SKYWARN Storm Spotter Guides Online

This module compiled with information courtesy of the official NOAA Storm Spotters Guide.

SECTION FIVE:

As in the other sections, you can click on the glossary image wherever you see it, and the glossary will open in another window. Just close that window when you are ready to continue.

THUNDERSTORM COMPONENTS:

At first glance, it may seem difficult to tell a severe thunderstorm from a "garden variety" thunderstorm. There are, however, a number of visual clues which can be used to gain an idea of a thunderstorms potential strength and organization, and the environment in which it is developing. Many of these visual clues are interrelated, but for discussion's sake, we will classify these as upper-level, mid-level and low-level features of the storm.

UPDRAFTS AND DOWNDRAFTS :

All thunderstorms require instability, moisture, and lift. When moisture and instability are both present, it is like mixing gasoline and air - the mixture has explosive potential. The lift is like the spark that ignites the mixture. Lift is produced by such things as fronts and low pressure troughs, or by air rising upslope. We say that the atmosphere is unstable when air rising in a cloud is warmer than its environment, like a hot-air balloon. It is the moisture condensed within a cloud that permits the rising air to stay warmer than its surroundings, and thus to be buoyant through great depths. In the same way, air that is cooler than its environment tends to sink as long as it can stay cooler than its surroundings. The atmosphere can be unstable for updrafts but stable for downdrafts, stable for updrafts but unstable for downdrafts, stable for both, or unstable for both.

The degree of atmospheric instability is one of the two major factors in determining the strengths of thunderstorm updrafts and downdrafts. Furthermore, vertical draft strengths basically determine the degree of storm severity. If we consider a "generic" storm, there are four possible combinations of weak and strong draft strengths. When the low level air is unstable but relatively dry and adequate mid-level moisture is present, a storm may develop with a weak updraft but a strong downdraft with the latter the result of strong negative buoyancy and cooling through evaporation of precipitation into the dry air. This high-based storm resembles high terrain, western U.S. storms which occasionally produce dry microbursts. Significant hail and rain are unlikely.

A storm which contains a strong updraft and weak downdraft; will not produce wind damage, but can foster heavy rains and/or damaging hail. Single and multicell storms comprise this category. They include storms that dump heavy rain, but little or no hail because of warm conditions aloft, and multicell storms that are capable of producing hail because of lower environmental freezing levels. Strong updraft, weak downdraft storms often form in very moist atmospheres where there is little, if any, dry air and evaporational cooling to drive downdrafts.

Relatively weak updrafts and downdrafts are found with non-severe showers and thunderstorms. The last possible combination is a storm with strong updrafts and downdrafts. These storms frequently produce destructive downbursts, hail, heavy rain, and tornadoes. As one would expect, the most severe storms, including supercells, have strong vertical drafts and occur in the most unstable atmospheres.

UPPER LEVEL FEATURES:

Most of the upper-level clues are associated with the storm's anvil. Recall that the anvil is a flat cloud formation at the top of the storm. Air and cloud material rising in the updraft reaches a point where it begins to slow down. This level is called the equilibrium level. The air and cloud material rapidly 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 surrounding anvil. This will form a "bubble" of cloud sticking up above the rest of the anvil. This bubble is known as an overshooting top. Most thunderstorms will have small, short lived, overshooting tops. However, if you observe a storm with a large, dome-like overshooting top that lasts for a fairly long time( more than 10 minutes), chances are that the storm's updraft is strong enough and persistent enough to produce severe weather.

The anvil itself provides clues to the storm's strength and persistence. If the anvil is thick, smooth edged, and cumuliform (puffy, like the updraft tower), then the storm probably has a strong updraft and is a good candidate for severe weather.

If the anvil is thin, fuzzy, and glaciated (wispy, similar to cirrus clouds), then the updraft is probably not as strong, and the storm is less likely to produce severe weather.

If the anvil is large and seems to be streaming away from the storm in one particular direction, then there are probably strong upper level winds in the storms environment. The storm will be well ventilated, meaning precipitation will be blown downstream away from the updraft rather than fall through it.

MID LEVEL FEATURES:

Most of the mid level features are associated with the storms main updraft tower. If the clouds in the main updraft area are sharply outlined with a distinct cauliflower appearance, then the clouds are probably associated with a strong updraft which may produce severe weather.

If they have a fuzzy, "mushy" appearance to them, then the updraft probably is not as strong. If the updraft tower itself is vertical, then the storm probably has an updraft strong enough to resist the upper level winds blowing against it. On the other hand, if the updraft leans downwind, the the updraft is weaker. The glaciated anvil (in the above pic) and soft updraft tower suggest a lack of severity.

Thunderstorms with good storm-scale organization typically have a series of smaller cloud towers to the south or southwest of the main storm tower. These smaller towers are called a flanking line and usually have a stair-step like appearance as they build toward the main storm tower. The above shows the flanking line from behind the storm.

Some supercells, as their mesocyclones develop, will show signs of rotation in the updraft tower. You may see striations on the sides of the storm tower. Striations are streaks of cloud material which give the storm tower a "corkscrew" or "barber pole" appearance and strongly suggest rotation. A mid level cloud band may also be apparent. The mid level cloud band is a ring of cloud material encircling the tower like a ring around a planet. This is another sign of possible rotation within the storm. As you can see, the visual results can be quite dramatic!

As a storm increases in size and intensity, it will begin to dominate its local environment. If cumulus clouds and other storms 5-15 miles away from the storm of interest dissipate, it may be a sign that the storm of interest is taking control in the local area. Sinking motion on the edges of the storm my be suppressing any nearby storms. All of the instability and energy available locally may be focused into the storm of interest which could result in its continued development.

LOW LEVEL FEATURES :

Some of the critical cloud features for assessing thunderstorm severity and tornado potential are found at or below the level of the cloud base. While there is a lot of information to be discerned in these low level cloud features, most of the confusion and frustration associated with storm spotting/chasing stems from attempting to interpret these similar appearing but meteorologically distinct cloud formation.

Perhaps the easiest low level feature to identify is the rain-free base. As its name suggests, this is an area of smooth, flat cloud base beneath the main storm tower from which little or no precipitation is falling. The rain-free base is usually just to the rear (generally south or southwest) of the precipitation area. The rain-free base marks the main inflow area where warm, moist air at low levels enters the storm. Some have called the rain-free base the "intake area" of the storm. The below image shows that it may sometimes be difficult to get good contrast on viewing the rain-free base.

We earlier discussed the domination by a storm of its local environment. Besides suppression of any nearby storms or clouds, this local domination can also show itself through the presence of inflow bands, ragged bands of low cumulus clouds which extend from the main storm tower to the southeast or south. The presence of the inflow bands suggests that the storm is gathering low-level air from several miles away. The inflow bands may also have a spiraled nature to them, suggesting the presence of a mesocyclone.

The beaver's tail is another significant type of cloud band The beavers tail is a smooth, flat cloud band that extends from the eastern edge of the rain free base to the east or northeast. It usually skirts around the southern edge of the precipitation area. The beavers tail is usually seen with high precipitation supercells and suggests that rotation exists within the storm.

Lowerings of the rain free base and "accessory clouds", such as shelf clouds and roll clouds, mark important areas of the storm.

As you can see in this image, low level features are completely obscured by dust picked up by inflow into the storm. This storm was possibly tornadic for several hours, producing only one confirmed tornado northwest of Cannon A.F.B. in New Mexico on June 4th, 2003

WALL CLOUDS AND OTHER LOWERINGS :

The wall cloud is defined as an isolated cloud lowering attached to the rain-free base. The wall cloud is usually to the rear (generally south or southwest) of the visible precipitation area. Sometimes, though, the wall cloud may be to the east or southeast of the precipitation area. This is usually the case with high precipitation supercells where the precipitation has wrapped around the western edge of the updraft. Wall clouds are usually about two miles in diameter and mark the area of strongest updraft in the storm.

As the storm intensifies, the updraft draws in the low level air from several miles around. Some low level air is pulled into the updraft from the rain area. This rain-cooled air is very humid; the moisture in the rain-cooled air quickly condenses to form the wall cloud.

	About 5 minutes after the above picture, the wall cloud/updraft area began lifting dust the cloud base. This was the beginning of tornadic cycle of the storm that lasted several hours. Shortly after this picture the dust did a brief tornadic circulation before dissipating and reforming later.
	This storm exhibits several good features that are visual clues to the spotter/chaser. In the mid-levels we can see the "barber pole" striations indicating visually that this storm is rotating. More importantly is the development of the massive wall and tail cloud underneath. Note that this particular wall cloud does NOT slope, but your clue here is the tail cloud forming back into the rain area. This storm later produced an F1 tornado near Guthrie, OK on June 13, 1998.
	Another dramatic image taken just outside of Lubbock, TX in June 2002. This is a good example of excellent contrast. This huge wall cloud was very visible to the storm chaser that took the image, and if a tornado were forming, it would have been very easy to see. This storm however did not produce a tornado, but did cause some extensive damage from rear flank downdraft winds.
	Sometimes being in closer it is a little more difficult to distinguish what you are seeing. This is a large wallcloud with developing funnel. This produced a brief tornado near Anthony, KS in May 1997 shortly after this was taken. Notice that while we did have some good side lighting and could see what was going on, it is very dark behind the funnel cloud, and not as good of viewing contrast for spotting.

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 clouds are long, wedge-shaped clouds associated with the gust front. Roll clouds are tube shaped clouds and are found also near the gust front.

Shelf/roll clouds can develop anywhere an area of outflow is present. Shelf clouds typically form near the leading edge of the storm or squall line. A shelf cloud can form under the rain free base however, and take on the appearance of a wall cloud. A shelf cloud may also appear to the southwest of a wall cloud in association with a phenomena called the rear flank downdraft (RFD).

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 wall clouds. Remember that shelf clouds signify an area of downdraft and outflow while wall clouds indicate an area of updraft an inflow. If a shelf cloud is observed for several minutes, it will tend to move away from the precipitation area. A wall cloud, though will tend to maintain its relative position with respect to the precipitation area. Shelf clouds tend to slope downward away from the precipitation while wall clouds tend to slope upward away from the precipitation area.

Only a few of the lowerings that will be seen when spotting/chasing wall be legitimate wall clouds, and only a few of these will actually produce tornadoes. Once a wall cloud has been positively identified, the next challenge will be to determine its tornado potential. There area four main characteristics usually observed with a tornadic wall cloud. First, the wall cloud will be persistent. It may change its shape, but it will be there for 10-20 minutes before the tornado appears. Second, the wall cloud will exhibit PERSISTENT rotation. Sometimes the rotation will be very visible and violent before the tornado develops. Third, strong surface winds will blow in toward the wall cloud from the east or southeast (inflow). Usually surface winds of 25-35 mph are observed near tornadic wall clouds. Fourth, the wall cloud will exhibit evidence of rapid vertical motion. Small cloud elements in or near the wall cloud will have these characteristics (and some tornadoes do not form from wall clouds), but these four characteristics are good rules of thumb to follow.

NON-TORNADIC SEVERE WEATHER PHENOMENA:

DOWNBURSTS

Recall that a downburst is defined as strong downdraft with an outrush of damaging winds on or near the ground. Downbursts are subdivided based on their size. If the swath of damaging winds is 2.5 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 dangerous to aviation. Microbursts are sub-classified as dry or wet microbursts, depending on how much or little rain accompanies the microburst when it reaches the ground.

Here we see the life cycle of a microburst. The formative stage of a microburst occurs as the downdraft begins its descent from the cloud base in the first picture. The microburst accelerates downward, reaching the ground a short time later. The highest wind speeds can be expected shortly after the microburst impacts the ground, in the second picture. As the cold air of the microburst moves away from the center of the impact point, a "curl" will develop. Winds in this curl will accelerate even more, resulting in even greater danger to aircraft in the area. After several minutes, the microburst dissipates, 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 aviation interests that these microbursts be identified and reported.

Some of the clues that a microburst may be forming are:

Patches of virga mark potential microburst areas. As the precipitation evaporates, it cools the air and starts a downdraft. If the conditions are right, the downdraft may accelerate and reach the ground as a microburst. Localized areas or rings of blowing dust raised from the ground usually mark the impact point of dry microburst.
A small, intense, globular rain area, with an area of lighter rain in its wake, may mark a wet microburst. A rain foot, a marked outward distortion of the edge of the precipitation area, is usually a visual indicator of a possible wet microburst. As the microburst reaches the ground and moves away from its impact point, a plume of dust may be raised from the ground. This plume is called a dust foot and also marks a possible microburst.

FLASH FLOODS:

For many years, flash floods were the leading cause of death and injury among weather phenomena. Although the death rates are decreasing, people still fall victims to flash floods.

The atmospheric conditions which cause flash floods have been somewhat different from those with severe thunderstorms. The typical flash flood environment has abundant moisture through a great depth of the atmosphere. Low values of vertical wind shear are usually present. Flash flooding commonly occurs at night, rather than in the late afternoon or evening. Flash flooding is typically produced by either large, slow moving storms or by "train effect" storms. The "train effect" occurs when several storms sequentially mature and drop their rainfall over the same area. This can occur when multicell cluster of squall line storms are present.

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