Excessive Rain/Flash Flood Event, Wednesday-Thursday Morning July 23-24, 2008

A unique atmospheric set-up for July resulted in excessive tropical rains developing across the region beginning on Wednesday July 23 and continuing into the morning on Thursday July 24. It could be argued that the batch of downpours and t-storms which rolled through the entire region during the afternoon and early evening on Tuesday the 22nd should be included when considering total rainfall amounts from what amounted to a slow moving pattern, rather than any one individual storm. But, for the purpose of this discussion, I am going to concentrate on what was the more consolidated period of torrential rain which resulted in the bulk majority of the rainfall accumulations occurring from Wednesday morning the 23rd through Thursday morning the 24th. However, in the storm total rainfall table below, some WeatherNet 6 spotters did include Tuesday's rain in their totals and have thus been noted as three day totals.

Set-Up:
The main driving force creating the favorable environment for this rain event was at the jet stream level with the development and intensification of an upper air low pressure system over the eastern Great Lakes states. This low, as it deepened and eventually closed off from the main flow, effectively shifted the main jet stream steering flow from a zonal west to east pattern over the Northeast to a strong south to north flow which allowed a rich supply of tropical moisture, which had been lurking over the Southeast states and Atlantic coast, to surge north into the region. At the surface, a weak air mass boundary (front) which had stalled over the area during the preceding weekend, acted as a convergence zone for the incoming tropical flow which helped to focus areas of heavy rainfall. The final players were individual very small scale jet stream disturbances imbedded within the flow which produced discrete zones of enhanced atmospheric lift and were ultimately responsible for the periods of heaviest rainfall that developed throughout the event.

Strengthening closed upper air low pressure system patterns during the warm season are fairly uncommon and are often associated with excessive rains when they occur because of the strong wind fields that develop in association with the developing low center. Strong winds aloft produce very strong atmospheric lift which when combined with deep tropical moisture result in torrential rains. During the winter, spring, and fall when closed upper storms are more common, there is generally less moisture available than during the summer, and therefore, precipitation amounts are lower. This is why two and three foot snow storms are more the exception than the rule during the cold season. If this pattern had set up during the winter with the summer time levels of moisture available, the region would have easily been digging out from a widespread three foot snow storm.

Severe Weather Threat:
Severe thunderstorms were a concern over eastern New York and New England as strong wind shear (change in wind speed and direction with height) was present over the region due to the strong flow pattern which had evolved. Highly sheared environments are favorable for severe thunderstorm and tornado development providing sufficient instability is present to allow updrafts to grow. Fortunately, the first major batch of downpours and thunderstorms, which occurred through the early to mid morning on the 23rd, produced an extensive cloud shield which kept the sun from heating the surface and therefore prevented enough instability from developing, despite dewpoint temperatures ranging from 65° to 70°, to narrowly prevent severe thunderstorms from occurring. Numerous torrential, near severe thunderstorms, however, were common through the event and a severe thunderstorm watch was issued for the mid Hudson valley, Berkshire and Litchfield counties from 3pm through 12:30am on Thursday the 24th for the possibility. The only minor wind damage reportedly to have occurred from a thunderstorm was in Lanesborough, MA at 9:45pm.

Flash Flooding:
Locally torrential rains, coming in bands of training cells in the deeply southerly flow (storm cells that repeatedly track over the same areas) resulted in rainfall rates in excess of two inches per hour at times from Wednesday afternoon into the night, which resulted in widespread flash flooding throughout the Catskills, Mohawk valley, and Capital Region with numerous road closures, including the New York State Thruway, during the late afternoon. 1.55" of rain fell at the Albany Airport during in forty four minutes during the early evening with 2.49" of rain on the 23rd setting a 24 hour rainfall record for the date. Flash flood and general flood warnings as well as urban and poor drainage flood advisories were issued for every county in eastern New York and western New England at least once from the late afternoon on Wednesday through early Thursday morning. With four to six inches of rain in the eastern parts of Delaware County, major flash flooding occurred in and around Margaretville with every road in the village closed for a period of time due to high water and mudslides. Figures #1-5 are Albany NEXRAD base reflectivity snapshots of how the rain pattern set up across the region at various times during the height of the event on Wednesday.

Figure #1: Torrential rains in the Adirondacks, Mohawk and Schoharie valleys and western Catskills at 3:31pm Wednesday July 23, 2008

Heavy rains on Albany NEXRAD at 3:21pm July 23, 2008

Figure #2: Torrential rains in the Hudson valley at 6:30pm Wednesday July 23, 2008

Figure #3: Torrential rains moving through Washington, Rensselaer, and Columbia counties at 7:02pm Wednesday, July 23, 2008

Heavy rains on Albany NEXRAD at 7:02 pm July 23, 2008 

Figure #4: Torrential strong thunderstorms moving north through the Hudson valley and Berkshire County, MA at 9:08pm July 23. These storms produced a localized damaging wind gust which downed trees in Lanesborough, MA around 9:45pm.

Torrential strong thunderstorms moving up the Hudson Valley and through Berkshire County, MA, 9:08pm July 23, 2008 

Figure #5: Another round of torrential t-storms tracking north through Berkshire County with frequent cloud to ground lightning, 12:10am, Thursday July 24

Torrential strong thunderstorms moving up north through Berkshire County, MA, 12:10am July 24, 2008, 2008 

Click here for a few area flood images


This table is a listing of the storm total rainfall amounts reported by WRGB's WeatherNet 6 weather spotter force for this event. Reports marked as "Two day" totals are rainfall amounts that occurred on Wednesday and Thursday July 23 and 24. Reports marked "Three day" totals are rainfall amounts that also include what fell during the mid to late afternoon and the evening on Tuesday July 22.

Town

County Rainfall Amount

Time Period

Sharon, CT

Litchfield 1.50" 2 Day
Alford, MA Berkshire 1.95" 2 Day
Savoy, MA (2400') Berkshire 5.22" 3 Day

Lanesborough, MA

Berkshire 3.40" 2 Day
Albany (Airport) Albany 3.92" 3 Day
Latham Albany 3.42" 3 Day
Colonie Albany 3.47" 3 Day
Feura Bush Albany 2.85" 3 Day
Cohoes Albany 2.76" 3 Day
Green Island Albany 4.61" 3 Day
Preston Hollow Albany 2.25" 2 Day
Watervliet Albany 2.46" 2 Day
Taghkanic Columbia 2.55" 3 Day
Hudson Columbia 4.00" 3 Day
Livingston Columbia 2.58" 2 Day
Stuyvesant Falls Columbia 3.97" 3 Day
Chatham Center Columbia 2.90" 3 Day
North Chatham Columbia 3.24" 2 Day
Arkville Delaware 6.15" 3 Day
Margaretville Delaware 4.40" 2 Day
Roxbury Delaware 3.20" 2 Day
Gloversville Fulton 2.82" 2 Day
Broadalbin Fulton 2.35" 2 Day
Catskill Greene 4.93" 3 Day
Ashland Greene 3.40" 3 Day
Prattsville Greene 2.75" 2 Day

Maplecrest

Greene 3.20" 3 Day
Speculator Hamilton 3.30" 2 Day
Fonda Montgomery 3.16" 3 Day
Glen Montgomery 3.20" 3 Day
Amsterdam Montgomery 3.17" 3 Day
Worcester Otsego 2.37" 2 Day
Speigletown Rensselaer 2.70" 2 Day
Center Brunswick Rensselaer 3.53" 3 Day
Stephentown Rensselaer 3.40" 2 Day
Poestenkill Rensselaer 3.75" 2 Day
Schaghticoke Rensselaer 3.60" 2 Day
Saratoga Springs Saratoga 3.90" 3 Day
Edinburg Saratoga 2.73" 2 Day
Malta Saratoga 3.93" 3 Day
Charlton Saratoga 2.75" 2 Day
Clifton Park Saratoga 3.50" 3 Day
Porter Corners Saratoga 2.25" 2 Day
Corinth Saratoga 4.30" 3 Day
Scotia Schenectady 2.54" 2 Day
Glenville Schenectady 2.90" 2 Day
Jefferson Schoharie 3.75" 2 Day
Fulton Schoharie 3.25" 3 Day
Phoenicia Ulster 3.31" 2 Day
West Shokan Ulster 3.99" 3 Day
Kerhonkson Ulster 2.70" 2 Day
Saugerties Ulster 2.92" 2 Day
Kingston Ulster 2.64" 2 Day
Whiteport Ulster 2.40" 2 Day
Bolton Landing Warren 4.64" 3 Day
Brant Lake Warren 2.20" 2 Day
Lake Luzerne Warren 4.30" 3 Day
Cossayuna Washington 2.50" 2 Day
Kingsbury Washington 2.48" 2 Day
Woodford, VT Bennington 2.60" 2 Day
Landgrove, VT Bennington 2.91" 2 Day
West Rutland, VT Rutland 3.00" 2 Day

Flooding Reports received by the National Weather Service. These reports are gathered for the purpose of verifying the National Weather Service flash flood warnings that were issued and do not represent all of the occurrences of flooding that resulted from this event.

Town

County Flooding Report

Approximate Time of Flood

Colchester Delaware Route 206 as well as Cat Hollow Rd, Holiday Brook Rd. closed due to flooding 3:08pm
Stamford Delaware Road flooded throughout the village of Stamford 4:30pm

Gilboa

Schoharie Torrential rain, 2.00" reported in one hour from WxNet 6 4:32pm
Andes Delaware Flash flood, all roads closed in the town of Andes due to widespread flash flooding 4:35pm
Schoharie Schoharie Flash flood, widespread road closures 5:09pm
2 Miles w. of Hoffmans Montgomery East bound lane of the Thruway closed at mile marker 169 with two feet of water on the road ...significant water on the west bound lanes 5:17pm

Florida

Montgomery Flash flood, Peck Road washed out in Florida 5:18pm
Margaretville Delaware All roads closed due to widespread flash flooding and mud slides 5:20pm
Schenectady Schenectady Flash flood, Guilderland Rd. closed at Cleveland 5:37pm
Rotterdam Schenectady Flash flood, Route 160, Crawford and North Kelly Roads all washed out and closed 5:43pm
Charlton Saratoga Flash flood, Route 147 and Route 67 washed out and closed 6:15pm
Knox Albany Flash flood, Beebe road washed out and closed 6:32pm
Colonie Albany Torrential Rain, 1.55" measured at Albany ASOS in 44 minutes 6:35pm
Fort Ann Washington Flash flood, Pilot Knob Rd. washed out and closed 6:36pm
Schoharie Schoharie Flash flood, Creek Rd to Esperance to the Montgomery County line closed, Treadlemire Rd. to Middleburgh, Junction Rd, Zicha Rd, Sanatarium Rd. to Esperance and Schoharie all closed 7:22pm
Glenville Schenectady Flash flood, Route 147 closed between Charlton Rd. and West Glenville Rd. 7:29pm
Schoharie Schoharie Flash flood, Barton Hill Rd., Sheldon Rd. and Route 30 all closed due to flooding 8:06pm
Esperance Schenectady Flash flood, Junction Rd. Creek Rd, and Shun Pike Rd. closed due to flooding 8:06pm
Princetown Schenectady Flash flood, bridge at Kelly Station Rd. and Route 7 washed out 10:36pm