Terrain-influenced tornadogenesis in the Northeastern United States

Significant (F2-F5 intensity) tornadoes in mountainous environments are rare events, but seem to have a propensity for recurrence in particular topographically configured domains. The complex terrain of the Hudson Valley region of New York State and adjacent Catskill and Berkshire mountains, where prominent north-south oriented river valleys are flanked by highlands above 800 m, represents one such region where the potential for tornadogenesis appears locally enhanced on days when the synoptic-scale environment favors supercell thunderstorm formation. This behavior is illustrated through case studies of two events: the Great Barrington, Massachusetts, F3 tornado of 29 May 1995 and the Mechanicville, New York, F3 tornado of 31 May 1998.

In the Great Barrington case, intense tornadogenesis resulted when a supercell interacted with a sequence of prominent landforms on a day when the synoptic environment was favorable for supercell thunderstorm formation. Doppler radar observations show the storm was already supercellular but not tornadic while west of the Catskill Mountains, and that the storm’s mesocyclone intensified significantly as it passed over the eastern end of the Catskill escarpment and entered the Hudson Valley. Mesocyclone intensification and tornadogenesis appear closely related to the storm’s interaction with two separate terrain-channeled flows, both of which are clearly evident in Doppler velocity data. The initial 35-km track tornado (F2) dissipated when its parent mesocyclone weakened as it moved upslope into the Berkshires of western Massachusetts, before intensifying again in the Housatonic River Valley where tornadogenesis was repeated (F3), resulting in 3 fatalities at Great Barrington.

In the Mechanicville case, the coupling of two upper-level jets with a low-level jet in association with an unseasonably strong surface cyclone created an especially supportive environment for supercell formation and prompted issuance of the first-ever “High Risk” convective outlook for the Northeastern United States by the NOAA Storm Prediction Center. Tornadogenesis potential near Mechanicville, where a long-track tornado caused F3 damage, was likely increased by terrain channeling of the low-level southerly flow up the Hudson Valley through the enhancement of right-turning (clockwise) hodographs in the lowest 1 km of the atmosphere and by the funneling of warm, moist air up the Hudson Valley, thus increasing the instability. The storm that produced the Mechanicville tornado developed over central New York ahead of a line of severe storms to its west. Outflow from this line of storms was channeled east-southeastward down the Mohawk Valley toward the Hudson Valley. Tornadogenesis occurred as Mechanicville storm reached the more favorable environment in the Hudson Valley, and at about the time outflow from the line of storms to the west caught up to it.