Severe and Unusual Weather ESAS 1115

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Severe and Unusual Weather ESAS 1115

• Spotter Training and Radar Meteorology
• Part 1 Introduction to Severe Thunderstorms

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Meteorological Sensors

• Two types of two types of sensors
• Remote vs. In-situ
• Active vs. Passive
• Our passive eyes can only see features of the
  storm
• In order to see the inner workings of a
  thunderstorm, and to understand it better, we
  need an active remote sensor weather radar
• Radar will allow us to interrogate information
  about the storm by detecting precipitation and
  wind information within

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Convective HazardsWind, Hail, Floods, Lightning,
Tornadoes
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Severe Thunderstorms are Severe
Many times, the media uses the word severe when
the storm is something less than the actual
meaning of severe.
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Severe Thunderstorm Criteria

• Hail greater than ¾ in diameter
• Winds greater than 50 kts (58 mph)
• Tornadoes
• Lightning, although posing the greatest threat to
  life, is not a criterion to determining whether
  or not a storm is severe

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Lightning
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Lightning
25 million CG lightning strikes per year in the
US making it the deadliest aspect of
thunderstorms
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Lightning Safety

• Lightning Stats
• Lightning Safety Video
• Lightning Safety Tips

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Flash Floods Turn Around Dont Drown

• Water is an incompressible fluid
• 1000 kg/m3
• Results in buoyancy 2 feet of water can move an
  SUV
• Kills more people than lightning more than 100
  annually in the US.
• Flash flooding vs. river flooding

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Flash Flood Threat
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Flash Floods Turn Around - Dont Drown
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Hail Big Chunks o Ice
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Tarrant County, TX May 5, 1995

• 10,000 people at Mayfest
• Baseball hail 10 miles wide50 miles long
• 14 people killed by flooding and collapsed roofs
• 2 billion (estimated)

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We Know First Hand
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Damaging Winds

• Strong outflow from a thunderstorm enhanced by
  evaporative cooling and downward momentum
  transfer

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Downbursts

• A strong and potentially destructive thunderstorm
  downdraft
• Microbursts are less than 2.5 miles in diameter
• Macrobursts are greater than 2.5 miles

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Rain Foot and Dust Foot
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Dust Foot
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Microbursts
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Aviation Hazards
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Danger on Takeoff and Landing

• On the glide path, too much lift is generated
  with headwind gain
• With the loss of a headwind, lift is limited

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Beaufort Scale
Winds based on visual observations
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First Microburst
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Microburst within a Macroburst
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Microbursts
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Microburst Evolution
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Tornado and Hail Climatology
There are more than 1000 tornadoes in the US per
year. Hail results in over 1 billion of damage
annually.
Harold Brooks - NSSL
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Tornado Threats

• Violently rotating column of air in contact with
  the ground and pendant from a cumulonimbus cloud
  (thunderstorm)
• Tornadoes are ranked on the Enhanced Fujita
  damage scale from EF0-EF5
• Appearance is deceiving with small tornadoes
  sometimes having high destruction potential and
  large tornadoes having low potential

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EF0, EF1 Weak Tornadoes

• Approximately 70 of tornadoes are in this
  category
• Well-built houses offer adequate safety for these
  tornadoes
• Cars and mobile homes are still very vulnerable
  to these tornadoes
• Wind speeds65 110 mph

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EF2, EF3 Strong Tornadoes

• Approximately 28 of all tornadoes are considered
  strong
• Well-built houses will be severely damaged but
  still can provide adequate life-protection
• Wind speeds111 165 mph

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EF4, EF5 Violent Tornadoes

• Only 2 of all tornadoes are violent
• Will completely level a well-built house
• The last EF-5s to hit the US were May 3, 1999
  (Moore, OK,) May 24, 2007 (Greensburg, KS,) and
  May 25, 2008 (Parkersburg, IA.)
• Wind speeds166 - 200 mph

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Rating a Tornado
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Wind Damage vs Tornado Damage

• Microburst
• Damage is divergent
• Large or diffuse area
• No signs of rotation

• Tornado
• Damage is convergent
• Narrow path
• Rotation about vertical axis

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Swirl Marks