Cover Photo - Alan Moller
1

RODUCTION

vice (NWS) has a number of devices for

rms. Included in these are radar, satel

 networks. However, the most important

torms is the trained eye of the storm

 quite often in severe storm warnings,

 will only indicate areas and intensities

give any indication of cloud formations

th a storm. Doppler radar, which is

 country, will give some indication of air

ppler radar, though, will not give these

 level. It is impossible for any radar to

 event in its coverage area, and radar

e weather when, in fact, none is present.

ection networks provide general

 are extremely valuable in data-sparse

tainous terrain or over bodies of water).

tent thunderstorm areas and can be of

g. These systems provide little in the

e information, though, and are not

mes of fast-breaking severe weather.

perform an invaluable service for the

vations of tornadoes, hail, wind, and

s provide a truly reliable information

ection and verification. By providing

ng NWS staff members in their warning

NWS to fulfill its mission of protecting

elping to provide the citizens of your

y life-saving information.

Objectives of this Handbook

During the past several years, researchers have uncovered a

tremendous amount of information regarding severe thunderstormstructure and behavior. New theories regarding thunderstormformation and tornado development have been presented. Stormintercept teams have correlated these theories with observed visualfeatures. Our current understanding of the thunderstorm ismarkedly more complete than it was just ten years ago.

With this handbook and the Advanced Spotter Training Slide Set

which was released a few years ago, the time has come to pass thisnew understanding on to you, the spotter. Only by providing freshtraining material can the NWS expect to maintain what has becomea very important group of observers.

Prerequisites for Using this Guide

The information contained in this guide is not for the novice

spotter. It is recommended that spotters go through two or morebasic spotter training sessions and have some experience at actualstorm spotting before attempting the intermediate/advanced training material. Spotters should be comfortable with the basic conceptsof storm structure and storm spotting. Obviously, spotters shouldhave a desire to learn the latest concepts of tornado and severethunderstorm behavior.

II. REPORTING PROCEDURES

Primary and Secondary Contacts

It is essential that any spotter network have a clear set

of procedures for reporting severe weather and other observations.All networks should have a designated methodology for relayingreports from the field to the local NWS office. There should be aprimary contact for activation and operation of the spotter network.It is also suggested that a secondary contact be established for thoseoccasions when widespread severe weather is occurring or whenthe primary contact is not available.
2

comprise the backbone of many spotter

dio networks include an operator at the

of reports and direction of spotters in the

s proven to be an effective, efficient

weather observations. Other operators

on or radio stations in the NWS office’s

 department personnel also serve as

reas. Many of these groups report to a

ays reports to the NWS. These spotter

 secondary contact (such as the dis

nty agency) for those times when pri

mpeded.

so receive at least basic spotter training.

ally observing the storms in the field,

al link in the severe weather information

ith thunderstorm and spotting terminol

screen out less important observations

t reports to the local NWS office.

ulated areas, private citizens may have

hese individuals may not be as well

dio or law enforcement-based groups,

hed reporting procedures. Local law

management offices are candidates for

n

on job. It is difficult, if not impossible,

y observe all aspects of a thunderstorm.

otters and spotter groups to coordinate

h the NWS and with each other) to

essment of the storm.

Spotters with two-way radio communications should talk not only

with their dispatch/control personnel but with other spotters in thearea. Positioning spotter teams at several strategic locations arounda storm, with active communication between the spotters, shouldenable a great deal of information concerning the thunderstorm tobe relayed to the local NWS office.

If two or more spotter groups are working in the same area

(i.e., an amateur radio group and a law enforcement group), thenthese groups should share information regarding their observations.The NWS should also attempt to coordinate between spottergroups. As a storm moves from one spotter group’s area to another,the downstream spotter group should be notified well in advance toallow time for their activation and deployment.

Reporting Criteria

There are certain criteria for reporting severe weather. Recall that

a thunderstorm is defined as severe if it produces a tornado, hail 3/4inch in diameter or larger, and/or wind gusts 58 miles an hour orhigher. It would be desirable to report events associated with athunderstorm before they reach these severe levels. Use the following guidelines for reporting weather events.

Report hail occurrences when the hailstones have a diameter of 1/

2 inch, and report wind gusts when their speed reaches 50 miles anhour. See tables 1 and 2 for estimations of hail size and wind speed.Obviously, tornadoes and funnel clouds should be reported.A funnel cloud is defined as a violently rotating column of air whichis not in contact with the ground. It is usually marked by a funnelshaped cloud extending downward from the cloud base (hence itsname). If the violently rotating air column reaches the ground, it iscalled a tornado. An important point to note is that the visiblefunnel DOES NOT have to extend to the ground for a tornado to bepresent. Instead, look for a rotating cloud of dust and debris underneath a funnel cloud as evidence that the tornado’s circulation hasreached the ground.
3

il Size Estimates

25 Penny ...................... 0.75

00 Half Dollar .............. 1.25

75 Tennis Ball ............... 2.50

75 Grapefruit ............... 4.00

d Speed Estimates

Effects

s in motion; whistling in telephone wires

 motion

ff of trees; wind impedes walking

mneys and TV antennas; pushes over

d trees

ff roofs; windows broken; trailer houses

houses; weak buildings and trailer houses

ge trees uprooted

ns.

reported, but the reporting criteria are

severe weather events. A flash flood is

ater usually during or after a period

soil type, terrain, and urbanization

 the amount of runoff which will occur

ount of rain. Consult your local NWS

reporting procedures in your area.

When making a report, you (or your dispatcher/control person)

should include the following information:

(1) WHO you are, and the name of your spotter group.(2) WHERE the event is occurring. Use reports from other nearby

spotters to triangulate and pinpoint the event’s location.

(3) WHAT you have seen (the severe weather event).

(4) MOVEMENT of the event. When estimating movement, don’t

use the motion of small cloud elements for estimation. Instead,observe the storms a whole for estimates of motion.

III. SAFETY TIPS

Safety should be first and foremost on the mind of a spotter.

Remember, the NWS values your safety more than we do yourobservations. It is essential that spotters proceed into the fieldarmed not only with knowledge of the storms but also with anunderstanding of the dangers posed by thunderstorms.

When spotting, travel in pairs if at all possible. When moving,

this will allow the driver to remain focused on the chore of drivingwhile the passenger keeps an eye on the sky and handles anycommunication with the dispatcher. When stopped, two sets of eyesare available for observation.

Keep aware of the local environment at all times. When in the

vicinity of a thunderstorm, keep a 2-mile “buffer zone” betweenyou and the storm. Frequently check the sky overhead and behindto ensure no unexpected events (such as a new tornado) are developing. Always have an escape route available, in case threateningweather approaches or if you get within the 2-mile “buffer zone.”

Lightning is the number one killer among weather phenomena.

During atypical year, lightning kills more people than hurricanes,tornadoes, and winter storms combined. The two main threats
4

e intense heat (about 15,000 degrees

rrent associated with the stroke,

 (less than 1 ampere can be fatal).

est weather hazard facing the spotter.

er will usually be in a preferred light

n, on a hilltop, etc.). Whenever possible,

hicle to minimize the chance of being

must leave your vehicle, crouch as low as

less-favorable target.

t threat to life, but hailstorms are the

 affect the United States. Each year,

lion in damage primarily to crops,

hailstones (2 inches or more in diameter)

es an hour as they fall to earth. If such a

esults can be fatal. There have been only

d deaths in the United States, but a

ver 100 people in 1976. A vehicle will

ection from moderate-sized hailstones.

e may damage windshields, so avoid

ible.

ed thunderstorm threats. A downburst is

aft with an outrush of damaging winds

e. Downbursts are responsible for the

used a number of airliner accidents in

When people experience property damage

n do not believe that “just wind” could

nd they assume that they were struck by

gest downbursts have wind gusts to

are capable of the same damage as a

 based on their size. If the swath

iles or greater, it is called a macroburst.

miles across, it is called a microburst.

long-term, large-scale events, while

ck-hitting phenomena. Microbursts are

subdivided as wet or dry microbursts, depending on how much

rain falls with the microburst. If very heavy rain falls with themicroburst, it is called a wet microburst, while a dry microburst

has little or no rain reaching the ground. Chapter VIII discussesdownbursts in more detail and outlines some spotting tipsregarding downbursts.

Flash floods are another example of an underrated thunderstorm

threat. In the past few years, more people have been killed in flashfloods than in tornadoes. Two factors are responsible for this. First,we have urbanized. Where rain water used to have open fields inwhich to run off, it now has highway intersections, basements,streets, etc. Second, the public as a whole is apathetic about flashflooding. We simply do not treat flash flooding with the respect itdeserves. Many of the recent deaths associated with flash floodinghave occurred because people attempted to drive their vehiclesacross a flooded low-water crossing and were swept away by thefloodwaters. It takes less than two feet of moving water to sweepaway a vehicle.

When spotting in a flash flood situation, follow these common

sense safety tips. Remember that flash flooding is most dangerousat night when the effects of flash flooding are difficult to see. Sincemost flash floods occur at night, this problem is compounded.Avoid low water crossings and don’t drive into areas where watercovers the road. If you are caught in a flash flood, abandon yourvehicle and quickly get to higher ground.

Last but not least is the tornado. Again, a tornado is defined as

a violently rotating column of air in contact with the ground andpendant from a thunderstorm (whether or not a condensationfunnel is visible to the ground). If the violently rotating columnof air has not touched the ground, it is called a funnel cloud. We

will discuss the tornado in more detail in Chapter IX.

If a tornado is approaching your location, drive away from the

tornado IF you are in open country, IF the location and motion ofthe tornado are known, and IF you are familiar with the local roadnetwork. If you are in an urban area and escape is not possible for
5

 vehicle and get into a reinforced build

 is not available, get into a culvert, ditch,

und (that is not flooded).

y more difficult than during the day.

p you with night spotting. If possible,

 flashes to illuminate the important parts

ough, lightning strokes are very brief

ts of the storm from different angles,

lt to accurately report what is occurring.

most dangerous part of the storm is near

e overhead within a few minutes. If you

a tornado may be close to your location.

ot all tornadoes have a loud roar, and

may also possess a loud roar. Finally, if

 not far from your location (i.e., within

g the horizon for bright flashes of light

wer lines and transformers.

HUNDERSTORM

ding of the thunderstorm before you can

 and other phenomena produced by the

 is convenient to think of a thunder

ng in the sky. Actually, a thunderstorm

rocess that takes heat and moisture near

ports it to the upper levels of the atmo

this process are the clouds, precipitation,

with the thunderstorm.

re are roughly 2,000 thunderstorms in

of these storms are beneficial, bringing

percent) of these storms is classified as

 3/4 inch in diameter or larger and/or

ts of 58 miles an hour (50 knots) or

he severe storms produce tornadoes.

rstorm is theoretically capable of produc

ery few storms will actually produce

t winds, or tornadoes.

In the United States, the Florida peninsula and the southeast

plains of Colorado have the highest thunderstorm frequency.Relatively small thunderstorms occur about once a year in Alaskaand 2-3 times a year in the Pacific Northwest. Although the greatest

severe weather threat in the United States extends from Texas toMinnesota, it is important to note that no place in the United States

is completely immune to the threats of severe weather.

Atmospheric Conditions for ThunderstormDevelopment

All thunderstorms, whether or not they become severe, must

have three conditions present in order to form. The first necessarycondition is moisture in the lower to mid levels of the atmosphere.As air rises in a thunderstorm updraft, moisture condenses intosmall water drops which form clouds (and eventually precipitation).When the moisture condenses, heat is released into the air, makingit warmer and less dense than its surroundings. The added heatallows the air in the updraft to continue rising.

The second necessary condition is instability. If the airmass is

unstable, air which is pushed upward by some force will continueupward. An unstable airmass usually contains relatively warm(usually moist) air near the earth’s surface and relatively cold(usually dry) air in the mid and upper levels of the atmosphere.As the low-level air rises in an updraft, it becomes less dense thanthe surrounding air and continues to rise. This process is oftenaugmented by added heat due to condensation as discussed above.The air will continue to move upward until it becomes colder andmore dense than its surroundings.

The third necessary condition is a source of lift. Lift is a mechanism for starting an updraft in a moist, unstable airmass. The lifting

source can take on several forms. The most common source is calleddifferential heating. As the sun heats the earth’s surface, portionsof the surface (and the air just above the surface) will warm morereadily than nearby areas. These “warm pockets” are less densethan the surrounding air and will rise. If the air has sufficientmoisture and is unstable, a thunderstorm may form.
6

be mechanical in nature. Moist air

untain may reach a point where it is less

 and thunderstorms may develop. This

opes of the Rocky Mountains during the

onts, warm fronts, outflow boundaries,

nts also act as triggers by lifting moist,

here the low-level air is warmer and less

at which time thunderstorms can form.

ife Cycle

er or not they become severe, progress

may be divided into three main stages.

d the cumulus or towering cumulus

pdraft (Figure 1). As the updraft devel

ed in

orm. As

all out

 initi

 enters

The

a co

wn

n the

air

ooled

ound

sually

he gust

extreme

elop

usts.

t

 and

ed by

he gust

ance

the

he

dissipating stage of the thunderstorm (Figure 3). Even though thisthunderstorm has dissipated, its gust front may trigger new thunderstorms as it lifts warm, moist, unstable air.

Convective Variables

The three ingredients listed above are necessary for the development of thunderstorms. Recent research has found that

if the environment (wind, moisture, or instability) of a storm ischanged, then the type of storm (multicell, supercell, etc.) which isfavored to exist may change as well.

The amount of vertical wind shear in the storm’s environment is

critical in determining what type of storm will form. Vertical windshear is defined as a change in wind direction or speed with height.

Figure 1: Towering cumulus stageof the thunderstorm. At this time,all air is moving upward in thedeveloping storm.

Figure 2: Mature stage of thethunderstorm. Updraft anddowndraft are coexisting in thestorm at this time.

Figure 3: dissipating stageof the thunderstorm. Updrafthas weakened and the storm isdominated by downdraft.
7

nd shear is low (little change in wind

ulticellular storms with short-lived

w values of vertical wind shear result in

ause the inflow is weak, the outflow

rea will push the gust front out away

n, will cut off the storm’s source of

n short-lived updrafts. Precipitation

through the storm’s updraft and contrib

rt-lived. Figure 4 depicts a storm which

nvironment.

r increases, storms with longer lived

ronger vertical wind shear results in

m. The gust front will be “held” close to

ll have access to the source of warm,

time. As a result, the storm’s updraft

n the environment has strong vertical

ill tend to fall down-wind from the

 the updraft. This enables the updraft to

periods of time. Figure 5 shows a storm

hear environment.

wind shear concept is veering of the

est mile or so of atmosphere. Veering is

ng of the wind direction as it moves up

s possible to make a rough check of

ng. If there are two layers of clouds in the

ere, look closely at the directions in

moving. If the direction turns clockwise

er layers, veering is present.

d observational studies have suggested

l wind is instrumental in producing

peed is sufficiently strong (usually

ing of the wind with height is present,

rolls” may develop in the lower levels

orizontal “rolls” may then be tilted into a

by a storm’s updraft. The updraft can

tation and increase the rate of rotation.

Figure 4: Thunderstorm in a low-shear environment. Winds were below40 mph throughout the depth of this storm. Photo - Alan Moller.

Figure 5: Thunderstorm in a high-shear environment. Wind speedchanged 130 mph between the bottom and top of the storm, producingthe extreme tilt. Photo - Alan Moller.
8

as been established, a mesocyclone (see

ich may produce a tornado or significant

instability can also have an effect on

nt of moisture in the atmosphere is low

High Plains), the storms will tend to have

ounts of precipitation will fall from the

ly have strong downdrafts. If moisture

 high (as might be found in the South

low cloud bases. Copious amounts of

ground usually accompanied by weak

b to keep in mind is: the higher the

ance for dry microbursts. The lower the

ance for flash flood-producing rainfall.

which is present plays an important role

storm’s updraft and downdraft. If the

rm’s drafts will probably not be strong

weather. If the storm’s environment has

rm’s drafts will be stronger, and the

nce of producing severe weather.

in the storm’s environment, although

mentioned factors, is the presence of a

n. The mid-level capping inversion is

ween the low-level moist air and the

y) air. If the mid-level cap is weak or

will usually form early in the day before

 produce high amounts of instability.

rm, but the storms will generally be

 If the mid-level cap is strong, then

The very warm mid-level temperatures

venting updrafts from growing

ate strength is preferred for the develop

ms. A moderate cap will prevent weak

“saving up” the atmosphere’s instability.

lly in the mid to late afternoon, only the

strongest few updrafts will be able to break through the cap andcontinue to develop. These few storms can take advantage of thehigh instability which is present, with little competition fromnearby storms, and possibly develop into severe thunderstorms.

V. THUNDERSTORM TYPES

In earlier spotter training material, thunderstorms were classified

based on their destructive potential (non-severe, severe, and tornadic). A better way to classify storms is to base the categories ontheir actual physical characteristics. There is actually a continuousspectrum of thunderstorm types, but there are four broad categoriesof storms that will be discussed: single cell storms, multicell clusterstorms, multicell line storms, and supercell storms. The thunderstorm spectrum is shown in Figure 6.

Figure 6: The thunderstorm spectrum. The four main storm categories arelisted in the boxes. The bar graphs indicate the frequency and threat withstorms of various updraft strength.
9

m

have lifetimes of 20-30 minutes. They

gh to produce severe weather. A true

quite rare. Even with separate appearing

d shear, the gust front of one cell often

her cell some distance away.

 storms are non-severe, some single cell

evere weather events. These storms,

tend to form in more unstable environ

ingle cell storm. Pulse severe storms

speeds and typically produce margin

 microbursts. Brief heavy rainfall and

can also be expected (it should be

remembered that any thunderstorm is theoretically capableof producing a tornado). Figure 7 illustrates the life cycle of a pulsesevere storm. Because single cell storms are poorly organized, andseem to occur at random times and locations, it is difficult to forecast exactly when and where severe weather will occur.

dar intensity of a single cell storm (top) and

ere storm (bottom).

Figure 8: propagation of a multicell cluster storm. Cloud outlines and radarecho intensities are shown.
10

r Storm

e most com

The multicell

f cells, moving

 cell in a

derstorm life

along, each

minant cell in

o form at the

 southwest

ture cells are

of the cluster

downwind

tern) edge

and 9

multicell

ulticell cluster

(as with a

cell cluster itself may persist for several

 usually more intense than single cell

r than supercell storms. Multicell cluster

ainfall (especially if a number of cells

 downbursts (with wind speeds up to

derate-sized hail (up to about golf ball

ornadoes. Severe weather will tend

d downdrafts are close to each other

draft interface (UDI) associated with

orm

or “squall line,” as it is more commonly

e of storms with a continuous, well

leading edge of the line. The line of

e can be gaps and breaks in the line.

c diagram of a squall line. As the gust

front moves forward, the cold outflow forces warm unstableair into the updraft. The main updraft is usually at the leading(eastern) edge of the storm, with the heaviest rain and largesthail just behind (to the west of) the updraft. Lighter rain, associated with older cells, often covers a large area behind the activeleading edge of the squall line.

Squall lines can produce hail up to about golf ball size, heavy

rainfall, and weak tornadoes, but they are best known as prolificdownburst producers. Occasionally, an extremely strong downburst will accelerate a portion of the squall line ahead of the restof the line. This produces what is called a bow echo (Figure 11).

As Figure 11 illustrates, bow echoes can develop with isolatedcells as well as squall lines. Bow echoes are easily detected onradar but are difficult (or impossible) to observe visually. It isnot your job to detect bow echoes, but you do need to knowwhat you will be up against should you encounter a bow echocomplex: namely, very strong downburst winds.

Figure 9: Schematic diagram of a multicell cluster storm. Cloud outlines, radar intensities, and the area ofgreatest severe weather probability are shown.
11

As with multicell cluster storms,

squall lines usually produce severeweather near the UDI. Recall that thisis near the leading (eastern) edge ofthe storm. If tornadoes are associatedwith a squall line, they will usuallydevelop in cells that are just north of abreak in the line or in the line’s southernmost cell (sometimes called the“anchor cell”). Cells in these locationstend to behave more like supercellsthan typical squall line cells.

The Supercell Storm

The supercell is a highly organized

thunderstorm. Although supercellsare rare, they pose an inordinatelyhigh threat to life and property. Likethe single cell storm, the supercellconsists of one main updraft. However, the updraft in a supercell isextremely strong, reaching estimatedspeeds of 150-175 miles an hour. Themain characteristic which sets thesupercell apart from the other thunderstorms we have discussed is the element of

rotation. The rotating updraft of a supercell, calleda mesocyclone, helps the supercell to produce

extreme severe weather events, such as giant hail(more than 2 inches in diameter), strong downbursts of 80 miles an hour or more, and strong toviolent tornadoes.

Recall that the supercell environment is characterized by high instability, strong winds in the mid

and upper atmosphere, and veering of the windwith height in the lowest mile or so. This environment is a contributing factor to the supercell’s

of a squall line. Cloud outlines, radar intensities, and the area of greatest severe

wn.

of a bow echo. Strong downburst winds accelerate a portion of the

mma echo configurations shown.
12

on is produced in the updraft, the strong

blow the precipitation downwind.

n falls back down through the updraft,

 long periods of time with only minor

entioned earlier, the veering winds with

ne formation within the supercell.

ercell’s precipitation area is character

ain falls closer to the updraft with

hail immediately north and east of the

 the main updraft (typically towards the

ferred area of severe weather formation.

grams of a supercell storm.

we will examine the visual aspects of the

thunderstorms) in more detail. We will

d some variations in the supercell model

SPECTS OF SEVERE

DERSTORMS

m difficult to tell a severe thunderstorm

understorm. There are, however, a

ch can be used to gain an idea of a

ength and organization, and the

torm is developing. Many of these

 but for discussion’s sake, we will

r-level, mid-level, and low-level features

observed.

s

ues are associated with the thunder

e anvil is a flat cloud formation at the

 Air (and cloud material) rising in the

ere it begins to slow down. This level is

l. The air (and cloud material) rapidly

Figure 13: Overheadview of a supercell storm.The precipitation area,gust front, and cloudfeatures are shown.Figure 12: Side view of a supercell storm. View is to the northwest.Prominent features of the storm are indicated.
13

slows its upward motion after passing the equilibrium level.As the air (and cloud material) spreads out, the anvil is formed.

If the storm you are watching has a vigorous updraft, a small

portion of the updraft air will rise higher than the surroundinganvil. This will form a “bubble” of cloud sticking up above the restof the anvil. The bubble is called an overshooting top (again, seeFigure 14). Most thunderstorms will have small, short-lived overshooting tops. However, if you observe a storm with a large, domelike overshooting top that lasts for a fairly long time (more than10 minutes), chances are good that the storm’s updraft is strongenough and persistent enough to produce severe weather.

The anvil itself will also provide clues to the storm’s strength and

persistence. If the anvil is thick, smooth-edged, and cumuliform(puffy, like the lower part of the storm), then the storm probably hasa strong updraft and is a good candidate to produce severe weather.This is also shown in Figure 14. If the anvil is thin, fuzzy, and glaciated (wispy, similar to cirrus clouds), then the updraft is probablynot as strong, and the storm is less likely to produce severe weather(Figure 15). If the anvil is large and seems to be streaming awayfrom the storm in one particular direction, then there are probablystrong upper-level winds in the storm’s environment. The stormwill be well ventilated, meaning precipitation will probably beblown downstream away from the updraft rather than fall throughthe updraft.

Mid-Level Features

Most of the mid-level cloud features are associated with the

storm’s main updraft tower. If the clouds in the main updraft areaare sharply outlined with a distinct cauliflower appearance, thenthe clouds are probably associated with a strong updraft which mayproduce severe weather (Figure 14). If they have a fuzzy, “mushy”appearance to them, then the updraft probably is not as strong as inFigure 15. If the updraft tower itself is vertical (almost perfectlyupright), then the storm probably has an updraft strong enough toresist the upper-level winds blowing against it (again, see Figure14). On the other hand, if the updraft leans downwind (usuallynortheast), then the updraft is weaker (Figure 16).

 top, thick anvil, vertical updraft tower, and

tower suggest storm severity.

vil and the “”soft” updraft tower (behind the

ground) suggest a lack of severity.

rms laboratory (NSSL).
14

Figure 19.: The rain-tree base. This marks the primary area of updraft in the

storm. View is to the northwest. Photo - Alan Moller.

mental wind speed. Compare the vertical

 updraft of the towering cumulus at right.

percell. View is to the southeast.

Figure 18. Striations are evident as the cork-screw-type markings on the sideof this supercell updraft tower. View is to the west. Photo - Alan Moller.
15

d storm-scale organization typically have

wers to the south or southwest of the

maller towers are called a flanking line

p appearance as they build toward the

hown in Figure 17.

mesocyclones develop, will show signs

wer. You may see striations on the sides

ns are streaks of cloud material that give

w” or “barber pole” appearance and

igure 18). A mid-level cloud band also

evel cloud band is a ring of cloud mate

pdraft tower encircling the tower like a

s another sign of possible rotation within

and intensity, it will begin to dominate

n about 20 miles). If cumulus clouds and

y from the storm of interest dissipate, it

m is taking control in the local area.

s of the storm may be suppressing

stability and energy available locally

rm of interest which could result in its

loud features for assessing thunder

potential are found at or below the level

re is a lot to see in these low-level cloud

ion (and frustration) associated with

attempting to interpret these similar

ally distinct cloud formations.

vel feature to identify is the rain-free

e suggests, this is an area of smooth, flat

n storm tower from which little or no

free base is usually to the rear (generally

precipitation area. The rain-free base

marks the main area of inflow where warm, moist air at low levelsenters the storm. Some call the rain-free base the “intake area.”

We earlier discussed the domination by a storm of its local environment. Besides suppressing any nearby storms or clouds, this

local domination can also show itself through the presence ofinflow bands, ragged bands of low cumulus clouds which extendfrom the main storm tower to the southeast or south. The presenceof inflow bands suggests that the storm is gathering low-level airfrom several miles away. The inflow bands may also have a spiralling nature to them, suggesting the presence of a mesocyclone.

The beaver’s tail is another significant type of cloud band. The

beaver’s tail is a smooth, flat cloud band which extends from theeastern edge of the rain-free base to the east or northeast as shownin. It usually skirts around the southern edge of the precipitationarea. The beaver’s tail is usually seen with high-precipitationsupercells (which will be discussed later) and suggests that rotationexists within the storm.

Figure 20: A beaver’s tail is seen extending to the right of the main updrafttower. View is to the north. Photo - Alan Moller.
16

 base and “accessory clouds,” such as

 mark important storm areas. The next

ds and other lowerings in more detail.

L CLOUDS AND

R LOWERINGS

 as an isolated cloud lowering attached

all cloud is usually to the rear (generally

visible precipitation area. Sometimes,

 be to the east or southeast of the pre

ly the case with high-precipitation

itation has wrapped around the western

ouds are usually about two miles in

 of strongest updraft in the storm. See

all clouds.

As the storm intensifies, the updraft draws in low-level air from

several miles around. Some low-level air is pulled into the updraftfrom the rain area. This rain-cooled air is very humid; the moisturein the rain-cooled air quickly condenses (at a lower altitude thanthe rain-free base) to form the wall cloud. This process is shownin Figure 22.

Shelf Clouds and Roll Clouds

Shelf clouds and roll clouds are examples of “accessory clouds”

that you may see beneath the cloud base of a storm. Shelf cloudsare long, wedge-shaped clouds associated with the gust front(Figure 23). Roll clouds are tube-shaped clouds and are also foundnear the gust front (Figure 24).

Shelf/roll clouds can develop anywhere an area of outflow is

present. Shelf clouds typically form near the leading edge of astorm or squall line. A shelf cloud can form under the rain-freebase, however, and appear to be a wall cloud. A shelf cloud mayalso appear to the southwest of a wall cloud in association with aphenomena called the rear flank downdraft (to be discussed later).

loud is an isolated lowering of the rain-free base. It marks an area of strong updraft.

 Steve Tegtmeier.
17

Shelf Clouds vs. Wall Clouds

Perhaps your biggest challenge as a spotter will be to discern

between shelf clouds under the rain-free base and legitimate wallclouds. Remember that shelf clouds signify an area of downdraftand outflow while wall clouds indicate an area of updraft andinflow. If a shelf cloud is observed for several minutes, it will tendto move away from the precipitation area. A wall cloud, though,will tend to maintain its relative position with respect to theprecipitation area. Shelf clouds tend to slope downward away fromthe precipitation while wall clouds tend to slope upward away fromthe precipitation area. Table 3 summarizes these differences.

Wall Clouds vs. Shelf Clouds

Wall Clouds: Shelf Clouds:Suggest inflow/updraft Suggest downdraft/outflowMaintain position Move away from rainwith respect to rainSlope upward away from Slope downward awayprecip. area from precip. area

Table 3: Characteristics of Wall Clouds vs. Shelf Clouds.

Only a few of the lowerings seen when spotting will be legitimate

wall clouds, and only a few of these wall clouds will actually produce tornadoes. Once a wall cloud has been positively identified,the next challenge will be to determine its tornado potential. Thereare four main characteristics usually observed with a tornadic wallcloud. First, the wall cloud will be persistent. It may change its

shape, but it will be there for 10-20 minutes before the tornadoappears. Second, the wall cloud will exhibit PERSISTENT rotation.Sometimes the rotation will be very visible and violent before thetornado develops. Third, strong surface winds will blow in toward

n (top). As cool, moist air is pulled from the rain

denses to form the wall cloud (bottom).
18

the wall cloud from the east or southeast (inflow). Usually surface

winds of 25-35 miles an hour are observed near tornadic wallclouds. Fourth, the wall cloud will exhibit evidence of rapid verticalmotion. Small cloud elements in or near the wall cloud quickly willrise up into the rain-free base. Not all tornadic wall clouds havethese characteristics (and some tornadoes do not form from wallclouds), but these four characteristics are good rules of thumb.

VIII. NON-TORNADIC SEVERE

WEATHER PHENOMENA

Downbursts

Recall that a downburst is defined as a strong downdraft with an

outrush of damaging winds on or near the ground. Downbursts aresubdivided based on their size. If the swath of damaging winds is2.5 miles or greater in diameter, then it is termed a macroburst. If

the swath is less than 2.5 miles, it is called a microburst. In general,

microbursts are quick-hitting events and are extremely dangerousto aviation. Microbursts are subclassified as dry or wet microbursts,depending on how much (or little) rain accompanies the microburstwhen it reaches the ground.

Figure 25 shows the lifecycle of a microburst. The formative stage

of a microburst occurs as the downdraft begins its descent from thecloud base (Figure 26) . The microburst accelerates downward,reaching the ground a short time later. The highest wind speedscan be expected shortly after the microburst impacts the ground(Figure 27). As the cold air of the microburst moves away from thecenter of the impact point, a “curl” will develop (Figure 28). Windsin this “curl” will accelerate even more, resulting in even greaterdanger to aircraft in the area. After several minutes, the microburstdissipates, but other microbursts may follow a short while later.

While spotting microbursts may not seem as dramatic as spotting

tornadoes, it is important to the NWS, the public, and the aviationinterests that microbursts be identified and reported. Listed beloware some visual clues for identifying microbursts.)

elf cloud marks an area of thunderstorm

o the shelf cloud, the roll cloud also marks an

o the southwest. Photo - Gary Woodall.
19

ential microburst formation areas. Virga

which evaporates before reaching the

n evaporates, it cools the air and starts a

onditions are right, the downdraft may

und as a microburst. Localized areas or

d from the ground usually mark the

ursts.

rain area, with an area of lighter rain in

microburst. This is shown in Figure 28.

ard distortion of the edge of a precipita

dicator of a possible wet microburst

rst reaches the ground and moves away

me of dust may be raised from the

ground. This plume is called a dust foot and also marks a possible

microburst (Figure 30).

Flash Floods

Recall that a flash flood is defined as a rapid rise in water usually

associated with heavy rains from a thunderstorm. For many years,flash floods were the leading cause of death and injury amongweather phenomena. Although casualty rates from flash floodsare decreasing, many people still unnecessarily fall victim toflash floods.

Atmospheric conditions that cause flash floods are somewhat

different from those which produce severe thunderstorms. Thetypical flash flood environment has abundant moisture through agreat depth of the atmosphere. Low values of vertical wind shearare usually present. Flash flooding commonly occurs at night, ratherthan in the late afternoon or evening. Flash flooding is typicallyproduced by either large, slow-moving storms or by “train effect”storms. The “train effect” occurs when several storms sequentiallymature and drop their rainfall over the same area. This can occurwhen multicell cluster or squall line storms are present.

There are three types of flooding that may occur due to excessive

rainfall over an area in a short period of time. The main differencelies in the terrain on which the rain falls. The first type is the classic“wall of water” which occurs in canyons and mountainous areas. Inthis type of flooding, rainwater rapidly runs off and is funneled intodeep canyons and gorges, where it quickly rushes downstream. Thesecond type, called “ponding,” is common in relatively flat areas.The rainwater collects in drainage ditches and other low-watercrossings and is particularly a problem in rural areas. The third typeis “urban flooding.” Extensive concrete and pavement in urbanareas results in a large amount of rainwater runoff which collects instreet intersections, underpasses, and dips in roads.

As mentioned in Chapter II, it is difficult to set spotting and

reporting guidelines regarding flash flooding. Local differencesin geography, soil type and character, and urbanization result inwidely varying amounts of runoff for a given amount of rain.

burst.
20

a wet microburst. The downdraft (in the

accelerating toward the ground.

et microburst. This marks the most dangerous

Photo - Bill Bunting.

Figure 28: Dissipating stage of a wet microburst. The curl is still evident on theedges of the microburst’s impact area. Photo - Bill Bunting.

Figure 29: A rain foot, an outward deflection of the rain shaft, may alsosuggest a wet microburst. View is to the west. Photo - Charles Doswell III.
21

ice for guidelines regarding flash

urse, keep the safety rules outlined in

e flash flooding is a possibility.

HE TORNADO

e cycle of a tornado. Although not all

yclones, most of the larger and stronger

m supercell storms with mesocyclones.

ironment usually contains strong,

 mile or so of the atmosphere. These

uce horizontal vorticity (“rolls”) in the

the atmosphere. The thunderstorm’s

zontal “rolls” into vertically-oriented

ocyclone to form. The tornado circulation develops at mid levels (about 20,000 feet) in the storm where the storm’s updraft and mesocyclone are strongest. The circulation gradually builds down (and up) within the storm. At about the same time, a downdraft develops at mid levels near the back edge of the storm. This rear flank downdraft (RFD) descends to the ground along with the tornado circulation. Rapidly lowering barometric pressure near the ground is believed to be the primary means of drawing the tornado circulation and RFD down toward the ground. The RFD may reveal itself as a “clear slot” or “bright slot” just to the rear (southwest) of the wall cloud. Sometimes, a small shelf cloud will form along this clear slot. Eventually, the tornado and RFD will reach the ground within a few minutes of each other (Figure 31a). After the tornado touches down, an ample inflow of warm, moist air continues into the tornado/mesocyclone. The RFD, though, will begin to wrap around the tornado/mesocyclone after the RFD impacts the ground. The RFD will actually cut off the inflow to the tornado as it wraps around the tornado/ mesocyclone. Wind damage may result from the RFD’s gust front as it progresses around the mesocyclone (Figure 31b). When the RFD completely wraps around the tornado/mesocy- clone, the inflow to the tornado/mesocyclone will be completely cut off. The tornado will gradually lose intensity. The condensation funnel will decrease in size, the tornado will tilt with height, and the tornado will eventually take on a contorted, rope-like appear- ance before it completely dissipates (Figure 31c). Tornado Variations Not all tornadoes go through the life cycle outlined above. Some tornadoes proceed from the developing stage directly to the dissi- pating stage, with little time spent in the mature stage. Tornadoes take on quite different appearances as they develop, mature, and decay. Additional tornado examples are shown in Figure 32. Figure 33 illustrates a multiple-vortex tornado. Multiple-vortex tornadoes have, as their name suggests, two or more circulations (vortices) orbiting about each other or about a common center. 

ot, the dust foot indicates the presence of a 

r.
22

ending toward

- NSSL.

(a) Thin tornado.

(b) Mature tornado (note clear slot in front wallcloud.) Photo - NSSL.

(c) Dissipating (rope) stage of tornado.Photo - NSSL.

(b) large violent tornado.Photo - Institute for Disaster Research.

(c) Dust-tube tornado. Photo - George Kuydendall.
23

multiple-vortex tornadoes as “several

er to form one large tornado.” Most of

adoes the United States has experienced

1968; Xenia, Ohio, 1974; Wichita Falls,

lvania, 1985, to name a few) were

f you observe a multiple-vortex tor

 dispatcher/controller, and stay clear!

on

owned severe weather researcher at the

loped a scheme for rating tornadoes

 scale, called the F scale, gives tornadoes

o F5. F0 and F1 tornadoes are consid

and F3 tornadoes are classified as

and F5 tornadoes are categorized as

 summarizes the Fujita scale.

nado damage (primarily to buildings),

scale. For example, a tornado which

moves over open country will tend to receive a lower rating than atornado that strikes a populated area. Since buildings have a widevariation in age, quality of design, and quality of building materials,more uncertainties are thrown into the mix. Tornadoes over opencountry probably encounter varying types of vegetation, leading touncertainties in these cases. Still, the Fujita scale provides a goodbaseline for classifying tornadoes according to their intensities.

Tornado/Funnel Cloud Look-Alikes

Experienced spotters are aware that a number of features (both

natural and man-made) can resemble a tornado or funnel cloud.Some of these features include rain shafts and scud clouds. Some ofthe man-made features include smoke from oil flares and factories.If a suspicious looking cloud formation is observed, watch it for aminute or two. Look for organized rotation about a vertical or nearvertical axis. Figure 34 depicts a number of tornado look-alikes.

Another phenomenon which must be discussed is the gustnado.

Gustnados are small vortices which sometimes form along a gustfront. Gustnados are generally not associated with the updraft area

ado. Photo - Howard Bluestein.

F0 Gale Tornado weak 40-72 mphF1 Moderate Tornado weak 73-112 mphF2 Significant Tornado strong 113-157 mphF3 Severe Tornado strong 158-206 mphF4 Devastating Tornado violent 207-318 mphF5 Incredible Tornado violent 261-318 mph

Fujita Damage Scale

Table 4: The Fujita tornado damage scale.
24

of the storm and do not originate in mesocyclones, so in some waysthey are not “legitimate” tornadoes. They can cause damage tolightweight structures and are hazardous to people in the open,though, so they do pose a threat and should be reported to thecontroller/dispatcher.

X. SUPERCELL VARIATIONS

The supercell discussed in Chapter IV is considered a “classic”

supercell and serves as a baseline when discussing supercell types.Much has been made recently of “low-precipitation” (LP) and“high-precipitation” (HP) supercells, which might lead some tobelieve that these are truly different kinds of supercells. In actuality,all supercells are fundamentally the same. They all possess a mesocyclone, are all long-lived, and all are capable of producing extremely dangerous weather. The only difference in these supercellsis the amount of visible precipitation which falls out of the storm.

cloud look-alikes (a) Scud Clouds.

Figure 35: Gustnados are small votices that sometimes form along stronggust fronts. View is to the southwest. Photo - Charles Doswell III.
25

ipitation will pose different problems

s and for spotters, the underlying theme

rcell, be it LP, classic, or HP.”

P) Supercells

ells are most commonly found on the

e (sometimes called “dryline storms”),

nted in the Upper Midwest as well.

o detect on radar. The radar echoes are

w reflectivity values). There may not be

the storm as detected by conventional

agram of an LP supercell. LP storms are

ly. The typical low-precipitation super

precipitation area. The main storm tower

(flared out close to the cloud base), and

ns on the sides of the tower.

HP) Supercells

ells can occur in any part of the country.

P supercells only occurred in the South

cumented in the Great Plains as well.

tect on radar. They usually have a large

 rotation within the storm. Figure 38

upercell.

n supercells, the mesocyclone is

r east side of the storm. This displace

ious amounts of precipitation falling

upercells difficult for spotters to identify.

y obscure some (or all) of the “rain-free”

mportant cloud features that are found

upercells will usually have striations

er and will probably have a beaver’s tail

 Thus, although events under the cloud

ern, ample evidence will exist to confirm

 Figure 39 depicts the visual characteris

Hybrid Storms

It is rare for a storm to fit perfectly into one of the four storm

categories (discussed in Chapter IV) for its entire life. Rather, it iscommon for a storm to evolve from one storm type to another. It isalso common for a supercell’s precipitation rate to increase duringits life, resulting in its “evolution” from an LP to an HP supercell.

See Figure 40 for an example of an LP-to-HP “evolution.”

One of the more common evolutions a storm may undergo is a

multicell-to supercell transition. Figure 41 contains an example ofthis transition. As the multicell storm moves along, it may encounter an environment more conducive to supercell formation. One ofthe updrafts in the cluster may become dominant, and the stormmay evolve into a supercell. In fact, numerous supercells withmulticell characteristics have been documented!

The multicell characteristics in some supercells may give rise to

the cyclic nature of some supercells. A cyclic supercell is a supercellwhich undergoes the mesocyclone formation-tornado formationRFD formation process a number of times. In the April 3, 1974,tornado outbreak, one supercell produced eight tornadoes as ittracked across Illinois and Indiana. While it is rare for a supercell toproduce this many tornadoes, it serves to illustrate the extremelydangerous nature of cyclic supercells. Figure 42 contains an example of a cyclic supercell.

Besides the possibility of a storm “evolving” from an LP to an HP

storm, it is also possible for a supercell to have both LP and HPcharacteristics at the same time. Figure 43 shows an example of sucha storm. The main precipitation area, to the right of the storm tower,had a thin, translucent appearance. Beneath the base of the storm,however, a heavy precipitation curtain obscured any importantcloud features which may have been present. These LP-HP hybridsare yet another example of the continuous spectrum of storm typesthat may be encountered in the spotting arena.
26

of a low-precipitation supercell. The main

f the updraft tower is usually very light.

of a low-precipitation supercell. View is to the

Figure 38: Schematic diagram of a high-precipitation supercell.Figure 39: Typical appearance of a high-precipitation supercell. View is to thewest. Photo - John McGinley.
27

(a) LP supercell with developing wall cloud.

ase nearly totally obscured.

Figure 41: Multicell-to-supercell evolution. (a) Multicell storm with(at least) 6 updraft elements.(b) Supercell with one dominant updraft.Photos - Tim Marshall, NSSL.
28

tornado #1 dissipates, inflow is refocused into the new wall cloud (right). Tornado #2 then develops from the new wall cloud.

. The “main precipitation area” is translucent,

e beneath the updraft base.
29

Acknowledgments

ntinuing effort of the NWS and NSSL to provide improved training materials to storm spotters. Dr. Charles Doswell

, WFO Fort Worth, TX, provided fundamental input and guidance regarding the guide. Andy Anderson, WFO

pful review and comments. Dr. Jerry Jurica of Texas Tech University, Charles Brown, and Melody Woodall assisted

guide’s first edition. NSSL diagrams were provided by Joan O’Bannon. Special thanks go to Bill Alexander and Linda

vice Headquarters and to Sue Dietterle from NOAA Visual Arts for their painstaking reviewing and editing of the

ion was created by Melody Magnus.

Figure Credits

by Alan Moller.

well III, 1985:

gy of Convec

Memo ERL

985, Vol 2).

nalysis.

n Moller.

A. Doswell III,

 Storm

 Preprints, 15th

ms, Baltimore,

1985, Vol. 2.

SL.

 1985, Vol. 2.

Manual

 for Project

No. 156,

l III, 1985,

Figure 14 - Photo by Tim Marshall.Figure 15 - Photo by NSSL.Figure 16 - Photo by Howard Bluestein.Figure 17 - Photo by Charles Doswell III.Figures 18-20 - Photos by Alan Moller.Figure 21 (a, b & c) - Photos by NWS,David Hoadley, Steve Tegtmeier.Figure 22 - 23 Photos by Alan Moller.Figure 24 - Photo by Gary Woodall.Figure 25 - T.T. Fujita, 1981: Tornadoes andDownbursts in Context of Gen. PlanetaryScales. J. Atmos. Sci., 38, 1512-1534.Figures 26-28 - Photos by Bill Bunting.Figure 29 - Photo by Charles Doswell III.Figure 30 - Photo by Alan Moller.Figure 31 (a & b) - Photos by SteveTegtmeier.

Figure 31 (c) - Photo by NSSL.Figure 32 (a, b, c) - Photos by TimMarshall, Institute for Disaster Research,George Kuykendall.Figure 33 - Photo by Howard Bluestein.Figure 34 (a & b) - Photos by WilliamAlexander and NSSL.Figure 35 - Photo by Charles Doswell III.Figures 36 & 38 - From C.A. Doswell III,A.R. Moller, and R. Przybylinski, 1990:A Unified Set of Conceptual Models forVariations on Supercell Theme. Preprints,

16th Conf. on Severe Local Storms,Kananaskis, Alta, Canada, 40-45.

Figure 37 - Photo by Steve Tegtmeier.Figure 39 - Photo by John McGinley.Figure 40 (a & b) - Photos by Alan Moller.Figure 41 (a & b) - Photos by Tim Marshall.Figures 42-43 - Photos by Gary Woodall.