Weather 101 - a Recipe for Disaster (Tornadoes)

April 3-4, 2009 marks the 35th anniversary of the greatest 24-hour tornado outbreak (Figure 1) in recorded U.S. history (note: this link is to a somewhat technical paper). One hundred and forty-eight twisters (thirty classified as F4 and F5 on the older Fujita Scale [Figure 2] struck across a large area from the Mississippi Valley to Virginia and from the Gulf Coast to Canada. More than 330 people died in this historic event. According to Corfidi, Levit and Weiss (of NOAA's - National Oceanic and Atmospheric Administration - Storm Prediction Center, 2004), "...The Super Outbreak of tornadoes of 3-4 April 1974 remains the most outstanding severe convective (thunderstorm) weather episode of record in the continental United States. By nearly every metric imaginable, the outbreak far surpassed previous and succeeding events in severity, longevity and extent."

Many other major tornado outbreaks (at least 6 tornadoes in a localized region during a 24-hour period) have been documented during the past 60 years (during which time the Federal government has taken on the task of forecasting these capricious weather creatures). Prior to that there was no severe weather forecasting service in the U.S. Tornado researchers, government meteorologists and others have added older historic events to the database that date back to pre-1900 periods. Note that the 24-hour period does NOT have to match with a calendar day (e.g., April 3-4, 1974 Outbreak).

Meteorological Recipe

So, what helps to create outbreak conditions (Figure 3)? The typical recipe (which can be modified as any recipe can) includes the following:

A warm, humid and unstable airmass. In the U.S., this usually means that air from the Gulf of Mexico is drawn northward. But, instability can also come from surface heating (due to sunshine), colder air aloft and other temperature and moisture variations in the vertical column. Often temperatures much above seasonal average (or even at record levels) precede an outbreak.

A storm system (low pressure area). Storm systems that are more intense and/or further south than usual are more likely to lead to outbreaks.

A high altitude jet stream, especially one that exhibits a fanning or spreading apart pattern to its east. This favors exhaust (like a chimney) atop developing thunderstorms and enhances more rapidly rising air currents.

A companion low-level jet stream that blows at a significantly different direction than the high altitude jet stream. The differing wind flow directions add to atmospheric spin.

Sometimes, the presence of a "dry line" enhances upward motion and provides additional instability.

Localized wind and temperature variations linked to nearby thunderstorms (or land or water breezes) can focus low-level convergence of winds and foster intense thunderstorm development.

As you can see, the recipe is not simple. Yet, severe weather forecasters understand how the ingredients come together to create dangerous storm situations and have been doing an increasingly better job of alerting people and local emergency agencies about imminent storm risks.

And, while the discussion here is focusing on tornadoes, the recipe works for recognizing events that include storms capable of producing large hail, very strong straight-line winds, frequent intense lightning and even torrential rainfall. In fact, sometimes the atmosphere just can't support tornado development, but instead offers a large helping of hail and high wind reports. If tornadoes occur in a very moist environment, they can become wrapped in rain, making them hard or impossible to see by the human eye.

The weather situation on April 2, 2009 fit almost every one of these antecedent conditions. And as I was writing this article, there were numerous severe weather watches (conditions possible) and warnings (storms already occurring or imminent) across a large part of the southeast U.S. Although this situation did not generate a massive tornado outbreak, there were about a dozen tornado reports, there were nearly 200 large hail and high wind reports during a period of about 18 hours.

Meteorologists, much like sports junkies, have lots of statistics at their disposal. And when it comes to tornado events, meteorologists excel. So, if you want to read about some past tornado outbreak events here are two web sites to get you started.

Statistics and detailed information about killer tornadoes from 2000-2009
Historical Tornado Events Listing - courtesy of Roger Edwards at the SPC - listing of web sites that address specific tornado events, not necessarily outbreaks, including events that occur around the world - e.g., Canada, Philippines, Australia.

You can also read my companion article (to be posted shortly and with a link to be added here) that showcases a dozen of the most dramatic U.S. tornado outbreak events.

Human-factors Recipe

Tornadoes wouldn't be such a problem if we humans weren't in the way. But since we are, how do we minimize our exposure to tornado destruction? To start with, we need to understand the conditions under which most tornado fatalities occur.

Many scientists (including meteorologists, sociologists and others) have been pondering this question for the past 40 years. After almost every major event, a NOAA disaster survey team (Figure 4) interviews many of those most directly affected. Typically, the following surface (even for those events that were very accurately predicted and warned for in a timely manner):

Most fatalities occur in manufactured (a.k.a. mobile) homes. Research from the National Severe Storms Laboratory (NSSL) indicates that the risk of a fatality from a tornado for those inside a manufactured home is 15 to 20 times greater than for those in permanent structures. Only 7 percent of U.S. residents live in manufactured homes, but nearly 50 percent of tornado fatalities occur in manufactured homes.

Most of the fatalities occur at night; most of the areas affected by the deadly tornadoes have dense forests; and many killer storms are wrapped in rain. Any or all of these make it more difficult to see tornadoes coming.

Most people interviewed acknowledge that they associate tornado outbreaks with the springtime or summer months and mainly in "Tornado Alley." This causes many to minimize the threat of out of season and out of place events, especially in regions to the south and east of the main "alley" region, a zone known as "Dixie Alley," a secondary tornado maximum location (Figure 5).

In fact, Dr. Walker Ashley (a meteorologist at Northern Illinois University), wrote in the December 2007 issue of the American Meteorological Society's journal, Weather and Forecasting that, "...while the "Tornado Alley" region of the Great Plains has the most frequent occurrence of tornadoes, most tornado fatalities occur in the nation's Mid-South region, which includes parts of Arkansas, Tennessee, Alabama, and Mississippi. The states of Kentucky and Georgia also have a high frequency of tornado fatalities." Ashley focused the threat area further by noting that, "The country's most vulnerable region for tornado-related fatalities and killer tornado events basically stretches from Little Rock to Memphis to Tupelo to Birmingham."

Further, many people have an "it can't happen to me" attitude, while some are more fatalistic (believing that whatever happens is destined to happen).

There also remains some misunderstanding of key terms such as "watch" (meaning key an eye and ear to the skies) and a "warning" (meaning imminent danger). The fact that both start with the same two letters adds to the confusion.

When meteorological and human-factors recipes are combined, it's no surprise that with a growing southern population, many of whom are not used to southern twisters, that there is a recipe for disaster awaiting this Nation.

NOTE: To view all images associated with this feature, just click on any image and a pop-up image choice window will appear.

Sources:

www.spc.noaa.gov/climo/torn/fataltorn.html#2009

www.nws.noaa.gov/om/assessments/index.shtml

www.spc.noaa.gov/faq/tornado/torpages.html

www.spc.noaa.gov/publications/corfidi/3apr74.pdf

www.tornadoefscale.com/pages/t/tornadoefscale.com-index-nav-1.html

http://ams.confex.com/ams/11aram22sls/techprogram/MEETING.HTM