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
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.
||Wind damage, trees and power lines down, damage to buildings
||Wind damage, trees and wires down
||Wind damage, front porch of house ripped off
||Heavy damage to trees and power lines
||Large trees down
||.75" diameter hail on Guilderland and Schenectady
||Two foot diameter trees down at Heritage Park
||Wind damage at Avon Crest Estates
||Wind damage, one to two foot diameter trees down
||59mph wind gust
||Extensive wind damage due to 80-100mph downburst in the
Maywood neighborhood, 100 trees down at the Colonie golf course
|Vischer Ferry, NY
||Wind damage, trees down
||1" diameter hail (quarter sized)
||Skywarn spotter report of an 80mph wind gust, Roof off of a
||Wind damage, trees and wires down