National Oceanic and Atmospheric Administration
National Weather Service
(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.
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
PLEASE NOTE: THE FIGURES REFERRED TO IN THE TEXT ARE NOT AVAILABLE AT THIS TIME....WE ARE WORKING ON IT!
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.
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.
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.
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.
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.
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.
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.
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.
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.
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)
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.
| pea size | 1/4 inch |
| marble size | 1/2 inch |
| dime size | 3/4 inch |
| quarter size | 1 inch |
| golf ball size | 1.75 inch |
| baseball size | 2.75 |
| 25-31 | Large branches in motion; whistling heard in telephone wires |
| 32-38 | Whole trees in motion; inconvenience felt walking against wind |
| 39-54 | Twigs break off trees; wind generally impedes progress |
| 55-72 | Damage to chimneys and TV antenna; pushes over shallow rooted trees |
| 73-112 | Peels surface off roofs; windows broken; light trailer houses pushed or overturned; moving automobiles pushed off roads |
| 113-157 | Roofs torn off houses; weak buildings and trailer houses destroyed; large trees snapped and uprooted |
| 158 & up | Severe damage:cars lifted off ground |
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)