the 16th drifted south into Pennsylvania and weakened. As a consequence of the storm's weakening and shift in position south, the surface low also weakened and therefore the strong, moisture rich Atlantic inflow into the Northeast broke down through the early morning hours of the 16th which brought an end to the continuous rain. Very humid air, however, remained in place over the region with dewpoint temperatures in the mid 60's. Intervals of sunshine developed across much of the area which was a nice psychological boost to folks, having been shut in the day before. The atmosphere, however, aloft remained cold, so with the advent of sunshine, and surface heating, especially of the elevated terrain, scattered showers and t-storms developed, much as they had done on Friday as the parent upper storm was making its approach. Rains on the 16th were scattered, but once again torrential where they occurred due to a very weak steering flow. Like the 14th, t-storms developed over the mountains, and remained almost stationary producing between two and seven inches of additional rainfall in parts of Washington county, eastern Rensselaer and Columbia counties in NY, and Bennington county, VT. In fact in Bennington county, VT, four to seven inches of rain fell in three hours along route 7 between Shaftsbury and Bennington. Severe flash flooding occurred throughout southern Bennington county as a result of this additional rainfall on the 16th. By midnight, most of the stationary t-storms in the hills east of Albany had ended concluding this storm event. The following is a listing of some, but not all of the flooding produced by these t-storms.
The following table lists a number of rainfall measurements from the 14th through the 16th from the National Weather Service Cooperative observers network as well as the Channel 6 WeatherNet 6 weather watcher network.