Last time we discussed the rationale for severe weather spotters. We also discussed some of the limitations of the equipment used by the National Weather Service (NWS).
We also discussed the structure of the course and how the lessons would be presented. You should have decided how you want to end the course by now (which style of exam you want).
In this lesson we will discuss the general types of severe weather and what severe weather actually is.
The NWS has developed specific criteria for severe weather:
It is also important to know what is not severe weather:
You may have seen reporting criteria from amateur radio groups. These vary widely in what they tell their members to report. The reason is that there is no standard of skill or knowledge required of most amateur radio groups. This allows vastly more people to be able to report (just about anyone with an amateur license can participate in the severe weather nets), but it has the drawback that there is little in the way of quality control. We attempt to work closely with the amateur radio folks to get better training to their members. Having a more rigorous and difficult training regime, we have a more loose reporting criteria. Also, like the amateur radio community, we have a base where we collect information about what a storm is doing and we interpret the reports we get before simply passing them along. Here is what we want:
Discuss the definitions of severe weather and think about why we have the criteria we have listed.
Here is a side view of a thunderstorm. Whenever you see this view the storm is moving from left to right. Any updraft-downdraft couplet is called a thunderstorm cell.
If we look at your average thunderstorm, you will see that there are two kinds of wind associated with it: Inflow winds (where the storm is essentially breathing in warm moist air from its surroundings) and downdraft-generated outflow (where the storm is essentially exhaling). The downdrafts are of two broad types: FFD (Forward-Flank Downdraft) and RFD (Rear-Flank Downdraft).
Here is an oblique view (here the storm is moving from the upper left to the lower right):
The places where you are most likely to experience severe winds are in the vicinity of the FFD and the RFD. In extremely rare cases you might experience near-severe winds in the surface-based inflow region.
Here is an example of what can happen to the NWS when a severe RFD hits their radar:
As the updraft column gets higher, the temperature will tend to get colder. When the column gets high-enough the water vapor does not condense into droplets, it adheres to impurities in the air (called ice nuclei) and forms into crystals. By a process that we will discuss at great length in Lesson 5 hail can form at the top of the updraft column.
We will discuss how precipitation forms in Lesson 5. Here we are simply concerned with the threat posed by rain.
Heavy rain occurs when visibility drops to a quarter of a mile or less. This is most likely to occur just ahead of the region where the greatest risk of hail can be found.
Flash flooding occurs when so much rain falls within a period of time that existing waterways cannot handle it all.
If a storm has any combination of the following factors, it carries the risk of flash flooding:
Thunderstorm winds at the surface spread out from the thunderstorm, and can go quite a distance. We call this flow of air from a thunderstorm outflow. Air flowing into a thunderstorm is then inflow.
Think about what we have discussed and how it can help you to understand where you should look in thunderstorms for precipitation.
The official definition of a tornado is: A violently rotating column of air in contact with the ground and the thunderstorm base. Please note that nowhere in this definition is there mention of a funnel-shaped cloud. This is because you need not have such a cloud. Here is a photograph used by Rusty Kapela of the NWS in his training that illustrates this nicely (I believe that this photograph is by Gene Moore, but I don't know for sure):
Notice that there is no visible funnel at ground-level.
Ted Fujita, a meteorologist who was active in tornado meteorology research from the 1950's through the 1980s created a system for categorizing tornadoes by the damage they do. We will go into more detail about this in Lesson 7. Here is a brief introduction:
Unlike the other forms of severe weather we have discussed, the tornado requires a strong updraft in order to exist. This is most often found in the inflow region of the thunderstorm. We will discuss some of the MANY variations of this in Lesson 7.
Discuss the ramifications for spotters of not needing a visible funnel to have a tornado.
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