Tornadic  Thunderstorm Types in the Northeastern United States

Boundary Interaction

Tornadic Thunderstorm Types in the Northeastern United States

Introduction

Supercells

Appendages

Bow Echoes

Boundary Interaction

Miscellaneous Types

 Summary Table

References

Radar detectable boundary interactions appeared to have played a role in about one fifth of the tornadic storms. Horizontal vorticity generated along the boundaries may be an important vorticity source for low-level mesocyclones through tilting and stretching (Markowski et al. 1998). Use of additional data sources (satellite imagery and surface data) may have identified additional cases where boundaries played a role in tornado formation 

Case 1:   Bracken et al (1998) described the role a boundary interaction played in the development of the Memorial Day tornado that struck Columbia County, NY and Berkshire County, MA.  As the supercell that eventually spawned the F3 tornado moved out of the Catskills into the Hudson River Valley, outflow from old storms to the northwest moved towards it. 

Click on image to enlarge

Just prior to tornadogenesis, inferred convergence and horizontal shear beneath the storm increased due to the meeting of a northwesterly surge of down the Catskill Creek Valley and a poleward channeled flow up the Hudson River Valley. The increase in shear may have triggered a shearing instability which led to the formation of a low-level mesocyclone that intensified due to vortex tube stretching (Wakimoto and Wilson 1989)

 

Case 2:   A boundary interaction may have played a major role in the development of the F3 Mechanicville tornado on 31 May 1998.  

	Note in the picture to the left (Click on image to enlarge) the line of thunderstorms and the small group of thunderstorm cells located well in advance of it. This is was taken nearly 90 minutes before the tornado touched down.
	During the next 60 to 90 minutes the line  of storms moved east while the lead cells ahead of it consolidated into a single supercellular storm.  The gap between the line and lead cell closes as they approach the Hudson Valley.  The outflow from the line of storms to the west caught up with the supercell about the time of tornadogenesis. ( Click on image to the left to enlarge).

Click on the play loop button to see a reflectivity loop showing the line of storms catching up to the lead supercell. (need Windows Media Player are similar program to play *.AVI file)