The Moore Tornado, and Common Tornado-Hurricane Misperceptions

On May 20, approximately two dozen tornado reports were scattered from Texas into Oklahoma, Kansas, Missouri, and Indiana.  Among them, one in particular combined two deadly ingredients: very intense winds and a populated urban area.  A tornado that struck Moore, OK (a southern suburb of Oklahoma City) was rated an EF5 tornado, with peak winds of 200-210mph.  An EF5 tornado contains the most violent winds on the planet — such winds are capable of leveling virtually any man-made building.

The large-scale setup for a severe weather outbreak was forecast at least a week in advance.  A 2-3 day period of all the necessary ingredients coming together at the same time was anticipated, and the peak threat was expected on May 20.  Indeed, on May 20, a tornado watch was issued for central and eastern Oklahoma at 1:10pm CDT.  Thunderstorms formed about 20 minutes later, and rapidly became severe, rotating supercells.  At 2:40pm, a tornado warning was issued for Moore, then at 3:01pm, a rare tornado emergency was issued.  From approximately 3:15-3:25pm, the massive tornado cut a path of destruction through the city, demolishing everything in its way and killing at least 24 people.

Radar image of the parent supercell that spawned the tornado. This image is from 3:21pm, just as the tornado was tracking through Moore. The magenta-purple blob is called a “debris ball”… rather than the radar beam reflecting off of just hail and rain, it’s also hitting a concentrated airborne ball of debris from houses and other structures. The radar itself is located at the center of the black circle east of Moore.

The tornado that passed directly over Moore was on the ground for 50 minutes and for 17 miles, and was at times about 1.3 miles wide.  This suburban town has been hit by significant tornadoes five times in 15 years: the October 4, 1998 F2, the May 3, 1999 F5, the May 8, 2003 F4, the May 10, 2010 EF4, and now the May 20, 2013 EF5.  (The original Fujita Scale from 1971 was replaced by the more accurate Enhanced Fujita Scale in 2007, and as such, the shorthand tornado rankings switched from F5 to EF5, for example.)  Not surprisingly, the odds of being hit by a significant tornado are climatologically quite high in central Oklahoma in May as seen in this map.

How Do Tornadoes and Hurricanes Compare?

Sometimes people erroneously interchange these two types of storms.  The only thing they have in common is strong winds; outside of that, they are entirely different phenomena.

1) Geography

In the U.S., tornadoes are most commonly found in the Great Plains states, but have been known to occur in almost every state.  They require a parent severe thunderstorm, and a list of atmospheric conditions that is fairly well-known.  If a tornado forms or passes over water, it’s called a waterspout, but for the most part, tornadoes “prefer” land.  Hurricanes, on the other hand, require a warm ocean to form and strengthen.  Once over land, hurricanes quickly weaken.  Only certain islands and coastal areas can be hit by a hurricane, though sometimes side effects can extend further inland (strong winds, flash flooding, tornadoes).

2) Intensity

While both types of storms are capable of producing destructive winds, tornadoes can become stronger than hurricanes.  The most intense winds in a tornado can exceed 300mph, while the strongest known Atlantic hurricane contained winds of 190mph.  The scales used to categorize the two are also different, as shown below.  Tornadoes are ranked on the Enhanced Fujita Scale, while hurricanes are ranked on the Saffir-Simpson Scale.  Beyond about 120mph, winds are powerful enough to significantly damage or destroy structures.

A visual comparison of the scales used for tornadoes and for hurricanes. On the top, the Enhanced Fujita Scale uses three-second wind gusts to define its thresholds. On the bottom, the Saffir-Simpson Hurricane Wind Scale’s thresholds are defined by one-minute sustained winds.

3) Size

While a very large tornado might reach 2 miles across, typically they are much less than a half mile across.  Hurricanes, on the other hand, are several hundred miles in diameter.  Even the eyewall (the inner ring of the most intense winds) is typically about 25 miles across.  Rainbands in the outer circulation of a hurricane can spawn multiple tornadoes simultaneously, while there is no way for the opposite to occur.  Tornadoes are completely dwarfed when it comes to a size comparison.

4) Predictability & Warning

There is also a huge difference in the timescales involved between tornadoes and hurricanes.  While the large-scale environment that is favorable for tornado development can be predicted several days in advance, there is presently no way of predicting individual tornadoes even HOURS in advance.  Once a rotating thunderstorm forms, there is still no way of knowing whether or not it will spawn a tornado, or how strong that tornado will become.  A tornado warning is issued an average of 13 minutes prior to impact, giving people a very limited amount of time to take shelter.  Sometimes that lead time is longer, sometimes shorter.  Conditions that are favorable for hurricane development can also be predicted several days in advance.  But since they usually form over the open ocean, they don’t immediately affect people.  There can be anywhere from a day to well over a week before the storm hits land… if it ever does at all.  Hurricane warnings are issued up to 36 hours before strong winds are expected to affect land, giving people time to prepare themselves and their houses as best they can.  Also due to the difference in time scales, people can evacuate an area prior to a hurricane landfall, but there is no time to evacuate an area before a tornado strikes.

5) Preparation

In both cases, having a plan in place before a storm comes is very important.  When the time comes, putting that plan into action will be stressful enough.  For a tornado, the most critical part of a plan is knowing where you and your family will take shelter; it might be an interior closet or bathroom, a basement, or a storm shelter.  Tornadoes are such short-fuse violent events that you may not have time for much else than protecting life.  Hurricanes are much easier to prepare for and allow for more elaborate planning.  You will have time to protect your house with window coverings, buy supplies, organize important documents, and evacuate if necessary.  If you don’t evacuate, then it’s very similar to a tornado: find the safest location you can to stay for the duration of the storm.  While a tornado will pass over in a matter of seconds or minutes, a hurricane will take several hours to pass over.  In both cases, no shelter is perfect — the most severe tornado or hurricane is capable of such destruction that even the best plan and best shelter may prove insufficient.  But clearly, there are ways to minimize your exposure to danger, and FEMA has some valuable information and resources available at http://www.ready.gov/tornadoes and http://www.ready.gov/hurricanes.

Author: Brian McNoldy,Senior Research Associate in Meteorology and Physical Oceanography at the University of Miami