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Welcome to Advanced Spotter Training. In this video, we will delve into the world of radar
meteorology. This will be a fairly general overview, but there are a lot of excellent
resources online should you choose to go beyond this training.
So how exactly does radar work? Well this is a very simple explanation, but it will
suffice. The radar, located on the left side of the screen, sends out a signal. The signal
then propagates through the air until it encounters a scatterer. Scatterers can be any number
of things, from smoke particles to birds and bugs, to rain and hail. In this case, weíre
looking at a thunderstorm, so the radar beam is encountering raindrops and/or hail within
this thunderstorm (right side of screen). Once the signal hits these particles, a portion
of the emitted signal is sent back to the radar. The radar then retrieves this portion
of the reflected signal and uses it to piece together a radar image. The stronger the returned
signal, the heavier the precipitation. This example shows a single radar elevation at
a specific time, but radars are constantly scanning and retrieving over 360 degrees.
Once the radar makes a full revolution, it will increase the angle at which it is scanning,
giving a more 3D radar depiction. Once the radar has made several revolutions at several
angles, a radar volume is complete. This will be important in the next slide, so remember
it!
Radar Reflectivity We will begin with reflectivity. This is the
radar parameter that youíre probably most familiar with. Reflectivity is a depiction
of precipitation. Colors range from light blue or green to a dark red or purple. Of
course, this can vary depending on which source the data are coming from as color tables can
be easily manipulated. There are also two basic types of reflectivity ñ base and composite.
Radar works by sending out signals at varying elevations. Assuming there is something out
there to reflect that signal, a portion of the original signal will be sent back to the
radar. Larger and/or more closely spaced precipitation particles will reflect more of that energy
back to the radar, giving them a higher reflectivity. Base reflectivity is taken at a single elevation.
Composite reflectivity, on the other hand, is a conglomerate of all radar elevations
(volume) in which the greatest reflectivity for a specific location is displayed. The
two images here show how base and composite reflectivity vary as both images are from
the same radar and the same time. You may notice base reflectivity provides greater
detail and is higher in spatial resolution. Composite reflectivity tends to be more blocky
and may hide important low level radar features, though both can be beneficial.
Slide 4 In this image, which comes from the Oklahoma
Climate Survey (OCS), we see a zoomed view of one of the storms on the last slide. This
is the historic Moore, OK supercell of May 3, 1999. Note the base reflectivity image
and how it compares to the composite image. The base image from tilt 1 shows a much higher
resolution depiction of the supercell with a clear hook echo. The composite image does
not show this, but it does indicate the presence of an intense hail core and the likely presence
of severe hail. When looking at low level features iconic to supercells, composite reflectivity
is essentially worthless, but it does provide some idea of storm severity.
Radar Velocity Moving on to the second important radar attribute
ñ velocity. Reflectivity gives us an idea of where storms are located and whether they
contain light, moderate, or heavy rain and/or hail. But reflectivity does not tell us anything
about damaging wind potential, or whether the storm is rotating. For this, we must use
velocity. Velocity is determined by the movement of precipitation particles within thunderstorms.
Two primary colors are used to depict velocity ñ green and red. Green indicates that precipitation
particles are moving toward the radar, or that winds are blowing toward the radar. Conversely,
red indicates winds moving away from the radar. This gets tricky because the radar is a point
and the way that the radar beam sweeps out in a circle can make certain velocity signatures
difficult to discern based on where the storm is located with respect to the radar. Just
like reflectivity, there are two main types of velocity ñ base velocity and storm relative
velocity (SRM). Base velocity is simply the wind speed and direction with respect to the
ground. Storm relative, as you may have already guessed, is wind speed and direction taking
into account the movement of the thunderstorm. In the two images above, we are looking at
a radar couplet which indicates rotation. The base velocity shows some weak low level
rotation indicative of a mesocyclone, but perhaps not a tornado. However, if we take
the storms forward motion into account, the velocity couplet becomes much more impressive
and a tornado vortex signature becomes obvious. Weíre throwing around a lot of complicated
terms here, but you will become more familiar with them as we move through the presentation.
Just remember that base velocity is ground relative, and SRM is storm relative.
Here we have an image showing the difference in base and storm relative velocities for
the same storm. The image in the upper left portion of the screen is base velocity and
shows what appears to be a very weak circulation. We see two shades of red with the radar located
southwest of the storm. Reds indicate winds moving away from the radar (to the northeast).
The brighter the red shading, the stronger those winds are, so we see strong outbound
velocities adjacent to weaker outbound velocities. It make take some imagining here, but that
means there is some sort of circulation within the storm. In this case, it is a cyclonic
or counterclockwise circulation. But because we are looking at ground-relative winds and
this storm appears to be moving quite fast, the circulation is somewhat masked. Now, compare
this image to the storm relative velocity image in the upper right. Here we see a large
area of outbound (red) velocities adjacent to inbound (green) velocities. This is a much
clearer depiction of the mesocyclone and gives some idea of how strong the circulation is.
On big severe weather days when tornadoes are possible, we use SRM much more frequently
than standard velocity! Though base velocity can be very useful on days when damaging winds
are more likely.
Storm Types on Radar Alright, now that weíve covered our bases
on radar reflectivity and radar velocity, we can delve into what some of this data looks
like under varying conditions. Here we see the three main storm types as depicted in
radar reflectivity. The most basic type of thunderstorm is called the pulse storm. The
reason it is described this way is because these storms tend to develop and collapse
quickly ñ pulsing up and down. The second type of storm, which requires more organization
than pulse storms are multicell storms. These are not individual storms, but rather storm
systems made up of multiple smaller storms. Squall lines and mesoscale convective systems
would fit into this category of thunderstorm type. The final, and most severe storm type
is the supercell. This will be the main focus of this training, though we will touch briefly
on the other storm types before moving on to supercells. You can quickly see differences
between these storm types and there are really only a handful of environmental factors that
affect what kinds of storms will develop on a given day. Wind shear and instability are
the big ones! We will discuss this more toward the end of this lesson.
Pulse Storms As mentioned before, pulse storms collapse
almost as quickly as they go up. These storms form when there is little to no environmental
wind shear, but adequate instability and moisture. Notice the reflectivity image shows some fairly
high radar returns with the orange and red colors. Some pulse storms may contain hail,
but there is rarely ever rotation with these storms. In fact, the greatest threat from
pulse storms is damaging wind gusts. The image on the right side of the screen depicts two
divergent velocity signatures - areas where inbound velocities (green) are closer to the
radar than outbound velocities (red). We call this a downburst or microburst signature.
These signatures donít ALWAYS indicate severe winds, but given the right conditions, as
pulse storms collapse they may produce damaging winds as rain-cooled air reaches the ground.
Pulse storms are common in Oklahoma and Texas during the mid to late summer months. It is
not uncommon for these storms to produce 70 to 80 mph wind gusts as they collapse.
Multicell Storms Contrary to pulse storms, multicell storms
or storm complexes require at least some wind shear to develop. As individual storms collapse,
they produce what is called outflow. With successive thunderstorm development and collapse,
thunderstorm outflows merge creating a cold pool of rain-cooled air. The outflow acts
as a front, creating additional storms and as these storms collapse the cold pool and
outflow are strengthened. Multicell storms often produce sporadic severe weather including
wind damage and large hail. On the extreme end of the multicell spectrum are derechoes.
Derechoes are essentially squall lines that produce widespread significant wind damage.
Derechoes often contain bow echoes or areas of enhanced severe winds within the greater
complex. The diagrams here show the evolution of a multicell convective system (MCS). The
complex begins as a single cell or cluster of closely spaced cells. As outflows from
these storms merge, new storms develop and a cold pool develops. Storm complexes often
take on a comma shape over time as illustrated here. Sometimes individual storms within multicell
storm complexes can produce tornadoes, however they are normally brief and fairly weak.
Multicell Storms Here we see two examples of MCSs as they would
appear on radar. The animation at left is a reflectivity loop showing the coma shaped
area of thunderstorms extending from Broken Bow to Mullen Nebraska. To the left of these
storms is an area of light to moderate precipitation that we refer to as ìstrataform precipitationî.
This area is actually where something called a rear inflow jet develops. This rear inflow
jet is what keeps the storm complex going and also contributes to severe winds experienced
at the leading edge of the complex. We wonít go into this feature in detail, but itís
worth looking up if you find some extra time. The image at right shows both reflectivity
and velocity data for a fairly compact severe storm complex. This could be referred to as
a bow echo, though many bow echoes are smaller than this. Reflectivity shows an arc of heavy
precipitation and hail oriented southwest to northeast. At the same time, velocity shows
an area of inbound winds along the leading edge of the heaviest storms. These winds are
in the 60 to 70 mph range, so some wind damage is likely with this storm cluster. Note that
we are referring to base velocity, not storm relative velocity because we are more concerned
with damaging straight line winds than tornadoes in this scenario.
Supercells Reflectivity The final storm type, and the one that we
will spend the most time on in this module is the supercell. We will start with the overhead
schematic of the supercell that we have already looked at in detail. We can easily match this
diagram to an actual radar image of a supercell from central Oklahoma. Weíve already used
this radar image several times in recent trainings, so it should look familiar by now. This is
an ideal example of what a supercell looks like in reflectivity. Many are not this well
defined, thatís why it pays to understand radar reflectivity and velocity signatures
because itís not always going to look as clear cut as this storm does. There are many
clear differences between this reflectivity presentation and the ones we have recently
seen with pulse storms and multicell storms. For one, we see a clearly defined hook echo.
We will discuss how these develop in just a few minutes. We also notice a large rain-free
area arcing into this hook echo. This is called the inflow notch or inflow region of the supercell.
This is where rich moist and unstable air is being pulled into the thunderstorm updraft.
There is little to no precipitation in this area because it is where the strongest updraft
is located, so all the precipitation particles ñ including large hail, are being lofted
upward and suspended above the radar beam in this picture. Moving on to the most prominent
feature ñ the precipitation area, we note that most of the precipitation and located
to the north and east of the main updraft and hook echo. This is very typical of classic
tornadic supercells. Environmental wind shear acts to loft precipitation particles down
shear, which in this case is to the north and east. This ensures that precipitation
and associated rain-cooled air do not fall into the thunderstorm updraft. If this were
to happen, as it does with pulse storms, the storm would lose its source of inflow and
would be cut off, causing it to weaken and ultimately die. Because the heaviest precipitation
is located away from the updraft region, the supercell is able to last for hours and hours
and travel many miles without weakening. Finally, in extreme cases such as this one, a reflectivity
ball or debris ball may be noted at the end of the hook echo. In this image, the ball
of high reflectivity was associated with tornado debris being lofted far into the sky. The
radar detects the debris just as it would any other precipitation particle, and because
tornado debris is often much larger and more closely spaced than rain or hail, it generates
a higher reflectivity. These will not always be apparent even in tornado producing supercells.
Weak Echo Region and Bounded Weak Echo Region (WER/BWER)
Related to the thunderstorm updraft is an area known as the weak echo region (WER).
This area is coincident with the inflow notch and the area of most intense thunderstorm
updraft (upward motion). Because vertical winds are so strong here, precipitation particles
are lofted far into the atmosphere, resulting in an area of little to no reflectivity adjacent
to the hook echo. This is seen in the lower elevation scans of the radar. As we ascend
in elevation, more of these precipitation particles become apparent, and in the most
intense supercells, a bounded weak echo region (BWER) may be present. The BWER is an area
of weak or no reflectivity surrounded by high reflectivity as seen in the image at right.
BWERs resemble a donut. If observing a storm with a BWER, be very careful that you stay
well away from the precipitation core. BWERs indicate extreme vertical motions ñ enough
to suspend very large hail particles such as baseballs and softballs. BWERs also indicate
a greater likelihood of tornado development.
WER and BWER Continued Here we see a vertical cross section through
a BWER. Note that this supercell does not have the same classic look to it as the initial
example we saw. A small but noticeable WER is present on the southwest side of the storm
and as we look in the vertical, we see high reflectivity suspended above the area of weak
reflectivity. This indicates a very strong updraft and large hail is likely with this
storm.
Hook Echoes Now we will discuss hook echoes more in depth.
Here we have two examples of well defined hook echoes. The image at left is from the
standard radar the NWS offices around the country use for severe weather (WSR 88-D).
The image at right is a high-resolution image from a Doppler on Wheels mobile radar. Hook
echoes are sickle-shaped protrusions of precipitation that form due to the low level rotation pattern
with the supercell. Rear Flank Downdrafts (RFDs) are instrumental in forcing precipitation
around the mesocyclone in a counter clockwise fashion. This is what creates the appendage
that juts out into the inflow region. On right-moving supercells, hook echoes are normally located
on the southwest side of the storm, while left moving supercells display hook echoes
on the northwest side. We will go into more detail on right-moving and left-moving supercells
in a few minutes. Notice the high resolution image contains a small donut hole within the
hook echo itself. This is a tornadic signature. The high reflectivity surrounding the tornado
is likely a mixture of precipitation and small debris particles. Notice that the tornado
is located on the north side of the hook echo as this is where low level rotation is maximized.
Just because there is a hook echo DOES NOT mean there is a tornado! There are many supercells
that produce substantial hooks that never produce tornadoes, though damaging winds and
large hail are almost a certainty within the RFD.
Hook Echoes Continued So now that you have a better understanding
of hook echoes in reflectivity, letís look at an example of velocity. The radar image
above is from the May 24, 2011 central Oklahoma tornado outbreak. The two hook echoes circled
were indeed associated with tornadoes. Note the balls of heavy reflectivity. These are
debris balls associated with two violent tornadoes that occurred near Chickasha and Goldsby.
Analyzing the velocity image at left, notice the location of the radar to the northeast
of the storms. This means inbound winds will be from the southwest and outbound winds will
be from the northeast. And what type of velocity are we assessing here? You guessed it! Storm-relative
velocity because weíre interested in low level circulations and tornado potential.
In each of these hook echoes, we have extremely strong inbounds adjacent to moderate outbounds.
These are not only low level mesocyclone signatures, but are what is termed a tornado vortex signature
(TVS) ñ more on those later. Being able to quickly look at a radar image such as this
and point out the most important features is critical in storm spotting. It will not
only make you a better spotter, but also a safer one!
Velocity and Mesocyclones Weíve already touched on this briefly, but
a mesocyclone is essentially a rotating thunderstorm updraft. To detect mesocyclones, we refer
to velocity data, typically storm-relative, though mesocyclones can be detected through
standard base velocity as well, just not as clearly. Here we see two examples of mesocyclones.
The example at right would be called a relatively weak mesocyclone. Note the inbound and outbound
velocities arenít all that strong. This storm is likely to be severe, but tornado potential
is assumed to be relatively low unless visible confirmation is obtained. The example at left,
however, depicts an intense mesocyclone and TVS, though we havenít gotten to that just
yet. No only is it likely that a tornado is ongoing with this signature, but the tornado
is probably strong to violent based on the extreme velocities observed.
Mesovortices (mesovortex) Within the spectrum of mesocyclone size, strength
and longevity lie the mesovortices. These are small scale transient circulations that
are normally associated with squall lines or thunderstorm clusters. While these occasionally
produce very brief tornadoes, substantial swaths of significant damaging winds are more
common.
Tornado Vortex Signature - TVS This radar image should look familiar. This
is the velocity image we saw about two slides back on mesocyclone strength. When velocities
are this strong, we actually have a new classification known as the tornado vortex signature, or
TVS. In order for this signature to be present, the velocity difference across two adjacent
velocity pixels, or ìbinsî as we call them, must be at least 90 knots or around 103 mph.
In this example, we have inbound velocities of 80 knots adjacent to outbound velocities
of roughly the same speed. This means our velocity difference (gate to gate shear) in
this small area is approximately 150 to 160 knots, or 170 to 185 mph! For context, we
have a photo of the tornado that occurred with this signature. You may recall the Greensburg,
KS tornado of May 4, 2007 which wiped out 95% of the town and was rated an EF-5 on the
enhanced Fujita scale.
Other Velocity Signatures Other velocity signatures worth noting are
convergence and divergence. Convergence is simply winds coming together, often indicating
locations of fronts and boundaries or strengthening thunderstorms. Divergence is just the opposite
and sometimes indicates a collapsing storm and potential for wind damage. Sometimes these
signatures are mistaken for mesocyclones and itís very important to know where the radar
is located with respect to the area of interest as this can result in misidentification.
Splitting Supercells Weíre now going to move on to more dynamic
processes with supercells rather than just simple radar signatures. However, understanding
these dynamics is important in deciphering radar trends and supercell evolution. The
images above show a hypothetical three-dimensional rendering of a supercell thunderstorm. On
the left side of both images, a representation of wind with respect to the storm is displayed.
Note how winds increase with height indicating wind shear. This shear creates a horizontally
rotating tube of air, sometimes called a vortex tube. As the storm develops, its updraft tilts
the tube in the vertical. This forms a vortex pair ñ one rotating clockwise, and one rotating
counterclockwise. Assuming wind direction does not change that much with height, both
of these circulations will intensify. A downdraft will then develop between the two, leading
to a separation of thunderstorm updrafts. We call these splitting supercells. In the
northern hemisphere, severe weather conditions normally favor the counterclockwise circulation
(referred to as a right-moving storm) the clockwise rotating supercell is termed the
left-moving storm. Weíll see why in the next few slides.
Splitting Supercells Straight Hodographs We plot wind changes with height on something
called a hodograph. While the formation of the hodograph really isnít that important
to understand, it is important to know the three main types of hodographs that favor
supercells and what you can expect to happen when storms begin to form. When wind speed
increases with height, but there is little change in direction, we call these straight-line
hodographs. This type of wind pattern favors mirror image splitting supercells. One glance
at the time series above shows how one cell splits into two through time. One cells moves
to the north (left split) and the other moves to the south or east (right split). The right-moving
storm, however, persist and often lasts many hours.
Splitting Supercells Clockwise-curved Hodographs When winds both increase and veer (shift from
south or southeast to west or southwest) with height, we call the hodograph clockwise-curved.
As you can see in the time series above, this type of wind pattern favors the right-moving
supercell. This kind of wind shear is what we normally see on big severe weather and
tornado outbreak days. Notice that left splits tend to weaken rapidly once they move away
from the dominant southern storm.
Splitting Supercells Counter-clockwise curved Hodographs
Finally, we come to the very rare (in the northern hemisphere) counter-clockwise curved
hodograph. In this case, we would have north or northwest winds at the surface and west
winds in the mid and upper atmosphere. As you can see in the image, this type of wind
pattern favors the left splitting storm and the right split rapidly dies as it moves east.
A real world exampleÖ Hereís a real world examples of splitting
supercells. We see the dominant right-moving supercell in the center of the screen. Note
the large heavy reflectivity core and the broad hook echo. To the north of this storm
is a smaller supercell that split off the dominant storm to the south. The arrows indicate
the storm motion. So clearly based on the intensity of the main storm, we can expect
some rather large hail with this storm.
A real world exampleÖ So letís get more into what hazards one can
expect with both right and left moving supercells. The right mover is normally more prone to
produce significant severe weather including tornadoes. The left split may, on rare occasion,
produce tornadoes, but is normally more known for very large hail.
Safe Positioning ñ Splitting Sups Splitting storms and storm mergers are a real
challenge for storm spotters and safe positioning. The white arrows indicate storm direction.
Note the cell in the lower left portion of the image which will likely merge with the
storm in the center of the image at some later point in time. The Xs represent varying storm
spotting positions. If youíre positioned at the pink X, the left split is headed your
way and you can bet on some large hail. At the white X, you may be on a more interesting
storm, but because youíre looking into the precipitation core, youíre not going to be
able to see the interest area of the storm. The red X marks the best location to observe
the most intense area of the dominant supercell, but caution should be stressed here because
these storms often take hard right turns and may move toward this location ñ especially
as low level rotation is increasing prior to tornado formation.
Safe Positioning And
just to reiterate, remaining well to the south and east of the hook echo and inflow area
is the best place to observe supercells for rotation and tornado formation. Make sure
youíre paying attention to other storms around you as there is rarely only one significant
storm.
A Word on Outflow Boundaries With spotter positioning in mind, itís important
to discuss outflow boundaries and their role in enhancing tornado potential. Outflow boundaries
act as a source of low level spin for supercells and if these storms interact with the boundary
just right, they can become tornadic in a hurry. The image above illustrates how this
process works. Initially, the boundary is removed from the thunderstorm. As the storm
approaches the boundary, or the boundary drifts toward the storm, horizontal vorticity is
lifted along and behind the boundary. This is then drawn into the storm, increasing itís
rotation. At the same time, cloud bases lower because air along and just behind the boundary
is normally cooler than the air ahead of it. This causes clouds to form at a lower elevation,
ultimately lowering the cloud base. With lower cloud bases and stronger storm rotation, tornado
potential increases. Storms can become tornadic in a hurry along boundaries!
In this real world example, a storm intersects with an outflow boundary in the upper left
portion of the top image. We see a well defined hook echo in reflectivity, and a strong velocity
couplet in the bottom image, indicating the presence of a tornado. Note the area of convergence
along the boundary, indicated by the long stretch of inbound and outbound velocities
coming together.
Elevated Thunderstorms The last thing we will discuss are elevated
thunderstorms. We actually just saw an example of these kinds of storms on the last slide.
Elevated thunderstorms form above a layer of stable air where atmospheric instability
resides. On the last slide, the area behind the outflow boundary was stable at the surface,
but because there is still instability above the stable layer, these storm continue. These
are called elevated storms because they do not draw their energy from the surface airmass.
The image above shows an atmospheric sounding. The green line denotes atmospheric moisture
while the red line depicts the temperature. Donít get too caught up on the details of
the plot, just understand that the area below the steep temperature increase (called an
inversion) is where the stable airmass is located. The air above this, however, is still
unstable. Depending on the amount of instability, elevated thunderstorms can produce large hail,
but storm rotation is not able to penetrate the stable layer near the ground, so they
never produce tornadoes.
Dual Polarization Radar (Dual-Pol) We will finish up this lesson by discussing
a relatively new radar technology known as dual-pol radar. Standard Doppler radar only
scans in the horizontal. This is fine for most radar interpretation activities, but
only provides a two dimensional aspect. In order to obtain additional information, the
radar must send out both horizontal and vertical pulses. This is known as dual polarization,
or simply dual pol radar technology. Because we are scanning in two directions, we can
now discern three-dimensional properties of precipitation particles. Since this is still
somewhat new, weíre still looking at ways to incorporate this data into our radar interpretation
skills.
Tornado Debris Signature - TDS For now, we will just look at one simple example
of what dual-pol data look like. Here we have a four panel layout for a tornado-producing
supercell. Weíre going to focus on something called the Tornado Debris Signature (TDS).
The idea of the TDS is simple, the radar detects tornado debris as it is being lofted inside
the tornado through a parameter called correlation coefficient (CC). CC is a way of measuring
the similarity of precipitation particles within a storm. If the CC value is close to
1, the particles are very similar in size and shape (e.g. rain, snow, etc.), but when
CC values fall below 0.7, the particles are very dissimilar. Rain and snow will have very
high CC values because they are usually very similar in size and shape, but tornado debris
can vary wildly in shape and size. The example above shows both reflectivity and velocity
in the upper two panels. The reflectivity image contains a large hook echo and velocity
indicates a strong rotation signature collocated with this feature. Donít worry about ZDR
in the lower left, but look a the circle shaped area of very low correlation coefficient in
the bottom right portion of the image. This is the radar sensing debris within the tornado
itself and indicates that a tornado is ongoing and has caused some kind of damage as debris
is present. Take note! Debris signatures that last for a long time likely indicate the presence
of a strong or violent tornado. Storms with a persistent TDS, or several TDSs should be
given extra room to breath, as erratic storm movements may quickly put you in a dangerous
position.
Hereís a real life example from the tornado that struck Moore, OK on May 20th 2013. Note
the high reflectivity (upper left) and very strong couplet (upper right) in velocity.
In the bottom left panel, CC shows a circular area of very low values associated with debris
being lofted by the tornado as it moves east of Newcastle. After looking at several radar
scans in the vertical, it was noted that this tornado lofted debris up to around 4 miles
above the ground. The bottom right panel shows the final tornado path from the ground survey
which confirmed the tornado was among the strongest on the Enhanced Fujita scale ñ
an EF-5.
Putting It All Together A Brief Radar Simulation Alright, now weíre going to go through a
radar simulation to sum everything up. This will touch on almost everything weíve learned
in this module.
Many of you may recall the incredibly wide tornado that struck rural areas between El
Reno and Union City, OK on May 31 of 2013. In this series of images, we will only look
at the 0.5 degree reflectivity and storm-relative velocity images from around 6:00 pm CDT to
6:45 pm CDT. We will turn first to the reflectivity image on the right. Note the area of very
high reflectivity north of I-40 west of El Reno. This likely indicates an area of hail.
We also see this drops off dramatically to the south in an arc shaped fashion. If we
were to observe reflectivity at higher elevation scans, we would see a clear WER and BWER in
this area, consistent with supercell structure. We would expect very strong inflow in this
area, and turning to velocity at left, we see this is indeed the case. In fact, the
inflow at this level is so strong, the radar has interpreted it to be an inbound signal
(moving out of the storm). This is a velocity artifact that sometimes occurs with obscenely
strong velocities. Looking more closely at the velocity image, we do see a somewhat weak
but tight circulation developing. A brief tornado had been reported just a few minutes
prior to this scan and if a tornado had still been ongoing, this is where it would have
been.
Five minutes later, we have a much more impressive signature, both in reflectivity AND velocity.
Notice the hook echo forming on the southwest side of the storm just to the south of that
reflectivity gradient we spoke of earlier. This indicates a rapid organization of the
storm. Turning to velocity, we now have a clear TVS with impressive inbounds and outbounds
right next to each other. We also see the persistent area of inflow to the east of the
tornado. This should tell us that this supercell has a strong fuel source and is not likely
to weaken anytime soon. And given the moist unstable air moving into the storm, it may
produce more than just this one tornado. And as if you hadnít already guessed, hereís
where the tornado would be located.
As we move through timeÖ
we continue to see this very impressive
reflectivity and velocity structureÖ
The velocities actually become quite complex
at times, because this tornado is so large and contains multiple smaller circulations
within the parent tornado ñ a multiple vortex tornado (multi-vortex).
Finally, the tornado reaches I-40 and becomes nearly stationary. Note the TVS sitting right
on the interstate between El Reno and Yukon. Turning to reflectivity, it is apparent that
this tornado is now completely rain-wrapped. Nothing in the reflectivity image alone would
tell you a tornado is present. This is why velocity is so critical in the warning process
and in safe storm spotting! This is an example of a storm that should prompt you to keep
your distance. Erratic changes in speed and direction of the tornado resulted in several
fatalities as people were struck by the tornado in their vehicles.
The couplet finally weakens at around 6:42 pm CDT, though other weak circulations persist
well into the evening. We also see several other storms developing back to the west.
Putting It All Together ñ May 20, 2013 Weíll wrap up with an animation from the
historic May 20, 2013 tornado that struck Moore, OK. In this animation, we will only
look at reflectivity, but rest assured the velocities were just as impressive! Notices
the long arching hook that wraps into the high-reflectivity ball just southwest of Moore.
These high reflectivities are due to a high concentration of tornado debris being sensed
by the radar. Often, these are referred to as debris balls. As the animation plays, one
can see the individual reflectivity elements spiraling into this area where the tornado
resides. The tornado dissipates to the northeast of Moore, but as debris continues to fall
from the sky we see an area of high reflectivity where the tornado lifted.
Thanks For Watching! That bring us to the end of our presentation
on radar interpretation. Thanks for your attention! Hopefully youíre more equipped to understand
what you see on radar, ultimately keeping you and others around you safer while storm
spotting. The next section of the Advanced Spotter Training will focus on where to find
information for basic severe weather forecasting.