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Severe Weather Guide. How to Recognize, Identify, and Report Severe Weather. Definitions and Terms. Watch : conditions are favorable for severe weather Warning : severe weather is currently occurring in the area
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Severe Weather Guide How to Recognize, Identify, and Report Severe Weather
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
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
Thunderstorms • Developing Stage • Rising cumulus clouds; strong updraft • Little if any precipitation • Lasts about 10 minutes • Occasional lightning
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
Thunderstorms • Dissipating Stage • Weakened updraft • Rainfall lessens in intensity • Lightning and strong winds remain a threat
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
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
Single Cell Storm • 20-30 minutes • Rarely turn severe • Heavy rainfall and weak tornadoes are still possible • Poorly organized
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
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”
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
Dangers of Thunderstorms • Flash Floods • Lightning • Hail • Downbursts • Tornadoes
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, Don’t Drown” • Get to higher ground immediately
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
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
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”
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
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
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
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!
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
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
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
