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 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 1” in diameter (dime size) –Winds greater than 58 miles per hour –Tornado

Single Cell Storm minutes Rarely turn severe Heavy rainfall and weak tornadoes are still possible Poorly organized

Single Cell Storm

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 Cluster Storm

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”

Multicell Line Storm

Multicell Line Storm – 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

Supercell Thunderstorm (w/o Tornado)

Supercell Thunderstorm (w/Tornado)

Supercell Thunderstorm

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 –Pea0.25” –Penny/Dime (Severe Criteria)0.75” –Nickel0.88” –Quarter1.00” –Half Dollar1.25” –Ping Pong Ball1.50” –Golf ball1.75” –Hen Egg2.00” –Tennis Ball2.50” –Baseball2.75” –Grapefruit4.00” –Softball4.50” recordhail

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 Clouds versus Shelf Clouds Wall CloudShelf 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

Wall Cloud What is the tornado potential for a wall cloud? –It will be consistent, lasting 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!

Shelf Cloud

Roll Cloud

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