U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Weather Service

BASIC SPOTTERS’ FIELD GUIDE

BASIC SPOTTERS’ FIELD GUIDE

BASIC SPOTTERS’ FIELD GUIDE

BASIC SPOTTERS’ FIELD GUIDE

NOAA PA 97050

NAT OI NAL OCEANIC AND ATMOSPHERIC ADMINISTRAT OI N

U.S. DEPARTMENT OF COMMERCE
Cover Photo - National Severe Storms Laboratory
BASIC SPOTTERS’ FIELD GUIDE

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Weather Service
TO THE SEVERE LOCAL STORM SPOTTER:

SEVERE WEATHER!!! Its effects are felt by many of us during our lifetimes. To obtain critical weather information, the

National Weather Service of the U.S. Department of Commerce’s National Oceanic and Atmospheric Administration, and

cooperating organizations, have established SKYWARN Spotter Networks. Although SKYWARN spotters are essential

information sources for all types of weather hazards, your largest responsibility as a SKYWARN spotter is to identify and

describe severe local storms. In the average year, 10,000 severe thunderstorms, 5,000 floods, and over 900 tornadoes occur

across the United States. During the past 10 years, tornadoes, severe thunderstorms, and flash floods have killed nearly

2,300 people in the United States and injured thousands of others. Because of storm spotter reports, such as those you

provided, plus the addition of new technology and improved warning dissemination, this death toll was reduced by more

than 800 from the previous 10 years. While the figures still appear staggering, several thousand lives have been saved by

reports from storm spotters.

Your information, coupled with Doppler radar, satellite, and other data, has enabled the National Weather Service to issue

more timely and accurate warnings for tornadoes, severe thunderstorms, and flash floods. This guide has been designed to

assist you in the important task of observing and reporting hazardous weather and protecting yourself during your

encounters with hostile weather situations. I am pleased that you are part of the ranks of those who form the Nation’s first

line of defense against severe weather. There can be no finer reward than to know that your efforts have given a community

the precious gift of time...seconds and minutes that can save lives.

Sincerely,

Susan F. Zevin

Deputy Assistant Administrator for Operations

NAT OI NAL OCEANIC AND ATMOSPHERIC ADMINISTRAT OI N

U.S. DEPARTMENT OF COMMERCE
Use of this Guide:

The information contained in this guide is provided as a reference to supplement the National Weather Service’s spotter training film and

slide series. It is not sufficient to qualify you as a SKYWARN spotter. This guide has been compiled for use by trained spotters in the field

(both fixed and mobile spotters) and communication specialists who receive and relay the reports. The safety tips contained in this guide

are geared primarily for mobile spotters, but the technical concepts that are outlined are applicable to spotters of all types. This guide is

not a general handout for the public. As a result of its specialized subject matter, a number of technical terms are used. Their meanings are

explained in the text. Many of the photos used in this guide were taken in the Great Plains area of the United States where visibilities usually

are good. Spotters in other areas of the country, especially the southern states, may have difficulty seeing some of the thunderstorm’s

features because of poor visibility caused by terrain, trees, heavy rainfall, or low clouds.

Definitions and Terminology

Severe local storms occur in all parts of the continental United

States in an average year. As part of their training, storm spotters

should be aware of severe storm definitions and terminology used by

the National Weather Service.

Watch — Conditions are favorable for the severe weather event in

or near the watch area. Watches are issued for tornadoes, severe

thunderstorms, and flash floods.

Warning — The severe weather event is imminent or occurring in

the warned area. Warnings are issued for tornadoes, severe

thunderstorms, flash floods, and river flooding.

Severe Thunderstorm — A storm that produces hail 3/4 inch in

diameter or larger and/or wind gusts of 58 mph or more.

Tornado — A violently rotating column of air attached to a

thunderstorm and in contact with the ground.

Funnel Cloud — A rotating, funnel-shaped cloud extending

downward from a thunderstorm base.

Downburst — A strong downdraft with an outrush of damaging

wind on or near the ground.

Flash Flood — A rapid rise in water, usually within 12 hours of a

period of heavy rain or other causative agent (i.e., dam break).
Severe Weather Reporting Criteria

Many types of weather information are needed from storm

spotters; however, some types of information are much more

important than others. Strict adherence to the reporting criteria

allows vital information to be communicated as soon as possible.

Also, some of the reporting criteria should receive higher priority

communication than others. You should report the following weather

events.

Urgent Priority

Tornado

Funnel cloud

Rotating wall cloud

Flash flooding

High Priority

Hail 3/4-inch diameter or larger

Wind speed greater than 58 mph

Persistent non-rotating wall cloud

Rainfall 1 inch or more per hour

Lower Priority

Hail 1/2-inch diameter or larger

Wind speed greater than 40 mph

Cloud features suggesting storm organization

Other locally-defined criteria

NOTE: When reporting 1/2-inch diameter hail, do not use the term

“marble” since marbles can come in a variety of sizes. In areas prone

to severe weather, some offices may not need reports of 1/2-inch hail.

Contact your local NWS office for specific adjustments to the criteria

suggested above.

Receiving Hazardous Weather

Information

Spotters, both point and mobile, should have access to reliable

hazardous weather information. Many spotters have access to

amateur radio networks. These networks will likely have one or more

radio operators stationed at NWS offices for providing radar and other

meteorological information to the net. Spotter networks not affiliated

with amateur radio groups should consider having one or more

members monitor NOAA Weather Radio and other outlets for weather

information. These networks may wish to select a liaison person who

could work with nearby amateur radio groups or the local NWS office.

Another means of receiving hazardous weather information is the

Emergency Managers Weather Information Network (EMWIN). EMWIN

is a low-cost, low-speed data stream of NWS products (warnings,

observations, etc.) provided as a service to local emergency managers.

The signal may be received directly from the GOES weather satellites,

processed for local use, and/or rebroadcast to the surrounding area on

VHF radio. Numerous EMWIN systems are coming on line across the

country, which should allow both point and mobile spotters access to

critical NWS information. This in turn should allow for more efficient

operation of spotter networks in the area.

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Thunderstorm Hazards and

Safety Tips

Flash Floods

Flash flooding is a major killer. Many flash floods occur at night,

which makes them more difficult to see. As a storm spotter, you may

encounter flash floods at any time. Heeding the following flash flood

safety rules may save your life.

• DO NOT attempt to drive or walk across a flooded roadway

or low water crossing. Nearly half of all flash flood deaths are

vehicle-related. Moving water 2 feet deep will carry away

most cars.

• If your vehicle becomes caught in high water and stalls, leave it

immediately and seek higher ground if you can do so safely.

Rapidly rising water may sweep a vehicle and its occupants

away.

• Be especially careful at night when flash floods are harder

to recognize.

Lightning

Lightning occurs in all thunderstorms and is also a significant

threat to life (figure 1). Storm spotters are especially vulnerable to

being struck by lightning since they are often in prime strike

locations, such as in open fields or on hilltops. The following

lightning safety rules are important.

• Lightning tends to strike the tallest object in an area...make sure

it is not you. Remain in your vehicle or an indoor location

whenever possible.

• If you must go outside, crouch down to make yourself a poor

lightning target. Do not lie flat on the ground since you will

be more likely to be severely shocked if lightning strikes

close to you.

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Figure 1: Cloud-to-ground lightning. Photo - Courtesy Roger Edwards.
Hail

Although large hail rarely causes fatalities, it is the most destructive

element associated with severe local storms and can cause considerable

property damage (figure 2). If the storm you are observing produces

a tornado, it will likely form very near the shaft of large hail.

These hail safety tips can help minimize damage to your vehicle and

possible injury to you.

• Substantial structures and highway overpasses (out of traffic

lanes) offer the best hail protection.

• Hard-top vehicles offer fair protection from hail up to about golf

ball sized, but significant windshield and auto body damage can

result with hail larger than golf balls.

Downbursts and Outflow Winds

A downburst is defined as a strong downdraft with an outrush of

damaging wind on or near the ground. Downbursts are responsible for

most thunderstorm wind damage. Winds may exceed 100mph in very

strong downbursts (see figures 14 and 15). The following downburst

safety rules are important.

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

control. Wind speed and direction can change rapidly in a

downburst.

• Blowing dust or heavy rain may accompany downbursts. Be

prepared for sudden changes in visibility that may create

hazardous spotting conditions.

• Point spotters observing from a substantial building should

move away from windows as the downburst approaches.

Tornadoes

Tornadoes pose a significant threat to all spotters. High winds and

flying debris can result in hazardous spotting conditions and

significant damage to vehicles and buildings (figure 3). Be especially

alert for tornadoes when storm spotting. These safety rules could

save your life.

• Mobile spotters in high visibility areas, such as open rural areas,

may be able to drive away from an approaching tornado.

REMEMBER, THIS DOES NOT APPLY TO SPOTTERS IN URBAN

AREAS, INEXPERIENCED SPOTTERS, SPOTTERS IN LOW

VISIBILITY LOCATIONS SUCH AS IN HEAVILY WOODED AREAS,

OR MEMBERS OF THE GENERAL PUBLIC. Spotters should be

familiar with their area and have a planned escape route.

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Figure 3: Tornado. Photo - © Alan Moller.

Figure 2: Hail Damage. Photo - Courtesy James Purpura.
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the development stage may begin as higher based altocumulus

clouds. As moisture becomes more plentiful, the base of the storm

may lower.

b. Mature Stage (figure 5) — This is the strongest and most

dangerous stage of the storm’s life cycle. At this stage, the storm

contains both upward and downward moving air currents (updrafts

and downdrafts) with precipitation in the downdraft area. The

downdraft results from precipitation evaporating, which causes

cooling. To a lesser extent, the falling precipitation itself creates

downward drag. When the cool downdraft hits the ground, it spreads

out and forms a gust front, which may include damaging winds called

a downburst. At the top of the storm, the updraft rapidly decelerates

and clouds spread out and form an anvil. If the updraft is strong, a

“bubble” of cloud, called an “overshooting top,” will be pushed above

the anvil. Spotters should pay particular attention to a storm with an

overshooting top since the area beneath the top is a preferred area

for severe weather formation.

c. Dissipating Stage (figure 6) — Eventually, excessive precipitation

and downdraft will weaken the updraft. Downdrafts dominate the

storm and any overshooting top disappears. At the surface, the gust

front will move away from the storm and cut off the inflow of energy

into the storm. This is indicative of the dissipating stage.

• If you can’t avoid an oncoming tornado, you should take shelter

in a substantial building, ditch, ravine, or other low spot (but be

cautious of flash flooding).

Safe Viewing Tips

Mobile spotters should try to view a storm from its right flank.

This will usually provide the best viewing angle, the best contrast, and

it will generally keep spotters out of the storm’s path. For storms

moving to the northeast, the best viewing location is from the south

or southeast. With east or southeast moving storms, a viewing angle

from the south or southwest (respectively) is preferred although

spotters will need to be more conscious of the storm’s movement and

have an escape route available.

Thunderstorm Life Cycle

All thunderstorms, whether they become severe or not, proceed

through a 3-stage life cycle.

a. Cumulus Stage (figure 4) — Occurs when thunderstorm

development begins. At this stage, the storm consists only of

updrafts (upward-moving air currents). These updrafts reach heights

of around 20,000 feet above the ground. In the western United States,

Figure 4: Towering cumulus stage of a thunderstorm. Figure 5: Mature stage of a thunderstorm. Figure 6: Dissipating stage of a thunderstorm.
Visual Indications of Updraft Strength

and Organization

Several visual clues will help the spotter determine if a storm has

severe weather potential. These clues are evident in the upper,

middle, and lower levels of the storm. The spotter should pay

particular attention to these clues, especially when watching more

than one storm at a time.

a. Upper-Level Storm Clues — These clues are best seen at a

distance of 30–40 miles from the storm, so they may be difficult to see

in poor visibility areas. The primary clues are a large overshooting

top that persists for more than 10 minutes and an anvil with sharp

and well-defined edges (figure 8). Storms with weaker updrafts will

usually have an anvil that is thin, wispy, and fuzzy.

Depending on the type and number of cells, thunderstorms may be

divided into four main categories:

• Single-cell storms are generally weak, short-lived, and poorly

organized. “Pulse storms” are strong single-cell storms and thus

are quite rare.

• Multicell cluster storms are the most common type of storm and

consist of a series of cells moving along as one unit.

• Multicell line storms, commonly called “squall lines,” consist of

a long line of storms with a continuous gust front at the leading

edge.

• Supercell storms have a single updraft, are very strong, and

always produce significant severe weather. (See figure 7 for

additional information about each type of thunderstorm. These

thunderstorm types will be discussed in more detail in the

advanced storm spotter’s training program.)

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Figure 7: Overview of the thunderstorm spectrum, with characteristics of each

primary storm type.

Figure 8: Distant supercell storm showing upper-level visual clues of storm strength.

Photo - Courtesy Bill Martin.
6

Figure 9: Non-severe multicell storm. Photo - NSSL.

Figure 10: Distant supercell storm with flanking line building into main storm tower.

Photo - Courtesy Roger Edwards.

b. Mid-Level Storm Clues — These clues are best seen at a distance

of 10–20 miles from the storm, and again, some of these features will

be difficult to see in poor visibility areas. These features are

concentrated in the main storm tower area. The primary clues are the

following.

• A solid appearing updraft tower with a sharp, cauliflower

definition in the storm tower (again, see figure 8). Some storms

have a soft or “mushy” appearance indicative of a weaker

updraft and therefore are a poor candidate for producing severe

weather (figure 9).

• A flanking line — a row of small cloud towers that build up

(stair-step) into the main storm tower from the south or

southwest. The flanking line does not suggest updraft strength,

but it does indicate storm-scale organization necessary for

persistent severe weather (figure10).

c. Low-Level Storm Clues — These clues are best seen at a distance

within 10 miles of the storm and are the easiest clues to detect in

lower visibility areas. Low-level storm features can be the most

critical in determining a storm’s severe potential but can result in the

most confusion among storm spotters. The primary clues are the

following.

• The rain-free base — a low, flat cloud base from which little

visible precipitation is falling. However, the precipitation in this

area is often in the form of large hail. The rain-free base defines

the primary inflow and updraft area in the storm. The preferred

area for severe weather formation is near and just north/east of

the rain-free base (figure11).

• The wall cloud — an isolated lowering of the rain-free base. It is

always attached to the cloud base. It indicates the storm’s

strongest updraft area, and it is the primary location for severe

weather development. Wall clouds with persistent rotation

(10 minutes or more) are especially significant since they denote

a very dangerous storm that may produce large hail, strong

downbursts, or a tornado (figure 12).
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Figure12: Wall cloud. Photo - NSSL.

Non-Tornadic Severe Weather Events

There are other types of severe weather events that spotters

should report besides tornadoes. These include downbursts and large

hail. Spotters should continue to report these phenomena even if a

tornado is in progress since this information is important to the NWS,

public, and aviation interests.

A downburst is defined as a strong downdraft from a thunderstorm

with an outrush of damaging wind on or near the ground. Damaging

downbursts, although relatively rare themselves, are much more

common than tornadoes. Because of their small size and short

lifespan, it is difficult to detect and warn for downbursts. Downbursts

are divided into two categories.

• Macroburst — Swath of damaging wind is 2.5 miles or more wide.

• Microburst — Swath of damaging wind is less than 2.5 miles wide.

Figure 13 shows the life cycle of a downburst. The initial stage

begins as the downburst starts to descend from the cloud base. The

second stage, called the “impact” stage, occurs when the downburst

makes contact with the ground and begins to spread outward. Expect

the strongest wind speeds shortly after the downburst hits the

ground. The impact stage is also the most dangerous stage for

aviation as aircraft caught in the strong winds may see wing lift

Figure11: Rain-free base. Photo - NSSL.

Figure13: Life cycle of a microburst.
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Figure 15: Microburst dust foot. Strong winds blowing left to right away from rain area.

Photo - NWS.

decreased, possibly causing the plane to stall and crash.

“Dissipation,” the final stage, occurs when the downburst spreads

out and weakens. Beware, other downbursts may form later.

One of two primary ways to detect a downburst is to spot a rain

foot. The rain foot is a pronounced outward deflection of the

precipitation area near the ground (figure 14), marking an area of

strong outflow winds. The second identification method is the

presence of a dust foot, a plume of dust raised as the downburst

reaches the ground and moves away from the impact point (figure 15).

The dust foot is most common in the High Plains, western states, and

over plowed fields.

Large hail is a common occurrence in strong thunderstorms,

especially supercells. Hail forms as supercooled water is carried aloft

by the updraft and freezes. Hail size is determined by the updraft

strength, i.e., the stronger the updraft, the larger the hailstones.

Single cell storms can produce hail up to about nickel size, multicell

storms to about golf ball size, and supercells up to about softball size.

If you spot hail larger than golf balls, you are very near a supercell’s

main updraft and should go quickly to a safe place as described in the

Thunderstorm Hazard and Safety Tips section of this Guide.

Figure 14: Microburst rain foot, visible on left side of rain area. Photo - © Alan Moller.

Supercell Structure and Appearance

A supercell thunderstorm is a long-lived storm containing a

mesocyclone — an area of intense, storm-scale rotation extending

through much of the depth of the storm. Supercell storms are usually

separated from other thunderstorms or may even be isolated. This

separation allows them to feed upon warm, moist air from miles

around. Supercell occurrences are rare but pose a significant threat

to life and property.

Figure 16 is a schematic side view of a supercell as a spotter might

see it when he/she is looking west/northwest at a northeast moving

storm. This is the safest viewing angle for the spotter. For east or

southeast moving storms, spotters should position themselves to the

south or southwest (respectively) of the storm for a safe viewing

angle. The storm is generally moving from left to right. The main

updraft of warm, moist air is entering the storm at the cloud base

below the main storm tower. Strong winds aloft are blowing from the

southwest to northeast. Air in the upper portion of the updraft

eventually becomes colder than the surrounding air and upward

motion decelerates. The cloud spreads rapidly, forming an “anvil.”

As precipitation begins to occur, downdrafts are created.
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Figure 17: Plan-view diagram of a “classic” supercell storm.

Figure 16: Slide-view diagram of a “classic” supercell storm.

Table 1

Tornadic Wall Cloud Characteristics

➤Persistence

➤Persistent rotation

➤Strong surface-based inflow

➤Rapid vertical motion (both upward and downward)

Figure 17 is a “bird’s eye” view of the same storm and its associated

weather from above, looking down. The intense updraft, which is rising

out of the page, is located within the main storm tower generally above

the rain-free cloud base. The “front flank” downdraft (FFD) sinks to the

ground in the area where precipitation is falling in the forward position of

the storm (usually north or northeast of the updraft). A second

downdraft forms just southwest of the updraft. This is the area, near the

intersection of the updraft and this “rear-flank” downdraft (RFD), where a

tornado is most likely to occur. Large hail is likely to fall just outside the

updraft core, mainly northeast of the updraft. Tornadoes also may form

along the gust front and flanking line; however, these are usually weak and

short-lived.

As described earlier, some of the more important features associated

with supercells include the rain-free base and the wall cloud.

The rain-free base is an area of smooth, flat cloud base beneath the

main storm tower from which little or no precipitation is falling. The

rain-free base is usually just to the rear (generally south or southwest) of

the precipitation area and marks the main area of inflow where warm,

moist air at low levels enters the storm.

The wall cloud is an isolated cloud lowering attached to the rain-free

base. It is usually about 2 miles in diameter and marks the area of

strongest updraft in the storm. As the thunderstorm intensifies, the

updraft draws in low-level air from the precipitation area. This raincooled air is very humid, thus, the moisture in the air quickly condenses

to form the wall cloud. The wall cloud is sometimes to the rear

(generally south or southwest) of the precipitation area but at other

times may be on the east or southwest side of the precipitation. Table 1

(below) shows a list of tornadic wall cloud characteristics. Less than
half of the observed wall clouds will actually produce tornadoes, and

not every tornadic wall cloud will have all of these characteristics.

Nevertheless, they are good rules of thumb.

Wall clouds are not rare. They can form at the base of any

thunderstorm, supercell or otherwise, having a sufficiently strong

updraft and adequate moisture available in the sub-cloud layer. In

fact, spotters, especially mobile ones, may see several wall clouds

during an average severe weather season. Your key will be to use the

rules of thumb to determine which wall clouds are potential tornado

producers.

Typical Tornado Life Cycle

The typical tornado goes through a three-stage life cycle:

developing, mature, and dissipating. Figure 18 shows the developing

tornado. A rotating wall cloud is evident, with tighter rotation evident

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Figure 19: Tornado in mature stage.

Photo - © Alan Moller.

Figure 18: Developing stage of a tornado.

 Photo - © Alan Moller.

in the base of the wall cloud. As the tornadic circulation continues to

develop, the condensation funnel appears. It may not be a tornado

yet since the visible cloud is less than half way to the ground;

however, in some cases you will see a dust whirl on the ground

indicating a tornado before the condensation funnel touches down.

Figure 19 shows the mature tornado. The storm is still getting a

good inflow of warm, moist air, and the circulation is near its

maximum size and intensity. The inflow becomes disrupted a short

time later, which starts the dissipating stage.

Figure 20 shows a dissipating tornado. This stage is sometimes

called the “rope stage.” The condensation funnel becomes tilted and

shrinks into a contorted, ropelike configuration. The tornado is still

dangerous even at this late stage in its life. Some tornadoes,

especially larger ones, dissipate as the funnel lifts into a bowl-shaped

lowering of the cloud base.

Figure 20: Tornado in its dissipating stage.

Photo - Courtesy Gary Woodall.
Tornado Classification

Tornado intensities are classified by the Fujita Damage Scale

developed by Dr. T. Theodore Fujita, a renowned severe weather

researcher. The scale ranges from F0-F5, with F5 storms creating

incredible damage. The NWS also uses a broader, three-level

classification scale, consisting of “weak” (F0-F1), “strong” (F2-F3), and

“violent” (F4-F5). Figures 21, 22, and 23 are examples of these tornado

classifications. Dimensions shown in figures are upper limits, not

averages.

Weak tornadoes may not be associated with mesocyclones. They

are difficult to detect and forecast; thus, there is a heavy reliance on

spotters to identify and report these storms.

Strong tornadoes are typically associated with mesocyclones.

They are easier to infer from radar, but spotter reports of these

storms are still a very important part of the warning process.

Violent tornadoes are virtually always associated with a powerful

mesocyclone. Their signatures are often easily detectable on radar,

but spotter reports provide vital ground truth of actual storm

conditions.

Many strong and violent tornadoes develop as multiple vortex

tornadoes. They consist of one large circulation (vortex) with several

smaller circulations rotating around it. The smaller vortices usually

are responsible for the extreme winds and damage associated with

violent tornadoes.

As discussed earlier, storm spotters should NOT wait for a

condensation cloud to reach the ground before reporting a tornado.

Instead, spotters should look for a rotating dust/debris cloud on the

ground below the funnel. This rule is especially true in the western

states, where storm cloud bases are relatively high and the air below

the cloud base can be quite dry.

Although the tornadoes pictured here are typical of the intensities

shown, spotters should not attempt to judge tornado intensity based

only on size. Cases have been documented of small, violent

tornadoes and weak tornadoes with large condensation funnels.

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Figure 21: Typical weak tornado. Photo - © Tim Marshall.

Figure 22: Typical strong tornado. Photo - NWS.

Figure 23: Typical violent tornado.

Photo - Courtesy Jeff Formby.
Tornado Look-Alikes

One of the biggest challenges in tornado spotting is determining

whether you are seeing the “real thing” or a tornado look-alike. Two

key features present with a tornado are a debris cloud near the

ground and organized rotation about a vertical axis.

Rainshafts sometimes may be located in a thunderstorm where a

tornado normally would be found (figure 24). Rainshafts lack a debris

cloud near the ground and organized rotation about a vertical axis.

Figure 25 shows a smoke column and is one of the most convincing

look-alikes ever photographed. To properly identify tougher cases,

watch for a minute or so to look for rotation, both in the cloud and in

the debris near the ground. Talk with your colleague. Also, talk with

other spotters in the area; they may have a closer look or a better

viewing angle.

Scud clouds are small, detached, wind-torn clouds that often form

near thunderstorms (figure 26). Scud clouds can change shape

rapidly and sometimes take on the appearance of a wall cloud or

funnel cloud. Remember, though, that wall clouds are always

attached to the cloud base and funnel clouds always rotate. Roll

clouds form along the gust front as the cool outflow lifts warm, moist

air. They take on the shape of horizontal tubes, and in some cases,

you can actually see a horizontal rolling motion (figure 27).

Shelf clouds form in a process similar to roll clouds, but shelf

clouds take on more of a wedge shape. You probably will see

considerable motion in the shelf cloud’s base. The motion will be

turbulent in nature, though, without the persistent, organized rotation

seen in true wall clouds. Shelf clouds are more common than roll

clouds and can define areas of strong downburst winds but usually

are not favorable locations for tornado development (figure28). They

are associated with cool outflow (downward vertical motion) while a

wall cloud is associated with a warm updraft (upward vertical

motion).

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Figure 24: Distant rain shaft. Photo- © Tim Marshall. Figure 25: Distant smoke column. Photo- Courtesy Brian Curran.
Supercell Variations and Unusual

Situations

There are variations in the supercell model that spotters may see

from time to time. The spotter should be familiar with the different

types of supercells, plus other non-typical events that may occur.

This guide will discuss some of these variations briefly. More in-depth

discussions are included in the Advanced Severe Storm Spotters

series.

The Heavy Precipitation or HP supercell is quite common east of

the Great Plains and may be the most common type of supercell

nationwide. The HP supercell produces tremendous amounts of

precipitation around the mesocyclone, especially on the west and

southwest sides of the mesocyclone. Heavy precipitation can

produce poor contrast under the cloud base or completely obscure

important features. If a tornado or wall cloud is obscured, striations,

low level inflow bands, and mid-level cloud bands may be used to

infer storm rotation (figures 29 and 30). If any of these clues suggest

storm rotation, DO NOT DRIVE THROUGH PRECIPITATION TO SEE

IF A TORNADO IS PRESENT. Even without a tornado, large hail and

strong downburst winds are likely, and heavy precipitation will

obscure visibility and may produce flash flooding.

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Figure 27: Roll cloud. Photo - Courtesy Gary Woodall.

Figure 28: Shelf cloud. Photo - NSSL.

Figure 26: Scud clouds. Photo - Courtesy Roger Edwards.

Figure 29: Side view diagram of an HP supercell.
The Low Precipitation or LP supercell most often occurs in the

High Plains but has been documented in other areas. The LP

supercell produces very little liquid precipitation and outflow but is a

prolific hail producer. LPs can be difficult to detect on radar but are

fairly easy to identify visually. The main precipitation area will be

small and light, and the storm tower will be slender, striated, and bell

shaped or flared out close to the ground (figures 31 and 32). It is

unusual for the LP supercell to produce a large tornado.

Another non-typical event that occurs is the “gustnado” (figure 33).

Sometimes vortices may develop in a gust front’s outflow as it moves

across the ground. These vortices are called “gustnados.” They are

not associated with the updraft area of the storm and generally do not

extend up to the cloud base. Thus, in some aspects they are not

“legitimate” tornadoes, however, they do pose a threat to life and

property and should be reported.

Tornadoes that occur at night are obviously much more difficult to

observe and recognize than those occurring during the day (figure

34). Limited assistance may be provided by lightning (although

lightning may illuminate different parts of the storm at different

Figure 30: Side view of an HP supercell. Photo - © Alan Moller.

Figure 31: Side view diagram of an LP supercell.

Figure 32: Side view of an LP supercell. Photo - NSSL.

times), power line flashes, tornado roaring sounds (although strong

winds blowing through trees can produce a roaring sound similar to

that of a tornado), and the presence of large hail.

14
Summary

This guide has given you an introductory look at severe

thunderstorms, the hazardous weather they produce, and some of the

visual clues available to help determine a storm’s severity. It has

provided an overview of the fundamentals of severe storm spotting.

Use this guide and the presentation that accompanies it as a first step

in your severe thunderstorm education, not your last step. As your

experience base grows, we encourage you to attend regular refresher

spotter training courses and advanced spotter training programs.

These continuing programs will enable you to increase your

knowledge of severe thunderstorms and become an even more

valuable part of the severe weather warning system. Figure 33: Gustnado visible directly below shelf cloud lowering..

Photo - NSSL.

Figure 34: Lightning-illuminated supercell.

Note the suspicious feature just right of the lightning.

Photo - © Alan Moller.

15
Notes

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Figure Credits

Cover and Title Page - Photo by National Severe Storms Laboratory

(NSSL).

Figure 1 - Courtesy Roger Edwards.

Figure 2 - Courtesy James Purpura.

Figure 3 - © Alan Moller.

Figures 4 - 7 - NOAA.

Figure 8 - Courtesy Bill Martin.

Figure 9 - NSSL.

Figure 10 - Courtesy Roger Edwards.

Figure 11 - NSSL.

Figure 12 - NSSL.

Figure 13 - NOAA.

Figure 14 - © Alan Moller.

Figure 15 - NWS.

Figure 16 - NOAA.

Figure 17 - NOAA.

Figure 18 - © Alan Moller.

Figure 19 - © Alan Moller.

Figure 20 - Courtesy Gary Woodall.

Figure 21 - © Tim Marshall.

Figure 22 - NWS.

Figure 23 - Courtesy Jeff Formby.

Figure 24 - © Tim Marshall.

Figure 25 - Courtesy Brian Curran.

Figure 26 - Courtesy Roger Edwards.

Figure 27 - Courtesy Gary Woodall.

Figure 28 - NSSL.

Figure 29 - NOAA.

Figure 30 - © Alan Moller.

Figure 31 - NOAA.

Figure 32 - NSSL.

Figure 33 - NSSL.

Figure 34 - © Alan Moller.

16
Spotter Reporting Procedures

• From radio or cellular phone-equipped vehicles, report severe weather observations to a central collection point

and request them to relay the report to the National Weather Service.

• Law enforcement and Emergency Management spotters—report to your dispatcher or net controller via NAWAS,

radio, cellular phone, or other direct communications links as prescribed by your Emergency Operations Plan.

• When the telephone is your only communications method, call your primary or alternate contact, and ask him or

her to relay your report to the National Weather Service. If the call is long distance, you can make it collect.

Report promptly as the storm may interrupt communications.

Report Briefly:

What you have seen: tornado, funnel cloud, wall cloud, waterspout, flash flooding, etc.

Where you saw it: the direction and distance from a known location, i.e., 3 miles south of Beltsville. To avoid

confusion, make sure you report the event location and not your location.

When you saw it: make sure you note the time of your observation.

What it was doing: describe the storm’s direction and speed of travel, size and intensity, and destructiveness.

Include any amount of uncertainty as needed, i.e., “funnel cloud; no debris visible at the surface, but too far away to

be certain it is not on the ground.”

Identify yourself and your location. Give spotter code number if you have one.

Report:

1. Tornado, funnel cloud, waterspout, or wall cloud.

2. Large hail, as defined by your local NWS office.

3. Damaging winds (usually greater than 50 mph).

4. Flash flooding.

5. Other criteria as defined by your local NWS office.