early on the morning of Saturday, July 15, 1995 a rare and extremely powerful
thunderstorm complex developed over Ontario, Canada. The system exploded,
much like a bomb going off, rapidly intensifying and growing in aerial
coverage. A strong northwest jet stream flow propagated the thunderstorm
cluster at speeds of 50 to 60 mph through upstate New York and New England.
During its five hours of life, the leading edge of the complex produced
widespread winds ranging from 50 to 100 mph as well as several suspected
small tornadoes. At 6:43 am the cluster arrived at the Albany airport,
Albany, NY producing hail and Albany's second highest wind gust on record
to date of 77 mph. Less than an hour later a 92 mph wind gust occurred
in Otis, MA. Nearly a dozen people regionally lost their lives, dozens
more were injured, millions of trees were felled in the Adirondack State
Park alone, and thousands were left without electricity for up to a week
after the storm. Most counties in upstate New York and western New England
reported some type of damage from the immense line of thunderstorms.
Mesoscale convective systems are described, non-technically, as a large grouping of self sustaining thunderstorms which may cover several hundred miles over average time frames of six to twenty four hours as they propagate. Most MCS's effect the central and southern parts of the United States, rarely the Northeast, and account for much of the farm belt's growing season rainfall. Thunderstorms on the leading edge of a developing MCS can and often do produce widespread severe weather in the form of hail, straight line damaging winds and tornadoes. Derechos specifically produce sustained damaging straight line winds over a widespread area. In many cases MCS's are responsible for extremely heavy rains which cause flooding. In fact the great Midwest floods during the summer of 1993 were in part caused by daily development of MCS's over the same general areas.Meteorological Setup
A record breaking hot, humid day preceded the development of the ferocious thunderstorm complex which blasted much of New York and New England. Temperatures across the region climbed to the century mark in combination with excessively high levels of humidity. Dewpoint temperatures hovered in the middle 70's the evening prior to the outbreak. Albany, NY recorded a record high temperature of 99 degrees F after a morning record high minimum temperature of 74 degrees F on July 14.
High heat and humidity are basic ingredients to developing thunderstorms. However, thunderstorms did not form in the record heat of Friday, July 14 because a layer of even hotter air was located a couple of thousand feet above the ground. The presence of a warm layer of air above a colder layer is called a temperature inversion.
Everyone knows cold air is dense and sinks, and warm air being less dense tends to rise. For clouds and precipitation to form air must rise in the atmosphere. But, on July 14, 1995 the stifling air on the ground was actually less hot than the air directly above it so it could not rise. Therefore, no thunderstorms could form. In other words the atmosphere was capped. Had thunderstorms been able to form on July 14, they would have released some of the energy that had been building under the inversion throughout the day. The energy release would likely have precluded the massive thunderstorm development that occurred later. Think of the atmosphere in this case as a well shaken can of soda, ready to explode at the moment the cap was removed. So, in essence the Northeast was sitting under an atmosphere time bomb just waiting to go off like a shaken can of soda.
what triggered the bomb? A combination of satellite, surface, and upper
air analysis showed a small scale upper level disturbance (pocket of cold
air aloft) during the evening of July 14, moving out of northern Minnesota.
The upper air disturbance intersected an outflow boundary (a mini cold
front produced by rain cooled air flowing out of prior thunderstorms)
which was positioned south of a stationary front located over Ontario,
Canada. The slightly cooler air aloft was enough to overcome the cap and
break a hole in the atmosphere, so to speak, allowing the hot, humid air
at the ground and all of its phenomenal energy, to rush upwards at speeds
in excess of 100mph. The updrafts rapidly formed a line of thunderstorms.
Once the storms formed they quickly organized into the monster complex
that plowed through the northeast, propagating through and feeding on
the mid 80 degree heat and high levels of moisture. The extreme and widespread
wind damage was a result of the thunderstorm updrafts interacting with
and mixing down to the ground the fast jet stream winds, which were blowing
over the Northeast at the time.
Imagery - Northeast Radar Composite
The derecho had just formed over eastern Ontario in this image. The intense reds in the thunderstorm cluster over Ontario indicate extremely heavy rain and hail. The yellow box on the map is a severe thunderstorm watch.
This image depicts the derecho moving into the western Adirondacks, extending through Syracuse, NY to about Erie, PA. The most intense part of the complex in this image is over Herkimer and Hamilton counties, NY where extensive wind damage occurred. The bowing out, or in other words, curving in the direction of the storm's motion of the radar echo over Hamilton county, NY is an indication the complex was producing strong, damaging surface winds.
The derecho by this time had already moved through the Capital District producing hurricane force wind gusts and widespread power outages. The most intense part of the complex depicted in this image is over south central Vermont, through Rensselaer county, NY. The rest of the line extended to just east of Binghamton, NY. Within about fifteen minutes of this radar image the line went on to produce 92 mph winds in Berkshire county, MA.
This is the final image in the sequence of radar composites depicting the remnants of the derecho over southeastern New England. Although this radar image may not look all that impressive, the leading edge of the complex was still producing damaging winds over Rhode Island. The image shows most of Vermont, New Hampshire, and Massachusetts covered by an area of beneficial rain. The next image is a Doppler view of the line as it moved through the Capital District.
Albany National Weather Service Doppler Radar Display on a Weather Spectrum 9000 Graphics Computer System