How are tornadoes ranked?

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National Weather Service survey teams are assessing the damage this afternoon.

HOUSTON — The National Weather Service offices in Houston and Lake Charles, La., were very busy Thursday morning doing something they don’t often do: Surveying severe destruction from a violent, long-lived tornado.

The Lake Charles NWS has already reported that the same cell that moved through Seven Oaks, Onalaska and Jasper Wednesday night is the same cell that produced “solid EF-2 damage” just west of the town of Woodworth, La., which is just west of Alexandria in Rapides Parish. No other damage surveys have come in at this time.

The Onalaska area was heavily impacted by the tornado Wednesday night.

Three people were killed: A woman in her 20s, a man in his 50s and another man of unknown age, Polk County Office of Emergency Management officials said Thursday afternoon. 

More than 30 people were injured and 291 homes were affected, including 46 homes that were destroyed. More than two dozen residents whose homes were damaged have been provided shelter by American Red Cross.

RELATED: Tornado northeast of Houston kills 3 and injuries dozens; 46 homes destroyed

Are tornadoes rare in the Houston area?

Tornadoes in our part of southeast Texas or southwest Louisiana are a common occurrence — normally very weak and causing only cosmetic damage to trees, powerlines or shingles. An example of this occurred in Brazoria County on Sunday, where a weak, EF-0 tornado briefly touched down for less than two minutes along Highway 35. 

Strong, high-ranked tornadoes are not a common occurrence. In-fact, they’re exceedingly rare due to our very maritime environment.

If only looking at EF-4 and EF-5 tornadoes, only two F/EF-4 tornadoes have ever been observed in the county warning area (CWA) of Houston (all of our area counties): one F4 in November 1992 in Channelview, and another F4 in Galveston during Hurricane Carla, September 1961. No F5 or EF-5’s have ever been documented in this part of Texas.

The supercell that produced the Polk County tornado Wednesday (and many others along its path) was extremely unusual in that it organized west of Bryan/College Station and held together across east Texas, Louisiana and western Mississippi, and carried a tornado warning tag for 5 hours and 55 minutes, tracking 250 miles — with only a brief moment west of Alexandria, La., where the storm cycled and briefly weakened. 

For perspective, almost all tornado warnings last no longer than 30 to 45 minutes and with only a handful of storms requiring a re-issuance of the warning.

RELATED: ‘God was with us’ | Polk County tornado flipped trailer home with man and his dog inside

How are tornadoes ranked?

Tornadoes are ranked by the damage they inflict. While homes in Polk County were completely destroyed, many of them were mobile homes or older wooden structures. It takes less wind to destroy a mobile home than it does a solid brick home on a foundation. Therefore we’ll have to wait for the NWS to survey the damage to determine a ranking.

In order to obtain an EF-3 ranking, a well-built structure will usually be missing the entirety of the roof and a few exterior walls. 

To get an EF-4 ranking, a well-built structure will have to be completely torn down with little to no interior walls remaining. The key there being interior walls. 

Finally, to get the EF-5 tag, not only does a well-built structure have to be completely leveled off its foundation but the debris must be completely removed leaving only that of the slab.

Thankfully, there is no evidence of EF-4 or EF-5 from the pictures that have come in so far. 

Violent tornadoes (EF-4/5) only account for less than 5 percent of all tornadoes annually in the U.S. but are responsible for 90 percent of the deaths. 

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