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This module compiled with information courtesy of the official NOAA Storm Spotters Guide.
 
SECTION FOUR:
GLOSSARY 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.

MULTICELL LINE STORMS:

The multicell line storm, or squall line is it is more commonly called, consists of a long line of storms with a continuous, well-developed gust front at the leading edge of the line. The line of storms can be solid, or there can be gaps and breaks in the line. As the gust front moves forward, the cold outflow forces warm unstable air into the updraft. The main updraft is usually at the leading edge of the storm, with the heaviest rain and largest hail just behind the updraft. Lighter rain, associated with older cells, often covers a large area behind the active leading edge of the squall line.

Squall lines can produce hail up to about golfball size, heavy rain, and weak tornadoes, but they are best known as prolific downburst producers. Occasionally, an extremely strong downburst will accelerate a portion of the squall line ahead of the rest of the line. This produces what is called a bow echo. Bow echos can develop with isolated cells as well as squall lines. Bow echos are easily detected on radar but are difficult (or impossible) to observe visually.

An approaching multicell line or squall line often appears as a dark bank of clouds covering the western horizon. The great number of closely-spaced updraft/downdraft couplets qualifies this complex as multicellular, although storm structure is quite different from that of the multicell cluster storm.

Multicell line storms are better known as squall lines, which is the term that we will use from here on. The former name is for positioning squall lines in the thunderstorm spectrum. This particular storm evolved from a supercell into a short line of storms at the time of the photograph. We are looking west from about 5 miles, as the storm approached. Wind damage and large amounts of small hail were occurring within the squall line at this time. Photo by Moller
GLOSSARY

The squall line is a solid or broken line of thunderstorms with a continuous, well-developed gustfront on the leading edge. Thus, updrafts and new updraft development occur on the downwind (east) side, where the squall line is moving into unstable inflow air. The gustfront "scoops" warm moist air into the updraft, and the rainy downdraft lowers dry, relatively cool mid-level air to the ground. This stabilization process is quite efficient; therefore, squall lines are common, especially in vertically sheared environments where the mid-level winds are moderate to strong.

 

The most common severe weather element in squall lines, by far, is the downburst, with damaging winds possible from the time of gustfront passage, into the period of heavy precipitation. Hail may occur with the rain, with the heaviest rain and largest hail adjacent to the updraft and near the leading edge of the squall line. Dissipating elements at the rear of the squall line often result in a period of light rain before cessation of precipitation.

Intense storms, in rare cases even tornadic supercells, periodically occur in squall lines. The most likely locations for these more powerful storms are at an eastward bend, on the south end, or north of a significant break in the line. Note that all of these positions allow a storm to compete better with its neighbors for the low-level inflow air.

 

 

 

This doppler radar view of an eastward moving squall line in western Oklahoma and northwest Texas. In particular notice the isolated supercell ahead of the line at Childress. Cells that develop ahead of a squall line should particularly be watched as they can be quite intense. This one dropped several tornadoes before the squall line overtook it. Those with an online MBA degree in meteorology can see for themselves in the doppler radar image to the right.
GLOSSARY

This satellite photo shows a huge anvil cloud arising from a large cluster of storms. This is called a mesoscale convective system or "MCS". An entire MCS cannot be viewed from the ground and in some cases not even by a single radar, so we use the satellite perspective. It is a group of multicell storms, often dominated by a vigorous squall line on the downwind (east) side and a number of weaker multicell cluster storms in the interior.

An MCS often will bring severe weather and heavy rain with the squall line, and additional heavy rainfall with the interior storms. A number of major flash floods have resulted from MCS passage, making this large storm complex an extremely important grouping of multicell thunderstorms to recognize.

Looking down the shelf cloud, note the change in appearance from the ragged, outflow-torn clouds to the smoother elements ahead of the line. The outflow winds, hail and heavy rain were to arrive in minutes.
This strong gustfront was accompanied by 40 and 50 MPH winds and a shelf cloud with a highly-sloped concave shape to the underside. Near the light area on the southwest horizon, a downburst was resulting in damage at this time, as reported by Amateur Radio Spotters southwest of Fort Worth, Texas. The squall line was moving eastward (right to left). Photo by Moller
Here is an image of the back side of a shelf cloud as it has passed over. High winds have begun and would most likely be followed shortly by heavy rain and hail.
GLOSSARY
Continuing its eastward movement, a squall line is pictured at sunset, looking to the distant southeast. The largest tops near the south end of the line graphically illustrate the tendency for fresh, strong convection to build southward with time, towards the area of strong inflow. Photo by Doswell

Squall lines and multicell storms occasionally develop the appearance of a "bow echo" on radar. When the bow shape opens toward the strong mid-level winds (10 to 20 thousand foot level winds of 40 kts or greater), there is an excellent chance that the strong mid-level currents have been directed to the ground in a downburst, forcing a portion of the squall line or multicell storm to accelerate forward. Macroburst and microburst winds are common with these storms, and 100+ MPH winds have been reported in extreme cases.

Cyclonic rotation may result from horizontal wind shear north of the bow, leading to a rotating "comma-head" storm. Weak to occasionally strong tornadoes may occur with the comma head storm, while gustnadoes may form on the strong bow echo gustfront.

The bow echo weakens as the accelerating downburst outruns the storm complex and comma head storm rotation ends. Those who operate radar should be aware that rotating comma head storms occasionally have deceptively weak radar reflectivities while producing damaging winds and tornadoes.

Smaller scale bow echoes frequently can be detected from visual observations. This southward view shows the underside of a right to left (eastward) moving storm's shelf cloud, with the southern extent of the complex bowing radically eastward in the background. Damaging winds and a radar bow echo occurred within this area.

We need to stress that it is not the job of spotters to detect and report bow echoes, but spotters should know what they are dealing with and what the main severe weather threats are if Weather Service personnel ask them to check out a bow echo complex.

Here is another good example of a bow echo feature that can be visually seen, contributed by Matt Hartman. You can see here below how it looked after it passed by.

In this final example we again see visual evidence of a bow echo. While these pictures are dramatic, bow echos are sometimes not visually evident. Again, it's not generally the job of spotters to detect and report bow echos. However, for your own knowledge and safety, and if you are asked about it by those you are reporting to, it's important you know what you are seeing.

 

GLOSSARY

Squall lines most frequently produce severe weather near the updraft/downdraft interface at the leading storm edge. Downburst winds are the main threat, although hail as large as golf balls and gustnadoes can occur.

Flash floods occasionally occur when the squall line decelerates or even becomes stationary, with thunderstorm moving parallel to the line and repeatedly across the same area.

Squall lines with a confirmed severe weather history allow for the issuance of reliable warnings. Pilots should be extremely cautious, as they should for all thunderstorms, particularly near the squall line's leading updraft/downdraft interface.


SUPERCELL STORMS:

The last of the four major storm types is the supercell. The supercell is a highly organized thunderstorm. Although supercells are rare, they pose an inordinately high threat to life and property. Like the single cell storm, the supercell consists of one main updraft. However, the updraft in a supercell is extremely strong, reaching speeds of 150-175 mph. The main characteristic which sets the supercell apart from other thunderstorms is the element of rotation. The rotating updraft of a supercell is called a mesocyclone, and helps the supercell to produce extreme severe weather events, such as giant hail (more than 2 inches in diameter), strong downbursts of 80 mph or more, and strong to violent tornadoes. Recall that the supercell environment is characterized by high instability, strong winds in the mid and upper atmosphere, and veering of the wind with height in the lowest mile or so. This environment is a contributing factor to the supercell's organization. As precipitation is produced in the updraft, the strong upper level winds literally blow the precipitation downwind. Relatively little precipitation falls back down through the updraft, so the storm can survive for long periods of time with only minor variations in strength.

In fact, the major difference between supercell and multicell storms is not the number of cells but the coincidence of updraft and rotation in the supercell and lack of same in the multicell. As we shall see, circumstances keep some supercells from producing tornadoes, even with the presence of a mesocyclone.

Nevertheless, even though it is the rarest of storm types, the supercell is the most dangerous because of the extreme weather generated by the intense updrafts and downdrafts.

This storm was producing baseball hail east of Carnegie, Oklahoma, as it was photographed looking east from 30 miles. From right to left (south to north), we note the flanking line, main CB, and downwind anvil above the precipitation area.

The flanking line of the supercell behaves differently than that of the multicell cluster storm, in that updraft elements usually merge into the main rotating updraft and then explode vertically, rather than develop into separate and competing thunderstorm cells. In effect, the flanking updrafts "feed" the supercell updraft, rather than compete with it.

GLOSSARY

This is a horizontal, low-level cross-section of a "classic" supercell. The storm is characterized by a large precipitation area on radar, and a pendant or hook-shaped echo wrapping cyclonically around the updraft area.

 

Note the position of the updraft and the gustfront wave. The intense updraft suspends precipitation particles above it, with rain and hail eventually blown off of the updraft summit and downwind by the strong winds aloft.

Updraft rotation results in the gustfront wave, with warm surface air supplying a continual feed of moisture to the storm. Updraft rotation occurs when winds through the atmosphere are strong, and low-level turning is significant. As inflow air in the lower 5,000 feet approaches the storm from the south, the low level turning results in development of rotation about a horizontal axis.

As the air is lifted into the updraft, the rotation is "tilted" to that about a vertical axis. To see this rotation about a horizontal axis caused by wind shear, imagine rolling a tube along a tabletop with the palm of your hand.

The movement of your hand represents the strong winds above the surface, producing rotation because the winds near the ground are much weaker. This simple picture is complicated by the turning of the wind direction with height, but the concept remains similar. Lifting this "horizontal" vortex into the updraft results in cyclonic rotation.

A westward view of the classic supercell reveals the wall cloud beneath the intense updraft core and an inflowing tail cloud on the rainy downdraft side of the wall cloud. Wall clouds tend to develop beneath the north side of the supercell rain-free base, although other configurations occur.

Observe the nearly vertical, "vaulted" appearance of the cloud boundary on the north side of the CB and adjacent to the visible precipitation area. A sharp demarcation between downdraft and rotating updraft results in this appearance.

Note the anvil overhang on the upwind (southwest) side of the storm and the overshooting top, both visual clues as to the intensity of the updraft. The largest hail falls adjacent to, and occasionally through the updraft. Therefore, hail damage is most likely near the primary tornado threat area.

GLOSSARY

A close, westward view of a supercell updraft and adjacent precipitation cascade strikingly resembles the model we have just seen.

Wall clouds frequently slope downward towards the precipitation area, as shown. If you are a mobile spotter and encounter a view such as this, turn around and outdrive the storm by going eastward or, better yet, move away from the storm to the southeast.

This is very close to the fall area of large hailstones, and moving north or waiting at this location will put you in danger of hail damage, or worse!

In this rare photograph we can see both the parent cumulonimbus cloud (CB) and the tornado. Doppler radar studies indicate that most tornadoes form first in the mid-levels (about midway to the summit of this cloud) where updraft and low pressure circulation are strongest.

Increasing rotation then extends downward, with the tornado circulation intensifying towards the ground. As this is occurring, a secondary but very powerful downdraft develops near the back of the cloud. This rear-flank downdraft (RFD) descends to the ground with the funnel cloud, both drawn by rapidly lowering pressures near cloud base.

The indentation on the left side of the CB in this photo seems to verify the presence of the RFD, with a clear distinction between hard-textured updraft cloud and the ragged, dissipating cloud elements caught in the RFD. The tornado is at the intersecting point of the rotating updraft and RFD.

Photo by NWS

Looking east from about 60 miles away, we see a line of towering cumulus clouds and a large supercell storm. Note the great amount of anvil overhang and the large overshooting dome at the summit of the updraft.

Distant supercells frequently have this domed, "diffluent" anvil appearance, with the tremendous updraft velocities and outflow resulting in the intense upper-level divergence. The visual clues are strong, although we cannot be sure that this is a supercell simply from appearance. However, this particular storm was producing a tornado that stuck downtown Ft. Worth, TX on March 28, 2000.

By necessity, man and machine (i.e., spotters and radar) complement each other in the severe weather detection program.

GLOSSARY
This supercell featured a rock-hard, overshooting CB top and anvil overhang, looking southeast from about 40 miles away. Note that the supercell CB is more vertically oriented than the weaker updraft of the neighboring towering cumulus cloud. This is a valuable clue in estimating the strength of updrafts on a day with strong vertical wind shear. This storm produced baseball hail, but no known tornadoes, along a track in southeast Oklahoma and southwest Arkansas. Photo by Bluestein

There are ample signatures of updraft rotation in this westward view of a very intense supercell from 10 miles away. The circular mid-level cloud bands and the smooth, cylindrical CB strongly hint of updraft rotation.

Cloud elements moved along the flank into the main CB, with rapid vertical development occurring at the merger point. Close examination of the photo will reveal a wall cloud beneath the lower edge of the CB. Although this storm produce a couple of very brief tornadoes and funnels, it did not produce the large tornado it was certainly capable of.

Here is a classic supercell in Montana. You can see the anvil, backsheared anvil, and a well defined overshooting top, although slightly hidden by some foreground scud clouds. This is a good example of how you would need to be in the correct viewing location to see what is going on, but from this distance, we have some good visual clues that this storm might be severe. Notice the firmness of the updraft.
GLOSSARY

This image shows the problem that frequently arises in viewing a tornadic storm to the north -- lack of contrast. The dark precipitation area all too often blends in with wall clouds, tornadoes, etc.

 


Supercell storms come in different shapes and sizes, as observed on radar and by the human eye. Some are very prolific rain producers, whereas others are drier than the average supercell. This is a westward view of a "wet" supercell approaching in the evening light.
 
This is a low-level, horizontal cross section through a "wet" or "heavy precipitation" (HP) supercell. Basically, the HP supercell has a broad hook or pendant, usually with high radar reflectivities (VIP 5s or 6s). Occasionally, the HP supercell has an even more pronounced southwest flank precipitation area, with the radar echo taking the shape of a kidney bean or letter "C". The inflow/rotating updraft notch will face east, and with nearly equal size precipitation areas northwest and southwest of the mesocyclone. Whichever is the case, the rotating updraft is on the leading storm flank, with heavy precipitation falling into the west and southwest flanks of the mesocyclone. Note the inflow band in the vicinity of the pseudo-warm front east of the updraft.
GLOSSARY
A westward view of a composite HP storm model shows the position of this inflow cloud band, very similar to the previously mentioned "beaver's tail" cloud in the classic supercell. In fact, the HP storm has an appearance similar to the classic supercell, except for the opaque precipitation curtain wrapping around the west and southwest flanks of the wall cloud and/or updraft. Sometimes the precipitation is a solid visual curtain, and at other times there is a distinct break, shown here, between the precipitation falling from the anvil area and that descending from the southwest flank. The southwest flank precipitation shaft is often visually dark and blue-green in color, indicative of unusually heavy rain and hail.

This is a northward view of an HP storm near Lake Stamford, TX in 2003 compiled of video frames from a pan of the storm. Inflow bands are visible in the photo, feeding into the updraft area. The storm had a very well-developed wall cloud, with precipitation wrapping around the north and west flanks of the lowered cloud base. Note the subtle gustfront and shelf cloud extending eastward from the wall cloud. Spotters will have a difficult time with the HP supercell, since there can be poor visual contrast between the wall cloud and precipitation behind it. The strongest visual clues in identifying this type of supercell usually are the curving inflow bands and mid-level cloud bands which wrap around the updraft, both suggestive of storm-scale rotation. This dramatic storm produced large hail and several brief tornadoes.

High precipitation supercells frequently have been observed to develop or intensify as they moved parallel to and along a stationary outflow boundary from previous thunderstorms. This is a northward view of such an outflow boundary, with several large thunderstorms in the distant and extreme right side of the photo moving away from our position. Note the long shelf cloud that has been left behind the storms and along the boundary. Photo by Doswell
In summary, supercells are extremely dangerous, but excellent warnings are possible once the storm has been properly identified. The demarcation between supercell and multicell storms is most important, obviously much more so than that between single cell and multicell storms, or between multicell and squall line storms. As mentioned earlier, it has been suggested that thunderstorms simply be classified as "supercells" and "ordinary" storms.
GLOSSARY

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