Thunderstorms, Supercells, and Tornadoes.

With all the severe weather over the last few weeks; I've been seeing a lot of interest in Severe thunderstorms and especially tornadoes. So I thought I would answer the questions I've been fielding in a blog post. This is all a very complex subject. I've tried to keep as much of the technical and scientific jargon  out of it as possible...when I couldn't...I included a brief explanation on what the term means......This  isn't met to make you experts...but it should give you a good understanding of what is going on and how things form.   

How does a thunderstorm form?

A thunderstorm is formed when rising warm air (updraft) cools as it moves aloft. As the warm air rises, the water vapor cools.  The cooling water vapor condenses and forms clouds.  As the updraft of warm air continues the storm gets taller and taller. At this point the air in the upper part of the cloud is quite cold. At this time all the dust, dirt and other things collects moisture and forms precipitation. At some point the precipitation can no longer stay aloft. The act of the falling precipitation creates cool downdrafts in the storm. The intermixing of the precipitation with dust and dirt, forms electrical charges. When these charges dissipate we see and hear it in the form of lightning. 

How is a rotating (supercell) thunderstorm formed?

To get a rotating thunderstorm (mesocyclone) the developing condensation of the storm  gives off heat to the area around it.  This allows it sucks up warm moist air (this is inflow). The inflow of warm moist air supplies the storm with the energy it needs to develop.  As the thunderstorm continues to develop the updraft gets stronger . The stronger the updraft the taller the thunderstorm. The higher the storm the more prone it is to wind shear. 

For a mesocyclone to form you need wind shear ( a change in wind direction and/or speed with height).

For example,  low level winds coming in from the South, strong midlevel winds Coming in from the West Southwest, with very strong upper level winds coming out of the Northwest.

Moderate to strong directional and speed shear from the surface up to around 20,000 feet. is the most important factor in the development of a supercell.  .

Sometimes as the  mechanics of this process  go on, there are spinning horizontal tubes of air (horizontal vorticity) formed closer to the ground from the Surface up to around 5,000 feet. This tube of air is drawn up and is aligned with the flow into the storm's updraft (vertical vorticity).  This forces the vertical vortex to start spinning (Helicity) (Helicity is the measurement of just how much rotation is wrapping around the updraft). Low pressure in the Mesocyclone's core, makes an inward pointing  gradient force that keeps this inflow vortex spinning in the midlevel of the storm. This gradient force balances out the fast vertical vorticity in the midlevel .

As the thunderstorm moves along, the right side will normally move faster than the air closer to and along the ground, this is where the updraft begins (This is the wind shear I outlined above). The warm air in the updraft will enter the storm in the bottom front side of the thunderstorm.  Because of the process, the rising air will end up in the back top edge of the thunderstorm. The result of this is the updraft becomes tilted.

The warm moist air mingles with the cool dry air higher up in the thunderstorm. The wind shear gets the vertical tube of air to spin faster, as the tube spins faster the tube shrinks in diameter. This increase in helicity is vital if a supercell is going to form. As the storm gets stronger, more and more moist air is drawn up, as the thunderstorm gets taller the air aloft gets colder, and more and more cool dry air is pushed toward the ground; the tube of air spins and shrinks faster and faster.  It is vertical wind shear that makes the thunderstorm tilt and rotate. A supercell is a thunderstorm with a deep persistent rotating updraft.  Once the storm becomes saturated with moisture, leading to more and more cloud formation. It can cause the formation of what is called a wall cloud, but not always.  Most strong to violent tornadoes are associated with a strong supercell with a wall cloud.

Here are two images that show what is basically going on.

The tilt of a severe or supercell thunderstorm is important to its lifecycle. The tilt helps to separate the updraft and the downdraft from each other. This separation is key to the longevity of the storm, it keeps the precipitation cooled and more stable air in the downdraft away from the updraft feeding energy to the supercell, keeping things unstable.  The tilt also plays a big role in the formation of large and giant hail. The tilt keeps the hail inside the thunderstorms for a much longer time than a typical garden variety thunderstorm.

A little on the tornado:

There is a lot we still don't know about tornadoes, but there is also a lot that we do know.

While most supercell thunderstorms produce severe weather, not every supercell will produce a tornado. In fact only around one out of four supercells will form a tornado. No one is exactly sure why that is the case.

Tornadoes are rated by the Enhanced Fajita Scale. It goes from EF0 the weakest to EF5 the strongest. The ratings are assigned after a damage survey has been completed by the National Weather Service.  

For a tornado to form updraft and downdraft are essential.    

When I first started chasing tornadoes. it was clear that tornadogenesis (tornado development) was highly dependent on the dynamics inside the storms structure.

There has to be a strong updraft and a source of vertical vorticity for a tornado to form.

When forecasting tornadoes, you have to look at a hodograph (This is a chart that shows the speed and direction of vertical wind shear at different levels of the atmosphere). What you're looking for is a substantial amount of curvature (a change in wind direction over a horizontal distance)  from the surface up to around 6,500 feet.

The vast majority of the time tornadoes move from Southwest to Northeast. This has to do with the fact that most tornadoes happen along a cold front.  Most of the time, the winds ahead of the cold front move from the Southwest to the Northeast.

Like thunderstorms tornadoes also go through a life cycle. The tornado life cycle is divided into five stages.

1)  the whirl stage...this is when the condensation funnel starts to drop out of the thunderstorm.

2) the organizing stage..... This is where the condensation funnel touches the ground, the base solidifies and broadens. It starts to suck up dirt and dust, making the tornado become darker.

3) the mature stage..... this is when the tornado is at its most powerful and is very destructive.  Many times this is when the tornado will take on the wedge shape most of us have heard of.  The tornado usually vertical, and is thick and most of the time appears wider than it is tall.  

4) the shrinking stage..... This is when the tornado is starting to dissipate and weaken.  The tornado will start to tilt and stretch out.  A tornado in stage 4 is still dangerous. 

5) the decaying stage.... .  The tornado will rope out ( takes on a rope like appearance).  Soon after the tornado will lift and dissipate back into the base of the thunderstorm.

A tornado doesn't have to go through all of these stages; it can go from stage 2 to stage 5.

Here is an image showing this five stages.  I will also include an image from "Tempest Tours" that shows the various shapes a tornado can take on.

Tornadogenesis is completely dependent on the storm scale processes inside the mesocyclone.  A thunderstorm is a breathing thing, it intakes air and expels air.  a big part of tornado development involves the Forward Flank Downdraft (FFD).  This is the part that causes the horizontal vorticity close to the ground, and draws it quickly up into the storm, where it is tilted and accelerated vertically into the updraft.

Once the mesocyclone forms, cool dry seeking air is pulled into the storm and wraps around the back of the mesocyclone. This starts a process called the Rear Flanking Downdraft (RFD). RFD also plays a huge role in tornadogenesis. The RFD makes a huge temperature difference between the outside of the storm and the inside of the storm. All of this greatly increases local instability, local wind shear, and helicity. This not only strengthens the mesocyclone, it also vastly increases the odds for tornadogenesis.  Once the local wind shear is enhanced and maintained to be self supporting, tornadoes are possible.  RFD is likely the reason for the movement of air downward, and very well could be the reason for the downward movement of the outside of the Funnel.

When you see wisp of rain moving left to right it is often a tornado is about form.

After all of this a tornado could form. This is when the condensation funnel will start to drop toward the ground from the thunderstorm.

What is a condensation funnel?

A condensation funnel is made of water droplets that extend downward from the base of the thunderstorm.  A funnel cloud becomes a tornado when the condensation funnel makes contact with the ground.  It is possible for a condensation funnel to be invisible most of the way up to the cloud. This is especially true with tornadoes that have just formed, or in quick spin-ups and short-lived tornadoes.  The dust, dirt, and debris will be visible a few hundred feet up, then  mid and upper part of the condensation funnel is invisible.  Once the tornado is more established, The rotation and interior mechanics like pressure drop and temperature differences between the tornado and the air around it, will turn the water vapor in the  condensation funnel  into clouds that form the typical visible tornado we see in videos. But as, I've said before in my storm chasing experience, I've seen evidence that some tornadoes might form from the ground up.....  

The size or shape of a funnel is no indication of the tornadoes strength.

Winds in a tornado:

The winds in a tornado can be from around 65mph to over 200mph. 

There are three things going on that make up the winds in a tornado. Forward speed, the circulation around the tornado, and the speed of individual vortices inside the tornado itself. 

1) The faster the tornadoes forward speed the stronger the winds will be on one side of the tornado. 

2) The faster the circulation around the tornado the stronger the winds will be.

3) The stronger the internal vortices (they're like a mini tornado inside of the larger parent tornado)  The stronger the wind rating for the tornado. These are interrelated with the circulation of the parent tornado...Not all tornadoes contain these small vortices. But I do believe most strong and all violent tornadoes contain them.  When present these mini vortices can be responsible for small areas of incredible damage. But it is also true,  that single vortex tornadoes can be just as intense as multiple vortex tornadoes.

The Hook Echo:

When a thunderstorm develops that rotating updraft; it can have a distinctive radar reflectivity signature called a hook echo. I have been having quite a few questions on hook echoes.   As its name implies it usually looks like a hook. This hook usually is found in the vicinity of the updraft; typically it is found in the right rear part of the mesocyclone.  Remember doppler radar sees precipitation and solid objects, not wind.

The hook echo is caused by the rear flank downdraft as it warps around the backside of the updraft.  What we see on radar is the precipitation that wraps around the mid level mesocyclone. The hook shape comes from the fact that the mesocyclone normally is rotating counterclockwise.

 The updraft and inflow notch part of the storm is found inside the hook echo.

The hook on radar indicates the presents of a mesocyclone. The hook echo is a function of the mesocyclone, and not really the tornado vortex; it doesn't mean there is a tornado or that a tornado is going to touchdown. All it means is that there is the potential for a tornado.

Many times ( maybe most of the time) a classic hook echo won't appear on doppler radar. There are many reasons this can happen.  The tornado is rain wrapped (the radar can't distinguish the tornado from the surrounding precipitation and mesocyclone, The storm is too far away for the radar to see that kind of detail, the radar beam is shooting over the top of the low level feature, and many others. When chasing tornadoes, I noticed that several times a tornado formed north of the hook. A few formed a good distance north well inside the precipitation shield. There are also signature's that appear on radar that are just a false hook echo. Using radar to find tornadoes involves a lot of guesswork.  So for all these reason, when a tornado warning is issued, don't waste time trying to find the tornado on radar, instead find safe shelter.   

 When looking at the radar reflectivity scan, the big looking round shape at the end of the hook, does not equate to a debris signature; it is simply a low level part of the mesocyclone that is rain wrapped.  The best way to see and find a debris signature using a dual polarization radar product using correlation coefficient data.  The depris signature must be located near the hook echo. Normally the objects being lofted have low differential reflectivity values.

There is also something called a Tornado Vortex Signature (TVS); that shows up on a radar velocity scan.  A TVS appears in the mesocyclone  in the mid level to upper level of a mesocyclone.  It shows where an intense area of very concentrated rotation is occurring.  While it doesn't mean there is a tornado on the ground; It does highlight where there is an elevated risk of a tornado occurring.

Here are some radar images from an EF1 in Pennsylvania from August 22, 2018.   

 Most of the processes involved in tornadogenesis aren't really a part of the general environment.  So you can't really see some of these things on a sounding.  Tornadogenesis is caused by the interaction between the storm and the environment. This is why, you can't just look at data and say a tornado is going to happen.  

Well that covers all the questions and comments that I have seen and been getting. I hope it clears up some things.