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Thunderstorms

Objective​

Understand the formation, classification, and lifecycle of thunderstorms, identify associated aviation hazards, interpret relevant weather products, and apply established safety strategies for avoidance and inadvertent encounters.

Timing​

  • 45 minutes

Format​

  • Whiteboard
  • ForeFlight weather products

Overview​

  • Conditions Required for Thunderstorms
  • Thunderstorm Lifecycle
  • Air Mass Thunderstorms
  • Steady State Thunderstorms
  • Squall Line Thunderstorms
  • Embedded Thunderstorms
  • Frontal Thunderstorms
  • Hazards Associated with Thunderstorms
  • Forecasts Associated with Thunderstorms
  • Radar Summary Chart
  • Convective SIGMETs
  • Thunderstorm Avoidance Strategies
  • Inadvertent Thunderstorm Encounter Strategies

Elements​

Conditions Required for Thunderstorms​

Three essential ingredients:

  1. Water vapor
  2. Unstable air
  3. A lifting mechanism
  • Conditionally unstable air allows for release of convective energy
  • Lifting mechanisms: converging winds, fronts, upslope flow, drylines, or local wind circulations
  • A rapid decrease in temperature with altitude may indicate an unstable air mass

Thunderstorm Lifecycle​

Three distinct stages: Towering Cumulus, Mature, and Dissipating

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  1. Towering Cumulus stage features strong updrafts exceeding 3,000 fpm
  2. Mature stage features both updrafts and downdrafts; hazards at peak intensity
  3. Dissipating stage dominated by downdrafts as moisture supply is cut off

mature thunderstorms

Air Mass Thunderstorms​

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  • Form on scattered basis due to uneven surface heating and instability
  • Also called popcorn convection
  • Typically small, short-lived, and rarely severe
  • Usually dissipate shortly after sunset and heating of the ground stops

Steady State Thunderstorms​

  • Supercells are long-lived (more than 1 hour) thunderstorms
  • Characterized by significant instability, strong surface winds
  • Supercells are typically "tilted"
    • Rain falls ahead of the storm, outside of the updraft, preventing the storm from collapsing on itself
  • Updrafts of supercells are commonly larger than a typical thunderstorm
    • Can be as large as 10 miles in diameter and up to 60,000 feet tall
    • Updraft speeds can reach 9,000 fpm or 100 knots

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Squall Line Thunderstorms​

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  • Narrow band of active cells extending hundreds of miles
  • Often form ahead of cold fronts associated with midlatitude low pressure systems
  • Generate approximately 25 percent of all U.S. tornadoes
  • Most effective barrier to air traffic due to height and length

Embedded Thunderstorms​

  • Thunderstorm cells obscured by massive cloud layers
  • Difficult to circumnavigate visually
  • Differentiated in SIGMETs as EMBD TS

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Frontal Thunderstorms​

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  • Triggered by cold, warm, or occluded frontal lift
  • Cold fronts act as a snowplow, forcing air upward abruptly
  • Warm fronts produce gradual lift, leading to widespread stratiform clouds and embedded cells
  • Can occur in a narrow band along the frontal boundary

Hazards Associated with Thunderstorms​

  • Severe turbulence exists between updrafts and downdrafts
    • Greatest turbulence is found around the perimeter of the storm, where the stormy air is mixing with the air around it
    • Usually the turbulence is strongest on the front side of the storms
    • 20 fpm gusts: Moderate turbulence, 30 fpm gusts: Severe turbulence
    • Gust fronts can extend 15-20 miles in front of a strong storm; surface winds can change rapidly in speed and direction
  • Hailstones ≥0.75 inches cause significant structural damage
  • Lightning strikes most common at temperatures between -5°C and +5°C
  • Static electricity (P-static) can cause complete loss of VHF communications
  • Tornado wind speeds can exceed 200 knots
  • Rapid pressure changes can cause altimeter errors > 100 feet

Microbursts​

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Microbursts produce downdrafts up to 6,000 fpm and headwind losses of 30–90 knots

  • Microbursts are small-scale, intense downdrafts that spread out symmetrically when reaching the ground

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Forecasts Associated with Thunderstorms​

  • Terminal Aerodrome Forecast (TAF) provides point-specific thunderstorm projections
  • Convective Outlook (AC) forecasts severe potential over 8 days
  • Graphical Forecasts for Aviation (GFA) website displays forecast TS areas

Radar Summary Chart​

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  • Graphically depicts collection of radar weather reports (SDs)
  • Shows precipitation intensity, echo tops, and cell movement
  • Identifies weather watches using heavy dashed lines
  • NE indicates no echoes detected; NA indicates no information

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Convective SIGMETs​

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  • Coded as WST
  • Issued hourly at H+55 for Eastern, Central, and Western U.S.
  • Valid for up to 2 hours
  • Criteria:
    • Surface winds >50 knots, hail ≥3/4 inch, or tornadoes
    • Tornadoes, lines of thunderstorms, embedded or severe thunderstorms
    • Area of very strong storms affecting 3000 square miles and covering at least 40% of the area
  • Based on a convective outlook, which is also available

Thunderstorm Avoidance Strategies​

Avoid thunderstorms by at least 5 miles, and 20 miles for severe storms.

  • Avoid by at least 20 mi any thunderstorm identified as severe or giving an intense, heavy, or extreme radar echo
    • Especially true under the anvil of a large cumulonimbus
    • Large echoes should be separated by at least 40 mi before flying between echoes
    • Separation distances may be reduced for avoiding weaker echoes.
  • Regard any storm with tops >35,000 feet as extremely hazardous
  • Circumnavigate areas with 6/10 or more thunderstorm coverage
  • Do not land or take off in the face of an approaching thunderstorm
  • Do not fly under the anvil of a thunderstorm due to clear-air turbulence (CAT) risk

Aviation Weather Handbook pg. 22-21

Inadvertent Thunderstorm Encounter Strategies​

  • Maintain a straight course through the storm to minimize time in hazards
  • Set power for recommended turbulence penetration speed (VA)
  • Focus on maintaining a level attitude; allow altitude and airspeed to fluctuate
  • Disengage Altitude Hold and Speed Hold modes on autopilot
  • Turn cockpit lights to maximum to reduce lightning blindness

Aviation Weather Handbook pg. 22-22

References​

  • Aviation Weather Handbook: 22-1 to 22-21, 24-3, 24-5, 24-36, 26-2 to 26-10, 27-7 to 27-21
  • Instrument Flying Handbook: 2-11, 10-24 to 10-25, 11-2
  • Instrument Procedures Handbook: 4-5, A-1
  • Aeronautical Information Manual: 7-1-26, 7-1-27
  • Backseat Pilot CFI-I Lesson Plans: III.A (Weather Information), III.A (Weather Theory)
  • Purdue Thunderstorms