Monday, April 17, 2017

Why Rating Tornadoes Can Be Difficult in Rural Locations

Chris Dolce
Published: April 17,2017

A large EF3 tornado that struck near Dimmitt, Texas, last Friday illustrated the difficult task the National Weather Service sometimes faces when rating the strength of tornadoes.
Tornadoes are rated from EF0 to EF5 on the Enhanced Fujita Scale based on the damage that they cause, not by their appearance or any other real-time metric. But when a tornado strikes a mostly rural area with few structures, the damage may be so sparse that it can be difficult to determine the exact strength of the winds.
(MORE: How Tornadoes Are Rated)
The tornado near Dimmitt stayed mostly over open land along its 4-and-a-half-mile-long path, but EF3 damage was done to one building and some homes nearby suffered EF2 damage. Had those structures not been impacted, there would have been very little evidence for the NWS to use in order to provide a maximum rating for the wedge tornado which was documented by storm chasers.
"Elsewhere along the path, a lack of damage indicators made classification difficult though numerous power poles and center pivots were damaged," the NWS said in its survey report for the tornado.
(MORE: The Different Types of Tornadoes)
The tornado moved on a north-northeast path west of Dimmitt, Texas, across mainly rural locations.
This is illustrated in the satellite image above, where the approximate start and end points for the tornado are marked. Very few buildings are seen along the tornado's north-northeast path west of Dimmitt. Had the tornado's same track been a few miles farther east, the town of Dimmitt may have suffered disastrous damage.
A similar situation unfolded May 31, 2013 near El Reno, Oklahoma, where two mobile radar systems sampled winds of more than 200 mph in a large and deadly tornado. Those wind speeds fall in the EF5 category, but that was not the rating the El Reno tornado received.
Since the massive tornado spared El Reno from a direct hit and moved through mainly rural areas, EF3 damage was the maximum found in the NWS survey.
"Despite the measured wind speeds, surveyors could not find any damage that would support a rating higher than EF3 based solely on the damage indicators used with the EF scale," the NWS said in its post-storm analysis.
For occasions when there is no tornado damage found but evidence a tornado existed in a given location, the NWS now has the ability to give an EFU rating for "unknown," which can be used in official records. One example occurred in eastern Ohio on Aug. 16, 2016; video confirmed a tornado existed near Barton, but the NWS could not find damage.
Changes could be coming soon for the Enhanced Fujita Scale to further improve the accuracy of damage assessments. That may include additional guidance for how various tree types are impacted by tornadoes.
MORE: Tornadoes Hit the Midwest

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

No comments:

Post a Comment