Severe Weather Guide

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Severe Weather Guide

• How to Recognize, Identify, and Report Severe
  Weather

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Definitions and Terms

• Watch conditions are favorable for severe
  weather
• Warning severe weather is currently occurring in
  the area
• Watches and warnings are issued for severe
  thunderstorms, hail, flash floods, and tornadoes

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Thunderstorms

• Every Thunderstorm Needs
• Moisture
• Unstable Air (warm air to rise rapidly)
• Lift (cold front)
• There are three stages in the life cycle of a
  thunderstorm
• Developing (Cumulus) stage
• Mature stage
• Dissipating stage

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Thunderstorms

• Developing Stage
• Rising cumulus clouds strong updraft
• Little if any precipitation
• Lasts about 10 minutes
• Occasional lightning

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Thunderstorms

• Mature Stage
• Updraft and downdraft
• Most likely time for heavy rain, frequent
  lightning, strong winds, and hail
• Tornado development is possible
• Averages 10-20 minutes, but can last a few hours

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Thunderstorms

• Dissipating Stage
• Weakened updraft
• Rainfall lessens in intensity
• Lightning and strong winds remain a threat

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Types of Thunderstorms

• Single Cell (Pulse)
• Generally weak, short lived, and poorly organized
• Multicell Cluster
• Most common type
• Series of cells moving as one unit
• Multicell Line
• AKA Squall Line
• Long line of storms with gust front at leading
  edge
• Supercell
• Very strong and produce severe weather

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Types of Thunderstorms

• A thunderstorm is classified as severe if it has
  any of the following characteristics
• Hail greater than 0.75 in diameter (dime size)
• Winds greater than 58 miles per hour
• Tornado

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Single Cell Storm

• 20-30 minutes
• Rarely turn severe
• Heavy rainfall and weak tornadoes are still
  possible
• Poorly organized

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Single Cell Storm
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Multicell Cluster Storm

• Most common type of thunderstorm
• Each cell in the cluster is at a different stage
  of the thunderstorm life cycle
• Each cell may last 20 minutes, but each cluster
  can last several hours
• Heavy rain, downbursts, moderate sized hail,
  occasional weak tornadoes

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Multicell Cluster Storm
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Multicell Cluster Storm
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Multicell Line Storm

• Squall Line
• Long line of storms with a continuous, well
  developed gust front at leading edge of the line
• Heaviest rain is at center of line
• Produce heavy rain, hail, and tornadoes
• Strong downbursts can cause line to bend and
  become a bow echo

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Multicell Line Storm
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Multicell Line Storm
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Multicell Line Storm Bow Echo
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Multicell Line Storm Bow Echo
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Supercell Thunderstorm

• Rarest type of thunderstorm, but the most
  dangerous
• The updraft rotates (called mesocyclone)
• Large hail
• Heavy downpours
• Strong downbursts
• Strong to violent tornadoes

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Supercell Thunderstorm (w/o Tornado)
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Supercell Thunderstorm (w/Tornado)
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Supercell Thunderstorm (w/Tornado)
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Supercell Thunderstorm
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Dangers of Thunderstorms

• Flash Floods
• Lightning
• Hail
• Downbursts
• Tornadoes

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Flash Floods

• 1 cause of death associated with thunderstorms
• An average of 140 fatalities every year (in US)
• Definition a rapid rise in water (creeks,
  streams, drainage ditches) within 12 hours of a
  period of heavy rain
• As little as 6 inches can knock a human over
• Two feet of water can move a car
• Turn Around, Dont Drown
• Get to higher ground immediately

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Lightning

• Lightning occurs in all thunderstorms
• Causes an average of 80 fatalities and 300
  injuries per year (in the US)
• Lightning strikes the tallest object
• If caught outside crouch down in a ball
• 30/30 Rule
• Go indoors if you hear thunder before counting to
  30 after you see lightning
• Wait inside for 30 minutes after you last hear
  thunder

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Hail

• Rarely causes fatalities, but causes significant
  damage to property and crops
• Can fall at rates up to 100 miles per hour
• Created by strong updrafts in thunderstorm

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Hail

• Sizing Chart
• Pea 0.25
• Penny/Dime (Severe Criteria) 0.75
• Nickel 0.88
• Quarter 1.00
• Half Dollar 1.25
• Ping Pong Ball 1.50
• Golf ball 1.75
• Hen Egg 2.00
• Tennis Ball 2.50
• Baseball 2.75
• Grapefruit 4.00
• Softball 4.50

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Downbursts

• A strong downdraft with an outrush of damaging
  winds at the surface
• Winds can reach 100 miles per hour or more
• Straight line winds
• Winds speed and direction can change rapidly

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Downbursts
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Downbursts

• Wind Speed Estimates (mph)
• 25-31 large branches in motion whistling in
  telephone wires
• 32-38 whole trees in motion
• 39-54 twigs break off of trees wind impedes
  walking
• 55-72 damage to chimneys and TV antennas pushes
  over shallow rooted trees
• 73-112 peels surface off roofs windows broken
  trailer houses overturned
• 113 roofs torn off houses weak building
  destroyed large trees uprooted

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Tornado Look-A-Likes

• Several cloud formations are associated with a
  thunderstorm that can be confused with an actual
  tornado
• Wall clouds
• Shelf clouds
• Roll clouds
• A roll cloud is similar to a shelf cloud, but it
  is detached from the main parent cloud whereas
  a shelf cloud is part of the main storm cloud
• Scud Clouds
• Detached and wind torn similar shape to
  wall/funnel clouds

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Wall Clouds versus Shelf Clouds
Wall Cloud Shelf Cloud
Suggest Inflow/Updraft Suggest Outflow/Downdraft
Maintain position with respect to rain Move away from rain
Slope upward away from precipitation Slope downward away from precipitation
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Wall Cloud
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Wall Cloud

• What is the tornado potential for a wall cloud?
• It will be consistent, lasting 10-20 minutes
• It will have persistent rotation
• Strong winds will blow into the wall cloud from
  the south or southeast (25-35 mph)
• It will exhibit evidence of rapid vertical motion
• These are rules of thumb there are always
  exceptions!

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Shelf Cloud
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Roll Cloud
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Scud Cloud
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Tornadoes

• A tornado is a violently rotating column of air
  in contact with the ground extending from a
  thunderstorm
• May appear transparent until dirt and debris are
  picked up in the vortex or until a condensation
  cloud forms
• A tornado that forms over a body of water is
  called a waterspout

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Tornadoes

• Tornadoes can occur at any time, any day, and in
  any state
• They are most common
• In tornado alley Texas north to Nebraska and
  east to Indiana
• During the spring and summer months
• During the late afternoon and early evening

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Average Number of Tornadoes Per Year
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Average Number of Tornadoes Per Month
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Tornadoes By Hour of Day
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Tornado Life Cycle

• 1. Funnel Cloud extending from wall cloud, but
  not yet in contact with the ground
• 2. Mature Tornado
• 3. Rope Stage the dissipating stage
• Tornadoes are dangerous during all stages

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Funnel Cloud
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Mature Stage
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Rope Tornado
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Tornado Characteristics
Weak Tornadoes Strong Tornadoes Violent Tornadoes
of Tornadoes 88 11 lt1
of Tornado Deaths lt5 30 70
Duration 1-10 minutes 20 minutes Can exceed 1 hour
Path Length Up to 3 miles 15 miles 50 miles
Winds lt110 mph 110-205mph gt205mph
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Tornadoes

• Tornadoes are always dangerous regardless of
  shape, size, or color
• Large cities and mountains are just as prone to
  tornadoes as wide open fields
• Average tornado speed is 30mph, but they can move
  as fast as 70mph
• Do not attempt to outrun a tornado, find shelter
  immediately

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Tornado Fujita Scale
F Scale Type Strength Winds
0 Gale Weak 40-72 mph
1 Moderate Weak 73-112 mph
2 Significant Strong 113-157 mph
3 Severe Strong 158-206 mph
4 Devastating Violent 207-260 mph
5 Incredible Violent 261-318 mph
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Severe Weather Alerts

• During periods of severe weather it is important
  to keep an eye on the sky
• Local television, radio, and the Internet are
  vital sources of information
• A S.A.M.E NOAA weather radio is essential for
  immediate watches and warnings