A Case Account
Saturday July 15, 1995 New York - New England Super Derecho (Severe Thunderstorm Outbreak aka "The Adirondack Blow Down")

Very 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.

The thunderstorms that moved through the Northeast early on Saturday, July 15 were not the typical types of storms that normally affect New York and New England during the spring and summer. The storms on July 15 were part of a highly organized complex. The technical term for this type of thunderstorm grouping is a mesoscale convective system (MCS.) The specific type of MCS that hit the Northeast on July 15 is called a derecho.

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.

So, 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.

Derechos rarely effect the Northeast because the necessary ingredients to form such monster complexes do not often simultaneously come together over the region. The combination of high heat and humidity, high atmospheric instability above a capping inversion and a well aligned, fast, west northwesterly jet stream flow along with a trigger mechanism are rarely set-up to produce the widespread convective winds. Severe weather in the Northeast normally is less organized and smaller in scale affecting localized areas.

Radar Imagery - Northeast Radar Composite
July 15, 1995

The Young Derecho

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.

Mature Derecho

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.

Mature Derecho Hitting Western New England

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.

The Complex Reaches the Coast

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.

NEXRAD Base Reflectivity

Albany National Weather Service Doppler Radar Display on a Weather Spectrum 9000 Graphics Computer System

Reflectivity Display
1.5 degree Elevation Tilt

6:50am, Saturday, July 15, 1995

This radar image was taken at the exact time the Albany County airport reported a 77 mph wind gust. The leading edge of the line extends from Rutland county, VT to just east of Binghamton. Notice the curved nature to the leading edge of the radar echoes...this is called a Line Echo Wave Pattern (LEWP) and suggest several small scale circulations along the line. Each point where the line bows outwards is an indication of particularly strong damaging winds at the surface.