9 October 2018

Last week the National Weather Service published an analysis of six of the many tornadoes that hit our area on Tuesday, October 2.  There were 14 tornadoes in Pennsylvania that day!

The most damaging tornado hit a nursing home in Conneautville, PA.  The most unusual one touched down in the Valley Green Road area of Westmoreland County.  NWS Pittsburgh describes what made the Valley Green Road EF1 tornado so interesting:

This tornado was unusual not only for its northward movement in an eastward-moving storm, but especially because it formed on the northern side of the parent thunderstorm, removed from the typical hook echo region.

NWS Damage Surveys for 10/2/2018 tornado event, National Weather Service Pittsburgh

So what is the hook echo region?

According to Wikipedia, “A hook echo is a pendant or hook-shaped weather radar signature as part of some supercell thunderstorms.”  

USTornadoes.com describes how it forms: “This “hook-like” feature occurs when the strong counter-clockwise winds circling the mesocyclone (rotating updraft) are strong enough to wrap precipitation around the rain-free updraft area of the storm.”

The annotated radar image below shows the hook at bottom left, curling around the back of the storm with a tornado at the tip. 

Tornadoes usually form in the hook echo and they move with the storm.  Storm chasers use these facts to find and safely chase tornadoes.

But not at Valley Green Road.  That tornado formed on the north edge and traveled north (See region on the annotated example of an Oklahoma tornado below.)

Sneaky tornado!  Fortunately it was not very powerful (EF1) and there were only trees in its path.

(photo and hook echo images from Wikimedia Commons. The tornado photo was taken in North Carolina (not in PA); click on the captions to see the originals)