UPDATE: Thanks to a commenter, here is an article with more videos from this thunderstorm.
This YouTube video showed up in my mailbox this morning, as a "Top 5 Most Viewed" video, and it now has over one million views:
INCREDIBLE FOOTAGE Sign blowing around on Deerfoot
I did some research (for our news story) to determine what wind conditions caused this incredible event. On Aug. 14, winds were sustained as high was 41 mph gusting to as high as 60 mph from the north-northwest during a thunderstorm between 2:45 and 4:00 p.m. local time at both Calgary International (graphed below) and Calgary/Springbank airports:
According to Google Maps & Street View, the location of the driver would have had a north-northwest wind behind him, which meant it was blowing perpendicular to the street sign's horizontal support bar -- not unlike what happened when winds perpendicular to the Tacoma Narrows bridge ("Galloping Gertie") caused it to crash in 1940 (except then it caused twisting, not simple bouncing up and down). This is my guess anyway, I'm by no means an expert on Aerostatic Fluttering; if you are, leave me a comment below. One YouTube commenter said that the bad engineering of signs spanning too many lanes contributed to the problem.

Sharx Security has sponsored a recent upgrade of our statewide AccuCam Weather Camera Network and I've been reviewing the hardware and software.
Derecho or no derecho, last night's storm was plenty severe in many places from Chicago to New Jersey, including here at my house in State College, Pa.
Was the incredible photo of lightning striking the new World Trade Center last Sunday real, or was it a hoax? I investigate.
During an unprecedented large tornado outbreak last week in Oklahoma City, Storm Chaser & Engineer Tim Samaras was killed, along with his son Paul, and Carl Young.
I met Sean Casey and toured his TIV2 Tornado Intercept Vehicle in 2011. Yesterday, that vehicle found itself in a large, violent tornado, and the video is incredible.
Snow was reported in Pennsylvania and New York on May 24, as viewers looked forward to temperatures in the 20s on Memorial Day Weekend.
Jesse Ferrell
Comments
Comments left here should adhere to the AccuWeather.com Community Guidelines. Profanity, personal attacks, and spam will not be tolerated.