The Greater Miami Chapter of the AMS presents
Thursday April 16, 2009 at 3:00 pm.
NWSFO/TPC Media room
The National Weather Service introduced the Enhanced Fujita (EF) Scale in 2007 in order to better determine tornadic wind speeds based on observed damage patterns. Rob Molleda, Severe Weather Warning Coordinator for the Miami NWSFO, will go over the basics of the EF Scale as well as show examples of its application with regards to South Florida tornado events.
As Warning Coordination Meteorologist for the Miami NWSFO, part of Molleda's job involves evaluating reports of tornadoes in the south Florida area and assigning strength values to them according to the Enhanced Fujita (EF) scale. He began his talk with a historical recap of the development of the standard Fujita (F) scale in the early 1970s. Dr. Ted Fujita attempted to formulate a wind scale that bridged between the Beaufort wind scale and the Mach speed of sound scale, which he divided into twelve ranges. He estimated that tornado wind speeds fell within the F0 to F5 ranges of this scale and developed descriptions of the types of damage typical of these wind speeds. The Super Outbreak of tornadoes in 1974 spurred wide use, as well as public recognition, of the F-scale. With time, however, problems became apparent with the F-scale, such as consistency in evaluating damage given differing construction standards.
In March of 2001 a steering committee was established to modify the F-scale to account for these problems as well as to re-evaluate the wind speeds assigned to each level. The result was the Enhanced Fujita (EF) scale. Still ranging from EF0 to EF5, the wind speeds in the lower ranges were increased while those of EF3 and above were lowered to match current studies of wind damage and construction testing.
The new system uses 28 different Damage Indicators (DI), ranging from trees and poles to residential homes and commercial buildings. For each DI there are several Degrees of Damage (DoD) with a range of wind speeds associated with each. These wind speeds can be adjusted by evaluating the level of construction's compliance with code. Using a computer based kit and several corroborating DIs, an evaluator can more objectively determine the EF value of a tornado along its track. Molleda stressed several limiting factors in the use of the EF-scale, including that it may not perfectly match previous F-scale evaluations and introduce artifacts into the historical record.
He then showed several examples of south Florida twisters and how there were classified. A detailed examination was offered of the Wellington tornado from 2008 with interactive use of the EF computer kit to create an analysis of the tornado's estimated strength along its path. This involved a preliminary broad survey of the damage, followed by a careful evaluation of several different DIs and adjustment of their DoDs based on the evaluator's on site determination of the quality of construction. Differing DoDs are then resolved with preference to higher rated DIs to give a resulting wind speed evaluation at various points along the track, and a final assignment of EF numbers at these points.
Several websites that provide more information on the EF-scale are available at: here and here
Several questions came up during the talk, so there were few left for the following Q&A period. The results of the By-Law vote were announced with the By-Laws being adopted unanimously and the meeting closed at 4:15 PM.
Neal Dorst for Dan Dixon
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