Isolated Damaging Supercell Thunderstorm Event
Friday Evening August 13, 1999

An unusual and somewhat unexpected severe weather event occurred in the heart of the Capital District during the evening and early nighttime hours of Friday, August 13.  A strong supercell thunderstorm (a thunderstorm with a rotating updraft and central area of low pressure) developed in the eastern Mohawk valley and tracked relatively slowly through the heavily populated communities a few mile north of the Albany metropolitan area east into Troy, and Grafton in New York, and Bennington in Vermont.  Widespread wind damage and power outages as well as hail and almost continuous lightning occurred under and to the north of the storm's circulation center.  Tornado warnings were issued for Schenectady, Albany, and Rensselaer counties.  Storm damage surveys conducted by the Albany National Weather service, however, determined that all the damage was caused either by strong straight line winds, up to 100 mph, or from the winds rotating around the mesocyclone (the area of low pressure at the center of the supercell t-storm) and not from a tornado.

Storm Set-Up: A tropical air mass was in place over New York and New England on August 13.  Dewpoint temperatures climbed into the mid 70's in the Capital District under the influence of rather light south winds.  The nearest cold front was located over central Michigan at 7pm on the 13th, with only small scale boundaries lying in the humid air over the Northeast.  A cluster of showers and t-storms formed on the leading edge of a very weak upper level disturbance (a pocket of slightly cool air aloft) during the late afternoon.  The cluster of showers and t-storms tracked east through the Adirondacks and Saratoga county, passing well north of the immediate Capital District through 6pm.  A narrow line of t-storms extended south from the cluster moving through the Adirondacks into western Otsego county at 6pm as well.  None of the t-storms during the evening were severe.
An outflow boundary of slightly cooler air and a wind shift into the NNE, generated by the northern convection, propagated south into the Capital District.  By 7:30pm the narrow line of t-storms previously over Otsego county had moved east into Montgomery and northern Schoharie counties, intersecting the outflow boundary.  The intersection of the weak thunderstorms with the outflow boundary where the surface winds were converging produced an area of enhanced upward motion in the atmosphere and greatly enhanced the low level wind shear. (Wind Shear: a change in direction and speed of the wind with height.)  Environments of high wind shear and instability support rotating (supercell) thunderstorms.  The collision of the weak storms with the outflow boundary caused the updraft in the southern most storm in the line to rapidly intensify and the shear environment caused it to rotate developing an isolated supercell.   The supercell developed it's own storm environment feeding on the high dewpoint air and the pocket of slightly drier and cooler air aloft.  All other t-storms in the area dissipated leaving this single monster to cut a path of damage through the Capital District.  On average 60% of supercell thunderstorms go on to produce tornadoes.   95% of all supercells produce at the very least large hail and straight line wind damage.  Supercell thunderstorms, to say the least, are dangerous.

Storm Time Line:

7:34pm: The Albany National Weather Service issued a severe t-storm warning for Montgomery and Schenectady counties, valid through 8:30pm.  The supercell was forming at this time.

7:45pm: The National Weather Service issued a severe t-storm warning for Schoharie county valid until 8:05pm.  The rotating center of the storm pushed south into extreme northeast Schoharie county and began producing damage.

7:55pm: The National Weather Service issued the first tornado warning of the night for Albany and Schenectady counties valid until 8:40pm.  The rotation of the supercell intensified to a point where the radar indicated a possible tornado.The storm tracked east at between 20 and 25 mph along the Albany, Schenectady county line producing damage.

8:35pm: The National Weather Service issued a second tornado warning for Albany and Rensselaer counties valid until 9:15pm.  The rotation continued at a velocity which indicated a possible tornado with the supercell.  The storm tracked through Colonie, Latham, and Troy, producing widespread damage.

9:15pm: The National Weather Service issued the third and final tornado warning for Rensselaer county valid until 10:00pm.  Strong rotation continued in the supercell as it moved through Brunswick and Grafton.  The storm moved into Bennington county, VT between Pownal and Bennington and weakened.  The cell finally dissipated at 11:00pm in Windham county, VT.

The table below is a lists some of the reported damage this single storm produced in it's two and a half hour trek through the Capital Region.

Town, State County Time Damage Report
Esperance, NY Schoharie 8:00pm Wind damage, trees and power lines down, damage to buildings
Burtonsville, NY Montgomery 8:05pm Wind damage, trees and wires down
Delanson, NY Schenectady 8:05pm Wind damage, front porch of house ripped off
Knox, NY Albany 8:15pm Heavy damage to trees and power lines
Altamont, NY Albany 8:30pm Large trees down
Rotterdam, NY Schenectady 8:40pm .75" diameter hail on Guilderland and Schenectady Avenues
Colonie, NY Albany 8:40pm Two foot diameter trees down at Heritage Park
Niskayuna, NY Schenectady 8:45pm Wind damage at Avon Crest Estates
Guilderland, NY Albany 8:45pm Wind damage, one to two foot diameter trees down
Albany Airport Albany 8:52pm 59mph wind gust
Colonie Albany 8:54pm Extensive wind damage due to 80-100mph downburst in the Maywood neighborhood, 100 trees down at the Colonie golf course
Vischer Ferry, NY Saratoga 8:52pm Wind damage, trees down
Pleasantdale, NY Rensselaer 9:00pm 1" diameter hail (quarter sized)
Brunswick, NY Rensselaer 9:15pm Skywarn spotter report of an 80mph wind gust, Roof off of a barn
Bennington, VT Bennington 10:05pm Wind damage, trees and wires down