Transcript Ch11Pres - Leornian.org

AMS Weather Studies
Introduction to Atmospheric Science, 4th Edition
Chapter 11
Thunderstorms and
Tornadoes
© AMS
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Case-In-Point
 Major severe weather outbreak of 3 May 1999
– More than 70 tornadoes were reported in Oklahoma,
northern Texas, and south central Kansas, and 26 of
these occurred in or around Oklahoma City
– An F5 tornado took 38 lives in Oklahoma City suburbs
– An F4 tornado claimed 6 more lives in Haysville, Kansas
– Essential ingredients for this outbreak:
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 Warm, humid surface air layer was initially capped (capping
inversion) with much drier air aloft
 Temperature and humidity contrast between low-level and
upper-level layers grew throughout the day, increasing the
potential for deep convection and severe weather
 Sounding indicated strong vertical wind shear
 Afternoon arrival of a jet streak lifted the air column and
eliminated the capping inversion
 Massive supercell thunderstorms developed explosively and
spawned violent tornadoes
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Driving Question
 What conditions in the atmospheric favor
development of severe convective weather
systems?
– Tornadoes are the most intense of weather
systems, but less than 1% of all thunderstorms
spawn tornadoes
– This chapter covers thunderstorms and
tornadoes, their characteristics, life cycles,
geographical and seasonal distributions and
associated hazards
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Thunderstorm Life Cycle
 A thunderstorm is a meso-scale weather system that is accompanied
by lightning and thunder, affects a relatively small area, and is shortlived. It is the product of vigorous convection extending high into the
troposphere.
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Thunderstorm Life Cycle
 Towering Cumulus Stage
– Cumulus clouds build vertically and laterally, and surge upward to
altitudes of 8,000-10,000 m (26,000-33,000 ft) over a period of 1015 minutes
– Produced by convection within the atmosphere
 Free convection – triggered by intense solar heating of Earth’s
surface
– Generally not powerful enough to produce thunderstorms
 Forced convection – orographic uplift or converging winds
strengthen convection
– This is generally the cause of thunderstorms
– Latent heat released during condensation adds to buoyancy
– During the cumulus stage, the updraft is strong enough to keep
water droplets and ice crystals suspended
 As a result, precipitation does not occur in the cumulus stage
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Thunderstorm Life Cycle
 Mature Stage – maximum intensity
– Stage typically lasts about 10-20 minutes
– Begins when precipitation reaches Earth’s surface
– Features heaviest rain, frequent lightning, strong surface
winds, and possible tornadoes
– Weight of droplets and ice crystals overcome the updraft
– Downdraft created when precipitation descending
through the cloud drags the adjacent air downward
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 Entrained dry air at the edge of the cloud leads to evaporative
cooling, which weakens the buoyant uplift and strengthens the
downdraft
 At the surface, the leading edge of downdraft air resembles a
miniature cold front and is called a gust front
 Ominous-appearing low clouds associated with a gust front
include a roll cloud and a shelf cloud
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Thunderstorm Life Cycle
Roll cloud
Thunderstorms can develop along
gust fronts ahead of the main storm
Shelf Cloud
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Thunderstorm Life Cycle
 Dissipating Stage
– Precipitation and the downdraft spread
throughout the thunderstorm cell, heralding the
cell’s demise
– Subsiding air replaces the updraft and cuts off
the supply of moisture
– Adiabatic compression warms the subsiding air
and the clouds gradually vaporize
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Thunderstorm Classification
NOAA classification of thunderstorms, and the likelihood of severe weather.
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Thunderstorm Classification
 Thunderstorms are meso-scale convective systems (MCS)
and are classified based on the number, organization, and
intensity of their constituent cells
 Single-cell thunderstorms
– Usually a relatively a weak system forming along a boundary within
an air mass (i.e., gust front)
– Typically completes its life cycle in 30 minutes or less
 Multicellular thunderstorms
– Characterizes most thunderstorms. Each cell may be at a different
stage in its life cycle, and a succession of cells is responsible for a
prolonged period of thunderstorm weather.
– Two types:
 Squall line
 Mesoscale convective complex
 Either can produce severe weather
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Thunderstorm Classification
A thunderstorm may track at some angle to the path of its constituent cells,
complicating the weather system motion. In the above idealized situation,
the component cells of a multicellular thunderstorm travel at about 20
degrees to the eastward moving thunderstorm. As they travel toward the
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northeast, the individual cells progress through their life cycle.
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Thunderstorm Classification
 Multicellular thunderstorm types
– Squall line – elongated cluster of thunderstorm cells that is
accompanied by a continuous gust front at the line’s leading edge
 Most likely to develop in the warm southeast sector of a mature
extra-tropical cyclone, ahead of and parallel to the cold front
– Mesocyclone convective complex (MCC)
 A nearly circular cluster of many interacting thunderstorm cells
with a lifetime of at least 6 hrs, and often 12-24 hrs
 Thousands of times larger than a single cell
 Primarily warm season phenomena (March – September)
 Usually develop at night over the eastern 2/3 of the U.S.
 Is not associated with a front
 Usually develops during weak synoptic-scale flow, often
develops near an upper-level ridge of high pressure, and on the
cool side of a stationary front
 A low level jet feeds warm humid air into the system
– Supercell thunderstorms are long-lived single cell storms
 Exceptionally strong updraft, with rotational circulation that may
evolve into a tornado
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Thunderstorm Classification
Radar image of a squall line
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Infrared satellite image showing a MCC
over the south-central U.S.
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Where and When
 Conditions necessary for thunderstorms to
develop include:
– Humid air in the low- to mid-troposphere
 Often mT air when that air mass is destabilized
– Atmospheric instability
 mT air becomes unstable when lifted to the convective
condensation level
– A source of uplift
 Along fronts, up mountain slopes, or via horizontal
convergence of surface winds
 The more humid the air, the less uplift needed to
destabilize it
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Where and When
 Solar heating drives atmospheric convection
– Thunderstorms are most frequent when and where solar
radiation is most intense
– Also storms are most frequent during the warmest part of
the day
 There are many exceptions
– Example - the low-level jet stream up the
Missouri/Mississippi River Valleys at night contributes to
nocturnal thunderstorm maximum
 Thunderstorm frequency is often expressed in
thunderstorm days per year
– This is merely a count of the number of days in which
thunder is heard
– This does not account for days with multiple lines of
© AMS thunderstorms passing over a weather station
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Where and When
 In the tropics and subtropics, intense solar heating may
combine with converging surface winds to trigger
thunderstorm development
– This combination characterizes the ITCZ
 In North America, thunderstorm frequency increases from
north to south
– Highest frequency over central Florida due to convergence of sea
breezes
– Second highest frequency over portions of the Rocky Mountain Front
Range due to topographically related differences in heating
 Thunderstorms are unusual over coastal areas downwind
from relatively cold ocean waters (i.e., coastal California)
 Infrequent in Hawaii due to trade wind inversion
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Severe Thunderstorms
 A severe thunderstorm is accompanied by locally
damaging surface winds, frequent lightning, or
large hail
– Surface winds stronger than 50 kts (58 mph) and/or
hailstones 0.75 in. (1.9 cm) or larger in diameter
– May also produce flash floods or tornadoes
 What causes some thunderstorms to be severe?
– Key is vertical wind shear, the change in horizontal wind
speed and direction with increasing altitude
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 Weak vertical wind shear favors short-lived updrafts, low cloud
tops, and weak thunderstorms
 Strong vertical wind shear favors vigorous updrafts, great
vertical cloud development, and severe thunderstorms
 With increasing vertical wind shear, the inflow of warm humid air
is sustained for a longer period because the gust front cannot
advance as far from the cell. Also, most precipitation falls
alongside the titled updraft, sustaining the updraft.
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Severe Thunderstorms
A synoptic weather pattern that favors development of
severe thunderstorms. A dryline is the western boundary of
the mT air mass and brings about uplift in a manner similar
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to a cold front.
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Severe Thunderstorms
 The polar front jet stream produces strong vertical
wind shear
– This maintains a vigorous updraft
– This supports great vertical development of
thunderstorms
– The jet contributes to stratification of air that increases
the potential instability of the troposphere
 A jet streak induces both horizontal divergence and
convergence of air in the upper troposphere
 Convergence occurs in the right front quadrant of a jet streak,
causing weak subsidence of air
 Sinking air is compressionally warmed and forms an inversion
(capping inversion) over the mT air mass
 The underlying air mass becomes more humid
 Contrast between air layers mounts
 All that is needed is a lifting mechanism for severe weather to
occur
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Severe Thunderstorms
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A temperature sounding that favors the development of severe
thunderstorm cells. A capping inversion separates subsiding
dry air aloft from warm, humid air near the surface.
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Severe Thunderstorms
Mammatus clouds occur on the underside of a thunderstorm anvil and
sometimes indicate a severe storm system. Their appearance is caused by blobs
of©cold,
cloudy air that descend from the anvil into the clear air beneath the anvil.
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Thunderstorm Hazards
 Lightning
– A brilliant flash of light
caused by an electrical
discharge within a
cumulonimbus cloud or
between the cloud and
Earth’s surface
– Direct hazard to human life
– Ignites forest and brush fires
– Very costly to electrical
utilities
– Lightning detection network
provides real-time
information
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Thunderstorm Hazards
 Lightning
– What causes lightning?
 Large differences in electrical charge develop within a cloud,
between clouds, or between a cloud and the ground
– Upper portion and much smaller region of the cumulonimbus cloud
become positively charged, with a disk-shaped zone of negative
charge in between. A positive charge is induced on the ground
directly under the cloud
 Lightning may forge a path between oppositely charged regions
 Charge separation within a cloud may be due to collisions
between descending graupel striking smaller ice crystals in their
path. At temperatures < -15 °C (5 °F) graupel become
negatively charged while ice crystals become positively
charged. Vigorous updrafts carry ice crystals to upper portions
of the cloud.
 Positive charge near cloud base also due to graupel-ice crystal
collision, but temps > -15 °C (5 °F) induce positive charge to
graupel and negative charge to ice crystals
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Thunderstorm Hazards
 Lightning
– A cloud-to-ground lightning flash involves a regular sequence of
events
 Stepped ladders: streams of electrons surge from the cloud base to the
ground in discrete steps
 Return stroke: forms as an ascending electric current when the positive
and negative charges recombine; often emanates from tall, pointed
structures
 Dart leaders, subsequent surges of electrons from the cloud, follow the
same conducting path
 Sequence takes place in < two-tenths of a second
– Lightning causes intense heating of air so rapidly that air density
cannot initially respond
 Shock wave is generated and propagates outward, producing sound
waves heard as thunder
– Flash-to-bang method: Thunder takes about 3 seconds to travel 1
km (or 5 seconds to travel 1 mi)
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 If you must wait 9 seconds between lightning flash and thunderclap,
the lightning is about 3 km (1.8 mi) away
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Thunderstorm Hazards - Lightning
Steps in a cloud-to-ground lightning discharge
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Thunderstorm Hazards - Lightning
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Thunderstorm Hazards
 Downbursts
– Exceptionally strong downdrafts that occur with or without rain
– Starburst pattern causes ground destruction
– Also very dangerous to aircraft because they trigger wind shear
 Aircraft have warning systems that use the same principle as Doppler radar
– A macroburst cuts a swath of destruction > 4 km (2.5 mi) wide with surface
winds that may top 210 km per hr (130 mph)
– A microburst is smaller and shorter lived
– Derecho: a family of straight-line downburst winds that may be hundreds of
kilometers long; sustained winds in excess of 94 km per hr (58 mph)
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Thunderstorm Hazards
 Flash Floods
– Short-term, localized, often unexpected rise in stream
level usually in response to torrential rain falling over a
relatively small geographical area
– Caused by excessive rainfall in slow moving or
stationary thunderstorm cells
– Atmospheric conditions that favor flash floods:
 More common at night and form in an atmosphere with weak
vertical wind shear and abundant moisture through great depths
 Precipitation efficient atmosphere has high values of
precipitable water and relative humidity and a thunderstorm
cloud base with temperatures above freezing
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Thunderstorm Hazards
A hydrograph
showing
changes in
gauge level and
discharge in
response to a
heavy rain
event. On the
top graph,
precipitation and
runoff
are shown in 6hr intervals.
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Thunderstorm Hazards
 Flash Floods
– Especially hazardous in mountainous
terrain
 Big Thompson Canyon, CO flood on 31
July 1976
 Fort Collins, CO flood on 28 July 1997
– Urban areas are prone to flash floods
during intense downpours
 Concrete and asphalt city surfaces
impervious to water and elaborate
storm sewer systems may be unable to
handle excess runoff
– Flash floods can also be caused by
breaching of a dam or levee, or by the
sudden release of water during
breakup of a river ice jam
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Thunderstorm
HazardsFlash Floods
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Thunderstorm Hazards
 Hail
– Frozen precipitation in the form of balls or lumps of ice >
5 mm (0.2 in.) in diameter, called hailstones
– Almost always falls from cumulonimbus clouds that are
characterized by a strong updraft, great vertical
development, and an abundance of supercooled water
– Develops when an ice pellet is transported vertically
through portions of the cloud containing varying
concentrations of supercooled water droplets
 Composed of alternating layers of glaze and rime
– Grows by accretion (addition) of freezing water droplets
and falls out of cloud base when it becomes to large and
heavy to be supported by updrafts
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Thunderstorm Hazards - Hail
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Thunderstorm Hazards
 Hail
– May accumulate on the ground in a long, narrow strip
known as a hailstreak; typically 2 km (1.2 mi) wide and
10 km (6.2 mi) long
– The figure below is a model of hailstreak development
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Thunderstorm Hazards
 Hail
– In the U.S. each year, hail causes an average $1 billion in damage,
mostly to crops, livestock, and roofs
 Farmers cope with hazard by purchasing insurance
– The figure below shows that the annual number of severe hail
reports has increased exponentially due to greater public
awareness, easier report filing, and other factors
– In the U.S., severe hail is most likely in tornado alley
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Tornadoes
 About 10% of the annual
10,000 U.S. severe
thunderstorms produce
tornadoes
 A tornado is a violently
rotating column of air in
contact with the ground
 Most are small and shortlived and often strike
sparsely-populated regions
 The most prolific tornado
outbreak on record occurred
over the Great Plains and
Midwest on 29-30 May 2004
– 170 tornadoes were
reported
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Tornado Characteristics
 The most striking characteristic is the funnel-shaped cloud
composed of tiny water droplets
– If the vortex remains aloft, it is called a funnel cloud
– If the cloud extends to the ground, it is called a tornado
– The funnel cloud forms in response to the steep pressure gradient
directed from the tornado’s outer edge towards its center
 A weak tornado’s path is typically < 1.6 km (1 mi) long and
100 m (330 ft) wide with a lifetime of a few minutes
 A violent tornado can have a path > 160 km (100 mi) long
and 1.0 km (3000 ft) wide with a lifetime of 10 min to > 2 hrs
– Wind speeds may be up to 500 km per hr (300 mph)
 Most are spawned by and travel with severe thunderstorms
 Usually track from SW to NE, but may go any direction
– Average forward speed is 48 km per hr (30 mph)
 An exceptionally great horizontal air pressure gradient is
responsible for a tornado’s vigorous circulation
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Where and When
 Tornadoes have occurred in all 50 states, but most occur in
tornado alley, a N-S corridor stretching from eastern Texas
and the Texas Panhandle northward to southeastern South
Dakota
 Weak tornadoes are more likely over flat than rough terrain,
but strong to violent tornadoes are largely unaffected by
terrain
 U.S. typically has 1300 tornadoes/yr
 Peak activity in May and June due to in part to:
– Relative instability of the lower atmosphere
– Favorable synoptic weather conditions: well-defined polar front and
intense cyclones
 Center of maximum tornado frequency follows the sun
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Where and When
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Average number of tornadoes per 10,000 square
miles by state, 1999-2008
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Tornado Hazards and the EF-Scale
 Tornadoes are a threat to people and property
because of
– Extremely high winds




Blow down structures
Flying debris main cause of death and injury
Multi-vortex tornadoes are most destructive
It is no longer recommended that windows be opened; most
buildings have sufficient air leaks so that a potentially explosive
pressure differential never develops
– A strong updraft
– Subsidiary vortices
– An abrupt drop in air pressure
 Some tornadoes consist of two or more subsidiary
vortices that orbit each other or a common center
– These multi-vortex tornadoes are the most destructive
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Model of tornado with multiple subsidiary vortices
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Tornado Hazards and the EF-Scale
 The F-scale was revised in 2007 as
the EF-scale (Enhanced F-Scale)
– The EF-scale is based on
rotational wind speeds
estimated from property
damage
– Ranges from EF0 to EF5
 EF5 tornadoes are rare
 About 77% of tornadoes in the U.S. are
considered weak (EF0 to EF1) and 95% are
below EF3
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Tornado Hazards and the EF-Scale
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Tornado Hazards and the EF-Scale
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Path of the Wichita-Andover, Kansas tornado of 26
April 1991. Numbers along the path indicate rating on
the F-scale.
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The Tornado-Thunderstorm Connection
 Most violent tornadoes come from supercells
– Very energetic with updraft speeds sometimes in excess
of 240 km per hr (150 mph); they can last for several
hours and produce more than one tornado
 Supercell formation and characteristics
– Horizontal wind exhibits strong speed and directional
shear, which causes air to rotate about a horizontal axis.
The updraft tilts tube of rotating air to vertical and a
mesocyclone is formed.
– A roughly circular lowered portion of the rain-free base
of a thunderstorm, called a wall cloud, often
accompanies a mesocyclone
 Most wall clouds do not produce tornadoes
 Tornadic wall clouds have strong and persistent rotation before
the appearance of a tornado
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The Tornado-Thunderstorm Connection
 Supercell formation and characteristics, cont.
– A mesocyclone circulation is most intense at 6100 m
(20,000 ft); in a tornadic supercell it narrows and builds
downward towards the ground
 As the spinning column of air narrows its circulation increases,
similar to an ice skater spinning faster as she pulls in her arms
 A tornado typically appears near the updraft and toward the rear
of a supercell
– As the tornadic circulation descends to the surface, a
downdraft develops near the rear edge of the supercell.
Eventually the downdraft surrounds the tornado and the
tornado dissipates
 Potentially destructive tornadoes can also develop
in multi-cellular thunderstorm clusters and
hurricanes
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The Tornado-Thunderstorm Connection
Schematic view of a tornadic supercell
thunderstorm
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Plan view of a tornadic supercell
thunderstorm
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The Tornado-Thunderstorm Connection
A thunderstorm wall cloud may accompany a mesocyclone,
but most do not produce a tornado
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Monitoring Tornadic
Thunderstorms
 Direct monitoring is generally not feasible
 Instead, storm chasers rely on photography, balloon-borne
instruments that monitor surrounding atmospheric
conditions, and portable Doppler radar to detect circulation
within supercells
 Doppler radar
– In the reflectivity mode, can show a hook echo when the parent
mesocyclone is present
– In the velocity mode it monitors circulation; a tornado circulation
may show up as a tornado vortex signature (TVS), a small region of
rapidly changing winds within a mesocyclone
 Storm spotters and visual surveillance of thunderstorms
are still essential
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Monitoring Tornadic Thunderstorms
A
B
On 3 May 1999, an F-5 tornado devastated Moore, OK. (A) is the
weather radar image showing the hook echo associated with this
tornado. In (B), a tornado vortex signature (TVS) is visible near
Moore. [NOAA/NWS/Storm Prediction Center]
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Monitoring Tornadic Thunderstorms
The annual number of reports of tornadoes in the U.S.
1950-2008
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