Storm Event Summary and Preliminary Analysis of the Derecho that struck the Adirondacks in July of 1995


During the early morning hours of July 15, 1995 a series of severe thunderstorms crossed the Adirondacks and much of eastern New York. The meteorological phenomena which struck the region can be identified using the Spanish term "Derecho." A Derecho event stems from a larger family of storms called the Mesoscale Convective System (MCS).

Radar loop of reflectivity images taken at an elevation angles of 1.4 degrees from the KRMX 88-D Doppler Radar. The loop begins at 0814 UTC and ends in 0906 UTC on 07/15/95.



3:32 am EDT 3:50 am EDT 4:07 am EDT
4:36 am EDT 4:48 am EDT 5:06 am EDT


As water vapor rises and cools, thunderstorms physically release warm air (latent heat) into the atmosphere by a process called condensation (as opposed to evaporation which is a cooling process). Warm, moist air is lighter than cool air. Therefore, this column of moist air becomes an area of low pressure relative to its surroundings. Cooler air, which is heavier (high pressure), rushes towards the MCS center from all directions, creating an environment in which winds converge much like approaching automobiles into a toll booth. Since air cannot be forced into the ground, it is reflected upward, thereby causing more air to rise, cool and condense (releasing more latent heat) within the MCS. The self sustaining process is actually similar to the processes within a hurricane.


Schematic of radar view of a Bow echo squall line.
Image from http://covis.atmos.uiuc.edu/guide/stormspotters/html/multiline.html


Once a MCS has developed, bow shaped lines of convection (thunderstorms) may form. This is referred to as a bow echo. Inside of the of the bow, faster winds from mid levels of the atmosphere are transported towards the surface. The updrafts and downdrafts within the thunderstorm complex provide a conveyor belt mechanism of transporting high velocity winds to the surface. The system is labeled a Derecho if a bow echo is followed by an extended area of damaging winds. The following two radar images depict the bow echo signature along with a vast area of damaging winds.

Distinguished shape and pattern associated with a Bow Echo. Composite Reflectivity image is from the KRMX 88-D Doppler Radar at 0906 UTC on 07/15/95.



Very large area of damaging winds across extreme eastern Lake Ontario and Jefferson County NY. Pale green color is indicative of northwest winds in excess of 63 Kts. Base Velocity image is from the KRMX 88-D Doppler Radar at 0906 UTC on 07/15/95.



On July 15th, a MCS developed over Ontario Province. Doppler Weather Surveillance Radar (WSR- 88D), then identified the formation of a Bow Echo east of Kingston. In addition, the WSR-88D archived color information displaying wind motion. This "Velocity Imagery" indicated the presence of a Derecho, as high velocity winds were contained over a large area inside the bow shape. In essence, strong jet stream winds were transported to ground level as the Derecho passed.

These storms continued moving southeast, reaching Thousand Island Park shortly after 4:00 am with wind estimates exceeding 60 knots (69 mph). During this period a Lightning Detection System (LDS) displayed flash rates of 3000 strokes per hour.
Graph shows that over 600 lightning strikes were observed in a 10 minute period during the storms peak intensity. 600 lightning strikes in 10 minutes translates into 60 strikes a minute or one strike every second.

Satellite photography depicted expanding cloud tops to 75,000 feet. The Derecho had a width of 50 to 100 miles and a nearly continuous path until reaching southern New England by 8:00 am.

Path of "Bow Echo" as it crossed southern Ontario, New York And western New England. Time in UTC.


The unmodified Skew-T atmospheric sounding taken at Sault Ste. Marie, MI, where the derecho originated and developed illustrate how unstable the atmsophere was. The lifted index was -8 and the CAPE near 3000 J/Kg. The Hodograph shows there was a large amount of turning directional shear.



Skew-T atmospheric sounding taken at Sault Ste. Marie, MI (where the derecho originated) at 00 UTC on 07/15/95.


Hodograph from atmospheric sounding taken at Sault Ste. Marie, MI (where the derecho originated) at 00 UTC on 07/15/95.



A detailed investigation into this complex phenomena will be completed at the National Weather Service Forecast Office in Albany, New York (NWSFO ALY) with a goal of publishing our findings in an atmospheric journal.



Credits


Various images were taken from the WATADS software package. WATADS(WSR-88D Algorithm Testing And Display System).

Bow Echo diagram supplied by Storm Spotters Guide -- Guide to Meteorology (UIUC)




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