"What does ''EF-4'' mean?' and other tornado questions answered

This frame grab provided by KWTV shows a tornado in Oklahoma City Monday, May 20, 2013. Television footage shows flattened buildings and fires after a mile-wide tornado moved through the Oklahoma City area. (AP Photo/Courtesy KWTV)

With tornadoes in the news lately I figured it'd be a good time to post answers to some frequently asked questions about the powerful storms:

What does "EF-4" mean?

WIth the devastating tornado in Moore, Oklahoma, you'll be hearing a lot of about "EF" ratings -- that's from the Enhanced Fujita Scale that rates tornadoes on a scale of 0 to 5, 5 being the strongest.

The Moore tornado was given a preliminary rating of EF-4 ("Devastating") with estimated tornadic wind speeds of up to 200 mph, although many are thinking that rating could be increased to an EF-5 ("Incredible") once more damage assessment is done.

Sadly, it's not the first time Moore has had to deal with such a catastrophic storm. On May 3, 1999, Moore was struck by an EF-5 tornado which recorded the strongest wind speed ever registered near Earth's surface. this map provided by the National Weather Service in Norman, Oklahoma shows just how close the two tracks were.

How rare are tornadoes here?

It used to be that Washington averaged just one tornado per year in the state, but factoring in more recent data, our official average is now up to two -- most likely helped by our 1997 season that had a record 14 tornadoes.

Still, tornadoes are typically very weak here, typically rating an EF0 or EF1 on the Enhanced Fujita Scale

Strong tornadoes need severe thunderstorms fed by large changes in temperature in the upper atmosphere. Severe thunderstorms typically need much colder air moving in aloft to make the air very unstable.

The so-called "Tornado Alley" in the Midwest is ripe for severe weather due to frequent battles between cold, arctic air marching south of out Canada colliding with very warm, moist air moving north from the Gulf of Mexico.

But in the Pacific Northwest, the cool waters of the Pacific Ocean are a great moderating force that keeps temperature changes from being too drastic, and thus tornadoes are quite rare.

What are more common here are what we call "cold core funnels"

They are different from typical devastating Midwest tornadoes in that these are spawned from non-severe storms and can occur when you get a tightly wrapped rush of rising air that can appear as a funnel.

They get their name from the usual pattern when you have a storm bringing much colder air into the higher altitudes -- a common occurrence around here in spring and fall.

Cold-core funnels rarely reach the ground, and if they do, are very weak. They are not all *that* rare with well-formed Convergence Zones -- especially in spring and autumn.

Has there ever been a bad tornado here?

There has only been one deadly tornado in recorded history in Washington -- an F3 tornado that touched down in Vancouver on April 5, 1972. Six people were killed and 300 were injured in that tornado.

There have been two other storms in Washington that rated an F3 -- one was the same date as the Vancouver tornado, but in Lincoln County. One person was injured there.

An F3 tornado also struck the Kent Valley on Dec. 12, 1969. One person was injured there as well. Finally, one tornado that touched down near LaCenter in Clark County on June 29, 1989 injured one person when their car was lifted six feet.

Those are the only tornadoes that have injured anyone since records have been kept in 1880.

In Oregon, there were reports of a tornado that killed three and injured 5 on June 14, 1888 near Lexington in Morrow County, and another one on June 3, 1894 in Grant County that killed three and injured 10. There have been no tornado-related deaths in Oregon since that 1894 tornado.

What's the difference between a funnel cloud and tornado?

A funnel cloud is basically a tornado that doesn't touch the ground. They don't do any damage by itself, but a funnel cloud can certainly become a tornado, so they need to be reported and monitored. Once the funnel touches the ground -- even for an instant -- it gets classified as a tornado.

You might have also heard of "waterspout" which is a tornado that is over water. There have been documented cases where waterspouts have been known to make it rain frogs or fish, having been sucked up out of the water.

Then there is a "gustnado" which is not a tornado at all. This is a swirling vortex along the ground that is caused by straight-line winds. These don't connect to the clouds and are more visually along the lines of a dust devil (although not the same!). Gustnadoes do minor, if any damage.

More tornado information:

Storm Prediction Center's Online Tornado FAQ

National Tornado Statistics

List of all tornadoes in Washington, 1950-2008

List of all tornadoes in Washington State, broken down by county, 1880-2000