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Hello, I'm Gary Woodall with the National Weather Service in Phoenix. Over the past
few weeks, we've heard the statement "National Weather Service meteorologists will survey
the damage area" all too frequently in the wake of damaging storm events. This video
will explain the goals and procedures of the damage surveys conducted by NWS staff.
The primary objective of a damage survey is documentation. First, we must determine what
type of event, such as a tornado or a downburst, caused the damage. We then need to determine
specifics regarding the event, including when it occurred and what areas were impacted.
Most importantly, the survey will determine how strong the event was. In the case of a
tornado, the Enhanced Fujita (or EF) scale rating will be determined, and in straight-wind
events, the maximum wind speeds will be estimated. This documentation is accomplished using all
possible tools, including radar data, eyewitness reports, ground and aerial surveys, and photos
and video of the event and resulting damage. When conducting a ground survey, the team
will start at the beginning of the damage path if possible. Most of the time will be
spent closely examining the most severely-damaged structures. We will investigate the connections:
how well the roof was anchored to the walls, and how well the walls were anchored to the
floor. In this example, notice the single bolt which connected the base of the wall
to the foundation. Investigation of nearby structures can support the wind speed estimates
associated with the worst damage. Unusual items such as projectiles can also provide
evidence nto the severity of the winds. If video or photos of the event are available,
such as the videos of the recent tornadoes, determining what type of event caused the
damage is straightforward. If such photos or video is not available, then the survey
team will study the damage path to determine whether a tornado or straight winds caused
the damage. Tornadoes will typically have a narrow, well-defined track. Debris and damage
will be blown toward the center of the track. This is called "convergence". The damage may
exhibit evidence of rotation about a vertical axis. Straight wind events, or downbursts,
typically have a track that is more broad and diffuse. Damage will spread out from the
center of the track, called "divergence". The debris will exhibit evidence of no rotation,
or rotation about a horizontal axis. The NWS has been using the Fujita Scale, or
F-scale, to rate tornadoes since the mid-late 1970s. The original F-scale was developed
by noted tornado research Dr. Ted Fujita of the University of Chicago. The original F-scale
contained photographic examples of documented structures and a description of expected damage
for each rating from F0 to F5. A shortcoming of the original F-scale is that it focused
on buildings, and framed residential structures in particular.
In the 1990s and early 2000s, it was recognized that more damage indicators were needed to
accurately assess the severity of a wind event. Engineering studies had noted that the wind
speeds needed to cause F-3 to F-5 damage were probably not as high as those in the original
F-scale. Construction variability, a highly subjective part of the original F-scale, needed
to be evaluated more objectively. There was also a need for flexibility in the scale,
to allow for the rating of hurricane and downburst wind damage. To address these needs, the Enhanced
Fujita (or EF) scale was developed. The EF-Scale committee identified 28 Damage
Indicators (DIs) based on their experience with tornado damage. The framed house is one
DI, a mobile home, and small retail buildings constitute other DIs. Other commercial structures,
light and power poles, and various trees are also classified as DIs. Additional DIs may
be added in the future. For each DI, the steering committee decided that the DOD should range
from the initiation of damage all the way up to devastation of the structure.
Based on input from the engineering community, the F-scale rating wind speed ranges were
converted to the new EF-Scale. In this way, there should be continuity in the way wind
damage is rated, regardless of the source. Note that the F-scale wind speed ranges you
see here are maximum 3 second gusts. The 3 second gust is thought to represent how the
wind damages structures a little better than the fastest average sustained wind speed.
To rate a tornado or a high wind event, the overall rating is based on the highest DOD
rating for any DI in the area. The DI and DOD information is used to estimate a maximum
wind gust speed. The maximum gust speed is, in turn, used to determine the EF-Scale rating.
However, there should be other nearby DIs to support the maximum rating. This helps
ensure that the construction of the structure which generated the maximum DOD is similar
to that of surrounding DIs. If another nearby DI cannot confirm the maximum estimate, then
non DIs such as smaller bushes, fences, and vehicles are checked for consistency with
the maximum DOD. If no evidence is available to support the maximum DOD, then the associated
DI is not used. When a survey team heads out, the team has
a job to do, and they must stay focused on their task. This is easier said than done,
however, given the emotional and human factors which come into play. The event that's being
surveyed is likely the most traumatic event that the victims have ever experienced in
their lives. In some cases, the surveyors may be investigating familiar neighborhoods
or even interviewing friends. While these are difficult experiences for everyone involved,
the survey teams often find uplifting stories of survival, assistance, and community through
their inverviews. Damage surveys provide valuable documentation
of a storm and its impacts, and they provide important information about the people and
structures impacted by a storm. If no video/photo images are available, studying the entire
damage area is necessary to determine the cause of an event. The Enhanced Fujita, or
EF scale provides a more quantitative and more accurate means of estimating maximum
wind gust speeds. Construction practices will affect, sometimes dramatically, how a building
responds in high winds. All of the information gathered in damage surveys is catalogued,
shared, and hopefully used to help save lives and property in the future.
Thank you for viewing this video. If you have any questions, please contact us. For the
latest on weather in south central and southwest Arizona and southeastern California, keep
up with us using the links here.