|Oregon Tornado Climatology & Aumsville Tornado Analysis|
January 20th, 2011
On January 20, 2011, with approximately 140 guests in attendance, Oregon AMS Vice-President Steve Pierce lead the chapter’s technical meeting located at the Oregon Museum of Science and Industry (OMSI) at 7pm. Steve stepped in for President Bobby Corser who had a previous engagement that evening. Steve introduced the evening’s guest speakers after touching on upcoming AMS events, our recent flooding event on Mt. Hood and a little weather trivia.
George Miller, retired NWS Meteorologist, historian and author recapped his recent Weatherwise article “Oregon Twisted History” with descriptions of past Oregon tornados and their destructive paths. Miller explained that the term “tornado” was not permitted to be used in the early days of the US Weather Bureau’s reports or assessments. Terms like; freak wind, tornado-like wind, cyclonic-like wind, freak blow, freak twister and freak storm were instead used. During his research, Miller noted that a tornado near Aurora, Oregon on 3/22/67 may have been linked to a death, but not reported to the Weather Bureau. Miller further explained seasonal and topographical influences for the number and location of tornados east and west of the Cascades.
Jonathan Wolfe, a Meteorologist from the Portland office of the National Weather Service (NWS) then gave a technical analysis of the December 14, 2010 Aumsville, Oregon tornado which detailed the atmospheric conditions and formation. Wolfe said that the original creation of the thunderstorm which eventually dropped the tornado into the town of Aumsville was from a convergence of three different wave boundaries. The collision or “tornadogenisis” provided enough energy to enhance local wind shear and create a “spin-up” tornado. Wolfe explained the three main types of “tornadogenisis” which include Supercell Thunderstorm, Spin-up (boundary interaction) and Cold Core Funnels that touch the ground.
The final speaker of the evening was Tyree Wilde, Warnings Coordination Meteorologist, also from the Portland NWS office. He presented a lecture on the old and new Fujita Scale used to survey and categorize tornados. Wilde explained that the new Enhanced Fujita Scale recently created by building engineers and scientists, uses 28 damage indicators for various structure or vegetation types. Each indicator is then assessed by Degree of Damage (DOD) indices that range from no damage to total destruction. The DOD indices associate known wind speeds to their affect on typical building construction methods, as well as above and below standard construction. Additionally, Wilde described the walking survey process and assessment for the Aumsville event.
The meeting adjourned at 9:00pm with a final reminder of upcoming AMS events and meetings.
Minutes submitted by John Rinier & Steve Pierce