Tornado !

• A column of air that rotates violently ... visible due to dust and debris in the vortex.

• It is by definition a tornado when a funnel cloud touches the ground.

• 99% spin cyclonically (CCW) in Northern Hemisphere ... the remaining 1% that spin anticyclonically (CW) usually form along gust fronts and are called gustnadoes.

• 90% of all tornadoes move from SW toward the NE.

• Tornado Alley: E/N TX, OK (central OK is the maximum), KS & NB.

• Tornado Season: Gulf States FEB/MAR, Great Plains MAR/APR/MAY, Northeast MAY/JUN/JUL

• 75% of all tornadoes occur from March through July.

• Peak time of day for tornado formation: 10 AM until 6 PM.

• "Average Twister": path less than 1 mile long, 328 feet wide, last for 1-3 minutes, have maximum winds less than 113 mph and a forward speed close to 34 mph.

• "Extreme Twister": path 100 miles or longer, up to a mile wide, can last for 1-2 hours, have winds up to 318 mph and a forward speed of up to 150 mph!

• Tornadoes (usually F0 or F1) can form in the right-front flank of a hurricane ... especially if the hurricane is moving up along a coastline ... due to increased friction as the winds blow from water to land.

• In a typical year, about 79% of all tornadoes are "weak" (F0 & F1), about 20% are "strong" (F2 & F3) and only about 1% are violent (F4 & F5).

• "Tri-State Tornado" is considered to be the most devastating in U.S. History, killing 695, injuring an additional 2000, while leaving 11,000 homeless. The twister moved from SE-MO across S-IL to SW-IN (219 mile path of destruction) at up to 73 mph lasting about 3.5 hours.

• "Super Outbreak" of April 3-4, 1974 saw 148 tornadoes (30 classified as F4 or F5) in 13 states from one storm system ... there were 307 deaths and another 5500 injured ... $600 million damage.

Spring is "Tornado Season"

• Daylight is increasing each day during Spring ... more insolation ...more heat is transported upward.

• Mid & upper troposphere retains winter-like cold well into Spring.

• Lower troposphere become very unstable, due to resulting steep lapse rate.

• Thermodynamic: weak from January to April; strong from May through July

• Hydrodynamic: strong from January to April; weak from May through July

"Tornado Recipe"

• Warm, moist air need the ground.

• An approaching cold front and/or low pressure area (low-level convergence) and upper level disturbance (upper level divergence) ... provides vertical wind shear in both direction and speed (veering winds).

• A layer of warm, dry air around 6500 feet above the warm, moist air ... cT air over mT is best.

• Tornadoes form in rain-free portion of T-Storm and develop in strong updraft.

• Meso-cyclone begins in mid-troposphere ... then builds up and down ... can result in up to a 50% "false alarm" warning on Doppler Radar.

• Speed increases as the funnel stretches vertically (increased vorticity ... similar to an ice skater).

• Will appear as a "hook echo" on conventional radar and as a "singularity" on Doppler Radar.