The skies are darkening, lightning flashes across the sky, the loud rumble of thunder seems to shake you to the core. These are the familiar signs of thunderstorms – one of the most fascinating atmospheric phenomena to occur. While you may be familiar with the terms “tornado watch” and “severe thunderstorm warning”, do you know why the National Weather Service uses the language in these products to get us to better understand what they mean? Perhaps you have read other weather blogs and/or watch The Weather Channel on a regular basis and have come across some strange weather terms.
The original intent of this site is to help discern the way language is used to describe the weather or how language affects our understanding of the weather. This objective is fairly broad so in order to not bombard you with a ton of weather language in one post, I will break it up into smaller chunks, grouped by topic. This first installment will cover the various weather terminology used during instances of severe weather.
Single Cell Storms
To start, it is important (and interesting) to note that there exists a storm spectrum, which attempts to classify the different varieties of storms that exist. Sure all thunderstorms have the same basic structure (an updraft and a downdraft) but storms can vary by duration and certainly by size. In fact the first type of storm I’ll mention are relatively small in scale and have a relatively short lifespan. They are called “single cell storms” . They rarely produce any type of severe weather, but heavy rainfall and small hail can accompany these storms.
Multicell Cluster Storms
Multicell cluster storms occur when thunderstorms form in a cluster, with numerous storms in various stages of development, or thunderstorm life cycle merging together. There is a higher probability of severe weather associated with multicell cluster storms and flash flooding can be a possibility if additional storms developing upwind of the main cluster of storms moves over the same location. This is referred to as “training”.
Multicell Line Storms
Sometimes thunderstorms can form a line that will extend laterally for hundreds of miles. These multicell line storms, or the more common term “squall line” can pack quite a punch. New storms develop along the leading edge of the system, with heavy rain and hail following. Are you familiar with the feeling of a sudden temperature drop and winds picking up as you see storms heading your way? Those sensations you are feeling are from the thunderstorm downdraft. Strong winds and large hail are the most common severe weather elements to occur with these storms. Long-lived squall lines, which can travel many hundreds of miles and produce considerable damage are called derechoes.
The final and most important type of storm are supercell storms. These storms are the most organized, persist for many hours, and are responsible for the majority of injuries, fatalities, and property damage than all other types of thunderstorms. These storms are unique because they consist of a special ingredient the other storms do not have, wind sheer. Wind sheer is when winds are veering, or turning clockwise with height. When winds change in speed and direction with height, it creates storm scale rotation which can lead to the development of tornadoes and large hail. In fact supercell storms are responsible for nearly all of the significant tornadoes in the United States and instances of large hail (for example baseball size hail). Flash flooding can also accompany supercell storms.
How to Interpret Storm Watches and Warnings
Most of us are familiar with the term “severe thunderstorm watch” or “tornado watch” and likewise with warnings. At times you may hear of a tornado watch being termed a “PDS watch”. This stands for Particularly Dangerous Situation. These types of watches are issued occasionally and meteorologists at the Storm Prediction Center will issue a PDS tornado watch when a major tornado outbreak is possible, and there is a potential for EF 4 or EF 5 tornadoes.
Over recent years, the National Weather Service has warned the public of an approaching violent tornado in an alarming but effective way. Instead of issuing a standard tornado warning which warns those in the path that a tornado has been sighted or detected by Doppler Radar, meteorologists have added the rhetoric “tornado emergency”. Given the heightened urgency in the use of “emergency”, this is reserved for when a violent tornado is expected to impact a densely populated area. Remember the EF 5 tornado that destroyed most of Greensburg, KS, the EF 4 tornado that hit Tuscaloosa, AL, or the EF 5 tornadoes that tore through Oklahoma City, including the town of Moore? All of those were preceded by a tornado emergency.
If you are interested in additional information on the thunderstorm spectrum along with some nice photos of storms, click here. This compilation of information was gathered from the National Weather Service storm spotting presentations. I urge you to attend a storm spotting class near you when applicable. They are held during the late winter/early spring in different locations across the country. Chances are there is a class near you. (Just click on your part of the map to access your local National Weather Service office and they should have information on available storm spotter classes in the late winter/early spring.) I have taught numerous storm spotting classes while I worked for the National Weather Service and these classes are designed to teach you the basics of thunderstorm development, how to tell if a storm is severe or not, and what to report to the National Weather Service if you witness any severe weather. Next I’ll offer a rhetorical perspective on basic weather terminology.