## Section8.2Types of thunderstorms

NOAA Jetstream: https://www.weather.gov/jetstream/tstrmtypes

### Subsection8.2.1Multi-cell thunderstorms

• composed of several individual single-cell storms at different stages of development (cumulus, mature, dissipating)

• moderate wind shear tilts storm, preventing quenching from rainfall into updraft

• gust fronts may produce additional lifting along the surface and trigger the formation of new cells (the gust front can produce phenomena such as shelf clouds)

• multicell thunderstorms may grow or join to produce mesoscale convective systems

### Subsection8.2.2Mesoscale convective systems

Squall Line

• thunderstorms arranged in a line or band

• along boundary of unstable air, often ahead of cold front

• Have life spans of 6 to 12 hours or more

• Extend over several states simultaneously

• A shelf cloud is often observed above the gust front

• Divergence aloft and a broad, low-level inflow of moist air favor development of squall lines

Mesoscale Convective Complex

• An MCC is a complex of individual storms that covers a large area in an infrared satellite image and lives more than 6 hours

• may form under ridge of high pressure if unstable air conditions at the surface

• Often form in late afternoon and evening

• In satellite images give the appearance of a large circular storm with cold cloud-top temperatures

• Often form underneath a ridge of high pressure; because upper-level divergence can occur in a ridge

• Do not require as much vertical shear as squall lines

• Can be maintained by the low-level jet

### Subsection8.2.3Supercell thunderstorms

Conditions that favor violent thunderstorms…

• high vertical wind shear

• low level (900 mbar) jet of mT air (sometimes called the nocturnal jet)

• capping inversion caused by movement of cT air at around 700 mbar level; increases LI at the surface

• updrafts tilt this shearing and can develop into a supercell

Supercell Thunderstorms

• The supercell thunderstorm is a large single-cell storm, sometimes 32 km or more across, that almost always produces dangerous weather

• Strong wind gusts, large hail, dangerous lightning and tornadoes

• Require a very unstable atmosphere

• Require strong vertical wind shear (in direction and speed)

• Vertical wind shear causes supercell thunderstorms to rotate around a vertical axis

• strong westerly flow aloft and southeasterly surface winds; vortex tube can be carried vertical by a strong updraft

• updraft and downdrafts do not interfere with each other: storm may survive for several hours

Supercell Structure

• mesocyclone: spinning updraft (5 to 20 km across); downdrafts caused by heavy precipitation

• Like a miniature extratropical cyclone, 5 to 20 km across; 2-5 mbar pressure drop

• Narrows and rotates more quickly when it stretched, but too large and too slow in rotation to be a tornado

Downburst/Microburst

• rain evaporates below cloud, leading to rapid cooling; cold, heavy air plunges to surface

• RFD (rear flank downdraft) is cool and relatively dry