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Severe and Unusual Weather ESAS 1115: 

Severe and Unusual Weather ESAS 1115 Spotter Training and Radar Meteorology Part 1 – Introduction to Severe Thunderstorms

Meteorological Sensors: 

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

Convective Hazards Wind, Hail, Floods, Lightning, Tornadoes: 

Convective Hazards Wind, Hail, Floods, Lightning, Tornadoes

Severe Thunderstorms are Severe: 

Severe Thunderstorms are Severe Many times, the media uses the word “severe” when the storm is something less than the actual meaning of “severe”.

Severe Thunderstorm Criteria: 

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



Lightning : 

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

Lightning Safety: 

Lightning Safety Lightning Stats Lightning Safety Video Lightning Safety Tips

Flash Floods – Turn Around Don’t Drown: 

Flash Floods – Turn Around Don’t 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

Flash Flood Threat: 

Flash Flood Threat

Flash Floods – Turn Around - Don’t Drown: 

Flash Floods – Turn Around - Don’t Drown

Hail – Big Chunks o’ Ice: 

Hail – Big Chunks o’ Ice

Tarrant County, TX May 5, 1995: 

Tarrant County, TX May 5, 1995 10,000 people at Mayfest Baseball hail – 10 miles wide 50 miles long 14 people killed by flooding and collapsed roofs $2 billion (estimated)

We Know First Hand: 

We Know First Hand

Damaging Winds: 

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


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

Rain Foot and Dust Foot: 

Rain Foot and Dust Foot

Dust Foot: 

Dust Foot



Aviation Hazards: 

Aviation Hazards

Danger on Takeoff and Landing: 

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

Beaufort Scale: 

Beaufort Scale Winds based on visual observations

First Microburst: 

First Microburst

Microburst within a Macroburst: 

Microburst within a Macroburst



Microburst Evolution: 

Microburst Evolution

Tornado and Hail Climatology: 

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

Tornado Threats: 

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

EF0, EF1 – Weak Tornadoes: 

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 speeds: 65 – 110 mph

EF2, EF3 – Strong Tornadoes: 

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 speeds: 111 – 165 mph

EF4, EF5 – Violent Tornadoes: 

EF4, EF5 – Violent Tornadoes Only 2% of all tornadoes are violent Will completely level a well-built house The last F5 to hit the US was May 3, 1999 in Oklahoma Wind speeds: 166 - >200 mph

Rating a Tornado: 

Rating a Tornado

Wind Damage vs Tornado Damage: 

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

Swirl Marks: 

Swirl Marks

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