Thunderstorms & Severe Weather Block III

Tornadoes Ch. 12

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Tornado Definition

• Rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground

• Circulation present either as a funnel-shaped cloud or as a swirling cloud of dust and debris

• Most have wind speeds of less than 135 mph but can exceed 250 mph

• A funnel cloud is a tornado whose circulation has not reached the ground

Tornado Rotation

• Almost all __ rotate counter-clockwise around their central core of low pressure

• a few have been observed to rotate clockwise but they are rare

Tornado Diameter

• Common diameter is 300ft to 2,000ft

— Some have been measured less than 20ft

— Still others have been larger than a mile

Tornado Average Lifespan

The average path/time is a few minutes and about 4 miles

Tornado Lifecycle

1. Dust-whirl

2. Mature Stag

3. Shrinking

4. Decay Stage

Dust-whirl Stage

• 1st

• Swirling upward from the surface marks the tornado’s circulation on the ground and a short funnel often extends down from the thunderstorm

Mature

• 2nd

• Normally is most severe as the funnel reaches its greatest width and is almost vertical

Shrinking

• 3rd

• During this stage the funnel shrinks and tilts

• damage swath narrows at the surface

Decay

• 4th

• Tornado shapes like a rope

• Shape typically greatly distorts

Where/When tornados are most likely

• US experience most tornados (averages more than 1,000 annually)

• Tornado Alley of the Central Plains (warm, humid surface; cold dry air aloft)

— most susceptible because it has the right conditions for severe T-storms

• Dixie Alley is the tornado max that stretches across Alabama and Mississippi

• Highest spring, lowest winter

Winter’s Role

• Southern Gulf States

• Polar-front jet is above the region

• Contrasts between warm and cold air masses is the greatest

Spring’s Role

• Humid Gulf air surges north

• Contrasting air masses and the jet stream also move north

• Tornadoes become more prevalent from the Southern Atlantic States west into the Great Plains

Summer’s Role

• Contrast between air masses lessens

• Jet Stream is near the Canadian border

• Tornado activity tends to be concentrated from the Northern Plains eastward to New York State

Tornado Frequency Info

• About 70% of all __ happen between March-July

• May has the greatest number of __ with an average 6 per day

• Most Violent __ are in April when vertical wind shear tends to be present as well as horizontal and vertical temperature and moisture contrasts are the greatest

• Most frequent between 4pm to 6pm

• Least frequent before sunrise

(Have occurred at all times of the day and night)

Perfect conditions for tornados

• Conditionally unstable atmosphere with strong vertical shear for supercell development

• Wind speeds increase with height and change direction

Where the strongest winds of a tornado are

• If facing a __ the strongest winds will be on the left side

• A __ approaching from the SW heading NE

— If it is going 50 mph with a 100 mph rotational speed

— The forward speed will add 50 mph to the SE side (position D)

— Subtract 50 mph from the NW side (B)

• Most destruction = SE