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 1” 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
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 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
Multicell Line Storm – Bow Echo
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 (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
–
–
–
–
–
–
–
–
–
–
–
–
Pea
Penny/Dime (Severe Criteria)
Nickel
Quarter
Half Dollar
Ping Pong Ball
Golf ball
Hen Egg
Tennis Ball
Baseball
Grapefruit
Softball
http://www.srh.noaa.gov/oun/?n=events20110523-recordhail
0.25”
0.75”
0.88”
1.00”
1.25”
1.50”
1.75”
2.00”
2.50”
2.75”
4.00”
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
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 Cloud
Shelf Cloud
Suggest Inflow/Updraft
Suggest
Outflow/Downdraft
Maintain position with
respect to rain
Move away from rain
Slope upward away from Slope downward away
precipitation
from precipitation
Wall Cloud
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!
Shelf Cloud
Roll Cloud
Scud Cloud
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
Average Number of Tornadoes Per Year
Average Number of Tornadoes Per Month
Tornadoes By Hour of Day
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
Funnel Cloud
Mature Stage
Rope Tornado
Tornado Characteristics
Weak
Tornadoes
Strong
Tornadoes
Violent
Tornadoes
% of Tornadoes
88%
11%
<1%
% of Tornado
Deaths
<5%
~30%
70%
Duration
1-10+ minutes
20+ minutes
Can exceed 1
hour
Path Length
Up to 3 miles
15+ miles
50+ miles
Winds
<110 mph
110-205mph
>205mph
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
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
261-318 mph
http://www.spc.noaa.gov/faq/tornado/efscale.html
Violent
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