There often are many natural indicators that conditions are right for a tornado, or that the arrival of one is imminent. You should know and heed these, because warnings by human technology sometimes fail or come too late. Be aware that NONE of these warning signs presages every tornado, nor does every tornado always have all--or even some--of these warning signs. If you observe any of these indicators, however, you should probably be extra vigilant and prepared to seek shelter on short notice. In increasing order of urgency:
ROTATING THUNDERHEAD TURBULENCE: A thunderhead having areas of visible spinning within it, especially if rapid and localized to only a small portion of the cloud base, indicates conditions that could lead to tornados. Thunderhead rotation may occur 1-6 hours before tornado development, and often becomes noticeably more vigorous as tornadoes become more imminent.
MAMMATUS (Latin for "breast-shaped") CLOUDS: The leading edge of an arriving thunderhead sometimes has very distinctive bulbous clouds hanging from its base. These mammatus clouds indicate very unstable conditions and spinning turbulence within the thunderhead above, which could produce funnel clouds. Typically mammatus occurs 1-90 minutes before possible tornado development.
GREENISH SKY COLOR: A very large thunderhead with a dense, turbulent interior and hail may prevent all but the peak wavelengths (green) of sunlight from reaching the ground. Such powerful thunderstorms may produce tornadoes 1-45 minutes after the daylight color changes.
CONTINUOUS LIGHTNING: Extremely strong "supercell" thunderstorms such as produce most tornadoes may also have lightning discharges so frequent as to become almost continuous near the center of the storm. Tornadoes may occur 0-30 minutes after lightning frequency so increases. Remember that lightning itself is just as dangerous as a tornado; seek shelter immediately.
LARGE HAIL: Only the most powerful thunderstorms produce hail; the bigger the hail, the stronger the storm. Hail typically occurs as a band of precipitation following a band of torrential rain that has begun abating. The hail band often preceeds, but can be concurrent with, the tornado-bearing rear portion of a passing thunderstorm. Tornadoes may follow the hail band 0-20 minutes later.
WALL CLOUD: Most tornadoes descend from within a rotating collar-shaped cloud structure, called a wall cloud, that protrudes from the base of the thunderhead. If you observe such a structure, start taking stock of nearby cover and begin moving towards shelter; a funnel may drop within 0-15 minutes.
TRAIN- OR JET-LIKE ROAR: Fast moving wind usually causes a roaring noise, but if this is audible above the normal noisiness of a thunderstorm you may be very close to a funnel. Because of driving precipitation and forest cover, or because the tornado hasn't yet touched down or hasn't yet picked up much debris, you may not see a funnel. Take cover anyway; it may arrive in 0-10 minutes.
FLYING DEBRIS: Even if you can't see a funnel, if you observe large local concentrations of flying debris--especially if it is spinning and rising--take cover. A tornado likely is near you, and may arrive in 0-3 minutes. Even if it misses you, nearby flying debris such as garage doors or window shards may cause injury. LEAVE YOUR CAR NOW!
"ANGRY BEES" BUZZING NOISE: Within very large tornadoes there often are multiple miniature funnels that are particularly dangerous. Wind speeds in such mini-funnels are much higher than the main funnel. Not only does this cause even greater damage, but raindrops and debris spin so violently as to cause a loud "bee buzzing" noise above the ambient thunder and wind roar. IMMEDIATELY TAKE SHELTER; you probably have 0-60 seconds. LEAVE YOUR CAR NOW!
SUDDEN WINDOW OR WALL BREAKAGE: The outer edge of the funnel is very near, and possibly is on top of you already. You have 0-20 seconds to protect yourself. Huddle or crouch into any low shelter within a few feet of you, pull any material within reach (a mattress, your roommate's laundry, or any sort of padding) over your head and body. LEAVE YOUR CAR NOW!
SUDDEN CALM DURING SEVERE THUNDERSTORM: You may be directly at a descending funnel's point of touchdown. Remote as the chances for this seem, this does happen occasionally. If so, you will have had virtually no warning, and you have 0-5 seconds to DIVE to the lowest place possible. As you do so, ball up your body and tuck your head forward to reduce your exposure, and fold one arm over the nape of your neck and the other across your face and forehead.
TORNADO MYTHS; DEADLY MISREADS
Many "facts" one hears about tornadoes are totally, or in other cases partially, incorrect. Relying and acting upon such ideas could make a bad situation ever so much more dangerous. Nine of these myths include:
"I'll see an approaching tornado in time to take cover." In torrential rain or in forested areas you may never see the funnel, even if it hits you. Further, many tornadoes do not have the classic "Wizard of Oz" shape; many appear to be little more than a thunderhead base that is touching the ground. Finally, tornado "families" (of several separate funnels) may have one member that strikes from behind while you are watching another one. You won't always see a tornado!
"Tornadoes always travel southwest to northeast." A majority do, but not all. Some years ago Amherst WI had a funnel that traveled southeast to northwest. You may badly mispredict what direction a tornado travels toward, and falsely assume safety based on presumed passage of a funnel actually coming at you.
"Tornadoes only occur in the spring, so there won't be one in this [summer, autumn, or winter] thunderstorm." Tornadoes are most common in the spring, but they have occurred during every month of the year in Wisconsin. The Amherst tornado occurred in December; the Big Flats WI tornado was in August.
"Tornadoes can't cross rough terrain, nor occur at high elevation." A few hundred feet of topographic difference does not impede tornadoes, which can be thousands of feet tall. One tornado also occurred at 12,500 feet on Long's Peak CO in 1981; there is no restriction on ground elevation for a type of storm that forms in the sky. Being in rough topography or at high elevation does not assure your safety.
"I can drive faster than a tornado's typical forward velocity." The thunderstorms that carry tornadoes typically advance forward at 30-70 miles per hour. Yes, your car can go that fast, but it must remain on the road to do so. Tornadoes don't have any such restriction, so they can take shortcuts and catch motorists.
"Tornadoes [and lightning] never strike twice in the same place." Tell that to the good folks of Topeka KS, and many other cities in the central and southern USA, who have often been hit and some of whom have had to rebuild the same house lot on several occasions. Once on the ground, there is very little that dictates where a tornado can go, including previous occurrences.
"The safest place in a tornado is the southwest corner of a basement." Half true; the lowest floor or basement usually is safest, because wind speeds are greater at higher altitudes above the ground (including upper stories). What is wrong on two counts is the idea of a southwest corner. The southwest is not always the shielded direction, and lethal flying debris tends to collect in corners. Go to the center of a basement, avoid the areas having widest ceiling distances, and get underneath heavy furniture for additional protection.
"I should open the windows so the building doesn't explode." Air pressure can't drop so fast as to cause a building to explode, because buildings just aren't that airtight. The battering effect of flying debris and broadside wind force is what destoys buildings. Exposure to flying debris, which may hit a person trying to close a window, is a vastly more severe danger that arises from the erroneous notion of explosion.
"Modern weather radar will always allow plenty of advance warning." Modern radar is a tremendous improvement, but it cannot always detect a tornado or give enough warning time. The modern NEXRAD system gave Oakfield WI no warning, and the warnings at Big Flats did not get issued until after the tornado had hit (and, incidentally, the sirens also had failed). Although it constantly gets better, there is no 100% guarentee of any warning from technological systems.
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