Tornadoes....The recent Oklahoma experience.

With a Tribute to Moore in May 99

Charles Stewart MD EMDM

Tornadoes....The recent Oklahoma experience.

Evolution of the tornado...

• When the ground warms and the high air is cold… the warm air near the surface rises. As it cools, the water vapor will condense, forming clouds

Formation of a Thunderstorm

Three stages have been identified in ordinary thunderstorms:

1.an unstable atmosphere and vertical updrafts keep precipitation suspended2.entrainment of dry air that causes cooler air from evaporation, triggering downdrafts and falling precipitation and gust fronts3.weakening updrafts and loss of the fuel source after 15 to 30 minutes.

Ordinary Thunderstorms

Severe Thunderstorms

Severe thunderstorms produce a minimum of

3/4 inch hail and/or

wind gusts of 50 knots and/or

tornado winds.

In ordinary storms, the downdraft and falling precipitation cut off the updraft.

In severe storms, winds aloft push the rain ahead and the updraft is not weakened and the storm can continue maturing.

The single supercell storm shown here maintained its structure for hours

Cool downdrafts leaving a mature and dissipating storm may offer relief from summer heat, but they may also force surrounding, low-level moist air upward.

Hence, dying storms often trigger new storms, and the successive stages may be viewed in the sky.

Multicell Storms

Pre-frontal squall lines identify major storms triggered by a cold front that may contain several severe thunderstorms, some possibly supercells, extending for more than 1000 kilometers.

This 1989 storm spawned 25 tornadoes, the worst killing 25 people.

Pre-frontal Squall Lines

An organized collection of thunderstorms extending across a large region is a mesoscale convective complex (MCC).

MCC's can regenerate new storms and last for upwards of 12 hours and may bring hail, tornadoes, and flash floods.

They often form beneath a ridge of high pressure.

Mesoscal Convective Complex

Gust front and Microburst

Turbulent air forms along the leading edge of the gust front, which can generate tumbling dust clouds.

Such gust fronts and associated cold dense air often feel like a passing cold front, and may cause a 1 to 3 mb local rise in pressure, called a mesohigh.

Shelf Cloud

When unstable air is prevalent near the base of the thunderstorm, the warm rising air along the forward edge of the gust front is likely to generate a shelf, or arcus, cloud.

Trailing Stratified Clouds

An extensive region of stratified clouds may follow behind a squall line.

This figure shows a loop of rising and falling air that supplies the moisture to the stratiform clouds and associated light precipitation

This is wind shear formation --- the ‘roll cloud’.... more later.

•When winds aloft blow in one direction and winds on the surface blow in another direction, they create a horizontally rotating mass of air

Wind Shear Formation

Abrupt geographic changes from moist to dry dew-point temperature, called drylines, form in western TX, OK, and KS in the spring and summer.

Cool air pushes hot and dry air over the warm moist air, at the height of the central plains. Such mixing causes large scale instabilities and the birth of many supercell storms.

Dry Line Formation

Middle troposphere winds control individual thunderstorms.•Dying storm downdrafts spawn new storms so the storm system moves rightwards relative to the upper level winds. •Here upper level winds move storms to the northeast, but downdrafts generate new cells to the south, which eventually cuts off moisture to the old cell.

Thunderstorm Movement

Lightning & ThunderLightning & Thunder

Charge differences between the thunderstorm and ground can cause lightning strokes of

30,000°C, and this rapid heating of air will creates an explosive shock wave called

thunder, which requires approximately 3 seconds to

travel 1 kilometer.

Lightning Stroke DevelopmentLightning Stroke Development

Charge layers in the cloud are formed by the transfer of positive

ions from warmer hailstones to colder ice crystals.

When the negative charge near the bottom of the cloud is large enough to overcome the air's

resistance, a stepped leader forms.

A region of positive ions move from the ground toward this

charge, which then forms a return stroke into the cloud.

Nearly 90% of lightning is the negative cloud-to-ground type, but positive cloud-to-ground lightning

can generate more current and more damage.

Several names, such as forked, bead, ball, and sheet lightning describe forms of the flash.

Distant, unseen lightning is often called heat lightning.

Types of Lightning

Lightning Rods & Fulgurite

Metal rods that are grounded by wires provide a low resistance

path for lightning into the earth, which is a poor conductor.

The fusion of sand particles into root like tubes, called fulgurite, may result.

A rapidly rotating column of air often evolves through a series of stages, from dust-whirl, to organizing and mature stages, and ending with the shrinking and decay stages.

Winds in this southern Illinois twister exceeded 150 knots.

Tornado

Tornado Occurrence

Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes annually, but the highest frequency is observed in tornado

alley of the Central Plains.

Nearly 75% of tornadoes form from March to July, and are more likely when warm humid air is overlain by cooler dryer air to cause strong

vertical lift.

The 4 “New Tornado Alleys”

Recent researchshows that there

are really 4 separatetornado alleys… Tornado alley Hoosier alley

Dixie alley Carolina alley

How we get from....

• To here:

Transition...

Remember wind shear formation --- the ‘roll cloud’ -

Wind Shear initiates rotation...

Rising air elevates the roll cloud

The first sign that a supercell may form a tornado is rotating clouds at the base of the storm, which may lower and form a wall

cloud, shown in this picture.

Spinning horizontal vortex tubes created by surface wind shear may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often

stretching rotating air may then turn into a tornado.

This changes the roll cloud into a vertical formation: A tornado

Rotation Moved From Horizontal to Vertical

And... A tornado is spawned

As the tornado moves along a path, the

circular tornado winds blowing opposite the

path of movement will have less speed.

For example, if the storm rotational speed

is 100 knots, and its path is 50 knots, it will have a maximum wind

of 150 knots on its forward rotation side.

Tornado Wind Speed

A system of tornadoes with smaller whirls, or suction

vortices, contained within the tornado is called a multi-vortex

tornado.

Damage from tornadoes may include its low pressure

centers causing buildings to explode out and the lifting of

structures.

Human protection may be greatest in internal and

basement rooms of a house.

Suction Vortices...

Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes annually, but the highest frequency is observed in tornado alley of the Central Plains.

Nearly 75% of tornadoes form from March to July, and are more likely when

warm humid air is overlain by cooler dryer air to cause strong vertical lift.

Tornado Occurrence

Tornado watches are issued = tornadoes are likelyTornado warning = a tornado has been spotted.

Once the storm has passed, the magnitude of the storm is classified based on damage done by the storm. – This is the ENHANCED Fujita scale.

Tornado Watch... or Warning?

Supercell thunderstorms may have many of the features illustrated here, including a mesocyclone of rotating winds formed when horizontal

vorticity was tilted upwards.

Tornado Breeding Superstorms

Supercell thunderstorm development may create an area where the updraft and counterclockwise swirl of upper winds converge into a rear flank downdraft.

This downdraft can then interact with lower level inflow winds and spawn a tornado.

Rear Flank Downdraft

If a pre-existing wall cloud was not present, than any tornado formed is not from a supercell storm.

These tornadoes are often not as strong as those formed by supercells.

Non-Supercell Tornadoes

A single Doppler radar unit can uncover many features of thunderstorm rotation and movement, but cannot detect winds parallel to the antenna.

As such, data from two or more units might be combined to provide a complete view of the storm.

Doppler lidar (light beam rather than microwave beam) provides more details on the storm features, and will help measure wind speeds in smaller tornadoes.

Doppler Radar

NEXt Generation Weather RADar (NEXRAD) •Uses Doppler measurements to detect winds

•moving toward (green)

•moving away (blue)•Which shows areas of rotation and strong shear.

NEXRAD Wind Analysis

Fast Scan of Radar 3May99

Moore 1999 (from space)

1 May 2010

Warm, shallow coastal water is often home to waterspouts, which are simply a tornado over water

The waterspout does not draw water into its core, but is a condensed cloud of vapor.

A waterspout may, however, lift swirling spray from the water as it touches the water surface.

Waterspout

Just how bad is this tornado???

Fujita Tornado Scale

1 2 3 4 5 6 7 8 9 10 11 120Beaufort Scale

1 2 3 4 5 6 7 8 90 10 11 12Fujita Scale

0.6 1.0

Mach Scale

B1 B3B5

B7B9

B11

F0F1

F2

F3

F4

F5

F12M1.0

M0.6

M0.7

M0.8

B17

Beaufort: V = 1.870B3/2

mph F – scale: V = 14.1(F+2)3/2 mphMach scale: V = (742 +1.3)M mph

These scales have wind speed defined first, impacts/damage were assigned to wind speeds.

Comparing Beaufort, Fujita, and Mach Scales

The Fujita scale isdesigned to show the wind speed.

Damage assessment was derived after the fact… this led to a few problems.

Fujita Tornado Scale

The Enhanced Fujita scale is a damage assessment scale that is related to wind speed.

Damage assessment drives the calculation and assignment of the ‘EF’ number.

The Enhanced Fujita Scale

Residences

Commercial/retail structures

Schools

Professional buildings

Metal buildings/canopies

Towers/poles

Vegetation

EF has 28 Damage Indicators

DOD Damage Description EXP LB UB

1 Threshold of visible damage 63 53 80

2 Loss of roof covering material (<20%), gutters and/or awning; loss of vinyl or metal siding 79 63 97

3 Broken glass in doors and windows 96 79 114

4Uplift of roof deck and loss of significant roof covering material (>20%); collapse of chimney; garage doors

collapse inward or outward; failure of porch or carport 97 81 116

5 Entire house shifts off foundation 121 103 141

6 Large sections of roof structure removed; most walls remain standing 122 104 142

7 exterior walls collapsed 132 113 153

8 Most walls collapsed except small interior rooms. 152 127 178

9 All walls collapsed 170 142 198

10 Destruction of engineered and/or well constructed residence; slab swept clean 200 162 220

Example DODs for a Framed House DI (FR12 or DI2)

Note some consecutive DODs have larger overlap than others

DOD Damage Description EXP LB UB




4Uplift of roof deck and loss of significant roof covering material (>20%); collapse of chimney; garage doors

collapse inward or outward; failure of porch or carport 97 81 116







Each indicator has “Degrees Of Damage”

DOD Damage Description – Framed House EXP LB UB




4Uplift of roof deck and loss of significant roof covering material (>20%); collapse of

chimney; garage doors collapse inward or outward; failure of porch or carport 97 81 116







Expected wind 97 mph

“Degrees Of Damage”

F ScaleF Scale Wind SpeedWind Speed EF-ScaleEF-Scale Wind SpeedWind Speed

F0F0 45-7845-78 EF0EF0 65-8565-85

F1F1 79-11779-117 EF1EF1 86-10986-109

F2F2 118-161118-161 EF2EF2 110-137110-137

F3F3 162-209162-209 EF3EF3 138-167138-167

F4F4 210-261210-261 EF4EF4 168-199168-199

F5F5 262-317262-317 EF5EF5 200-234200-234Wind speeds in mph, 3-second gust

F to EF Conversion

•BEFORE…

Picher, OK10 May 2008

Picher, OK10 May 2008

Population (year 2000): 1,640. Estimated population in July 2006: 1,633 (-0.4% change)

Males: 800 (48.8%)Females: 840 (51.2%)

Ottawa CountyMedian resident age: 36.8 years

Oklahoma median age: 35.5 years

Zip codes: 74360.Approximately 60% of houses are abandoned.

Picher, OKVital Statistics

Lone Grove Tornado

Lone Grove Tornado

EF4 Tornado

First violent February tornado since 1950

Killed 8, Injured 46

Part of complex that hit OKC and Edmond

6 reported tornadoes in OK that day.

OKC-Edmond

Lone Grove ‘hook’

?? Lone Grove Tornado ??

Lone Grove Tornado Path

Lone Grove

Power line hazards

Trailer Damage

DOD Damage Description – Framed House EXP LB UB




4Uplift of roof deck and loss of significant roof covering material (>20%); collapse of

chimney; garage doors collapse inward or outward; failure of porch or carport 97 81 116







All walls collapsedExpected wind 170 mph

“Degrees Of Damage”

Frame House

Frame House Damage

An EF-4 tornado, with winds estimated by the National Weather Service at 180 mph to 185 mph

2009 2008 2007 2006 3Year

preliminary Actual Actual Actual Average

Jan 10 84 21 47 51

Feb 44 147 52 12 70

Mar 33 so far 129 170 150 149

Apr 189 167 245 200

May 461 252 139 284

Jun 294 128 120 181

Jul 93 69 71 77

Aug 101 75 80 85

Sep 111 52 84 82

Oct 21 86 76 61

Nov 15 7 42 21

Dec 46 19 40 36

Tot 1691 1098 1106 1297

Monthly Tornado Deaths

1May2010 Ark – 1 dead

1May2010 Ark

20 minutes vs 60 Seconds?

•Data analysis of 18,000 tornadoes between 1986 and 2002.

•On average advanced warning reduced expected injuries by about 32 percent.

•Overall, when people were notified of a tornado up to about 15 minutes ahead of time, deaths decreased.

However, lead times greater than 15 minutes seemed to increase fatalities compared with no warning.

> 15 Minute Warning

"There is anecdotal evidence that came out of the tornadoes in Oklahoma and Missouri in

February. Out of the 23 fatalities, eight were people in cars. I don't know if those people were trying to outrun the storm, or if they just

happened to be in their cars."

> 15 Minute Warning

When people don't know what to do in a dangerous situation, many times they do the wrong thing. I am surprised the authors didn't take that approach. It is almost like they are saying that advance warning is a bad thing,

when in reality it is a GREAT thing; it's just that people are not well educated enough to know

how to respond.

WE NEED TO FIX THIS!

It may be a busy season…

In the event our luck does run out, please put me down for as much warning as

possible.

I have things to do...

Thank You...Chuck Stewart MD EMDM

Professor of Emergency Medicine,University of Oklahoma

email [email protected]@storysmith.net

Cell - 918-344-4557Work - 918-660-3828

2E24 Schusterman Center4502 E. 41st Street

mailto:[email protected]

mailto:[email protected]

Moore 1999

Tornadoes....The recent Oklahoma experience.

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