An exceptionally unusual tornado struck the mid Cape today with substantial damage in parts of Yarmouth, Dennis, and Harwich. The storm formed along a warm front that was draped along the South Coast of New England. This is a classic setup for tornadoes along our shoreline.
The first sign of trouble was just after 9 a.m. when a supercell thunderstorm developed just south of the Hamptons. A supercell is a fancy term for a thunderstorm that is rotating. There certainly could have been a waterspout just off Long Island. The same storm tracked east-northeast toward Martha's Vineyard when the storm started to take off again. There was a very narrow corridor when moist and unstable air just offshore was juxtaposed with an area of strong low level wind shear over the Cape. Where the two intersected - along the warm front - the rotating storm and tornado occured.
This radar image shows the strong rotation moving through Yarmouth. In addition to a tornadic circulation there was a long swath of damaging wind that stretched from Mashpee east through Chatham and included a measured wind gust of 90 mph at Kalmus Beach in Hyannis!
I have a few annoying weather sayings in the summer for severe weather that I say all the time to anyone who will listen. "Always watch the triple point" and "south coast special" for events just like this.
The National Weather Service did a masterful job with this tornado with timely warnings that included 18 minutes of lead time. Additionally, they recognized a suble signature on radar that indicated tornado debris had been sucked up into the storm's updraft which allowed them to confirm a tornado touchdown before any damage reports came in.
Thankfully this has been an unsuually quiet severe weather season in New England with the exception of today's Cape Cod tornadoes. Let's keep it quiet through the fall!