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U.S. Department Of Commerce
National Oceanic and Atmospheric Administration
National Weather Service

Spotter's Guide for Identifying and Reporting Severe Local Storms

(PLEASE NOTE: THIS GUIDE WAS WRITTEN IN 1981 AND IS NOW OUT OF PRINT. HOWEVER THERE ARE STILL SO MANY REQUESTS FOR THIS EDITION THAT WE HAVE DECIDED TO MAKE IT AVAILABLE HERE ON THE OM HOME PAGE)

In the Wichita Falls, Tex., tornado of April 10, 1979, spotters played an essential role in the advance warning. There were more that 18,000 people in the tornado's damage path and yet only 44 persons died. Loss of life would have been much higher without warnings and knowledge of safety rules.

TO THE SEVERE LOCAL STORM SPOTTER:

The National Weather Service of the U.S. Commerce Department's National Oceanic and Atmospheric Administration, and cooperating organizations have established Severe Local Storm Spotter Networks in order to secure critical information on severe local storms.

During the past 10 years tornadoes, severe thunderstorms, and flash floods have killed about 3,000 people and injured thousands of others. While this figure may appear staggering, we believe that thousands and possibly tens of thousands of lives have been saved by reports from storm spotters, such as yourself.

Your information, coupled with 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 this important job. I am pleased to welcome you to the ranks of the tens of thousands of your counterparts who form the Nation's first line of defense against tornadoes, flash floods, and other severe local storm hazards. There can be no finer individual reward than to know that your efforts have given a community the gift of time-seconds and minutes that save lives.

Sincerely,

Elbert W. Friday, Jr.
Director
National Weather Service
NOAA

PLEASE NOTE: THE FIGURES REFERRED TO IN THE TEXT ARE NOT AVAILABLE AT THIS TIME....WE ARE WORKING ON IT!

Use of this Guide:

The information which is contained in this guide is not sufficient to qualify you as a SKYWARN spotter. Rather, it is provided as a reference source to supplement the National Weather Service's spotter training film and slide series. It has been compiled for use by trained spotters in the field and communications specialists who receive and relay the reports. It 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 or in the glossary.

I. Severe Thunderstorms

All thunderstorms are capable of producing tornadoes. In coastal areas, waterspouts and weak funnels frequently form in areas of weak shower activity. While these are dangerous and should be reported, they usually are not preceded by the cloud structures described in this guide. Strong to violent tornadoes are usually associated with what we shall call the Tornadic Thunderstorm. Other significant tornadoes do occasionally occur with Squall Line Thunderstorms. In the following sections, we will review the extremely important cloud features which commonly precede tornado and severe weather events.

A. The Tornadic Thunderstorm:

These thunderstorms are usually separated from other thunderstorms or may even be isolated. This separation allows them to feed upon warm moist air from miles around. Figure 1 is a representation of a tornadic thunderstorm as a spotter might see it when located east of the storm, looking west. The storm is generally moving from the left to the right. The main updraft of warm moist air is entering the storm at 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. At this level the cloud spreads out rapidly, forming an Anvil Clould. As precipitation begins to occur, downdrafts are created.

Figure 2 is a view of the same storm and its associated weather from above, looking down. The intense updraft, which is rising out of the drawing, is located within the main storm tower as shown by the scalloped lines in a semicircle surrounding the medium blue areas. The downdraft air sinks to the ground in the area where precipitation is falling, mainly to the north and 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, where the tornado is most likely to occur. Large hail is likely to fall just outside the updraft core, mainly northeast of the updraft. Tornadoes may also form along the Gust Front and Flanking Line, these are usually weak and short-lived.

Looking back at Figure 1, we can see the primary features to look for at the cloud base. The Flanking Line Towers, Collar Cloud, and Tail cloud are explained in the glossary. The three features of primary importance are the Rain Free Base, Wall Cloud, and Precipitation Area. In Figure 3, we are west of the storm looking east-southeast and the storm is moving to the left. The heavy rain area to the left tells us where a main downdraft is located. The Rain Free Base extends from under the Main Storm Tower (which is tilted slightly to the left by the upper level winds) to the Flanking Line Towers. The lowering of the cloud base near the center is a Wall Cloud. The Wall Cloud is usually in the southwest portion of the storm within several miles of the rain area. Not all Wall Clouds rotate. Nevertheless, a rotating Wall Cloud usually precedes significant tornado development from minutes to over an hour. Be alert to the fact that some tornadoes may develop from the Rain Free Base itself.

B. The Tornado:

Tornadoes vary greatly in appearance and intensity, ranging from the violent type (pictured on the cover) to the very weak and short-lived ones that last only a few seconds. In addition, a tornado's appearance frequently changes during its life cycle. Figure 4 shows a multiple vortex tornado which is composed of several small but intense vortices which revolve around a common center. These small vortices may develop and dissipate very quickly. At times, rainfall is drawn into the tornado's circulation, making it very difficult to see. While this is common in the southeast United States, it is not limited to that area as illustrated by the Kansas tornado in figure 5. In Figure 6, we see a tornadic dust whirl. It should no be confused with a dust devil which usually occurs on nearly cloudless and warm days having light winds. The first stage of tornado development is often a dust whirl at the ground with or without a funnel aloft. The tornadic dust whirl in Figure 6 did not develop beyond this initial stage. It is common for flanking line dust whirls to form along the gust front; these generally remain weak and short lived.

C. The Life Cycle of a Tornado:

In Figure 7, we are looking west and can see the typical pattern: Rain Free Base, Wall Cloud, and Precipitation Area. The Wall Cloud is located in the southwest quadrant of the storm, and formed 30 minutes prior to the tornado. In Figure 8, what appears to be a funnel cloud is visible under the left portion of the Wall Cloud. In fact, this is a tornado because damage was occurring at the ground even though it is too far to see the surface debris. In such a situation, you should properly report this as a funnel cloud, along with the fact that it is too far away for you to determine if it is on the ground. Report what you see, along with any additional remarks which would make the report more useful. In Figure 9, the circulation becomes visible all the way to the ground. The thin, "needle-tipped" shape is common for a tornado in this early stage. The tornado reaches its mature stage in Figure 10, with its width at the surface expanding to 1/4 mile. Note the Tail Cloud forming to the right. We have changed position in Figure 11 and are looking southeast as the tornado moves away from the town of Union City, Oklahoma. The tornado is shrinking rapidly into the "rope stage," but is still very destructive. During the latter portion of a tornado's life, it is common for it to decrease in size (not necessarily in intensity) and become increasingly tilted.

D. The Squall Line Thunderstorms:

Figure 12 is a view of a squall line as seen from above. Precipitation is shown in the diagram's central area, the gust front is the white line with teeth, and the anvil edge is in light blue. While any hail which may occur is usually smaller that occurring in the tornadic thunderstorm and considerably fewer tornadoes result, strong straight-line winds are common with squall lines. The strongest winds usually occur a few minutes after the gust front passage, just before or just after rain and hail begin. If tornadoes occur, they are generally weak, short-lived, and are found along the gust front. Occasionally, a tornadic thunderstorm will develop in association with a squall line. The most distinctive cloud associated with the squall line is the Shelf Cloud, shown in Figure 12. It is usually located above the squall line gust front. An individual tornadic thunderstorm or even an isolated non-severe thunderstorm may at times develop a shelf cloud associated with its gust front. Before we look closer at the shelf cloud, let's take a look at a vertical cross section of the squall line from point A to B in Figure 12. This is shown in Figure 13. Note that the squall line thunderstorm has a significantly different structure than the tornadic thunderstorm. In Figure 13, the squall line is moving from left to right. Updrafts form a nearly continuous curtain along the leading edge above the gust front. Downdrafts are located in the precipitation area to the rear. Thus the updraft-rainy downdraft orientation is reversed from that of the tornadic thunderstorm.

In Figure 14, we are looking west at 3 layers of cloud near the south end of a gust front. The lowest band of cloud is a shelf cloud. This cloud is wedge-shaped and smooth and sometimes appears layered. As the squall line passes overhead, shown in Figure 15, gusty winds and a sharp drop in temperature can be expected. Upward motion is along the leading edge and downward motion along the trailing edge of the shelf cloud. Cloud base behind the gust front is often very turbulent, as shown here. Less common than the shelf cloud is a Roll Cloud. In Figure 16 it is seen as the detached tube shaped cloud. It can often be seen to rotate slowly about a horizontal axis. Like the shelf cloud, it may indicate the leading edge of a zone of strong straight-line winds. Because of its horizontal rotation, it may be mistaken for a tornado. Let's look at some other structures that are frequently falsely reported as tornadoes.

E. Tornado Look-Alikes:

Virga or rain shafts are often reported as tornadoes. In figure 17, we see a developing rain shaft, which is fuzzy in appearance. An intense rain column, as in Figure 18, can be more difficult. Although it looks similar to the violent tornado on the cover of this guide, its edges are much less distinct. Mammatus clouds, as seen in Figure 19, are often mistaken for tornadoes. While mammatus often accompany severe thunderstorms (see figure 1), they are not severe in themselves and may also accompany non-severe thunderstorms. The feature most often mistaken for a tornado is a scud cloud. These are ragged, low cloud fragments which usually are not attached to the cloud base. When the scud is attached to the cloud base as in Figure 20, it is very difficult to distinguish it from a wall cloud or tornado. In this example, and for all tornado look-alikes, the key is to LOOK FOR ORGANIZED AND SUSTAINED ROTATION ABOUT A NEARLY VERTICAL AXIS.

Spotter Reporting Procedures

From radio-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 Civil Defense spotters--report to the National Weather Service via NAWAS, radio, 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 them to relay your report to the National Weather Service. If you are unable to reach the primary or alternate contact, place an emergency call through the telephone operator to the National Weather Service. If the call is long distance, it can be made collect. Report promptly as the storm may interrupt communications.

REPORT BRIEFLY:

What you have seen: wall cloud, tornado, funnel cloud, waterspout, heavy rain, etc.
Where you saw it: the direction and distance from a known location, e.g., 3 miles south of Beltsville.
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 one has been assigned.

Report:

1. Tornado, Funnel Cloud, Waterspout, or Wall Cloud.
2. Hail, 1/4 inch or larger
3. Damaging Winds (usually 50 mph or greater)
4. Flash Flooding
5 Rain, (rate of 1 inch per hour or more)

Additional Tips for Skywarn Spotters

The first sign of a tornado may not be a funnel at the cloud base. Your first clue may be debris or dust at the surface, so be alert to events at ground level, as well as in the clouds. At night, lightning flashes can aid in identifying the Rain Free Base, Wall Cloud, and Precipitation Area. Although a loud roar is frequently associated with a tornado, strong straight-line winds can also produce such a sound.

If you spot from a fixed location, use a map to determine distances and directions to known landmarks such as water towers, TV towers, etc. This will help you estimate distance and direction in your reports. Mobile spotters should always have up-to-date maps and be familiar with the area in which they are operating.

When available, use binoculars to look for rotation and other cloud features. Once you spot a funnel, tornado, or wall cloud, be alert for the formation of others in the area.

If you find yourself in large hail, remember you are in or near the area where tornado formation is most likely in a tornadic thunderstorm.

Always follow the basic safety rules. In open country, a spotter may be able to use his knowledge of the tornado's motion and available escape routes to drive away from the tornado safely. In urban area this is usually not possible because of traffic congestion. Make certain your family knows what to do in tornado emergencies as you may not be available to assist or direct them.

Spotter Aids

National Weather Service Storm Spotter's Glossary and Supplemental Guide

Accessory Clouds

Clouds that are dependent on a larger cloud system for development and continuance. Accessory clouds associated with the thunderstorm include roll, shelf, mammatus, and wall clouds.

Anvil

The spreading of the upper portion of a cumulonimbus cloud into an anvil-shaped plume usually of fibrous or smooth appearance. Strong or severe thunderstorms often have thicker anvils with the side and bottom having a cumuliform or slowly boiling appearance in the immediate vicinity of the parent cumulonimbus.

Collar Cloud

Frequently used as a synonym for a wall cloud although it actually is a generally circular ring of cloud surrounding the upper portion of a wall cloud.

Cumulonimbus Cloud

The parent cloud of a thunderstorm. The cumulonimbus cloud towers above ordinary cumulus clouds, with stronger or severe storms often having a more sharply outlined "hard" appearance with relatively rapid rising motions visible. The cloud's upper portion includes the anvil. Accompanying precipitation is often heavy and the usual occurrences of lightning and thunder with these clouds leads to the popular names of thunderhead or thundercloud. See also Thunderstorm.

Cumulus Cloud

A column of rising air that has condensed into a dense, nonfibrous cloud with distinct outlines, appearing much like a rising mound, a dome, or cauliflower. The base of the cloud is relatively flat and dark, while the tower is usually white and sunlit. The cumulus cloud is the first stage of a developing thunderstorm, although most cumulus do not form thunderstorms.

Downdraft

A column of generally cool air that rapidly sinks to the ground, most often accompanied by precipitation in a shower or thunderstorm. Areas of downdraft usually contain little cloud, and what clouds that may be present are typically dissipating.

Flanking Line

A line of cumulus connected to and extending outward from the most active portion of a parent cumulonimbus, usually found on the southwest side of the storm. The cloud line has roughly a stair step appearance with the taller clouds adjacent to the parent cumulonimbus. It is most frequently associated with strong or severe thunderstorms.

Flash Flooding

Flooding that develops very quickly on streams and river tributaries usually as a result of thunderstorms. Sometimes the onset of flash flooding comes before the end of heavy rains. There is little time between the detection of flood conditions and the arrival of the flood crest. Swift action is essential to the protection of life and property.

Front

A transition zone between two differing air masses. Basic frontal types are (1) COLD FRONT where cooler air advances replacing warmer air; (2) WARM FRONT- warmer air advances replacing cooler air; (3) STATIONARY FRONT- warmer air meeting cooler air with neither air mass moving appreciably. Thunderstorms can form in association with any of these fronts. However, fronts are not necessary for thunderstorm development.

Funnel Cloud

A funnel-shaped cloud extending from a towering cumulus or cumulonimbus base. It is associated with a rotating air column that is not in contact with the ground. The cloud is a tornado if a ground-based debris or dust whirl is visible below the funnel aloft.

Gust Front

The leading edge of the thunderstorm downdraft air. The gust front is most prominent beneath the rain-free base and on the leading edge of an approaching thunderstorm. It is usually marked by gusty cool winds, and sometimes blowing dust. The gust front often precedes the thunderstorm precipitation by several minutes. The shelf or roll cloud sometimes accompanies the gust front, especially when the gust front precedes a line of thunderstorms.

Hail

Precipitation in the form of balls or clumps of ice, produced by thunderstorms. Severe storms with intense updrafts are the most likely large hail producers.

Hook Echo

A radar pattern sometimes observed in the southwest quadrant of a tornadic thunderstorm. Appearing like the number six or a fishhook turned in toward the east, the hook echo is precipitation aloft around the periphery of a rotating column of air 2-10 miles in diameter. The hook echo is often found in a local area favorable for tornado development. However, many tornadoes occur without a hook echo and not all hook echoes produce tornadoes.

Lightning

Any and all of the various forms of visible electrical discharge caused by thunderstorms. Severe thunderstorms usually have very frequent and sometimes nearly continuous lightning. However, some non-severe thunderstorms also contain frequent and vivid electrical displays, while some severe storms are accompanied by little lightning.

Mamma Clouds

Also called mammatus, these clouds appear as hanging, rounded protuberances or pouches on the under surface of a cloud. With thunderstorms, mammatus are seen on the underside of the anvil. These clouds do not produce tornadoes, funnels, hail, or any other type of severe weather, although they often accompany severe thunderstorms.

Precipitation Shaft

A visible column of rain and/or hail falling from a cloud base. When viewed against a light background, heavy precipitation appears very dark gray, sometimes with a turquoise tinge. This turquoise tinge has been commonly attributed to hail but its actual cause is unknown.

Rain-Free Base

A horizontal, dark cumulonimbus base that has no visible precipitation beneath it. This structure usually marks the location of the thunderstorm updraft. Tornadoes most commonly develop (1) from wall clouds that are attached to the rain-free base, or (2) from the rain-free base itself. This is particularly true when the rain-free base is observed to the south or southwest of the precipitation shaft.

River Flood

Usually occurs on rivers, after flash flooding has occurred on streams and tributaries. River floods develop and reach their peak more slowly than flash floods. In many cases the river flood peak occurs after the rain has ended.

Roll Cloud

A relatively rare, low-level, horizontal, tube-shaped accessory cloud completely detached from the cumulonimbus base. When present, it is located along the gust front and most frequently observed on the leading edge of a line of thunderstorms. The roll cloud will appear to be slowly "rolling" about its horizontal axis. Roll clouds are not and do not produce tornadoes.

Scud Clouds

Low cloud fragments often seen in association with and behind thunderstorm gust fronts. These clouds are ragged and wind torn and are not usually attached to the thunderstorm base. Scud clouds do not produce severe weather. In some cases, when scud clouds are attached to the thunderstorm base they can be mistaken for wall clouds or tornadoes.

Severe Thunderstorm

A thunderstorm that goes from the mature stage to the severe stage before dissipating. Severe thunderstorms are most efficient "machines" because the updraft remains strong for a long time. They also occasionally contain rotations on a broad scale. Because of its structure, the severe storm may last for hours beyond the lifetime of a normal thunderstorm while producing large hail, high winds, torrential rain, and possible tornadoes. Officially, a thunderstorm is classified as severe if 50 knot (58 MPH) winds are measured, 3/4 inch or larger hail occurs, or funnel clouds or tornadoes develop.

Straight Winds

Winds associated with a thunderstorm, most frequently found with the gust front. These winds originate as downdraft air reaches the ground and rapidly spreads out becoming strong horizontal flow. Damaging straight winds, although relatively rare themselves, are much more common than are tornadoes.

Shelf Cloud

A low-level horizontal accessory cloud that frequently appears to be wedge-shaped as it approaches. It is usually attached to the thunderstorm base and forms along the gust front. The leading edge of the shelf is often smooth and at times layered or terraced. It is most often seen along the leading edge of an approaching line of thunderstorms, accompanied by gusty straight winds as it passes overhead and followed by precipitation. The underside is concave upward, turbulent, boiling, or wind-torn. Tornadoes rarely occur with the shelf cloud.

Squall Line

Any line or narrow band of active thunderstorms. The term is usually used to describe solid or broken lines of strong or severe thunderstorms.

Tail Cloud

A low tail-shaped cloud extending outward from the northern quadrant of a wall cloud. Motions in the tail cloud are toward the wall cloud with rapid updraft at the junction of tail and wall cloud. This horizontal cloud is not a funnel or tornado.

Thunderstorm

A local storm (accompanied by lightning and thunder) produced by a cumulonimbus cloud, usually with gusty winds, heavy rain, and sometimes hail. Non-severe thunderstorms rarely have lifetimes over two hours. A typical, non-severe thunderstorm life cycle consists of three stages: (1) CUMULUS STAGE-- warm, moist air rises (updraft) and condenses into tiny water droplets which make up the visible cloud. (2) MATURE STAGE-- the cloud grows above the freezing level; precipitation forms and becomes heavy enough to fall back to earth. This precipitation generates cool air which also sinks back to earth with the precipitation. (3) DISSIPATION STAGE-- Cool rain and downdraft spread throughout the storm replacing the updraft which is the lifeblood of the thunderstorm. The visible cumulonimbus cloud becomes softer in appearance, less distinctly outlined or "fuzzy" and dissipates, sometimes leaving only the high anvil cloud, as the storm rains itself out.

Tornado

A violently rotating narrow column of air in contact with the ground and extending from a thunderstorm base. The tornado is most often found in the southwest quadrant of the storm, near the trailing edge of the cumulonimbus cloud. Tornadoes and funnel clouds are usually pendant from (1) wall clouds, or (2) directly from the thunderstorm base, within a few miles to the southwest of the precipitation shaft. The spinning motion of a tornado is most often left to right on the front side and right to left on the backside (counterclockwise). Tornadoes have been called twisters and cyclones, but these words are all synonyms for the most violent storm on earth, with estimated wind speeds up to 300 mph.

Updraft

Warm moist air which rises and condenses into a visible cumulus or cumulonimbus cloud. Once the cloud forms, it depends on the updraft for continuance and further development.

Virga

Wisps or streaks of rain falling out of a cloud but not reaching the earth's surface. When seen from a distance, these streaks can be mistaken for funnels or tornadoes.

Wall Clouds

A local and often abrupt lowering of a rain-free cumulonimbus base into a low-hanging accessory cloud, from 1 to 4 miles in diameter. The wall cloud is usually situated in the southwest portion of the storm below an intense updraft, marked by the main cumulonimbus cloud and associated with a very strong or severe thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and rotation in the same sense as a tornado, except with considerably slower speed. A rotating wall cloud usually develops before tornadoes or funnel clouds by a time which can range from a few minutes up to possibly an hour. Spotters should key on any lowering of the cumulonimbus base as suspect wall cloud, particularly when it is located southwest of the precipitation shaft. Wall clouds should be reported. NOTE: Sometimes other low-hanging accessory clouds are mistakenly identified as wall clouds.

Warning

(Issued for tornadoes, severe thunderstorm, flash flood, river flood.) A warning is issued when severe weather has already developed and has been reported by spotters or indicated by radar. Warnings are statements of imminent danger and are issued for relatively small areas near and downstream from the severe storm or flood.

Watch

(Issued for tornado, severe thunderstorm, flash flood.) A watch identifies a relatively large area in which flash floods or severe storms might occur. Watches are quite often issued before any severe weather has developed. Severe thunderstorm and tornado watches usually include an area 140 miles wide by about 200 miles long. The watch is only an indication of where and when the severe weather probabilities are the highest, and should not be confused with a warning.

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration
Rockville, Maryland 20852



When in doubt, make your report anyway!

When a destructive storm is observed, report to the primary contact

______________________________________________

at_____________________________________________
               (telephone)
Alternate______________________________________
at______________________________________________


_______________________________________________
               (your name)

_______________________________________________
               (your address)

_______________________________________________
               (telephone)

_______________________________________________
               (your spotter code number)


Document: NOAA/PA 81011
(Rev. April 1982)

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