🌦️Atmospheric Science Unit 10 – Severe Storms: Thunderstorms and Tornadoes

Key Concepts and Definitions

• Thunderstorms are convective storms characterized by the presence of lightning and thunder
• Tornadoes are violently rotating columns of air extending from a thunderstorm to the ground
• Supercell thunderstorms are the most severe type of thunderstorm with a persistent rotating updraft (mesocyclone)
• Wind shear is the change in wind speed or direction with height, a key ingredient for thunderstorm organization and tornado formation
• Instability refers to the atmosphere's tendency to promote vertical motion, necessary for thunderstorm development
  • Measured by indices such as Convective Available Potential Energy (CAPE)
• Downdrafts are descending currents of cool, dense air within a thunderstorm
  • Responsible for strong surface winds and the formation of gust fronts
• Hail consists of layered ice particles formed by repeated updrafts and downdrafts within a thunderstorm

Formation and Development

• Thunderstorms form when three conditions are met: moisture, instability, and a lifting mechanism
• Moisture is necessary for cloud formation and latent heat release during condensation
• Instability allows air parcels to rise freely through the atmosphere
• Lifting mechanisms, such as fronts, boundaries, or terrain, initiate vertical motion
• Thunderstorm development follows three stages: cumulus, mature, and dissipating
  • Cumulus stage: characterized by updrafts and cloud growth
  • Mature stage: both updrafts and downdrafts are present, along with heavy precipitation, lightning, and strong winds
  • Dissipating stage: downdrafts dominate, leading to the weakening and eventual dissipation of the storm
• Tornadoes typically form from supercell thunderstorms when strong wind shear and instability are present

Structure and Anatomy

• Thunderstorms consist of an updraft, downdraft, and various cloud regions
• The main updraft is responsible for the vertical growth and sustenance of the storm
  • Updrafts can exceed speeds of 100 mph in severe thunderstorms
• Downdrafts form as precipitation falls through the storm, cooling the air and causing it to sink
• The gust front is the leading edge of the downdraft, characterized by strong, gusty winds and a shift in wind direction
• Supercell thunderstorms have a rotating updraft (mesocyclone) and a distinct hook echo on radar
• Tornadoes have a central core (eye) surrounded by the rotating wall cloud
  • The funnel cloud becomes a tornado when it reaches the ground
• Rear Flank Downdraft (RFD) is a region of descending air that can help tighten rotation and lead to tornadogenesis

Classification and Types

• Thunderstorms are classified based on their severity, organization, and duration
• Ordinary cell thunderstorms are short-lived, isolated storms with a single updraft and downdraft
• Multicell thunderstorms are clusters of ordinary cells in various stages of development
  • Multicell lines (squall lines) are organized lines of thunderstorms that can persist for hours
• Supercell thunderstorms are the most severe, characterized by a persistent rotating updraft (mesocyclone)
  • Supercells are responsible for the majority of significant tornadoes, large hail, and damaging winds
• Tornadoes are classified using the Enhanced Fujita (EF) Scale, which ranges from EF0 to EF5 based on damage severity
• Waterspouts are tornadoes that form over water, often from non-supercell thunderstorms or cumulus clouds

Severe Weather Indicators

• Severe thunderstorm warnings are issued when storms are capable of producing hail ≥ 1 inch in diameter or wind gusts ≥ 58 mph
• Tornado warnings are issued when a tornado is imminent or occurring based on radar or spotter confirmation
• Strong low-level wind shear is a key indicator of potential tornado development
• High values of CAPE (Convective Available Potential Energy) indicate a highly unstable atmosphere conducive to severe storms
• The presence of a "hook echo" on radar is a strong indication of a supercell thunderstorm and potential tornado
• Wall clouds are lowerings of the cloud base that can signify the presence of a mesocyclone and imminent tornado formation
• Hail size can be an indicator of storm severity, with larger hail often associated with more intense updrafts
• Lightning frequency and type (e.g., cloud-to-ground, intracloud) can provide insights into storm intensity and structure

Forecasting and Prediction Methods

• Numerical weather prediction models are the primary tool for forecasting thunderstorms and tornadoes
  • Models simulate atmospheric conditions and provide guidance on the likelihood and location of severe weather
• Skew-T log-P diagrams are used to assess the vertical profile of temperature, moisture, and wind in the atmosphere
  • Skew-Ts help forecasters evaluate instability, wind shear, and the potential for severe weather
• Radar is a crucial tool for detecting and tracking thunderstorms and tornadoes in real-time
  • Doppler radar can measure wind speed and direction, allowing for the detection of rotation within storms
• Satellite imagery provides a broad overview of cloud cover, moisture, and atmospheric circulation patterns
• Surface observations and upper-air soundings provide essential data on temperature, humidity, wind, and pressure
• Storm spotters and chasers provide ground-truth observations and confirmation of severe weather events
• Ensemble forecasting involves running multiple models with slightly different initial conditions to assess uncertainty

Impacts and Hazards

• Thunderstorms can produce a variety of hazards, including lightning, heavy rain, hail, and strong winds
• Lightning is a significant threat, causing injury, death, and property damage
  • Lightning can ignite wildfires and cause power outages
• Heavy rainfall from thunderstorms can lead to flash flooding, particularly in urban areas or regions with steep terrain
• Hail can cause extensive damage to crops, vehicles, and structures
  • Large hail can also cause serious injury to people and animals
• Strong winds associated with thunderstorms can uproot trees, damage buildings, and create flying debris
• Tornadoes are one of the most destructive weather phenomena, causing catastrophic damage and loss of life
  • Tornadoes can destroy homes, businesses, and infrastructure in a matter of seconds
• Severe thunderstorms and tornadoes can disrupt transportation, communication, and power systems

Safety Measures and Preparedness

• Develop and practice an emergency plan for thunderstorms and tornadoes
  • Identify safe locations within your home, workplace, or school (e.g., interior rooms, basements)
• Stay informed about weather conditions and warnings through local media, weather apps, or NOAA Weather Radio
• When thunder roars, go indoors: seek shelter in a substantial building or hard-topped vehicle
• Avoid open areas, tall objects, and conductive materials during thunderstorms
• If caught outdoors during a tornado, seek shelter in a sturdy structure or lie flat in a low-lying area
• Prepare an emergency kit with essential supplies (e.g., water, non-perishable food, first-aid kit, flashlight)
• Secure important documents and valuables in a safe, easily accessible location
• Consider installing a safe room or storm shelter in your home for added protection during severe weather events