Supercell & Squall Line Severe T-Storm Outbreak
A widespread and very significant outbreak of severe thunderstorms developed over eastern New York during the mid afternoon hours on Thursday, May 18 leading to widespread wind damage, several reports of large hail, and several brief tornado touchdowns.
The event was not particularly classic in nature, primarily due to the lack of significant heating. However, extremely strong winds aloft and very strong surface convergence along an advancing cold front, as well as cold air aloft, lead to the development of an intense squall line which produced widespread wind gusts of 50 to 80 mph and and isolated gust to 126mph.
The atmosphere during Thursday morning was rather cool and stable, having been affected by a batch of showers and thunderstorms that moved through eastern NY and western New England during the previous night. A strong cold front, however, was lined up along the New York Canada border during the morning and a highly diffuse warm front extended from western New York to western PA. The pressure gradient between the advancing cold front and high pressure off the New England coast tightened, which created a very strong south wind through the morning and afternoon across New York and New England. Wind gusts frequently reached between 30 and 35 mph. The strong southerly flow began transporting higher dewpoint air into eastern New York by early afternoon. Dewpoint temperatures rose from the low 50's during the early morning to the low 60's by mid afternoon (The more than ten degree rise in dewpoints in six hours represents fairly strong moisture convergence which is a severe weather ingredient.) Simultaneously, breaks in the clouds allowed for some sunshine and minimal heating in the Capital Region. Temperatures moved up into the low 70's, which combined with dewpoint temperatures in the low 60's along with the cold air aloft, was sufficient to destabilize the atmosphere enough to support strong vertical air currents and thus very strong thunderstorm updrafts.
Winds aloft, from the 5000 foot level and higher were blowing between 50 and 80 mph from the west over eastern New York and western New England. Typically in spring severe weather cases, winds aloft are rather strong, compensating for relatively weak atmospheric instability. However, the winds aloft in this event were atypically strong, coupled with moderate instability, which ultimately lead to the widespread high wind events at ground level. The wind set-up with this event is described in meteorology as being one of high wind shear in both speed and direction. (Wind shear being a change in wind speed and direction with height.) As thunderstorm updrafts and downdrafts interacted with the high winds aloft, some of the high momentum air was mixed down to the ground in the form of damaging thunderstorm downbursts which resulted in extensive wind damage in some communities. The directional shear, which was enhanced due to the Hudson valley acting as a channel for the southerly surface wind, supported up to three individual supercell thunderstorms that developed and moved east just ahead of the main squall line. (Supercell thunderstorms have rotating updrafts and sometimes produce tornadoes.)
A strong supercell developed in Chernango county at around 3pm and moved east at up to 50 mph through Otsego county from Pittsfield to Worcester, then into Schoharie county through Jefferson and Gilboa, then through northern Albany county where a funnel cloud was reported at the National Weather Service office on Fuller road at 4:40pm. The squall line overtook the supercell in Albany county at 5pm and absorbed it.
Another supercell thunderstorm developed simultaneously in western Montgomery county and tracked over Canajoharie where wind damage occurred, east into central Saratoga county before being absorbed by the squall line at 5pm. Widespread wind damage was reported in much of Montgomery county with the passage of this storm. Large hail, and wind damage was also reported in Saratoga county as well with trees being sheared off at the ten to twenty foot level in Ballston Lake at 4:30pm, indicating a possible brief tornado touchdown. Golf ball sized hail fell for two minutes in Burnt Hills covering the ground. Rain with this supercell came down at a blinding rate with reports from both Mechanicville and Saratoga Springs of 1.5" of rain in ten minutes!
The third supercell developed further south in Ulster county tracking to the ESE at up to 50 mph through southern Dutchess county producing widespread wind damage through Kerhonkson, Phoenicia, Kingston and Marbletown as well as a possible tornado that damaged one hundred homes in Poughkeepsie.
The squall line produced extensive straight line wind damage in eastern Greene county in Catskill and Cairo with widespread reports of structure, tree, and power line damage. 68 mph winds were reported in Pittsfield, MA as the squall line moved through producing damage to structures, trees, and power lines there.
Due to the very fast steering winds aloft, the storms moved east through the area at up to 50 mph. Typically spring severe weather moves very quickly due to stronger jet stream winds that are typically not present during the summer months. The event began shortly after 3:00pm in central New York and ended by 6:15pm as the squall line moved out of western New England.
The table below is a listing of damage reports received by the National Weather Service and through the Channel 6 WeatherNet 6 weather watcher network.