More Thunderstorms

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More Thunderstorms
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Today

• Homework in
• Wind shear
• More multicellular storms

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Importance of Wind Shear

• Single cell
• Storm is vertically stacked
• Outflow boundary may outrun the motion of the
  storm cell
• New storms that develop may be too far from the
  original to be a part of it
• Multicell
• Storm motion keeps up with outflow
• New development forms adjacent to the older cells
  and connects with the old cell

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Wind Shear

• Example of little or no vertical wind shear

Height
7 kts
There is very little change in the speed or
direction of the wind with height.
6 kts
7 kts
6 kts
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Wind Shear

• Definition
• The change in the direction or speed of the wind
  over a distance.
• Vertical Wind Shear
• The change in the direction or speed of the wind
  with height.
• Low wind shear indicates little change in
  direction or speed of the wind over a distance.

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Multicellular Thunderstorms

• As each cell matures, the updraft holds the
  precipitation aloft. Eventually the
  precipitation unloads and a gush of heavy rain
  occurs at the ground.

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Multicellular Thunderstorms

• At low levels
• The outflow produces the surface convergence to
  form new updrafts
• New updrafts often form at the right or
  right-rear flank of the storm
• At high levels
• As the cell matures, the updraft may pass the
  equilibrium level
• This forms a penetrating updraft of an
  overshooting top

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Multicellular Thunderstorms

• As the next cell grows and develops, the top of
  the older cells tends to collapse.
• This enhances the upper-level divergence which
  helps to expand the anvil
• The collapsing air mixes with the downdraft air
  and enhances the downdraft.

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Multicellular Thunderstorms

• Some multicellular storms show little or no
  motion
• Very localized rainfall that can cause
  significant flooding
• Rapid cell development on the rear of the storm
• New cells move over the same land region as the
  previous cells
• The individual cells move with the mean wind but
  the storm is stationary

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Hazards of Multicellular storms

• Same as single cells
• Greater hazard of flooding due to cell
  regeneration over the same area
• Greater hazard of strong winds when gusts combine

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Supercells

• What are they?
• How do they form? What conditions are necessary?
• Severe weather

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Thunderstorms

• Supercell Thunderstorms
• Relatively long lived intense thunderstorm.
• Consists of one large cell usually with
  exceptionally strong updrafts.
• The structure of the storm is such that the
  updraft and downdraft are separated from each
  other.
• Often the entire storm can begin to rotate.
• May produce large tornadoes.

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Thunderstorms
Supercell Thunderstorms
The updraft and downdraft are tilted and are
thereby separated from each other.
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Severe Thunderstorms
Figure 15.5
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Severe Thunderstorms

• Severe Thunderstorm or Tornado Watch
• Conditions are favorable for the development of
  severe weather or tornadoes.
• Does not mean that severe weather or tornadoes
  will occur at your location.
• Severe weather or tornadoes are likely in and
  around the watch area.

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Severe Thunderstorms

• Severe Thunderstorm or Tornado Warning
• Severe weather or tornadoes are occurring or have
  been indicated by Doppler weather radar.
• Take immediate action to protect life and
  property.

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Classic Severe Weather Conditions

• Idealized Classic conditions for severe weather.

• Low level moisture
• Instability -- Cold air aloft associated with the
  upper level trough
• Inversion (Cap) -- Warm air near 800 mb
• Strong Wind Shear
• Lifting Mechanism -- Cap Breaker

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Classic Severe Weather Conditions
Severe Storm Setup
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Tornado Breeding Supercell Storm
Figure 15.38
Supercell thunderstorms may have many of the
features illustrated here, including a
mesocyclone of rotating winds formed when
horizontal vorticity was tilted upwards.
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Radar Image of Supercell
The area of precipitation and winds in the
mesocyclone is known as the bounded weak echo
region (BWER) which the radar is unable to detect
and displays as a black core to this storm. The
cyclonic flow of precipitation on the radar
screen is often shaped like a hook echo.
Figure 15.39
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Thunderstorms

• Criteria for Severe Weather
• Winds ? 50 kts
• Hail Diameters ? ¾ inch (20mm)
• Tornado

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Flash Flood

• A sudden overflow of a river channel or other
  drainageway, often caused by torrential rains
  associated with thunderstorm activity.
• Slowly moving thunderstorms.
• Training thunderstorms
• Snowfall runoff
• Slowly moving tropical storms
• Example Big Thompson Canyon
• 31 July 1976