Overview
The lecture covers the development, structure, and hazards of severe thunderstorms, including ordinary, multi-cell, and supercell storms, as well as associated phenomena like hail, damaging winds, and tornadoes.
Ingredients for Severe Thunderstorms
- Severe thunderstorms require three ingredients: moisture, instability, and lift.
- Instability refers to a situation where rising air remains warmer than its environment and continues to rise.
- The National Weather Service defines severe weather as storms producing hail ≥1 inch, wind gusts ≥50 knots (58 mph), or tornadoes.
- Other important hazards not in the official definition include lightning and flash flooding.
Thunderstorm Life Cycle and Types
- Ordinary cell thunderstorms have three stages: cumulus (rising moist air), mature (updrafts and downdrafts with precipitation), and dissipating (downdraft cuts off updraft).
- Ordinary thunderstorms form when winds are uniform with height and typically last 15-30 minutes.
- Multi-cell thunderstorms consist of several cells at different life stages and are organized by gust fronts.
- Vertical wind shear (change in speed/direction with height) allows storms to become organized, last longer, and sustain convection.
Mesoscale Convective Systems (MCS)
- When multi-cell storms grow, they form mesoscale convective systems (MCS): squall lines, bow echoes, and mesoscale convective complexes (MCCs).
- Squall lines are long lines of thunderstorms; bow echoes are curved lines that "bow out" and MCCs are large circular clusters.
- MCSs are responsible for about half of summer rainfall in the Midwest.
Supercell Thunderstorms
- Supercells have a rotating updraft (mesocyclone), can last many hours, and are the most dangerous thunderstorm type.
- Wind shear is crucial for supercell formation and rotation.
- Supercells produce most significant tornadoes (about 80%), large hail, extreme winds, and flash flooding.
- Classic, low precipitation (LP), and high precipitation (HP) supercells exist, with HP supercells sometimes obscuring tornadoes.
Severe Weather Hazards
- Hail forms in strong updrafts, with size increasing the longer stones remain aloft; severe hail is ≥1 inch in diameter.
- Microbursts are intense, localized downdrafts that spread out on hitting the ground, producing damaging straight-line winds hazardous to aircraft.
- Derechos are widespread, long-lasting windstorms from bow echoes, producing damaging winds over large areas.
- Both microbursts and derechos can cause tornado-like damage.
Tornadoes
- Tornadoes are violently rotating columns of air in contact with the ground beneath a cumuliform cloud.
- About 20% of supercells produce tornadoes.
- Tornado Alley in the central US and Florida have the highest frequency of tornadoes.
Thunderstorm and Severe Weather Distribution
- Thunderstorms are most frequent in the tropics, Florida, the Gulf Coast, and central US plains.
- Large hail is most common in the central plains.
- Thunderstorms move heat and moisture across regions.
Key Terms & Definitions
- Instability — Condition where rising air remains warmer than the environment, favoring thunderstorm development.
- Vertical Shear — Change in wind speed or direction with height.
- Updraft — Rising column of air within a storm.
- Downdraft — Sinking air that can organize or end a thunderstorm.
- Mesoscale Convective System (MCS) — Large group of organized thunderstorms, e.g., squall lines, bow echoes, MCCs.
- Supercell — Thunderstorm with a rotating updraft; most dangerous type.
- Microburst — Intense, small-scale downdraft causing damaging winds.
- Derecho — Long-lasting windstorm from a bow echo, producing widespread damaging winds.
- Tornado — Violently rotating column of air in contact with the ground.
Action Items / Next Steps
- Review next lecture focusing on tornadoes.
- Optional: Consider enrolling in ATMO 2011 for further study of weather processes.