Excessive Rain, Major Flash Flood, Isolated Severe Event
1pm Wednesday July 29 to 8am Thursday July 30

With the mean jet stream trough axis over the Midwest, further west than the trough axis had been during June and the first half of July, a broad strong southwest jet stream flow was directed up and over New York and New England. This retrogression of the trough allowed for a period of warmer and much more humid conditions to prevail over the Northeast during the last two weeks of July that had not been realized at any point earlier in the summer. The slight shift in the pattern also allowed fairly frequent frontal and jet stream disturbances to ride into the Northeast producing rounds of heavy rain. This pattern was key in setting the stage for the excessive rain event which lead to severe flooding in a narrow swath that included northeast Greene, northern Columbia, southeast Rensselaer, and central Berkshire counties, as well as flash flooding in a second narrow swath (a much lower population area) which extended from eastern Fulton through northwest Saratoga, southern Warren, and northern Washington counties. The image below is the Albany Doppler weather radar storm total rainfall estimate through 11:31pm Wednesday, illustrating the two distinct zones of excessive rainfall ranging from 3"-6".

Click here for a series of close up Doppler radar storm total rainfall estimate graphics

Set-Up:
Wednesday morning began with a mixture of hazy sunshine and clouds and a rapid surge in dewpoints into the lower 70s and air temperatures into the lower 80s. A capping inversion was present in the mid levels of the atmosphere (warm layer aloft) which suppressed thunderstorm development allowing potential instability to build ultimately setting the stage for explosive thunderstorm and rain development during the early afternoon. Rich moisture also flowed into the region aloft riding along the southwest to northeast jet stream flow providing abundant fuel for heavy rain. Moderate wind shear, due in part to the position of the jet and the slow approach of a surface cold front, both in direction and speed was present, creating a favorable environment for rotating thunderstorms in areas where sufficient instability existed, along with providing a favorable pattern for repeat or training thunderstorms over the same areas.

The atmosphere ripped open in two zones between 12:30 and 1:00pm with a rapid development of thunderstorms just ahead of an area of tropical stratiform rain that had been tracking through Pennsylvania and western New York during the morning. Brief damaging wind gusts occurred with a couple of rotating storms in northeast Ulster, Greene and Columbia counties during the early to mid afternoon as instability was maximized in those areas with CAPES running between 1500 and 2000 j/kg (CAPE: convective available potential energy). Once the initial storms formed and moved northeast, new ones redeveloped over the same areas and trained for several hours creating the southern swath of excessive rain depicted in the storm total rainfall radar estimate image above. The atmosphere also erupted over the eastern Mohawk valley and northwest Saratoga County with a similar rapid development of strong thunderstorms that also trained over the same communities creating the northern swath of excessive rainfall. Terrain likely played some role in why the initial heavy rain and thunderstorm development occurred in the areas that it did.

The radar image below depicts the scene fairly early in the event at 2:48 in the afternoon. Notice the discreet torrential thunderstorm cells remaining over the Mohawk valley and southern Adirondacks almost two hours after their initial development. Also notice the enhanced radar returns (the deep red colors) over Greene and Columbia counties. The strong radar returns were being created by the initial strong and locally severe thunderstorms (still ongoing at this time) which had already put down up to two inches of rain in what would become the zone of severe flooding. These thunderstorms at this time were in the process of being overtaken by the leading edge of the stratiform rain advancing from Pennsylvania. This merging of rain areas created a further enhancement to the downpours in the southern torrential rain zone which ultimately lead to the major flooding.

Click here for a series of additional radar images  

Wednesday Afternoon through 8am Thursday Morning:
After the initial burst of strong thunderstorms through the mid afternoon, the remainder of the event was best characterized as more of stratiform moderate to heavy rain scenario with embedded lines and clusters of torrential convection. This prolonged period of rain continued, especially over the Catskills, the mid Hudson valley to Berkshire County, into early Thursday morning July 30 due to the interaction of the slow moving cold front advancing from western New York, the approach and passage of a moderately strong jet stream disturbance, and the deep tropical moisture that remained in place. Basin wide rainfall totals for eastern New York and western New England by 8am Thursday averaged 1.50" to 2.00", with 3.00 to 6.00" on average in the two narrow flood zones.

Severe flooding, which continued through the day on Thursday, was reported in Coxsackie, Kinderhook, Stuyvesant, New Lebanon Ghent, Chatham, Nassau, and Stephentown from northeast Greene through northern Columbia and southern Rensselaer counties. Many roads were reported closed or washed out with at least two bridges over creeks destroyed including the Route 22 bridge into Stephentown. The Lebanon Valley Speedway was inundated and closed. States of emergency were declared and continued through Thursday July 30 in Kinderhook, Stuyvesant, New Lebanon, Chatham, Nassau, and Stephentown because of the ongoing flooding and infrastructure damage.

WeatherNet 6 Storm Total Rainfall Reports for the July 29-30, 2009 Excessive Rain Event

The tale below lists the flash flood and severe weather reports collected by the Albany National Weather Service office for the purpose of verifying the warnings that were issued. The listing is a sampling of what occurred across the region and is therefore not a complete list of all the flooding that occurred.

Storm and Flash Flood Reports for the Wednesday July 29-Thursday July 30 Event