Last week brought a very rare sight to Western Washington - a tornado that damaged two roofs near the Frederickson Boeing plant and toppled several trees around and into nearby homes.
The National Weather Service would later rate the storm an EF-1 on the Enhanced Fujita Scale with top wind speeds of 110 mph.
But obviously, they didn't have a lucky anemometer right in the tornado's path to record the wind speed, so how can they tell how strong the tornado was?
Just by analyzing the damage is the simple answer, but the process involved is a bit more complex. How do they know what damage was caused by 90 mph winds versus 120 mph winds?
I asked Ted Buehner, Warning Coordination Meteorologist with the Seattle office of the National Weather Service to give me a peek into how they figure this out. Buehner was among the storm damage survey team that went to the site of the Frederickson tornado.
Buehner says after a tornado strikes, two staff members from the local NWS office will join a local emergency manager (since it's their turf) to go out and survey the damage. They look to see where the track started, where it ended, how wide a path the tornado took and how long it was on the ground.
"We talk with witnesses, gather photos and/or video, and any other evidence we can find," Buehner said.
The data is then taken back to their office and reviewed with other staff members along with a piece of software that compares the damage to the Enhanced Fujita Scale and wind studies done over time.
It's that rather new "EF" scale that gives them a lot of guidance as to how strong a tornado really was.
The Enhanced Fujita Scale was put in use in 2007 to replace the old Fujita Scale. The new scale uses 28 categories of damage, such as to mobile homes, apartment buildings, business, large homes, and large trees, to help forecasters and tornado sleuths get an idea of just how strong the wind speeds were.
Inside each category are 10 levels of damage ("Degree of Damage" as they call it). Here is the example, for a typical 1-2 story family home: (EXP is expected winds, LB is lower boundary, UB is upper boundary)
So in this case, if a survey team finds there is a small loss of roof covering material or vinyl/metal siding, that would rate a '2' and give approximate wind range of 63-97 mph with an average speed of 79 mph for storm winds at that point of its track. (Remember, a tornado can strengthen and weaken during it's time on the ground.)
If exterior walls have collapsed, that's a 7 and rates an average 132 mph speed. If the slab is swept clean, as we saw in the Moore, Okla., and Joplin, Mo. tornadoes of late, that would be a 200 mph wind.
In the Frederickson case, the tornado didn't seem to directly hit any houses, although some were damaged by tree falls (which would count in the ratings as damage to the tree, but the house damage would not be considered since it wasn't at the hands of a direct hit by the tornado.) But perhaps the most visible damage came to the roof of the NW Doors building, probably rating a 2 or 4 on the warehouse building damage scale:
And several large tree falls:
So Buehner and his team noted all of the damage, went back to their office and plugged in into some computer software that knows all these EF damage categories and came back with a likely peak wind of 110 mph, giving it a high EF-1 rating (was almost an EF-2). The survey also determined the storms had a path 1 mile long, 75 yards wide and was on the ground for less than 5 minutes.
If you're curious about more of the EF categories and their degree of damage, you can either click on each category's number here or there is an all-encompassing 95-page .PDF file here.