Severe Storms

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Lecture 13 (11/25)

• Severe Storms

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Severe Storm

• A thunderstorm must have one or more of the
  following to be considered a severe storm (NWS
  classification)
• Winds 58 mph (50 knots) or more
• Hail 3/4 in diameter or larger
• Tornado

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Supercell Thunderstorms

• A supercell thunderstorm is a t.s. with a deep
  rotating updraft (mesocyclone)
• Updraft elements usually merge into the main
  rotating updraft and then accelerate rapidly
• Flanking updrafts "feed" the supercell updraft,
  rather than compete with it
• Small percentage of all t.s.s are supercells but
  they cause the majority of damage

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Diagram of a Supercell
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A Look from the SE
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Types of Supercells

• Low Precipitation - (LP) - high cloud bases,
  little precip, large hail and winds more likely
  than tornadoes, often form along dryline
• High Precipitation - (HP) - occurs in very moist
  air, precipitation often wraps around wall cloud
  and tornado, hard to see except by Doppler radar,
  dangerous to chase (and easy to get dents in car
  with)

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Classic Supercell

• Classic supercell - in between HP and LP
• Most of precip is separated from updraft region
• Good one to chase b/c you can usually see the
  wall cloud and tornado and stay out of its path

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Features of Supercells

• Mesocyclone (p.125) organizes updraft and
  downdraft and keeps them separate
• Updraft is slanted downwind (aloft) so hail/rain
  doesnt fall through it and kill it
• Supercell can last for hours and travel a hundred
  plus miles
• Often moves to the right of the mean flow - has
  to do with rotation (vorticity) and propagation
• What does propagation mean?

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How Supercells Move

• Movement Advection Propagation
• This little formula applies to pretty much
  everything in weather
• advection just the horizontal transport of the
  feature (like a supercell) along with the winds
• propagation development of the feature (usually
  happens towards inflow or flanking line in the
  case of a supercell)

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Overshooting Top

• Overshooting top - characteristic of a strong
  updraft
• The updraft goes higher than the rest of the
  clouds near it (in the anvil)
• Overshoots the tropopause or equilibrium level
  btwn the troposphere stratosphere
• Updraft penetrates stratosphere and then is
  forced back down to equilibrium level

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Overshooting Top
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Mammatus

• See bottom of p. 167
• Little puffy clouds extending downward from anvil
• Indication of high turbulence and strong
  updraft(s) in vicinity
• Remember, the harder and more defined cloud
  features are stronger than the soft and less
  defined features

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Mammatus pictures
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Pileus Clouds

• Can form immediately above a growing updraft if
  its humid at upper levels
• Dont be fooled by the soft appearance of pileus
  - they can hide the hard features of the actual
  updraft
• see p. 176 in text

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Pileus Picture
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Anvil

• Strong winds at upper levels usually accompany
  supercell thunderstorms (they help the updraft
  tilt)
• The winds push the anvil along with them at high
  speeds (50 mph)
• In a strong updraft, a small anvil will blow
  upstream too (called backsheared anvil)
• Again, the harder the feature, the stronger the
  updraft

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Backsheared Anvil
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Wall Cloud

• Wall cloud is a region of cloudiness beneath the
  rotating updraft region of a thunderstorm
• Usually slopes toward the rain and hail shaft
  (bottom of wall cloud closer to precip than top)
• Must be rotating for it to be classified as a
  wall cloud
• Often confused with shelf clouds scud

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Wall Cloud Picture
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Shelf Cloud

• Usually slopes away from precipitation
• Shelf cloud feature of outflow whereas a wall
  cloud feature of inflow

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Shelf Cloud
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Beavers Tail
Beavers tail is just a flanking line of inflow
winds
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Hail

• Hail can be a very destructive force
• Formation
• Starts as frozen raindrop or groupel
• Held aloft by thunderstorms updraft
• Grow from riming or supercooled droplets freezing
  to hailstone

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Hail images
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More hail
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Lightning
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Tornadoes Fujita scale

• F0 Light damage. Wind up to 72 mph.
• F1 Moderate damage. Wind 73 to 112 mph.
• F2 Considerable damage. Wind 113 to 157 mph.
• F3 Severe damage. Wind 158 to 206 mph.
• F4 Devastating damage. Wind 207 to 260 mph.
• F5 Incredible damage. Wind above 261 mph.

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Tracking Tornadoes
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For Next time

• Homework is posted
• No reading assignment