Thursday, June 2, 2011

Severe Thunderstorm Structure

          Hi  it's Rebecca again, with all the thunderstorms this afternoon; I thought it would be a good time for this segment on thunderstorm structure. From a storm chaser viewpoint,  It is important to know basic storm structure before attempting to chase. The reason is, people immediately draw a parallel to dark skies as being the worst part of the storm. They would not know the importance of a rain free base on the back side of a supercell. This is the location of the updraft region and the area that you want to look for the development of a wall cloud. However, reading storm structure is not just important to chasers. Everyone can benefit, there are times you won't be around a TV or radio, you could be hiking, boating, or just at a ballgame. By watching the storm, you would be able to tell what type of storm it was, its relative strength, and its general direction of movement. This information would help you know what to expect when the storm arrived at your location. 

Severe thunderstorm structure:

            Visual clues in and around the storm will tell you it's direction of movement. If you can see the anvil of the cloud(Image below)  it usually is stretched out in the direction of the upper level winds which is typically the direction the storm is moving. One other clue to look for is rain, if you can see the rain coming from the base of the cloud and it is slanted, you can determine the movement of the storm. For example, if the slant from the base to the ground is to the left, the storm is moving to the right. (If it is moving right at you or directly away from you, no slant would be noticed.) Sometimes you can notice other, closer cumulus clouds moving rapidly across the sky. The storm would tend to move in the same direction.

                 Color can sometimes give you an indication of how powerful a thunderstorm is: A very dark (black) thunderstorm or one taking an peculiar look ( green, yellow, or even brownish cloud colors) may be an indication of a severe thunderstorm. The colors and darkness of the cloud are caused by the storm's massive size and the blockage of sunlight.

                                               Image credit Skywarn

The diagram above shows a severe thunderstorm, in fact, it's a supercell. You may remember, in my last blog installment I said, the major difference between supercell and multicell storms is the element of rotation in supercells.


            The Anvil is the top flat portion on a storm, it is the most impressive part of a severe thunderstorm. how the  anvil looks  can give you a clue is to updraft strength.  If the updraft is weak, the anvil will have a fuzzy appearance. However, this doesn't mean the storm won't later strengthen.  On the other hand, if the edges of the anvil are very sharp you know there is a very strong updraft in the storm.
Overshooting Top:
             An overshooting top shows where the updraft is strongest. Generally the larger and the higher it is the more intense the updraft producing it is.
                                                Image of overshooting top

Back sheared Anvil:
             This is when the anvil cloud spreads upwind against the stronger flow aloft. It can be an  indicator of a intense updraft.  Mammatus clouds are found in this region of the storm. I will go into more detail on these cloud features in the next blog post.

Forward-flank Downdraft(FFD):
        The FFD is downwind of the updraft.  This is the outflow from the rain-cooled air of the storm's downdraft.  The FFD is the main downdraft of the storm.  Most of the precipitation falls in this region.
 Rear-flank Downdraft(RFD):
        The air in rear flank downdraft tends to be warmer than the forward flank downdraft.  The FFD and RFD interact with each other.  It works like this, because of the lack of evaporational cooling in the RFD area of the storm,  Shear is enhanced along these flanking downdraft boundaries and the shear can be magnified along where the two flanks merge. If you have a the right setup of shear and instability it can lead to a tornado. Because of this, the RFD can give you several clues that a tornado is about to happen. I will go into this in the last blog entry, which will be about the tornado.

Flanking Line:
            This can be seen as a line of developing cumulus clouds extending from the storm. The cumulus closer to the storm tend to be more mature and eventually merge into the parent storm. Like I said in the last blog post, it often looks likes steps leading up into the storm. The flanking line often feeds into the updraft of the storm. Remember that RFD behind the storm? That sinking air acts as lift that  gets new updrafts going. It is called the Gust Front. The flanking line is the new storms forming due to the new updrafts. It was this same basic process that caused the second tornado in Springfield, MA earlier tonight.
                                               Image of a flanking line
The diagram also shows areas of heavy rain and hail; these areas are called the rain/ hail core. the heaviest precipitation is found in the outer edge of the updraft and downdraft. There is often extreme turbulence in this spot; which can lead to large hail growth. As the hail drops a lot of it melts and hits the ground as rain. if the hail size started out extreme in size all of it won't melt so it will reach the ground as hail.

The wall cloud and rain free base are unique to supercells.

Wall Cloud:
             The wall cloud is located in the updraft area of a supercell. The base of the wall cloud is close to the ground.  The wall cloud will often be seen as rotating since directional wind shear (change in wind direction with height)  acts on the updraft as it rises. Tornadoes can occur under the wall cloud.
                               Image of an almost fully  developed wall cloud in Kansas.

Rain-Free Base:
             The updraft region in supercells will often lack precipitation. This is much more true of developing supercells and for classic/LP supercells, than it is for HP supercells.  The reason for this is:  most of the time rain will wrap around the updraft and eventually infiltrate into the updraft region. This  will allow the updraft to tilt with height. This will move most of the rain away from the updraft.  
 Well that's it for this installment of my series. I hope you found it informative. the next installment will be on different kinds of clouds and cloud features associated with severe weather.
Rebecca Ladd


  1. Thank you for posting this. I'm outside a lot during the summer and this knowledge will come in handy.

  2. You're welcome! Its always great to learn something new and it is also great to know what you are looking at especially when it comes to atmospheric phenomena. thank you for reading the blog! - AG


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