Severe Weather Event, Saturday May 31, 2008

The first widespread organized outbreak of severe convection erupted over the southern half of the local region during the mid to late afternoon on Saturday May 31. Thunderstorms brought localized high wind and hail to portions of southern Schoharie, southern Albany, Greene, Columbia, Berkshire, Ulster, Dutchess, and Litchfield counties, as well as southern Washington, northern Rensselaer and southern Bennington counties, generally between 3:15pm and 7:00pm. An even more significant and widespread outbreak of severe thunderstorms, including a possibility of tornadoes, was narrowly averted due to the late arrival and departure of a round of downpours and rumbles during the late morning and early afternoon. Had the initial area of rain been about three hours earlier to exit the region, yielding three additional hours of heating, a far more serious severe weather would likely have developed due to a combination of much higher instability combined with significant low and mid level wind shear (change in both direction and speed of the wind with height), both favorable parameters for severe weather development.

The Day's Players:
1) Warm Front
2) Strong directional and speed shear of the wind through the morning and early afternoon
3) Dewpoint Surge
4) Surface Pre-Frontal Trough
5) Surface Cold Front
6) Upper Air Low Pressure Trough (Cold pocket located around 25,000')

The severe weather forecast for Saturday, May 31 was complex due to issues with timing the interaction of the six key weather players that all impacted the outcome of the day's weather. First to affect the region was the warm front as it moved through during the morning helping to focus and direct an area of rain (a weakening mesoscale convective complex, MCS) through the region from the late morning through the early afternoon. The late arrival and departure of this batch of rain was integral in cutting down on the potential heating that otherwise would have developed, and thus somewhat limited the available instability in the atmosphere. The decrease in heating and subsequent decrease in available instability was the key in somewhat subduing the ensuing severe t-storms that would form a little later in the day.

With the passage of the warm front and the rain by the early afternoon, a fairly rapid clearing trend developed which did allow surface temperatures to quickly warm into the mid 70s, combined with a surge in surface moisture noted by dewpoints which climbed into the mid 60s. A low pressure trough, or wind shift/dewpoint front, slowly moved from central New York to eastern New York as skies partially cleared. This wind shift and dewpoint boundary acted as the focus for what was initially a short line of thunderstorms which formed just west of the western Catskill counties of Otsego and Delaware, between 2:15pm and 3:00pm. The slow forward motion of the trough, on top of acting as the t-storm focus, also allowed surface winds in the Hudson valley to remain out of the south and southeast which enhanced the low level directional wind shear, which coupled with the elevated speed shear, maintained a favorable environment for severe weather as the developing line of thunderstorms approached.

The combination of modest surface heating, and cooling aloft caused by the approach of the upper air trough, created sufficient instability for the t-storm updrafts with the initial line of storms to quickly grow stronger. As the intensifying updrafts interacted with the higher momentum air aloft, the downdrafts within the individual t-storm cells effectively mixed that fast moving air down to the ground resulting in damaging wind gusts as the line moved into Schoharie County at 3:30pm. As the line tracked further east, closer to and eventually into the Hudson valley, the strong updrafts were able to tap into the higher dewpoint air in the more strongly sheared environment allowing the line to expand and eventually develop into a significant squall line which extended from Berkshire County on south and west through Columbia, Greene, and Ulster counties by 5pm. By 7pm, the line moved south and east out of clearing the local area. The squall line produced pockets of large hail and damaging wind along with frequent lightning and locally torrential rain along its path through much of central and southern Berkshire County and the mid Hudson valley. Figures #1 and #2 show the squall line on the Albany National Weather Service NEXRAD Doppler radar as it affected Berkshire County and the mid Hudson valley.

Figure #1: Albany National Weather Service NEXRAD base reflectivity, 0.5" tilt elevation, Saturday May 31, 2008, approximately 5:15 pm. Squall line extending from central Berkshire County through the mid Hudson valley

ENX radar image of the mid Hudson valley Squall line on Saturday May 31, 2008 

Figure #2: Albany National Weather Service NEXRAD base reflectivity, 0.5" tilt elevation, Saturday May 31, 2008, approximately 5:55 pm. Squall line extending from southern Berkshire County through southern Columbia, northeast Ulster, and northern Dutchess County. Individual t-storm cells were producing wind damage in northeast Ulster and northern Dutchess County at this time.

ENX radar image of the mid Hudson valley squall line on Saturday May 31, 2008

Simultaneous to the development of the squall line were several discrete thunderstorm cells a bit further north in the Mohawk valley, Schenectady, southern Saratoga, Washington, northern Rensselaer, and southern Bennington counties. Also forming along the pre-frontal trough, these storms, with one exception, remained below severe limits, but did produce locally heavy rain, small hail, and occasional lightning during the late afternoon. The one exception was a storm that formed over southern Saratoga county that developed a mid level rotation taking on supercell characteristics, due to the moderately strong shear in the Hudson valley. The sustained rotating updraft allowed the storm to become severe and produce a non-continuous wind damage path across extreme southern Washington and extreme northern Rensselaer counties as well as throughout southern Bennington county, VT between 5pm and 6pm.

Figure #3: Albany National Weather Service NEXRAD base reflectivity image, 0.5" tilt elevation, Saturday May 31, 2008, approximately 5:30pm. Supercell thunderstorm located between Shaftsbury and Bennington, VT.

ENX radar image of a supercell severe thunderstorm on Saturday May 31, 2008 

By 7pm the severe weather had ended in much of the region with a few additional isolated strong thunderstorms popping up between 8pm and 10pm as the cold front and accompany upper air low pressure system moved into the region.

The table below is a listing of the wind damage and hail reports used by the Albany National Weather Service to verify the severe thunderstorm warnings that were issued for the Saturday May 31, 2008 event. This is not a list of all of the damage that may have occurred as a result of these thunderstorms, only what was reported for warning verification purposes.

Town County Severe Weather Report Time of Severe Weather
Jefferson Schoharie 0.88" diameter Hail 3:40pm
Jefferson Schoharie Thunderstorm wind damage, trees down 3:55pm
Coxsackie Greene Trees and wires blown down 4:35pm
Valatie Columbia Thunderstorm wind damage, trees blown down 4:45pm
Stuyvesant Falls Columbia Trees blown down 4:45pm
Chatham Columbia Wind damage, trees blown down 4:50pm
Kinderhook Columbia Trees blown down 4:55pm
Red Rock Columbia Thunderstorm wind damage, trees down 5:00pm
Austerlitz Columbia Trees blown down 5:00pm
Canaan Columbia Thunderstorm wind damage, trees down 5:00pm
Catskill Greene 0.88" diameter hail 5:10pm
Buskirk Rensselaer Wind damage, trees and wires blown down 5:14pm
Eagle Bridge Rensselaer Wind damage, barn blown 120 feet off foundation 5:18pm
Pittsfield, MA Berkshire Trees and wires down, limbs down on holmes Road by Miss Halls School, 4 hour power outage (5:22pm to 9:16pm, pea sized hail 5:22pm
Becket, MA Berkshire Trees and wires blown down 5:29pm
Bennington, VT Bennington Thunderstorm wind damage, trees and wires down 5:30pm
Glenerie Ulster 50 trees blown down, 12 power poles snapped, five homes damaged 5:33pm
Mount Marion Ulster Multiple trees blown down 5:33pm
Tivoli Dutchess Trees and wires blown down 5:35pm
Red Hook Dutchess Trees and wires blown down 5:40pm
Rhinebeck Dutchess Trees and wires blown down 6:00pm
Pine Plains Dutchess Thunderstorm wind damage, trees/wires down 6:00pm
East Fishkill Dutchess 0.88" diameter hail 6:45pm