Weather Spotter’s Field Guide

A Guide to Being a SKYWARN® Spotter

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Weather Service

June 2011

R

© Roger Edwards

Section 1

The Spotter’s Role

The SKYWARN® Spotter and 

the Spotter’s Role

The United States is the most severe weatherprone country in the world. Each year, people 

in this country cope with an average of 

10,000 thunderstorms, 5,000 floods, 1,200 

tornadoes, and two landfalling hurricanes. 

Approximately 90% of all presidentially 

declared disasters are weather-related, 

causing around 500 deaths each year and 

nearly $14 billion in damage.

SKYWARN® is a National Weather Service 

(NWS) program developed in the 1960s 

that consists of trained weather spotters who provide reports of severe 

and hazardous weather to help meteorologists make life-saving warning 

decisions. Spotters are concerned citizens, amateur radio operators, truck 

drivers, mariners, airplane pilots, emergency management personnel, and 

public safety officials who volunteer their time and energy to report on 

hazardous weather impacting their community. 

Although, NWS has access to data from Doppler radar, satellite, and 

surface weather stations, technology cannot detect every instance of 

hazardous weather. Spotters help fill in the gaps by reporting hail, wind 

damage, flooding, heavy snow, tornadoes and waterspouts. Radar is an 

excellent tool, but it is just that: one tool among many that NWS uses. 

We need spotters to report how storms and other hydrometeorological 

phenomena are impacting their area. 

SKYWARN® spotter reports provide vital “ground truth” to the NWS. 

They act as our eyes and ears in the field. Spotter reports help our 

meteorologists issue timely, accurate, and detailed warnings by confirming 

hazardous weather detected by NWS radar. Spotters also provide critical 

verification information that helps improve future warning services. 

SKYWARN® Spotters serve their local communities by acting as a vital 

source of information when dangerous storms approach. Without spotters, 

NWS would be less able to fulfill its mission of protecting life and property. 

1

R
SEVERE LOCAL STORMS

Spotter’s Field Guide

2

Spotter Reporting Procedures

Effective spotter reports are a critical component of NWS severe weather 

operations. NWS meteorologists use science, technology, training, 

experience, and spotter reports when making warning decisions. An 

effective spotter report is one that is timely, accurate, and detailed. Spotters 

should use the following guidelines when reporting:

 Follow the specific reporting guidelines for your area.

 Remain calm, speak clearly, and do not exaggerate the 

facts.

 If you are unsure of what you are seeing, make your 

report, but also express your uncertainty.

 Your report should contain the following information:

• WHO you are: trained spotter

• WHAT you have witnessed: the specific weather 

event

• WHEN the event occurred: NOT when you make 

your report

• WHERE the event occurred, (not necessarily your 

location) using well known roads or landmarks

Immediate, real-time reports, are most helpful for warning operations, but 

delayed reports are also important, even days after an event. Delayed 

reports are used for climatological and verification purposes. 

Weather events should be reported according to the instructions provided 

by your local NWS office. Here are some general guidelines on what to 

report.
Section 1

The Spotter’s Role

Tornadoes

 What damage did you observe?

 How long was it on the ground? When did it start 

and end?

 How wide was it? How far did it travel if known? 

Flash Flooding

 Report flooded roadways, rivers and streams, giving 

approximate water depth.

 Does the flooding consist of standing water or is it 

flowing?

 Is the water level continuing to rise, staying steady or 

falling? 

 Is the flooding occurring in a known flood prone area?

 Any damage from the flooding or mud slides? 

Wall Clouds

 Report if clouds are rotating and how long they have 

existed.

Funnel Clouds

 Watch for organization, persistence and rotation. 

Lightning

 Only report lightning when damage or injuries occur.

Winter Weather

 Report any occurrence of freezing rain, ice accumulation 

and damage. 

 How much heavy snow accumulation is there and is 

there any damage? 

 Do blizzard conditions exist: winds 35 mph or more AND 

visibility 1⁄4 mile or less? 

3
SEVERE LOCAL STORMS

Spotter’s Field Guide

Wind

 Report estimated or measured wind speed and 

wind damage.

 Wind speed estimation is difficult. A detailed description 

of moving objects or damage is often more useful. 

 Details to submit for tree damage:

• What is the height and diameter of the branch, limb 

or tree that was broken or blown down?

• Was the tree healthy or decayed?

• What type of tree was damaged, e.g., hardwood 

or softwood?

 Details to submit for damage to structures.

• Is the damage to a well-built structure or a 

weak outbuilding?

• What is the main building material for the structure: 

wood, brick, metal, concrete, etc.?

• If the structure is a mobile home, was it 

anchored down? 

Hail

 Report the size of the largest stone and any damage.

• To estimate size, compare hail to well known objects 

such as coins or balls, but not to marbles, or measure 

the hail with a ruler. 

4
Section 1

The Spotter’s Role

Marine Hazards

Report the following marine events:

 Waterspouts: you must observe rotation

 Squall lines

 Heavy freezing spray 

 Wave heights and winds that differ significantly from 

forecasted conditions

 Hydrometeorological phenomena that are not in the 

current marine forecast, e.g., thunderstorms, dense fog 

 Waves greater than twice the size of surrounding waves

 Tsunami inundation and any damage 

 Coastal Flooding: Inundation of people, buildings, and 

coastal structures on land at locations that under normal 

conditions are above the level of high tide 

 Lakeshore Flooding: Inundation of land areas along 

the Great Lakes over and above normal lake levels 

 High Surf: Large waves breaking in the surf zone with 

sufficient energy to erode beaches, move large logs, 

wash over jetties or exposed rocks, etc. 

Other Environmental Hazards

■ Dense fog: visibility 1⁄4 mile or less

■ Dust storms: visibility 1⁄4 mile or less

■ Volcanic ash accumulation and any damage

■ Any injuries or fatalities as a direct result of weather

5
SEVERE LOCAL STORMS

Spotter’s Field Guide

Spotter Safety Tips

The environment in and around severe storms is a dangerous place. 

Even though tornadoes are an obvious danger, other life-threatening 

thunderstorm hazards, such as lightning and flash floods can be just as 

deadly. Spotter reports are vital to your community and the NWS, but your 

safety should be your number one priority! 

Before venturing out, you need to be aware of the hazards of thunderstorms 

and the recommended practices to minimize risk. As a spotter, it is your 

responsibility to stay safe while spotting. Please following the guidelines 

below for your personal safety and for the safety of those around you.

 Personal safety is the primary objective of every spotter. 

 Adhere to the concept of ACES, defined below, at all 

times.

 Obey federal, state, and local laws and directives from 

public safety officials. 

 Never put yourself in harm’s way. This includes 

attempting to walk or drive over obstructions such 

as flooded roadways and downed power lines, and 

positioning yourself under objects that have a potential 

to fall or be blown over due to severe weather.

ACES stands for Awareness, Communication, Escape Routes, 

and Safe Zones. ACES is a concept commonly used by emergency 

management personnel. If you remember ACES, you can remain safe in 

any situation, including spotting.

6

Bolivar Peninsula, TX, September 20, 2008: Damaged houses, debris and downed power 

lines resulting from Hurricane Ike. Photo by Jocelyn Augustino, FEMA.
Section 1

The Spotter’s Role

Awareness means you are constantly observing the situation around 

you. This type of observation is sometimes referred to as situational 

awareness. Continuously monitoring the risks around you can save your 

life, especially in rapidly changing weather conditions. Knowing that there 

is a river crossing, or observing the street is lined with power poles and 

trees, can prepare you for the hazards of severe weather. When you are 

aware of the imminent threats, and you are thinking ahead about possible 

outcomes, you can position yourself better to minimize these threats.

Communicating your whereabouts to others on a regular basis and 

having multiple lines of communication available can keep you and others 

safe from hazards.

Escape Routes are vital when you are entering a potentially dangerous 

area. As part of awareness, note the escape routes available to you, 

making sure you always have more than one and the safest way to get to 

that escape route.

If the event you cannot get to escape routes due to rapidly changing 

conditions, find your closest safe zones or shelters. Safe zones are the 

areas where you will be safest if you need to get to immediate shelter. 

Knowing these locations will limit your risk.

Remembering ACES: to remain aware of your surroundings, have open 

lines of communication, know your escape routes, and know your safe 

zones wherever you are can increase your safety.

Here are a few basic tips that could save your life if you are watching a 

storm from your vehicle. 

 Keep a buffer zone between you and the storm to 

allow for changes in storm movement and to keep your 

options open for an escape route.

 Travel in pairs so the driver can concentrate on driving, 

and so you can observe multiple areas of the storm.

 Always know where you are in relation to the storm, and 

which way the storm is moving. Remember that storms 

can change direction and speed.

 Never drive through the core of the storm, e.g., through 

heavy rain and/or hail, to get a better vantage point. 

7
SEVERE LOCAL STORMS

Spotter’s Field Guide

 Have a source of current local weather information, such 

as a NOAA Weather Radio, to be sure you have critical 

storm information.

 If possible, observe a storm from a four-way intersection 

to facilitate escape in multiple directions.

 Nighttime storm spotting in a vehicle can be dangerous 

and is not recommended. It is difficult to observe key 

storm features at night and harder to safely maneuver 

around the storm. 

 Be alert for emergency vehicles, pedestrians, and other 

road and traffic hazards. 

 If you stop, remember to watch for traffic and to be 

aware of the potential effects of the storm. Pull out of 

traffic and keep away from trees, power lines, and signs. 

 Never stay in a vehicle under large trees or signs in high 

wind conditions. Get into a sturdy structure. 

 Always be prepared for all of the hazards associated 

with thunderstorms. 

 If close to or in the path of a storm, do not turn off 

your vehicle.

8

Photo taken on July 10, 2008, from the passenger seat of a moving vehicle on 

Interstate I-88 near Moline, IL. Photo by Terren.
Section 1

The Spotter’s Role

Tornado Safety

Tornadoes are violently rotating columns of air attached to a thunderstorm 

and in contact with the ground, whether or not a condensation funnel is 

visible to the ground. Debris or dirt swirling on the ground, under an area 

of cloud base rotation, may be a clue that it is a tornado and not a funnel 

cloud or gustnado. The high winds and flying debris associated with a 

tornado pose a significant threat to a spotter. Here are a few safety tips if 

you encounter a tornado:

 Watch for other tornadoes that could form in the vicinity 

of the tornado you are watching. 

 Never try to outrun a tornado in an urban or congested 

area. Immediately get into a sturdy structure after 

parking your car out of the traffic flow. 

 Do not take shelter under bridges or overpasses. These 

structures do not offer protection and could increase the 

chance of injury or death.

 If you are caught outdoors, seek shelter in a basement, 

shelter or sturdy building. If you cannot quickly get to a 

shelter, immediately get into a vehicle, buckle your seat 

belt and try to drive to the closest sturdy shelter. If flying 

debris occurs while you are driving, pull over and park. 

You have the following options as a last resort: 

• Stay in the car with the seat belt on. Put your head 

down below the windows, covering with your hands 

or a blanket if possible. 

• If you can safely get noticeably lower than the level 

of the roadway, exit your car, and lie in that area, 

covering your head with your hands. 

 Flying and falling debris is the biggest hazard in a 

tornado. To be safe, you should get inside, get down and 

cover up. Underground or in a Safe Room is your first 

choice. If no underground shelter is available, get to the 

center of a sturdy building on the lowest level. Put as 

many walls between you and the tornado as possible. 

Stay away from windows and doors. Cover up to help 

minimize being injured by flying or falling debris.

9
SEVERE LOCAL STORMS

Spotter’s Field Guide

Flash Flood Safety

Turn Around Don’t Drown! Thunderstorms can produce torrential rain 

over a short period resulting in flash flooding. Flooding is particularly 

dangerous at night when it is harder to see the road is flooded and even 

harder to tell how deep the water is. Flooding causes more fatalities each 

year than any other thunderstorm hazard. More than half of all flood-related 

drownings occur when a vehicle is driven into hazardous flood water. Use 

these safety tips and facts to avoid being a victim of a flash flood:

 Turn Around Don’t Drown! Do not attempt to drive 

or walk across a flooded road or low water crossing. 

You cannot be sure about the depth of the water or the 

condition of the roadway. The road might be washed out.

 Two feet of moving water will carry away most vehicles. 

 Six inches of fast-moving water can knock you off 

your feet.

 If your vehicle is suddenly caught in rising water, leave it 

immediately and get to higher ground.

 Be especially vigilant at night when flash floods are 

harder to recognize.

10

Street flooding in downtown Reno, NV, 

January 2006. Photo from NOAA.
Section 1

The Spotter’s Role

Lightning Safety

When Thunder Roars, Go Indoors! Lightning is an underrated killer. 

Nearly as many people lose their lives to lightning strikes as they do to 

tornadoes, but because lightning typically hits just one or two people at 

a time, fatalities due to lightning receive less publicity. Lightning occurs 

with every thunderstorm and is the most common weather hazard facing 

spotters. As a spotter, you are frequently positioned in the open or on 

a hill top, making you especially vulnerable to lightning. Here are a few 

important safety guidelines for dealing with lightning:

 Remain in a hard-topped vehicle or an indoor location for 

at least 30 minutes after you hear the last thunder clap. 

If you use radio equipment, avoid contact with it or other 

metal inside your vehicle to minimize the impacts should 

lightning strike.

 If you are out on the water and skies are threatening, 

get back to land and find a fully enclosed building or 

hard-topped vehicle. Boats with cabins offer a safer but 

not perfect environment. You are safer if the boat has a 

properly installed lightning protection system. If you are 

inside the cabin, stay away from metal and all electrical 

components.

 Do not use a corded 

phone during a 

thunderstorm. Use a 

cordless phone or cell 

phone for all calls. 

 Lightning victims do not 

carry an electrical charge, 

are safe to touch, and 

need urgent medical 

attention. If a person 

has stopped breathing, 

call 9-1-1 or your local 

emergency phone number 

and begin CPR if the 

victim is not breathing.

11
SEVERE LOCAL STORMS

Spotter’s Field Guide

Downburst Wind Safety

Damaging thunderstorm straight line winds known as downbursts are 

another hazard facing a spotter. A downburst is a strong downdraft 

with an outrush of damaging winds on or near the ground. Most of the 

wind damage done by severe thunderstorms is caused by downbursts. 

Downburst winds may exceed 100 mph in the most intense storms, and 

may cause damage similar to a tornado. Here are some tips to stay safe 

in and around downbursts:

 Keep a firm grip on your vehicle’s steering wheel to 

maintain control. Downbursts can occur suddenly with 

an abrupt change in wind speed and direction. 

 If you can do so safely, point your vehicle into the wind 

to minimize the risk of the vehicle being blown over.

 Be prepared for sudden reductions of visibility due to 

blowing dust or heavy rain associated with downbursts.

 Point spotters observing from a substantial building 

should move away from windows as the downburst 

approaches.

12

Damage from the Stokesdale tornado and a 79 mph 

downburst wind gust at the Raleigh-Durham International 

Airport during the passage of the remnants of Hurricane 

Ivan. September 17, 2004, Guilford County Emergency 

Management, Stokesdale Fire Department, Doug Schneider, 

Darin Figurskey and Jeff Orrock.
Section 1

The Spotter’s Role

Hail Safety

Large hail can cause serious injuries and damage to vehicles and 

buildings. Although fatalities attributed to hail are rare, it is the costliest 

weather element in the United States with an average of more than a 

billion dollars in agriculture and property damage each year. The costliest 

U.S. hailstorm caused around $2 billion dollars in damage in the St. Louis 

Metropolitan Area on April 10, 2001. Below are some points about hail that 

could minimize damage to your vehicle when storm spotting.

 Substantial structures and buildings such as a garage 

offer the best protection from hail. 

 If in a vehicle, avoid those parts of the storm where large 

hail is occurring.

 Hard-top vehicles offer good protection from hail up 

to about golf ball size. Larger hail stones will damage 

windshields.

A photo—or video—is worth a thousand words! 

Spotters often observe amazing weather phenomena. Whether it is a 

tornado in the distance or ice taking down power lines, your local NWS 

Forecast Office learns a great deal from spotter photos and videos. If you 

are willing to allow the NWS to use your photo or video in our education 

and outreach efforts, please state that NWS has permission to reprint 

when you submit the file to us. If you would like us to credit you on the 

image, we will gladly do that. Multimedia files are a tremendous resource 

when conducting our spotter training courses and our weather safety 

education efforts. 

Send NWS Your Home Weather Station Data Service

The Citizen Weather Observer Program (CWOP) 

is a private-public partnership that allows 

people with computerized weather stations and 

always on Internet access to send their weather 

information to a special data server that collects 

weather observations from around the country. 

Your data can then be used by computer 

forecast models to produce short term forecasts 

(3 to 12 hours into the future) of weather conditions 

in your region. 

13
SEVERE LOCAL STORMS

Spotter’s Field Guide

Visit http://wxqa.com to register for an ID and learn which weather stations 

and software packages work with the CWOP network. 

Check out http://www.met.utah.edu/mesowest from the University of Utah 

to see a plot of surface observations available from home weather stations, 

the National Weather Service, state Departments of Transportation, the 

Federal Aviation Administration, and the U.S. Forest Service.

CoCoRaHS: Community Collaborative 

Rain, Hail and Snow Network

CoCoRaHS is a unique non-profit, communitybased network of volunteers of all ages and 

backgrounds working together to measure and 

map precipitation: rain, hail and snow. By using 

low-cost measurement tools, stressing training 

and education, and accessing an interactive website, volunteers provide the highest quality data 

for natural resource, education and research applications. The program 

is open to anyone who has an interest in weather and would like to share 

their precipitation data with others. Requirements of the program include:

 A 4 inch diameter, high-capacity rain gauge purchased 

through the CoCoRaHS website or from other sources

 A ruler to measure hail size: stone diameter

 Hail pads (not used in all states)

 Internet access; some states have a phone number 

where volunteers without computers can leave a 

message with their precipitation information. The data is 

later entered online.

To join the CoCoRaHS program, go to: http://www.cocorahs.org.

The precipitation data is available for anyone to see online. 

Spotter’s Field Guide

14

SEVERE LOCAL STORMS
Section 2

Thunderstorm Basics

Thunderstorm Basics

Part of the fascination many people have with thunderstorms is the mystery 

that surrounds them. Leading researchers are still learning about many of 

the phenomena associated with thunderstorms. In order to understand 

thunderstorm-related spectacles like tornadoes, lightning, and hail you 

must have a basic knowledge of thunderstorm characteristics.

Thunderstorm Climatology 

At any given moment, there are 

thousands of thunderstorms 

occurring worldwide. Most of 

these storms are beneficial, 

bringing needed rainfall. A 

small percentage of the storms 

become severe, producing 

large hail—1 inch in diameter 

or larger—strong wind gusts of 

58 mph or greater, or 

tornadoes.

Although the area stretching 

from Texas to Minnesota 

has the greatest known 

occurrences of U.S. severe 

thunderstorms, no place in the 

United States is immune to the 

threat of severe weather. 

Thunderstorm Ingredients

All thunderstorms require the following three ingredients: moisture, 

instability, and lift. Organized severe thunderstorm events also require 

vertical wind shear.

Moisture forms the clouds and precipitation in thunderstorms. Primary 

moisture sources include the Atlantic and Pacific Oceans and the Gulf of 

Mexico. The Great Lakes also can provide moisture for thunderstorms. In 

the Midwest, evaporation from farmlands can enhance low-level moisture.

Average annual thunderstorm days per year

15
SEVERE LOCAL STORMS

Spotter’s Field Guide

Atmospheric stability is a measure of the atmosphere's tendency to 

enhance or deter vertical motion. In unstable conditions, a lifted parcel 

of air will be warmer than the surrounding air at that altitude. Because 

it is warmer, it is less dense and can rise more. Thus instability favors a 

storm’s updrafts and downdrafts.

Lift provides the mechanism for the air to rise, starting the thunderstorm 

process. Sources of lift include cold fronts, warm fronts, drylines, 

thunderstorm outflow boundaries, and flow up the slopes of topography.

Vertical wind shear is the change in wind speed and direction with height. 

This effect is typically strongest near the surface, though it can be very 

strong at higher levels in the atmosphere near upper level jets and 

fronts. Generally, the greater the instability, the stronger the updrafts and 

downdrafts may become. The greater the vertical wind shear, the better 

the chance of storms becoming organized and long-lived. Vertical wind 

shear through a deep layer (3-5 miles above ground) also can induce 

rotation—at times intense—in the storm’s mid levels. Storms developing 

in weak-shear environments still can produce brief hail and microbursts, 

and even weak tornadoes. 

Low-level vertical wind shear (in the lowest 1 mile or less of the atmosphere) 

can help to generate low-level rotation in a storm. Tornadoes and severe 

winds are most often related to the strength of the low-level rotation. When 

the low-level shear increases in strength, the strength of the low-level 

rotation increases, along with the likelihood of tornadoes. 

Examples of directional shear (left) and speed shear (right). 

16
Section 2

Thunderstorm Basics

Thunderstorm Life Cycle 

Thunderstorms generally last 30-60 minutes, but because they continue 

to form new updrafts, they can last for over 8 hours. Thunderstorms have 

three distinct stages: 

1. Developing Stage (Cumulus or Towering Cumulus) 

 Updraft, upward moving column of air, develops. 

 Storm begins to produce precipitation within the upper 

portion of the cloud.

2. Mature Stage

 Updraft and downdraft coexist.

 Downdraft reaches the ground as a spreading of raincooled air called the cold pool. The leading edge of the 

cold pool is called the gust front.

 Top of updraft forms an anvil-shaped cloud as air 

spreads outward.

3. Dissipation Stage

 Downdraft is dominating.

 Loses favorable inflow as the gust front moves out a 

long distance from the storm.

 Sometimes shows an “orphaned anvil,” the remnants of 

an anvil with the storm dissipated below.

Towering Cumulus Stage Mature Stage Dissipating Stage

17
SEVERE LOCAL STORMS

Spotter’s Field Guide

Severe Thunderstorms

A severe storm has at least one of the following: 

 Hail that is 1 inch or larger: the size of a quarter 

 Wind gusts of at least 58 mph or higher: 50 knots 

 Tornado

The Storm Prediction Center issues Tornado and Severe Thunderstorm 

Watches. Local weather forecast office issue Tornado or Severe 

Thunderstorm Warnings. 

Watch: Conditions are favorable in a region for severe 

thunderstorms or tornadoes to develop or to move into 

your area. Watches are generally issued for a 4-8 hour 

period. Be ready for storms in the near future. Take action 

to protect property before the storms arrive in your area. 

Warning: There is either an imminent threat or an actual 

occurrence of large hail, damaging winds or a tornado. 

When a warning is issued, respond immediately to protect 

life and property. Warnings are generally issued for 30-60 

minutes. 

Thunderstorm schematic

18
Section 2

Thunderstorm Basics

Features Indicating Strong/Severe Storms

Anvil: The anvil is the elongated cloud at the top of the storm that spreads 

downwind with upper level steering winds. The anvil will appear solid, not 

wispy, and will have sharp, well defined edges.

Overshooting Top: The overshooting top is the dome of cloud directly 

above the main storm updraft tower and the anvil. If the overshooting top 

is persistent and lasts for 10 minutes or longer, it is generally a sign of a 

very strong thunderstorm updraft.

Main Storm Tower: The “trunk” of the storm is the visible updraft of the 

storm from its base near the ground to just below the anvil. This part of the 

storm can show:

 Vertically oriented tower, with sharp, well defined edges

 Solid, cauliflower appearance

 Visible rotation of the middle and lower levels, and 

possibly striations evident in the clouds

Rain-free Base: The area below the main storm tower. It is generally on 

the south or southwestern flank of a storm. 

Shown is a severe thunderstorm in the distance with a visible thick anvil 

and large overshooting top. Photo by Gene Rhoden.

19
SEVERE LOCAL STORMS

Spotter’s Field Guide

Wall Cloud: A wall cloud is an isolated lower cloud attached to the rainfree base and below the main storm tower. Wall clouds often are on the 

trailing side of a storm. For example, with a storm that is moving to the 

north or northeast, the wall cloud typically is on the south or southwest 

side of the storm. With some storms, the wall cloud area may be obscured 

by precipitation. Wall clouds associated with potentially severe storms:

 Usually persist for 10 minutes or more

 Often, but not always, rotate visibly

 Sometimes are accompanied by obvious rising or 

sinking motion of cloud piece

Flanking Line: A flanking line 

is a row of towering cumulus 

clouds stair-stepping up to 

the main storm tower. New 

storm cells can develop 

from the flanking line, which 

usually extends south or 

southwest of a thunderstorm.

Wall cloud and rain free base. 

Photo by Brian Morganti.

Wall cloud with the rain free base and heavy rain 

behind the wall cloud. Photo by Roger Hill.

The flanking line of a thunderstorm. 

Photo by Matt Ziebell. 

20
Section 2

Thunderstorm Basics

Shelf Cloud: A shelf cloud is a low, horizontal, banded cloud attached to 

the base of the parent cloud, usually a thunderstorm. Rising cloud motion 

often can be seen in the leading, outer part of the shelf cloud, while the 

underside often appears turbulent and wind-torn. Generally, a shelf cloud 

appears on the leading edge of a storm.

Roll Cloud: A roll cloud is low, horizontal, tube-shaped and relatively rare. 

It differs from a shelf cloud by being completely detached from other cloud 

features. 

Severe/Strong Storm Features Checklist

Upper-level storm features visible at long distances from the storm:

 A solid-looking overshooting top persisting 

for 10 minutes or more

 Overshooting tops that may dissipate, followed by 

new ones

 A solid-looking anvil with sharply defined edges

Why? An overshooting top is a signal of a strong updraft. If the top persists 

for at least 10 minutes, it is a sign that the storm is continuing to strengthen. 

If the overshooting top suddenly collapses, a burst of precipitation, hail, or 

damaging wind may be imminent. As the storm weakens, it will take on a 

wispier, fuzzy appearance.

Shelf cloud in Illinois. Photo by Walker Ashley.

21
SEVERE LOCAL STORMS

Spotter’s Field Guide

Mid-level storm features that may be seen are:

 Solid or hard-looking storm tower with a cauliflower 

appearance

 Flanking line, with clouds building toward the main 

storm tower

Severe/strong thunderstorm in the distance. Note the overshooting top, 

sharply defined edges of the anvil, and the flanking line. 

Photo by Brian Morganti.

Looking northeast into a thunderstorm with the flanking line pointing 

south southwest. Photo by Tom Warner.

22
Section 2

Thunderstorm Basics

Why? A solid-looking storm tower indicates a strong updraft along with a 

favorable shear environment. A flanking line indicates the storm is drawing 

air from many miles away and likely will sustain itself or intensify for 

some time.

Low-level storm features can be seen when you are close to a storm:

 Rain-free cloud base with a large and solid-looking storm 

tower above

 Wall cloud persisting for 10 minutes or longer, especially 

if it is obviously rotating 

 Rapid vertical motion (up or down) within the wall cloud 

or other areas of the rain-free cloud base

Why? A rain-free cloud base indicates a strong updraft, where precipitation 

and hail is not heavy enough to fall to the ground. When a rotating wall 

cloud is present, there is a much higher potential for tornado development. 

Wall clouds begin to rotate as the larger scale circulation, or mesocyclone, 

2-10 miles in diameter, develops toward the surface. When combined with 

the proper atmospheric conditions, this pattern supports tornadoes.

Lower level storm features, including the updraft and downdraft area. 

Photo by Jim LaDue.

23
SEVERE LOCAL STORMS

Spotter’s Field Guide

Thunderstorm Types 

Thunderstorms can be categorized by their physical characteristics: the 

presence or absence of rotation, the number of location of updrafts and 

downdrafts present.

There is a continuous spectrum of storms in the sky. At times, it is difficult 

to place a storm into a specific category. A storm may move from one 

category to another. These five types are often useful in describing storms: 

 Ordinary or Single Cell Storm: Single cell storms are 

short lived, and usually not severe.

 Pulse Storm: A Pulse Storm is a single-cell 

thunderstorm that is usually not strong; when it is of 

substantial intensity, it produces severe weather for 

short periods of time. Such a storm weakens and then 

generates another short burst or pulse. 

 Multicellular Cluster: This type is the most common 

storm, consisting of a group of ordinary cells at various 

stages of the thunderstorm life cycle.

 Multicellular Line: This category is a long line of storms 

with a continuous, well developed gust front along the 

leading edge.

 Supercell: A supercell is a highly organized 

thunderstorm with an extremely strong updraft. They 

exhibit persistent storm-scale rotation of the updraftdowndraft couplet or mesocyclone.

24

During early evening of April 3, 2004, this supercell thunderstorm dropped 

2 inch diameter hail over Chaparral, NM. Photo by Greg Lundeen.
Section 2

Thunderstorm Basics

Ordinary or Single Cell Storm

This storm forms when there is weak shear in the atmosphere. 

Characteristics of this kind of storm include:

 Short life, generally 30-45 minutes 

 Downdraft that forms within 15-20 minutes after cell 

initiation

 Updraft that weakens in 25-30 minutes, outflow stabilizes

 Small hail, usually not severe

 Gusty winds, usually not severe

A series showing an ordinary storm cell’s life cycle. These photos were taken in a span of 

21 minutes. Photos by Phil Kurimski.

25
SEVERE LOCAL STORMS

Spotter’s Field Guide

The Pulse Storm

 Short life, generally 30-45 minutes

 Usually not severe, but given the right environmental 

conditions, these storms can create:

• Brief, small to moderate size hail

• Downburst winds, usually less than 70 mph

• A weak tornado

 Damage is isolated

The Multicell Cluster Storm

 Most common type of thunderstorm 

 May last for several hours 

 Consists of a group of cells moving as a single unit 

 Contains cells in different stage of the thunderstorm 

life cycle

 Occasionally may contain supercells

Multicellular thunderstorm. Photo by Gary Woodall.

26
Section 2

Thunderstorm Basics

The new updrafts in the cluster form in an area of persistent lifting where 

air converges in low levels, such as:

 Cold or warm front

 Dry line

 Outflow boundary from nearby storms

 Higher terrain features than the surroundings 

Typically, cells will develop in the lifting zone and move with the mid and 

upper level winds as it matures and dissipates, with new cells continuing 

to develop. In one typical scenario during the spring and summer months:

 New cells initiate on west or southwest edge of cluster 

 Dissipating cells weaken on east or northeast edge of 

cluster

 Each cell lasts 20-30 minutes

 Clusters as a whole often last an hour or more 

Given the right conditions, the cells can become severe within the 

multicellular cluster producing: 

 Brief, small to moderate size hail

 Downburst winds

 Weak tornadoes

 Heavy rainfall in a short time

The Multicell Line Storm

 Frequently called squall line

 A long line of storms with individual storm outflows 

merging to produce a continuous, well developed gust 

front marking the leading edge of rain-cooled air

 Line of storms often oriented north-south or northeastsouthwest and usually move toward the northeast, east 

or southeast

 May be embedded along the line

27
SEVERE LOCAL STORMS

Spotter’s Field Guide

Individual thunderstorm updrafts and downdrafts along the line can 

become severe, resulting in large hail and episodes of damaging outflow 

winds that move rapidly ahead of the system.

Given the right environmental conditions, multicell line storms can produce:

 Strong downburst winds

 Heavy rainfall

 Moderate-sized hail

 Occasional tornadoes

The Supercell

Classifying supercellular storms is subjective. There is a lot of research 

being done in this area. We do know the following about supercells: 

 Highly organized storm with rotation inside

 Updrafts can attain speeds more than 100 mph

 Can produce extremely large hail and strong, violent 

tornadoes

 Rear-flank downdraft can produce damaging outflow 

winds in excess of 100 mph

28

Multicellular Line 
Section 2

Thunderstorm Basics

It is essential to become familiar with the visual aspects of these intense 

thunderstorms. Two important characteristics that distinguish supercells 

from ordinary thunderstorms are:

 Persistent rotation at the rain-free base

 A rear-flank downdraft (RFD): region of dry air wrapping 

along the back portion of the circulation within the storm 

The presence of a strong rotation in the storm greatly enhances updraft 

intensity, persistence and overall storm organization. The temperature 

and moisture characteristics of the RFD and its evolution during a storm’s 

lifetime have been shown to play a crucial role in tornado formation.

Supercells can produce the following elements:

 Large hail and potentially torrential rainfall immediately 

adjacent to the storm updraft

 Smaller hail and lighter rainfall at greater distances from 

the updraft

 RFDs producing strong, sometimes damaging outflow

 Tornadoes

Simplified supercell diagram, 

rear-flank downdraft (RFD), 

forward-flank downdraft 

(FFD), and updraft (UD) areas 

are highlighted. Arrows show 

air flow. Blue lines with barbs 

represent gust fronts. Red 

line is the edge of the inflow 

air at ground level. Purple 

section of line is the occlusion 

where the surface inflow air 

gets cut off from the low-level 

mesocyclone. As a spotter, 

your location relative to this 

diagram will influence the 

winds you experience and the 

storm features you see.

29
SEVERE LOCAL STORMS

Spotter’s Field Guide

Supercell Variations

All supercells contain storm-scale rotation, which may give a striated or 

corkscrew appearance to the storm’s updraft. One supercell may appear 

quite different visually from another, depending on the following:

 The amount of precipitation accompanying the storm 

 Whether precipitation falls near or far from the updraft 

How a storm moves influences the winds that it “feels,” much like sticking 

your hand out a car window as you turn and change speeds. Winds around 

the storm play an important role in where precipitation exists in and around 

the storm’s updraft. 

Based on their visual appearance, supercells are often labeled as:

 Low precipitation

 Classic 

 High precipitation

Classic Supercell thunderstorm. Photo by Roger Hill.

30
Section 2

Thunderstorm Basics

Low Precipitation Supercell (LP) 

 Barber pole or corkscrew appearance is possible.

 Precipitation is sparse or well removed from the updraft 

below cloud base, which often is transparent. 

 Large hail is often difficult to discern visually. Although 

precipitation may not be apparent below the storm, 

sometimes very large hail is falling that can not be seen 

at a distance. 

Low Precipitation Supercell. Photo by Roger Edwards.

Artist rendition of a Low Precipitation Supercell.

31
SEVERE LOCAL STORMS

Spotter’s Field Guide

Classic (CL) Supercell

 Majority of supercells in this category

 Large, flat rain-free base

 Can have wall cloud 

 Barber pole or corkscrew appearance of updraft 

possible, as in LP supercell

 Heavy precipitation falls adjacent to the updraft 

 Large hail possible

 Potential for strong, long-track tornadoes

Classic Supercell. Photo by Bill Martin.

32

Artist rendition of a Classic Supercell.
Section 2

Thunderstorm Basics

High Precipitation (HP) Supercell

 Precipitation often surrounds updraft, and may hide it 

 Can have a wall cloud, but it may be obscured by the 

heavy precipitation

 RFD filled with precipitation

 May have an associated shelf cloud

High Precipitation Supercell. Photo by Al Moller.

Artist rendition of High Precipitation Supercell.

33
SEVERE LOCAL STORMS

Spotter’s Field Guide

 Tornadoes potentially obscured by heavy precipitation 

(rain-wrapped)

 Extremely heavy precipitation with flash flooding 

Visual Clues of Supercells

 Rotating wall cloud suggests the presence of a rotating 

updraft

 Striations on the sides of the storm, streaks of cloud 

or bands of cloud that give the storm a corkscrew or 

barber pole appearance, indicate the storm’s updraft is 

rotating, generally seen with Low Precipitation or Classic 

Supercells

 Inflow cloud bands, such as a “beaver’s tail,” feed into 

the storm. The beaver’s tail is a smooth, nearly flat cloud 

band extending out from the eastern edge of the rainfree base toward the east.

Inflow band. The smooth, flat cloud on the lower right side is often 

referred to as a beaver's tail. Photo by Roger Edwards.

34
Section 2

Thunderstorm Basics

Severe Hail and Winds

Hail: NWS issues a severe thunderstorm warning for hail of 1 inch across 

or larger. When reporting hail, it is best to measure the hail when safe to 

do so. If you are not equipped with a ruler or other measuring instrument, 

hail size can be related loosely to coins or athletic balls, as in this table:

BB Less than 1/4"

Pea 1/4"

Dime 7/10"

Penny 3/4"

Nickel 7/8"

Quarter 1"

Half Dollar 11⁄4"

Walnut or Ping-Pong Ball 11⁄2"

Golf Ball 13⁄4"

Lime 2"

Tennis Ball 21⁄2"

Baseball 23⁄4"

Large Apple 3"

Softball 4"

Grapefruit 41⁄2" 

Hail storm. Photo from NOAA.

35
SEVERE LOCAL STORMS

Spotter’s Field Guide

Damaging Winds: NWS issues warnings when the winds from a 

thunderstorm are expected to be 58 mph (50 knots) or higher. 

Downburst: This term refers to an area of strong, often damaging winds 

produced by air rapidly descending in a thunderstorm. Downbursts are 

sometimes described as a microburst when it covers an area of less the 2.5 

square miles and last 3-7 minutes, or macroburst when they cover larger 

areas or last for more than 7 minutes. On rare occasions, downbursts can 

have wind speeds in excess of 150 mph. Downbursts, both microburst and 

macroburst, are described as wet or dry.

■ Wet: Rainfall accompanies the damaging winds. 

■ Dry: Very little rainfall accompanies the damaging 

winds and sometimes all that is visible is the 

evaporating precipitation (see virga below) or the dust 

kicked up off of the ground. 

36

Microburst series. Photos by William Bunting.
Section 2

Thunderstorm Basics

Bow Echo: A bow-shaped line of convective cells, best seen on radar, is 

often associated with swaths of damaging straight line winds and small 

tornadoes.

Derecho (deh-REY-cho): A derecho is a widespread and usually fastmoving convectively induced episode of damaging winds. Derechos can 

result from bow echoes, supercells, multicell clusters or lines, or a blend of 

any of these. They can produce damaging straight-line winds over areas 

hundreds of miles long and more than 100 miles across. Due to the large 

area extent, an episode is usually not classified as a derecho until it is 

complete. 

Schematic showing the development of a bow echo.

37

Shelf cloud in front of a Squall Line (line of severe thunderstorms). This cloud brought 

damaging winds and caused significant damage on July 14, 2010. Source Wikimedia.
SEVERE LOCAL STORMS

Spotter’s Field Guide

Estimating Wind Speed: It’s often difficult to estimate wind speed, 

especially in the plains where there are few physical indicators to observe 

damage. Below is the Beaufort Wind Force Scale for estimating wind 

speeds. This is only a rough guide. Actual damage may occur at weaker 

or stronger speeds. 

Wind Speed (mph) Effects

 25-31 Large branches in motion

 32-38 Whole trees in motion

 39-54 Twigs break off trees, wind 

 impedes walking

55-72 Damage to TV antennas, large 

 branches break off trees

 73-112 Surfaces off roofs peeled off, windows 

 broken, trailer homes overturned

 113+ Roofs blown from houses, weak 

 buildings and trailer homes 

 destroyed, large trees uprooted, 

 train cars blown off tracks

38

Town near Desoto, MO, hit by thunderstorms that produced 20 to 30 

minutes of severe winds estimated to be between 80 and 100 mph, 

May 6, 2003. Photo from NOAA.
Section 2

Thunderstorm Basics

Spotting Downbursts 

There are several visual signs that a downburst is either underway or 

about to occur. 

Virga: Precipitation streaks from the cloud, but does not reach the 

ground. The atmosphere below the clouds tends to be very dry and rainfall 

evaporates before it touches the ground. Gusty winds occur in the area of 

the virga.

Rain foot: The rain foot is a pronounced outward bend of the precipitation 

area near the ground, marking an area of strong outflow winds.

Virga with a dry microburst. Photo by Brian Morganti.

Wet downburst with rain foot on the left of rain shaft area. 

Photo by Brian Morganti.

39
SEVERE LOCAL STORMS

Spotter’s Field Guide

Dust foot: A plume of dust/dirt that is raised as the downburst reaches the 

ground and moves away from the impact point.

Dust foot/microburst. Photo by Brian Morganti.

A wet microburst. Photo by Jim LaDue.

40
Section 2

Thunderstorm Basics

Flash Floods

 Water rises rapidly with little or no advance warning

 Occurs in steep terrain, but can also occur in urban 

areas when sewer systems cannot quickly drain the 

water running off large areas of pavement 

 Usually occurs due to heavy rainfall, storm surge, ice 

jams, dam breaks, or tsunamis 

Lightning 

 Lightning is an electrical discharge occurring in 

thunderstorms, but can occur in hurricanes, winter 

storms, volcanic eruptions and large wildfires.

 Lightning discharges can occur between oppositelycharged parts of the thunderstorm cloud or between 

opposite charges in the cloud and on the ground. 

 In the United States, there are about 20 to 25 million 

cloud-to-ground lightning flashes each year.

 Lightning kills an average of 30 to 50 people each year 

in the United States and injures several hundred more.

 Any lightning or thunder indicates a charged atmosphere 

and consequently a dangerous situation.

Fort Collins, CO, flash flood in 1997. Photo by John Weaver.

41
SEVERE LOCAL STORMS

Spotter’s Field Guide

 The initial stages of thunderstorm development often 

bring a short period of in-cloud lightning, heard as a 

crackling or rumbling aloft. As the thunderstorm continues 

to develop, you may see cloud-to-ground strikes.

 Most cloud-to-ground lightning is between negative 

charges in the middle to lower part of the cloud and the 

positively charged ground. A small percentage of lightning 

occurs between the positively charged upper part of the 

cloud and negatively charged ground. These positive 

flashes can strike over 10 miles from the parent storm.

 As the updraft of a storm weakens, the negatively 

charged area in the lower part of the storm diminishes 

and a larger percentage of positive flashes occur.

 The downdrafts from some strong and severe storms pull 

positive charge downward causing positive flashes near 

the most intense part of the storm. 

 Positive Charged Lightning: This type of lightning 

makes up less than 5% of all lightning strikes. The 

majority of positive lightning originates in the upper levels 

of the storm, and has a much stronger charge to it than 

most negative-charged lightning strikes. These strikes 

are very bright, and are known for being good fire starters 

when they strike grass, brush, or trees. They can strike 

more than 10 miles away from the parent thunderstorm.

 Negative Charged Lightning: Negative lightning makes 

up over 95% of all strikes. While the electrical charge isn’t 

usually as strong as a positive strike, it is still enough to kill 

people who are outdoors without proper shelter. 

Cloud-to-ground lightning. Photo by Tom Warner.

SEVERE LOCAL STORMS

Spotter’s Field Guide

42
Section 3

Tornado Formation

Tornado Formation

Tornadoes are one of nature’s most fearsome creations. Each year they 

constitute a major hazard around the United States. Storm spotters can 

help to increase warning lead time by recognizing and reporting clues 

associated with tornado development and the various stages of the 

tornado life cycle. This section describes the typically observed features 

before tornado formation, during the life of the tornado, and as the 

tornado dissipates.

Visual Clues of Tornado Formation

 Large, rounded rain-free base. This can indicate the 

presence of a mesocyclone. 

 Increasing spin in wall cloud and cloud base around 

wall cloud. This can suggest that the low level rotation is 

increasing. 

 Clearing skies working into the rain-free base, which 

suggests a part of the rear-flank downdraft is wrapping 

around the mesocyclone. This often precedes 

or accompanies tornado formation in supercell 

thunderstorms.

 Rapid vertical motions, scud (see p. 49), rising into wall 

cloud, sinking motion around wall cloud from rear-flank 

downdraft.

 Local burst of heavy rain/hail just west or southwest of 

wall cloud. Occasionally, this is a precursor to a tornado.

A tornado may form within a few minutes of these clues appearing! In 

other cases, outflow behind the gust front can spread out from storm and 

cut off the formation process. Not all of the signs listed above are required 

for tornado development; however, storms exhibiting most or all of these 

features have better odds of producing a tornado.

43
SEVERE LOCAL STORMS

Spotter’s Field Guide

Developing Stage

Incipient and early-stage 

tornadoes. Views to the 

west or northwest. Note 

visual clues of rotation, 

clear slots around edges of 

wall clouds, and developing 

condensation funnels. 

Photos by Joel Genung, 

Scott Blair, Roger Edwards.

 Tornado circulations can develop from either the ground 

upward or from the middle and low levels downward. 

 Rear-flank downdraft and precipitation southwest of a wall 

cloud may signal processes that help establish a tornado.

 Some circulations start in low levels, near the cloud base, with 

rapid accelerations of cloud material into an area of tightening 

rotation.

 Watch closely! The first sign of tornado development may be 

a dust whirl at the ground. If seen, closely examine this whirl 

to see if it is connected to the cloud base.

44
Section 3

Tornado Formation

Mature Stage

Mature tornadoes. Views to north and northwest. Note near-vertical orientation of 

condensation funnels and clear slots advancing around wall clouds. Photos by Scott Blair, 

Roger Edwards, Robert Prentice.

 Potentially the strongest and most dangerous stage 

of the tornado’s lifetime.

 Funnel often has a near-vertical orientation.

 Visible funnel may not extend all the way to the ground, 

or may become hidden inside the wrapping precipitation! 

 Often, rear-flank downdraft wraps around south and east 

side of the wall cloud gradually cutting off original 

inflow air.

 Rain-free base may take on a horseshoe-shaped 

appearance. The tornado and wall cloud may be found 

at the north end of this structure.

45
SEVERE LOCAL STORMS

Spotter’s Field Guide

Dissipating Stage

 Rear-flank downdraft wraps around tornado.

 The tornado is separating from the warm buoyant air that 

it needs for its survival.

 Tornado shrinks, tilts, and takes on a contorted snakelike 

appearance before finally dissipating. Sometimes this is 

referred to as the rope stage.

 Although the tornado is not as large as it was in the 

mature stage, it can still be dangerous. 

46

Dissipating or rope stage tornadoes. Note surface debris clouds or condensation on 

the ground, indicating continuation of damaging winds. Note also the tilted, contorted 

appearance of the condensation funnels. Photos by Chuck Doswell and Roger Edwards.
Section 3

Tornado Formation

Cyclic Supercells

Cyclic supercell, view to 

northeast toward Stockton, 

KS, June 9, 2005. Note old 

tornado and clear slot in left 

foreground and new tornado 

in right background. 

Photo by Scott Blair.

Simultaneous tornadoes 

from two different 

mesocyclones, looking 

west: left, fully condensed, 

late-stage tornado from 

older mesocyclone; 

right, tornado without 

condensation into the 

debris cloud becoming 

better organized in newer 

mesocyclone, Zurich, KS, 

June 9, 2005 . Photo by 

Chuck Doswell.

 With some supercells, inflow may be refocused a few 

miles east of the dissipating tornado. 

 If the environment is favorable, a new mesocyclone 

and wall cloud will form. 

 The new mesocyclone and wall cloud will become 

the dominant part of the storm, and a new tornado 

may form.

 For safety, check overhead often when near the inflow 

region of a supercell.

47
SEVERE LOCAL STORMS

Spotter’s Field Guide

Tornado Variations in Appearance

Tornadoes can come in various shapes and sizes. Below are some pictures 

of the various types that may be encountered.

Pencil-shaped tornado 

about the time of F5 

damage near Union City, 

OK, May 24,1973. 

Photo by Chuck Doswell.

Cone-shaped tornado 

with debris fan south of 

Hill City, KS, looking west, 

June 9, 2005. 

Photo by Elke Edwards.

“Wedge” tornado, where the 

visible funnel is wider than 

the distance from ground 

to surrounding cloud base, 

looking northwest near 

Argonia, KS, May 29, 2004. 

Photo by Robert Prentice.

48
Section 3

Tornado Formation

Deadly nighttime tornado 

near Westminster, TX, 

May 9, 2006. 

Photo by Eric Nguyen.

Bowl-shaped condensation 

funnel above well-developed 

tornado debris cloud near 

Aurora, NE, June 17, 2009. 

Photo by Roger Edwards.

Scorpion tail shape to roping 

tornado near Lake McClellan 

TX, May 20, 1999. 

Photo by Roger Edwards.

49
SEVERE LOCAL STORMS

Spotter’s Field Guide

Multi-Vortex tornadoes have two or more circulations or vortices orbiting 

about each other or around a common center. 

Multi-vortex tornado near 

Glen Elder, KS, May 29, 

2008. Photo by Scott Blair.

Rope tornado near Saint 

Peter, KS, May 22, 2007, 

looking northeast. 

Photo by Roger Edwards.

Non-supercell tornadoes 

developing on leading part 

of a squall line, where an 

updraft base has not been 

undercut yet by outflow, 

taken near Fort Dodge, IA, 

June 11, 2004. 

Photo by Roger Edwards.

Rope tornadoes often signify a tornado that is weakening or dissipating. 

Still, such tornadoes can contain deadly and damaging ground circulations, 

whether visible or not.

50
Section 3

Tornado Formation

Non-Supercell Landspout tornadoes do not arise from organized stormscale rotation, and therefore usually are not associated with a visible 

wall cloud or a radar-indicated mesocyclone. Typically, these tornadoes 

are observed beneath cumulus and towering cumulus clouds, often as 

no more than a dust whirl, and are the land-based equivalents of nonsupercell waterspouts.

Non-supercell tornado or landspout 

from a single high-based 

thunderstorm in Colorado in 1988. 

Photo by David O. Blanchard.

Non-supercell tornado from a 

high-based thunderstorm near 

Venango, KS, in 1997. 

Photo by David O. Blanchard.

Non-supercell tornado from the base 

of a line of towering cumulus clouds 

near Kit Carson, CO, June 19, 2008. 

Photo by Roger Edwards.

51
SEVERE LOCAL STORMS

Spotter’s Field Guide

A waterspout is a tornado over water. There are two types of waterspouts. 

The Type A waterspout forms in a supercell thunderstorm and is a violent 

and potentially destructive vortex that often moves quickly and is capable 

of significant structural damage if it moves over land. 

The Type B waterspout is usually less violent, slower moving, and less 

destructive. Type B develops quickly beneath a rapidly building line of 

cumulus clouds. The winds near the base of a Type B waterspout can reach 

and exceed gale force (34 knots), which is strong enough to swamp or capsize 

a small watercraft. All waterspouts pose a threat to boater safety and should 

be avoided.

Tornado/Funnel Cloud Look-a-Likes: Several atmospheric and man-made 

features may be mistaken for tornadoes. Some of the most common are:

 Scud Clouds ■ Smoke

 Rain Shafts ■ Communication Towers

 Gustnadoes ■ Grain Elevators

 Tail clouds ■ Dust Devils

To distinguish between a real tornado or funnel and one of the above looka-likes, study the feature and be able to answer the following questions:

1. Can I see it clearly?

2. Is the feature attached to a thunderstorm base?

3. Is the feature in the section of the storm where tornadoes/funnels 

typically develop, i.e., near the updraft?

4. Is there organized rotation present within the feature?

5. If it appears to be 

a tornado, is there 

debris?

If your answer to any of 

these questions is “no,” then 

the feature likely is not a 

tornado. If you have doubts, 

continue to observe the 

feature. Important: report 

only what you see, not 

what you think you see. Scud clouds near Julesburg, CO, June 1, 2008. 

Photo by Chuck Doswell.

52
Section 3

Tornado Formation

Scud clouds are low cloud fragments that may attach to a storm’s base 

and can strongly mimic the appearance of a ragged funnel. Some scud 

can rise from or near the ground leaving the impression of a tornado. 

Watch for persistent rotation of the suspicious feature to rule out scud.

Precipitation shaft from a 

high-based supercell near 

Wray, CO, July 1, 2005. 

Photo by Chuck Doswell.

Smoke being ingested into a wall 

cloud near Gotebo, OK, 1984. 

Photo by David O. Blanchard.

53
SEVERE LOCAL STORMS

Spotter’s Field Guide

A Dust Devil is usually a small, rapidly rotating wind that is made visible 

by the dust, dirt or debris it picks up. Also called a whirlwind, it develops 

best on clear, dry, hot afternoons.

Examples of dust devils over the 

western United States in 1985 and 1986. 

Photos by David O. Blanchard.

Spotter’s Field Guide

54
Section 3

Tornado Formation

Section 4

Technology and 

Storm Spotting

Technology and Storm Spotting

Doppler Radar

The most effective tool to detect precipitation is radar. Radar, which stands 

for Radio Detection and Ranging, has been used to detect precipitation, 

and especially thunderstorms, since the 1940s. Radar enhancements 

have enabled NWS forecasters to examine storms with more precision.

NWS radars use Doppler weather radar principles. All weather radars, 

including Doppler, electronically convert reflected radio waves into pictures 

showing the location and intensity of precipitation; however, Doppler radars 

also can measure the frequency change in returning radio waves which 

allows meteorologists to display motions toward or away from the radar.

This ability to detect motion has greatly improved the meteorologist's 

ability to peer inside thunderstorms and determine if there is rotation in the 

cloud, often a precursor to the development of tornadoes. 

 Doppler Weather Radar Images

Reflectivity is the amount of transmitted power returned to 

the radar receiver after hitting precipitation. It is measured 

in decibels (dBZ). Composite Reflectivity uses all radar 

elevation scans to create an image and displays the 

maximum reflectivity vertically at any point. Precipitation

images (1 Hour and Storm Total) are created by applying 

computer algorithms to reflectivity imagery to estimate 

rainfall.

Velocity imagery is a sample of wind data using Doppler 

principles. Red indicates outbound wind speed and green 

shows inbound wind speed in knots relative to the radar. 

Storm Relative Velocity (SRM) subtracts the storm 

motion from the overall wind to reveal winds relative to 

the storm. This image is useful in displaying small scale 

circulations within thunderstorms.

55
SEVERE LOCAL STORMS

Spotter’s Field Guide

Satellite Imagery

NWS satellites are capable of producing information on clouds and 

moisture in three primary forms: Visible, Infrared (IR) and Water Vapor.

Visible imagery shows the earth in visible light. This process is similar 

to that of a person taking a picture with a camera. The satellite detects 

sunlight reflected from objects within the viewfinder. In the case of the 

satellite, the objects are the upper surfaces of clouds. Thick clouds do a 

much better job of reflecting light and therefore appear brighter in visible 

photos.

The problem with visible imagery is that it is only available during the day. 

To combat this problem, the infrared (IR) sensor was devised. It senses 

radiant (heat) energy given off by the clouds. Warmer clouds, which are 

lower in the atmosphere, give off more energy than higher, cold clouds. The 

IR sensor measures the heat and produces several images based upon 

different wavelengths in the IR range of the electromagnetic spectrum.

Doppler velocity wind data showing a tornadic circulation near Van Wert, OH, on 

November 10, 2002.

56
Section 3

Tornado Formation

Section 4

Technology and 

Storm Spotting

Water vapor imagery is unique in that it can detect water in a gas state in 

addition to clouds. This type of image shows water vapor in the top onethird of the troposphere. Energy from moisture in the lower levels of the 

atmosphere is absorbed by the atmosphere and hidden from the satellite 

sensor. Upper level moist and dry areas are plainly observable and can 

show prominent air currents. Moist areas show as white, while dry areas 

show as black. 

Visible satellite image showing overshooting thunderstorm tops in Arkansas on 

January 21, 1999.

57
SEVERE LOCAL STORMS

Spotter’s Field Guide

Types of Satellites

The Geostationary Operational Environmental Satellite’s 

(GOES) path around the earth is at an altitude of 22,236 

miles. At this distance the satellite complete one orbit 

of the earth in 24 hours. The net result is the satellites 

appears stationary, relative to the earth. This allows it 

to hover continuously over one position on the surface. 

Because they stay above a fixed spot on the surface, they 

provide a constant look at atmospheric severe weather 

conditions. The United States operates two meteorological 

satellites in geostationary orbit, one over the equator at 

75 deg W with a view of the East Coast and the other over 

the equator at 135 deg W, with a West Coast view.

Polar Orbiting Satellites (POES) offer the advantage 

of daily global coverage by making nearly polar orbits 

roughly 14.1 times daily. Since the number of orbits per 

day is not an integer, the orbital tracks do not repeat on a 

daily basis. Currently in orbit are morning and afternoon 

satellites, which provide global coverage four times daily.

Infrared (IR) picture of a thunderstorm that produced a tornado near Xenia, OH, on 

September 20, 2000.

58
Section 3

Tornado Formation

Section 4

Technology and 

Storm Spotting

Common Storm Types on Radar 

Single Cell Thunderstorm 

This type of thunderstorm develops in weak vertical wind shear 

environments characterized by a single updraft core and a single downdraft 

that descends into the same area as the updraft. The downdraft and its 

outflow boundary then cut off the thunderstorm inflow, causing the updraft 

and the thunderstorm to dissipate. Single cell thunderstorms are shortlived. They only last about a half hour to an hour. These thunderstorms 

will occasionally become severe (1 inch hail, wind gusts in the excess of 

58 mph, or a tornado), but only briefly. In this case, they are called Pulse 

Severe Thunderstorms. 

Single Cell Thunderstorm viewed by Boston radar on June 9, 2008.

59
SEVERE LOCAL STORMS

Spotter’s Field Guide

Multicell Thunderstorm 

Multicell thunderstorms are organized in clusters of at least 2-4 shortlived cells. Each cell generates a cold air outflow that combine to form 

a large gust front. Convergence along the gust front causes new cells 

to develop every 5 to 15 minutes. The cells move roughly with the mean 

wind; however, the area (storm) motion usually deviates significantly from 

the mean wind due to discrete propagation (new cell development) along 

the gust front. The Multicell nature of the storm is usually apparent on 

radar with multiple reflectivity cores and maximum tops.

This multicell cluster of storms produced 60 mph wind gusts in several storms, resulting in 

large broken tree branches in western Montana near Missoula, MT. Image is from the NWS 

Missoula Doppler Radar on June 4, 2007.

60
Section 3

Tornado Formation

Section 4

Technology and 

Storm Spotting

Multicell Line 

A line of active thunderstorms, with or without breaks, including contiguous 

precipitation areas, resulting from the existence of the thunderstorms

Supercell Thunderstorm 

Supercell thunderstorms are potentially the most dangerous of the 

convective storm types. Storms possessing this structure have been 

observed to generate the vast majority of long-lived strong and violent 

(F2-F5) tornadoes, as well as downburst damage and large hail. This type 

of storm consists of one quasi-steady to rotating updraft that may exist for 

several hours. 

Multicell thunderstorm line without breaks, as viewed by Oklahoma City Radar on 

September 7, 2001. A bow echo is evident in the center of the line, southeast of KTLX, 

with a gust front leading the line south of the bow. Near the bow echo, wind speeds of 

60 knots were observed. Hail 1.75 inches in diameter was reported on the south end 

of the thunderstorm line in Oklahoma.

61
SEVERE LOCAL STORMS

Spotter’s Field Guide

Radar will observe one long-lived cell, but small perturbations to the cell 

structure may be evident. The stronger the updraft, the better the chance 

the supercell will produce severe weather, hail greater than 1 inch in 

diameter, wind gusts greater than 58 miles an hour, and possibly a tornado.

Severe Weather Radar Signatures

A hook echo is a radar reflectivity pattern that forms a hook shape, 

usually in the trailing portion of a Supercell storm. This hook shape forms 

when precipitation gets wrapped around the storm mesocyclone and is a 

favorable region for tornado development. 

Supercell thunderstorm with a hook echo viewed by Oklahoma Radar on May 3, 1999. This 

storm produced an F5 tornado that moved through Bridge Creek, Moore, and Oklahoma 

City with storm damage reported in a path up to 1 mile wide.

62
Section 3

Tornado Formation

Section 4

Technology and 

Storm Spotting

A bow echo occurs when part of a line of thunderstorms accelerates 

ahead of the rest of the line. This produces a bend, or bow, in the line. This 

acceleration of the radar echo is a reflection of strong localized “straightline” winds at or near the surface.

Observations and post-storm analysis show that the greatest threat for 

straight-line wind damage is typically found near the leading edge, or 

apex, of the bow.

Another radar characteristic of mature bow echoes is the region of 

weak reflectivity trailing immediately behind the bowing line of strong 

thunderstorms. This weakness in the reflectivity is caused by a descending 

flow of air from mid-levels of the atmosphere. 

Sometimes significant wind damage and even weak tornadoes also can 

occur on the northern end of the bowing line segment within the cyclonically 

(counter-clockwise) rotating comma head region.

Radar features of a bow echo in southeast Illinois, June 3, 2008.

63
SEVERE LOCAL STORMS

Spotter’s Field Guide

Large, organized, long-lived bow echoes can develop and move across 

several states producing long swaths of wind damage. This type of 

convective system is often referred to as a derecho. 

In a severe thunderstorm, large water-coated hail stones suspended aloft 

reflect the radar energy in a complex way. This effects causes a narrow 

spike of reflectivity to protrude from the intense reflectivity core on the 

image. This feature is referred to as a three-body scatter spike. The 

spike is along a radial, the radar beam at that particular azimuth. In basic 

terms, this is caused by the radar beam hitting the large water-coated hail, 

scattering the energy to the ground below, then scattering the energy back 

upward, and finally scattering the energy once again by the hail aloft. 

The three scatterings illustrate the triple reflection or Three Body Scatter 

Spike (TBSS). The presence of a hail spike is a reliable indicator that 

severe hail, greater than 1 inch in diameter, exists in the storm.

A severe squall line, known as a derecho, in northwest Ohio, May 21, 2004.

64
Section 3

Tornado Formation

Section 4

Technology and 

Storm Spotting

The bounded weak echo region (BWER) is a nearly vertical channel 

of weak reflectivity echoes surrounded on all sides and on top by higher 

radar reflectivities. The weak reflectivity core is a result of strong storm 

updrafts carrying hydrometeors upward so quickly they do not grow to 

radar-detectable size until at high storm levels. The BWER is associated 

with very strong storm updraft speeds and is typically found 3-10 km above 

the ground.

The strong updraft speeds associated with the supercells suspend the 

hail above the updraft until the hail grows too large for the updraft winds 

to support it. At that time, the hail falls to the ground. The largest hail falls 

next to the updraft area of the supercell, generally from the west through 

north side of the mesocyclone. 

A V-notch is a radar reflectivity pattern that forms a V-shape in the 

downwind part of a supercell thunderstorm echo. This V-notch is a sign of 

diverging flow around the very strong storm updraft.

Three Body Scatter Spike on June 6, 2005, Cannon Air Force Base, NM, radar.

65
SEVERE LOCAL STORMS

Spotter’s Field Guide

The Bounded Weak Echo Region (BWER) shown in a reflectivity cross-section of the 

May 3, 1999, Oklahoma City Tornado.

V-notch on a supercell storm on June 24, 2008, from the KLNX radar.

66
Section 3

Tornado Formation

Section 4

Technology and 

Storm Spotting

Storm Movement and Spotter Location

Storm spotters must be constantly aware of the storm’s location and 

movement. Since either of these can be hard to determine visually, it is 

a good idea to take advantage of radar data to help with these critical 

details. Once you know where the storm is and which way it is moving, 

you can determine where to position yourself to view the updraft region 

of the storm. For a storm moving northeast, the best observing location 

would be to the southeast of the storm. From this direction, you can get 

a clear view of the rain-free updraft region of the storm where wall clouds 

and associated tornadoes may form. In any other direction, rain and hail 

may block the view of the updraft region of the storm.

Never assume all storms move from the southwest to the northeast. 

Storms typically move in the same direction as the mid-level atmospheric 

winds, so you may experience storms moving from the west, northwest 

or even north. Supercell storms sometimes move (turn) to the right of 

the mid-level winds, and these storms typically have a higher potential 

of to become severe. Remember storms don’t move in a straight line at a 

constant speed. It is critical to have current information about the motion 

and behavior of storms in your area and to use this information to avoid 

the most dangerous parts of the storm.

67
SEVERE LOCAL STORMS

Spotter’s Field Guide

Supercell storms frequently have a deviant motion and may move to the right or left of the 

prevailing storms This propagation can be dangerous to unsuspecting spotters.

68

YPA-201154