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This
module compiled with information courtesy of the official NOAA Storm
Spotters Guide. |
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| SECTION SIX: |
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. |
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| THE TORNADO |
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The
difference between this storm and a tornadic supercell is
that the latter storm's vigorous rotation and surface low
pressure field cause a wave to form on the gustfront. |
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This
allows warm, moist surface-based air to feed continually
into the updraft and wall cloud. Cold air is "wrapped up"
by the strong circulation and does not immediately undercut
the wall cloud. Instead, the wall cloud becomes the location
of extreme convergence of warm and rain-cooled air. |
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| Schematic
of Mesocyclone Effect on Storm's Gust Front
The
two previous examples are combined for comparison's sake.
If the "undercut" storm is a relatively weak supercell (rather
than a severe multicell storm), then the circulation is
not strong enough to prevent cold outflow undercutting. |
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| LIFE
CYCLE OF A TORNADO: |
| These
3 pictures illustrate the life cycle of the tornado. Tornado circulation
develops at the mid levels (about 20,000 ft) in the storm where
the storm's updraft and mesocyclone are the strongest. The circulation
gradually builds down and up within the storm. At about the same
time, a downdraft develops in the mid levels near the back edge
of the storm. This downdraft, called a rear flank downdraft (RFD),
descends to the ground along with the tornado circulation. Rapidly
lowering barometric pressure near the ground is believed to be the
primary means of drawing the tornado circulation and RFD down toward
the ground. The RFD may reveal itself as a "clear slot" or "bright
slot" just to the rear (SW) of the wall cloud. Sometimes a small
shelf cloud will form along this slot. Eventually the clear slot
and tornado will reach the ground within a few minutes of each other. |
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The
first of three photos of a tornado in the Lake McClellan
National Grasslands, TX in May 20, 1999. Warm inflow and
cool RFD outflow arrows superimposed to help clarify the
low-level airflow around a tornado. Note inflow wrapping
into the tornado from the southeast, east, and northeast.
We are looking west-northwest from about 5 miles. A clear
slot is becoming visible behind the subtle "outflow lowering."
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A
few minutes later, the clear slot becomes more obvious,
as does the weak shelf-like outflow cloud at the base of
the flanking line. This is during the tornado's mature stage,
and the twister's warm inflow is already being cut off by
the left-to-right advancing cold RFD outflow. |
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Near
the end of the mature stage, we have a spectacular view
of the clear slot wrapping around the tornado. The gustfront
continues to advance and virtually has isolated the tornado
from the rest of the storm and has almost ended it's life.
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| TORNADO/FUNNEL
CLOUD LOOK ALIKES: |
| An
entire section could be devoted to this alone, and very may well
be at some point in the future. But for now, we will take a brief
look at it. Remember the one key thing. Look for sustained rotation! |
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Experienced
spotters/chaser are probably aware that a number
of features (both natural and man made) can bear
a resemblance to a tornado or funnel cloud. Some
of these natural features include rain shafts and
scud clouds. Some of the man made features include
smoke from oil flares and factories. If a suspicious-looking
cloud formation is observed, watch it for a minute
or two. Look for organized rotation and a vertical
or near vertical axis. Here we have a scud/shelf
cloud type formation that is getting almost vertical,
and could cause confusion to the untrained eye.
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In
this image we have a backlit rain shaft. However,
later this storm was briefly tornadic. |
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| A
strong gustnado formed along the leading edge of
the Rear Flank Downdraft boundary. Gustnados can
be particularly confusing if they occur near or
under the updraft base. Always try to look for some
sort of rotation at the cloud base when you see
a dirt "spinup" on the ground. |
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| A
non-tornadic lowering near Ruma, IL. Notice how
the bottom is blocked from view by the trees. This
was not touching the ground or rotating, however
it did prompt tornado sirens to be sounded in Ruma. |
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| Here
we have another dense, narrow rain shaft. From a
distance this could appear to be a wedge tornado.
Again, look for signs of persistent rotation, lofting
of debris, and other signs of rotation in the mid
levels of the storm, which the storm is not showing
here. |
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| The
images below are a series that shows a tornado always doesn't follow
the rules as far as the typical life cycle. This tornado in South
Dakota actually appeared more rope like in it's earlier life, and
skipped the dying rope stage at the end. The last picture in the
series was how it appeared shortly before dissipating. |
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Above
images contributed by Andrew Revering |
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