New Standard Aims to Improve Tornado, Severe Wind Estimates

By Jim LaDue, NWS Warning Decision Training Branch
For more than four decades, the go-to for rating tornadoes has been the Fujita Scale, and in the last eight years, the Enhanced Fujita Scale, or EF Scale. Soon, scientists and NWS field teams will have a new and powerful benchmark by which to gauge the extreme winds in tornadoes and other severe wind events.
The American Society of Civil Engineers (ASCE) has approved the EF Scale Stakeholder Group’s proposal to develop a new standard for estimating wind speeds in tornadoes. This standard will allow, for the first time, a rigorous process to improve not only the EF-scale but to adopt new methods to assign wind speed ratings to tornadic and other wind events.
The intent is to standardize methods. According to the ASCE blog, “The content of the standard would include improvements to the existing damage-based EF scale to address known problems and limitations.” ASCE went on to state that the data used for estimating wind speeds would be archived.
The EF Scale Stakeholders Group, composed of meteorologists, wind and structural engineers, a plant biologist, and a hydrologist, held a series of meetings over the past year to discuss methods available to provide wind speed estimations.  The consensus among the group is that many methods exist in addition to that used in the EF Scale today.  These include mobile Doppler radar, tree-fall pattern analysis, structural forensics, and in situ measurements.  The group also discussed ways that the EF Scale could be improved through the correction of current damage indicators and by adding new ones. The outcome of these discussions is available online.
The new standard will be housed under the Structural Engineering Institute (SEI) of the ASCE.  Users of the standard include but are not limited to wind, structural, and forensic engineers, meteorologists, climatologists, forest biologists, risk analysts, emergency managers, building and infrastructure designers, and the media.
Interested parties are encouraged to apply to the committee, selecting Membership Category as either a General member with full voting privileges or as an Associate member with optional voting capabilities. Membership in ASCE is not required to serve on an ASCE Standards Committee.
The online application form to join the committee is available at
The new committee will be chaired by Jim LaDue and cochaired by Marc Levitan of the National Institute of Standards and Technology. For more information or questions about joining the committee, please contact us at james . g . ladue @ noaa . gov and marc . levitan @ nist . gov.

Shovels, Cleats, and Fabrics: Just Another Snow Story

The Minnesota Vikings are hosting football outdoors for the first time in 29 years in their hometown, thanks to the collapse last week of the fabric roof of their home, the Metrodome. Last Sunday the football team decamped to a dome in Detroit, but tonight they’re expecting six inches of snow in Minneapolis to greet the Chicago Bears (themselves no strangers to snow and cold, of course).
The weather story this time is not just the falling snow, but the valiant efforts of workers and volunteers who have prepared the University of Minnesota’s FieldTurf synthetic field for this Monday night game. Not only does the snow need to be cleared, but the frozen field needs to be warmed sufficiently to prevent a slew of injuries. One player called the surface “hard as concrete.” Unlike NFL stadiums, which deal with a season that stretches into December, the university’s field is normally shut down by now, and does not have heating coils underneath to blunt the effects of freezing air temperatures. In addition the stadium as a whole was “winterized,” or put in cold storage with pipes dissembled to withstand freezing, so reawakening the facility for the game was quite a process.
The conditions of the game make one appreciate the need for a dome for winter sports in Minnesota, but last week’s spectacular roof collapse raises the architectural question: how to design a large roof for Minnesota’s famously varied climate.

The keepers of the Metrodome have good reason to believe that, an occasional roof collapse aside, fabric is still the right answer. The extremes of temperature in Minneapolis stretch and contract any covering, so in fact a flexible roof is ideal. And the maintenance of a fixed structure also means significant snow removal costs which may outweigh the occasional rip and fix for a fabric roof. (Thanks to the forecasts for heavy snow, workers were on the Metrodome roof trying to clear and melt snow last Friday before retreating in a lost cause .) The main downside of fabrics these days is the energy cost of the air pressure between the two sheets of the dome to keep the roof inflated.
Here’s a radio press conference discussing the climatic considerations of the stadium after a previous collapse of the Metrodome roof, also due to snowfall, back in 1982.

It takes a particular kind of storm to damage the roof. The design calls for warm air forced between the outer and inner layers to help melt the snow, but particularly cold storms can overcome that defense, especially if coupled with sufficient water content for a heavy accumulation and winds to drift the snow and cause particularly devastating loads in particular spots on the dome. Apparently even in Minnesota this doesn’t happen often enough to make other roof solutions less expensive or more convenient.