4B.1 RADAR characteristics of the July, 2008 New Hampshire tornado: precursor and tornadogenesis stages

Monday, 1 June 2009: 4:00 PM
Grand Ballroom West (DoubleTree Hotel & EMC - Downtown, Omaha)
John W. Cannon, NOAA/NWS, Gray, ME; and D. St. Jean and M. Cempa

On 24 July 2008, a long-lived tornado raced across eastern New Hampshire. The 52 mi (84 km) path was the longest continuous tornado track ever recorded in New England. This path length eclipsed other well known historical New England storms such as the Worcester, Great Barrington and Stockbridge tornadoes. One death occurred and hundreds of homes were damaged by the rain wrapped EF-2 (Enhanced Fujita) tornado. Detailed storm track information and storm damage photos can be viewed at http://www.erh.noaa.gov/er/gyx/.

An anomalously vigorous (three standard deviations from the atmospheric mean), cut-off, upper level low pressure system and strong low level winds provided favorable dynamics to produce the tornadic storm. Tornadogenesis occurred despite modest atmospheric instability and limited cloud to ground lightning. Rather, a series of storm scale interactions likely enhanced the evolving tornadic cell, which was evident in radar animations.

High resolution radar products from the National Weather Service WSR-88Ds in Brookhaven, New York, Taunton, Massachusetts and Gray, Maine were collected to further examine storm morphology. The imagery revealed two unique stages to the convective lifecycle of the storm, which will be defined as a “precursor stage” and a “tornadogenesis stage”.

The “precursor stage” was associated with storm scale dynamics that preceded the development of the tornadic cell. This period was initially characterized by a pulse of inbound velocities approaching Long Island, New York early on 24 July. Subtle, but distinct cell mergers occurred as this storm entered Connecticut. Radar reflectivity images showed this enhanced convection triggered the formation of a bowing segment which propagated to the right of the mean atmospheric flow.

During the formative “tornadogenesis stage”, the convective cell which eventually produced the tornado evolved. A circulation formed near the apex of the bow echo. Rotation increased as the cell intersected the junction of the Merrimack Valley and surrounding hill towns and continued to fluctuate as the storm traversed northeast at 40 kts across complex terrain. A final cell merger was then coincident with the development of a second bow echo and a rapid increase in low-level shear. A tornado touchdown soon followed.

Radar animations detailing the storm morphology which produced the long-lived tornado will be presented. A conceptual diagram will be displayed to demonstrate that complex and discrete storm scale interactions accompanied the organized convection and tornadogenesis.

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