| |
|
This
module compiled with information courtesy of the official NOAA Storm
Spotters Guide. |
| |
| SECTION FOUR: |
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 |
|
|
| |
| 
|
| 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. You can see a radar example of this on the right. |
|
|
| |
|
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. |
|
|
|
| |
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.
|
|
| |
|
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. |
|
|
|
 |
| 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.
|
|
|
| |
|
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. |
|
| |
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. |
|
|
| |
| 
|
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. |
|
| |
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. |
|
| |
|
|
| |
|
