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Topics in Advanced Storm Spotting Welcome to advanced spotter training, in this
video we will dive into the complicated, yet fascinating world of severe storm ingredients.
It is our hope, that after completing this training, you will have a greater understanding,
and respect of the complicated combination of ingredients that must exist for severe
storms to develop and thrive.
But before we begin Recall a few basic equations Alright, before we begin, we need to go ahead
and recall a few basic equations, first off, we’ll need our set of variables, our equations
of motion, different forecast equations, my favorite, the quasigeostrophic omega equation,
the basic equations of conservation of momentum, energy, mass, and water, not forgetting the
equation of state, and finally we’ll need the Hypsometric equation to complete the derivation
of the second law of thermodynamics. OK, I know, right now you’re thinking, they’re
kidding right? Well I’m not, or am I?
The Ingredients for Severe Storms Well, we’re all in luck, because we’re
not going to go that in depth in this training. In general, we can break down the ingredients
of severe storms into four separate, but easily understood components. Moisture, Instability,
Lift, and wind shear. With wind shear playing a vital role in the development of supercell
thunderstorms.
The Ingredients for Severe Storms So lets start from the beginning. Nearly all
moisture for thunderstorms across the plains is transported from the Gulf of Mexico. Commonly,
southerly winds ahead of a strong storm system moving off the western high plains drives
this transportation of moisture. To the west, behind a cold front and/or dry line (which
we will describe later) very dry air is transported off the desert southwest. So how do we measure
the available moisture? We;;. we use the dew point temperature. In short, the dew point
temperature, is the temperature at which water condenses, the higher the dew point, the more
water vapor in the air. The dew point can never be more than the observed temperature,
and if the temperature falls, the dew point will follow. When the temperature and the
dew point are equal, the air is completely saturated.
The Ingredients for Severe Storms Now that we have discussed moisture, lets
discuss how it plays a major role in the development of Instability. In short, instability is the
term meteorologists use to describe the atmosphere's tendency to encourage or deter vertical motion.
In the simplest sense, we need warm moist air to rise for thunderstorms to develop.
For instability to exist, we usually have warm moist air in the lowest levels of the
atmosphere, with generally cooler air aloft. When you think instability, one only needs
to picture a hot air balloon, which uses instability to its advantage. The air within the balloon
is much warmer than that of the surrounding air. Therefore, the warm air creates buoyancy.
Now working against buoyancy is gravity, when the buoyancy is greater than that of the downward
force of gravity, the net gain is your instability.
The Ingredients for Severe Storms Now that we know what instability is, how
do we measure it in the atmosphere, how do we quantify it? We use a measurement called,
CAPE, which stands for Convective Available Potential Energy. We measure CAPE by looking
at the atmosphere in the vertical, one of the best ways of doing this is by releasing
a weather balloon and actually sampling the atmosphere. The data that is returned by the
weather balloon is plotted on a Skew-t diagram, which we call a sounding, here are two basic
examples of a skew-t diagram. On the sounding we theoretically lift a parcel of air, either
from the surface or from a desired height in the atmosphere, which is represented by
the yellow line on the two soundings above. We then measure the area between the temperature,
the red line, and the yellow line, the trajectory of the parcel. That area is the computed CAPE,
which has units of Joules per Kilogram. The distribution of CAPE in the atmosphere plays
a critical role in thunderstorm development, both Sounding A and Sounding B have the same
value of CAPE, but notice their distribution is very different, the “Fatter” CAPE of
Sounding A may be more beneficial for development than the “skinny” CAPE of Sounding B.
The Ingredients for Severe Storms So, now that we have discussed what instability
is, lets go ahead and discuss the cap and it’s role in the development and sometimes
hindrance of thunderstorms. Given this is advanced training, many, if not all of you,
have likely heard one time or another about the cap. Either the cap is limiting thunderstorm
development, or the cap is eroding and development has started. So what is the cap? The cap is
basically a region in the atmosphere where air temperature increases with height instead
of decreasing, the more technical term is, temperature inversion. So lets say we have
a parcel of warm air, and it’s rising through cooler air within the environment, so it’s
unstable. But then our parcel rises into an environment that is warmer than itself. Once
the parcel reaches a level where its temperature equals that of the inversion, it stops rising,
our parcel has become stable.
Here is quick way to visualize the cap in three dimensions. We have the surface, with
our dry line and region of warm moisture laden air advecting into the region, and above,
at approximately 3000 feet, we have a layer of warm or hot air. This is our cap, or temperature
inversion if you will.
For forecasters and spotters, the easiest way to observe the Cap is to look at either
an operational or model sounding. Here we have a model sounding from April of 2013.
Notice the temperature begins to rise with height once you get to a height of approximately
3000 to 4000 feet, this is your cap.
The Ingredients for Severe Storms Sometimes you can actually visualize the temperature
inversion with your own eyes. I would imagine many of you watching right now, have seen
smoke stacks, either from factories or power plants where the rising smoke, which is unstable,
suddenly begins to flatten out and spread like it’*** a ceiling. The region where
the smoke begins to spread, is the location of the Cap, or temperature inversion.
The Ingredients for Severe Storms Ok, so we’ve discussed moisture, we’ve
discussed instability, and we’ve covered the role and impact of the cap. Now, thunderstorms
can form with only the lift available from instability, when no capping inversion is
present. We typically refer to these storms as pulse thunderstorms, but we’ll dive more
into thunderstorm types in another video. For the moment, lets focus on what many consider,
the most severe type of thunderstorm, the supercell. Now from the previous slides, you
might be thinking, well the Cap is bad, but au contraire, The cap plus instability play
a vital role in the development of strong, severe, thunderstorms, typically supercellular
thunderstorms. With that said, with a cap in place early in the day into the early afternoon,
you can “build up” CAPE at the surface and above the cap, this is especially true
on days when there is a fairly stout inversion. Now, this is where lift comes into play, we
need a source of lift, to give a “nudge” upward, to help our parcel of air “bust
through” the cap and rise into the unstable air above it. Lift comes in many forms, the
most common being that of warm and cold fronts, dry lines, and outflow from current or previous
thunderstorms.
The Ingredients for Severe Storms Let us now focus on the outflow scenario.
Thunderstorm development due to outflow is a particularly fascinating way of generating
lift. This is because, for outflow boundaries to be present, there must already be thunderstorms
or the remnants of thunderstorms in the general area. Take the example shown, we have a line
of thunderstorms shown on radar. Extending out from the storms is a visual boundary picked
up on the base reflectivity scan. The storm farthest to the left, likely developed along
the outflow boundary which was generated by the storm to its right. Another example would
be that of morning boundaries left by mesoscale convective systems or MCSs. In short, MCSs
are just a large cluster of thunderstorms that usually develop late in the evening and
persist through the morning hours, but as mentioned before, we’ll go more into thunderstorm
types in another video. However, sometimes the boundaries left by MCSs can linger through
the afternoon, providing a source of lift for thunderstorms.
Ok, now that I’ve bored you with all that talk about lift, instability, the cap… and…
what was it? Oh yeah, moisture… lets get into some serious fluid dynamics, recall those
equations? Right then, only kidding, but seriously lets talk about wind shear, one of the most
vital components in the development of severe thunderstorms. In the simplest terms, wind
shear is generated by increasing wind speed and/or direction with height. We need deep,
strong wind shear for the development of supercells. Wind shear is measured in knots and we usually
look for deep wind shear within a layer from the surface to 6 km above the ground. Now
for the development of tornadoes, we also need strong low level wind shear, and for
this we usually look at the surface to about 1 to 3 km. Here is a simple example of how
wind shear results in the development of rotation within storms. First off, you have very strong
winds aloft, notice the high clouds are tilted in the direction of the upper level flow.
Meanwhile, the winds near the surface are slower. The difference between these two wind
speeds, the shear, and since we’re only dealing with changes in speed with height,
we’d call this speed shear. This results in a rotational component, this rotation,
is vital for the development of severe supercellular thunderstorms.
The Ingredients for Supercells Ok, so lets take this a step further, what
if we add a directional component to our shear profile. So we not only have winds increasing
in speed with height, we also have a change in direction. Here our surface winds start
out from the southeast and as we move up in the atmosphere, our winds veer, or turn clockwise,
with height. So, as with the previous example, we still have our rotational component in
the horizontal. Now, if we add in updraft, we begin to tilt this rotation into the vertical.
This is a critical component to the development of supercellular thunderstorms.
The Ingredients for Supercells Alright, so now we have our established rotating
updraft. Recall from basic spotter training, that a supercell is defined as having a persistent
deep rotating updraft, which we call the mesocylone. In tornadic supercells, the mesocyclone is
the source of rotation for the tornado. Remember, all supercells have a mesocylcone, but not
all supercells result in a tornado. In fact, there are thousands of supercellular storms
every year across the world, but only a fraction of these generate a tornado. Now I know there
is a lot going on in this image, and we’ll dive into the nuts and bolts of supercells
in another video, but for right now, the major take away from this video is to understand
and respect all of the intricate ingredients that have to combine for supercells, and severe
storms to develop.
In closing, I just want to remind all of you that safety is your number one priority while
spotting! Don’t risk your life for a video, photograph, or report. Always report what
you see, don’t ever assume its already been reported.
Make sure you stay in touch, follow us on Twitter, like us on Facebook, and definitely
subscribe to our YouTube channel. We will continue to post more videos diving into advanced
spotter topics as the months move on. We have a lot of topics to cover, such as storm types
and structure, safe spotting practices, where to find and how to monitor data. The topics
are endless! And finally, thank you for your interest in advanced spotter training. Let
us know what you think of this video in the comments section below. Spotters are our eyes
in the field and we appreciate all you do. And with that, this concludes the Severe Storms
Ingredients Advanced Spotter Training video from the National Weather Service Norman,
Oklahoma Forecast Office.