How do tornadoes get their ratings and why hasn't Arkansas seen an E(F)-5 since 1929?

Yahoo

04-04-2025

FAYETTEVILLE, Ark. (KNWA/KFTA) — April 10, 1929, was the day the only documented F5 tornado occurred in Arkansas. It stretched from southern Batesville to north of Swifton and southern Alicia. How are tornadoes rated, and why haven't we seen any in Arkansas in such a long time?
The Fujita scale, created in 1971 by Dr. Ted Fujita and first used in 1973 by the National Weather Service, was used to classify tornadoes based on wind speeds. Through several years of rating tornadoes, it became clear that tornadoes couldn't be rated on wind speeds alone. For better rating accuracy, they introduced the Enhanced Fujita Scale on February 1, 2007. This takes the damage into account. This method is also better because as you get farther from a radar, the wind speed measured will be higher in the storm.
First, someone from the local National Weather Service office will identify the correct Damage Indicator (DI) from a list of 28 indicators based on the damage they see. Once they choose that, they pick one of the 8 Degrees of Damage (DOD) on the scale. The person evaluating the damage will then judge if the damage is within the upper and lower bounds of the wind speeds associated with the DOD. They go around to many structures in the path and then give it an official rating.
Damage Indicators
NUMBER(with links)
DAMAGE INDICATOR
ABBREVIATION
1
Small barns, farm outbuildings
SBO
2
One- or two-family residences
FR12
3
Single-wide mobile home
MHSW
4
Double-wide mobile home
MHDW
5
Apt, condo, townhouse (3 stories or less)
ACT
6
Motel
M
7
Masonry apt. or motel
MAM
8
Small retail bldg. (fast food)
SRB
9
Small professional (doctor office, branch bank)
SPB
10
Strip mall
SM
11
Large shopping mall
LSM
12
Large, isolated ('big box') retail bldg.
LIRB
13
Automobile showroom
ASR
14
Automotive service building
ASB
15
School – 1-story elementary (interior or exterior halls)
ES
16
School – jr. or sr. high school
JHSH
17
Low-rise (1-4 story) bldg.
LRB
18
Mid-rise (5-20 story) bldg.
MRB
19
High-rise (over 20 stories)
HRB
20
Institutional bldg. (hospital, govt. or university)
IB
21
Metal building system
MBS
22
Service station canopy
SSC
23
Warehouse (tilt-up walls or heavy timber)
WHB
24
Transmission line tower
TLT
25
Free-standing tower
FST
26
Free-standing pole (light, flag, luminary)
FSP
27
Tree – hardwood
TH
28
Free-standing pole (light, flag, luminary)
TS
EF Rating
3 Second Gust (mph)
0
65-85
1
86-110
2
111-135
3
136-165
4
166-200
5
Over 200
While the tornado rating is based on the damage it does, the damage indicators correspond to wind speeds given in the table above.
Example Rating
For example, let's say a tornado hit one of our local high schools. You would go to Damage Indicator No. 16 and read the description to make sure that's the right building. Then you read the DOD description that goes along with what you see. With each of the 11 DODs for this damage indicator, there's a lower and upper bound for possible wind speed and an expected value that corresponds to possible wind speeds. DOD 9, for example, says 'collapse of exterior walls in top floor.' Winds could be anywhere from 121-153 mph, with the expected to be 139 mph. This would likely be EF-3 damage. You then move on to the next structure.
Let's take a one and two-family residence (#2). The description for typical construction is:
Asphalt shingles, tile, slate, or metal roof covering
Flat, gable, hip, mansard or mono-sloped roof or combinations thereof
Plywood/OSB or wood plank roof deck
Prefabricated wood trusses or wood joist and rafter construction
Brick veneer, wood panels, stucco, EIFS, vinyl or metal siding
Wood or metal stud walls, concrete blocks or insulating-concrete panels
Attached single or double garage
To have EF-4 damage, you would need all walls collapsed or, at the very least, all walls except small interior room walls. To have EF-5 damage, the description reads 'destruction of engineered and/or well-constructed residence; slab swept clean.'
Basically, nothing can stand from a well-constructed home. Key words are well constructed, because it can come down to the types of nails and how they have been driven into the wood, and that could mean the difference between an EF-4 and EF-5. Typically, they'll bring engineers out to help iron out those kinds of wrinkles, as well has prove that the bolts or screws didnt't fail during EF-4 strength, but only during EF-5 intensity.
It's a complicated question to answer. You have to have high amounts of CAPE (t-storm fuel), strong wind shear (change in direction with height), a lot of warm, moist air, and strong lift along a boundary to get tornadic supercells.
The formation of most weather systems comes from the east side of the Rockies, known as lee cyclogenesis. All the warm, moist gulf air gets pushed into the south-central plains and meets up with the strong lift from these systems, which is why most tornadoes occur in states like Kansas, Oklahoma, and Texas.
Then, as that energy continues to move east into the overnight hours, yes, they may lose some fuel and heating, but the nocturnal low-level jet kicks in and can add an insane amount of spin, which might answer the question of why there are many nocturnal tornadoes in Northwest Arkansas and the River Valley.
Other times, we may see many tornadoes in Dixie Alley, which encompasses eastern Arkansas and northern Louisiana and stretches across the southeast United States. This typically happens when you have strong weather systems and intense lift along a boundary like a cold front. These will typically come in the form of lines of storms known as Quasi-Linear Convective Systems or QLCSs.
The shorter answer to the question is: It's hard for all the ingredients to come together at the right time and place for any tornado to form, but Arkansas is located in a place that's just a little more difficult for everything to come together.
Supercells are typically favored where its flatter, like central and eastern Arkansas, and QLCSs in Northwest Arkansas, but both can occur in either part.
There have been several outbreaks and violent tornadoes across Arkansas, including two recent EF-4s in Northeast Arkansas.
Below is a map of all the violent (EF-4 and 5) tornadoes in Arkansas and bordering states from 1950 to 2023. That's the only available data range from the Storm Prediction Center.
So while it has been a long time since we've seen an E(F) 5 in Arkansas, let's hope it stays that way.
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