Every year, new residents of tornado-prone areas of the United States have to learn very quickly what to look out for during storm season. Fortunately, there are professionals at the local and national level who keep a close eye on the weather and are always ready to inform us about any threats. If you have just moved into a possible “tornado alley,” here is what you need to know when they issue a tornado watch.
♦ Tornado alleys and their seasons
A tornado can happen almost anywhere in the United States, but there are three “alleys" where most of the 1,000 or more tornadoes that are reported each year seem to occur. The exact boundaries depend on the criteria used, but if you include all tornadoes from the weaker and more common EF0 and EF1 events up to the rare but very violent EF5 tornadoes, the classic “Tornado Alley” runs through the central United States and up into the Canadian plains. There is also the “Dixie Alley” in the southern states; and there is Florida, which is a special case because of its many thunderstorms as well the tendency for hurricanes to strike there.
A twister may occur any time, but there are seasons when tornadoes have historically happened frequently. In “Tornado Alley,” that would be from roughly May through early June in the southern plains and from June through July in the northern plains, upper Midwest, and Canada. The “Dixie Alley” has the distinction of being the only region of the country with two separate severe weather seasons, the first generally from late March through April, and the second most often in November and December. Florida has two seasons as well, with the first running from roughly February through April, but the second season there, from June through November, is really part of the hurricane season, as those tropical cyclones are often accompanied by tornadoes.
♦ When a tornado watch is needed
Tornadoes and other forms of severe weather happen most often when cold, dry air comes into contact with warm, moist air. As a result of the differences in temperature and humidity, the atmosphere where these two very different air masses meet can become unstable, with thunderstorms forming as warm air rises, sometimes very quickly (up to 90 mph in a convective column), and then falls after it has cooled at high altitude, its moisture condensing into rain. A storm cell can also pick up some rotation from a number of things, including shifting of the boundary between dry and moist air (the "dry line") back and forth, and shear from air at higher levels of the atmosphere that is moving in different directions and at different speeds.
A supercell thunderstorm is one in which all this internal hubbub has reached a point where it can sustain itself for a fairly long time, even several hours. Not all supercells form tornadoes, but those are where the tornadoes that do happen usually occur. Scientists are not sure of the detailed processes that lead to tornado formation inside these larger severe storms.
They have, however, learned what warning signs to look for. Besides the overall general picture of cold air, warm air, and a “dry line,” meteorologists at the National Weather Service’s Storm Prediction Center (SPC) in Norman, Oklahoma—who issue all tornado watches in the country—use a complex set of computer models and tools to monitor for conditions that, based on past records, have spawned tornadoes. Current station measurements from all over the country, as well as satellite and weather balloon data, provide information about helicity, kinematics, CAPE (convective potential) and other physical characteristics of the atmosphere for the SPC experts to work on.
♦ A tornado watch is issued
The SPC can’t predict each and every tornado in the United States. However, when conditions are such that multiple tornadoes could happen at any time over a three-to-eight-hour period, they will issue a tornado watch. A watch box is shaped like a parallelogram and typically covers an area about half the size of Iowa; about 1,000 are issued each year. When the atmosphere is particularly unstable, individual meteorologists may add a PDS (particularly dangerous situation) enhancement to the watch text; this is a subjective decision and doesn't happen too often in any given year.
A watch just means that a tornado is possible. If one is actually spotted, either by witnesses or through its unique weather radar signature, the local National Weather Service office will issue the tornado warning.
We need to always remember that the weather doesn't know about the watch box and will do its own thing regardless. People who are near an area that is under a tornado watch should also take precautions.
♦ Things to do during the watch
You will know about the tornado watch from news reports on the TV or the radio. There are also several online sites you can check, including (but not limited to) your local TV station’s website; your local National Weather Service office’s page, which can be found by typing in your city and state at the Service’s national website; and the SPC’s current watches page.
Before the watch was issued, you had already, hopefully, familiarized yourself with tornado basics like these. Now that there is a tornado watch, keep the TV or radio on. If you are at work or school and these are not available, a useful alternative is one of the free weather alert mobile phone notification services.
Turn on the weather radio, too, if you’ve got one, and make sure the alarm is set. Notify family, friends, and neighbors who might not have heard about the watch.
Take a minute or two to review your safety plan in case the sirens went off right now: make sure you’ve got a safe shelter nearby and clear access to it, get out the cat carrier or other pet necessities that might be needed, and so forth. Everyone's situation will be different; the important thing is to be ready to act quickly, with little notice.
♦ After the watch expires
If there was a tornado, check on loved ones, follow common guidelines, and listen to the instructions of your local emergency management people and law enforcement personnel.
If no twister roared through your neighborhood, don’t feel silly or disgusted that all your preparation and care were wasted. After all, a tornado watch only means that tornadoes are possible, not that they will definitely happen. Congratulate yourself on your luck this time around, and do exactly the same thing the next time a watch is issued.
Good mental and physical preparation before any emergency always makes a life-saving difference, and this is especially true when it comes to one of nature’s most violent storms.
Roger Edwards, Storm Prediction Center (December 10, 2010). “The Online Tornado FAQ”. Retrieved February 13, 2011, from http://www.spc.noaa.gov/faq/tornado/
National Climatic Data Center (April 10, 2008). “U. S. Tornado Climatology.” Retrieved February 13, 2011, fromhttp://www.ncdc.noaa.gov/oa/climate/severeweather/tornadoes.html
Wikipedia (n.d.) “Tornado Watch.” Retrieved February 13, 2011, fromhttp://en.wikipedia.org/wiki/Tornado_watch