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- Thunderstorms, also called cumulonimbus clouds, are tall, vertically
developed clouds that produce lightning and thunder.
- The majority of thunderstorms are not severe.
- The National Weather Service reserves the word “severe” for
thunderstorms that have potential to threaten live and property from
wind or hail.
- A thunderstorm is considered severe if:
- Hail with diameter of three-quarter inch or larger, or
- Wind damage or gusts of 50 knots (58mph) or greater, or
- A tornado.
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- A source of moisture
- A conditionally unstable atmosphere
- A mechanism to tiger the thunderstorm updraft either through lifting or
heating of the surface
- Vertical wind shear: a rapid change in wind speed and/or wind direction
with altitude
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- In cool season (late fall, winter, and early spring), lifting occurs
along boundaries between air masses č fronts
- When fronts are more distinct, very long lines of thunderstorms can
develop along frontal boundaries č frontal squall lines.
- In warm season (late spring, summer, early fall), lifting can be provide
by less distinct boundaries, such as the leading edge of a cool air
outflow coming from a dying thunderstorm.
- Thunderstorms developed along these subtle boundaries in the warm season
often undergo a self-organized process and lead to the a kind of severe
storm called “mesoscale convective system” (MCS).
- “Meso-scale” refers to atmospheric processes that occurs on a scale of a
few hundreds hundreds kilometers.
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- If winds increase rapidly with height ahead of a strong front or a less
distinct boundary, thunderstorms triggered along that boundary may
organize into violent storms call “supercell thunderstorms”.
- Supercells can occur along a line or individually, but always have their
own circulations that are related to the storm’s rotation and the
vertical shear in the atmosphere.
- Supercells develop most often when strong winds are present in the upper
troposphere and winds in the lowest kilometers or two of the atmosphere
increase rapidly and change direction with altitude.
- Supercells can occur in both cool and warm seasons
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- Frontal Squall Lines
- Mesoscale Convective Systems
- Supercell Thunderstorm
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