Buffeted By RFD

The storm chase team of Ryan Alliss, Ethan Milius, Bryce Link, Tyler Roney, Nick Carletta, Sam Schreier, Tristan Morath and myself stopped in the midst of Rear Flank Downdraft (RFD) winds of around 50 mph at this location on Keystone Avenue at 330th Street, about eight miles south of Ida Grove, Iowa on Saturday, April 9, 2011. These video frame captures looking north show the blasting winds and lightning flashes every few seconds. The top image, shot around 8:09 PM, includes chaser Ryan Alliss at lower left. The bottom image, shot about two minutes later, shows a CTG lightning bolt a two pieces of debris (arrows) swiftly blowing across the road from left-to-right. In the car monitoring the situation on his laptop was Ethan Milius. We didn't have long to wait to see another tornado--about ten minutes later to our north as we drove east (right in image) on 330th Street (County Road D54). RFD winds are descending regions of warm dry air forced down from mid-levels of the atmosphere that wrap around the back of a mesocyclone. They form when middle and upper troposheric winds encounter the storm's updraft. The winds, typically from the southwest and sometimes reaching 100 mph, yield evaporative cooling which ultimately leads to their formation. Some RFD can actually choke off the formation of a tornado. Due to our SE positions to the storm system as we followed it, we pretty much experienced RFD winds from 7:50 PM to the end of our chase just before 10:00 PM.