Severe and Unusual Weather ESAS 1115

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Severe and Unusual Weather ESAS 1115

• Spotter Training and Radar Meteorology
• Part 5 Supercell Thunderstorms

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Supercell Thunderstorms

• A supercell is a storm with a mesocyclone

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Wind Shear and Rotation

• A storm that forms in a sheared environment
  (winds change speed and direction with height)
  will acquire storm scale rotation
• This fairly strong and persistent area of
  rotation near the updraft results in increased
  longevity and potential severity large hail,
  damaging winds and tornadoes

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Mesocyclone A region of strong and persistent
rotation that is well-correlated with the updraft
of a thunderstorm.
Movie
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Supercells

• Supercells are storms that are usually larger
  than nearby storms
• Spotters often call large storms with some
  recognizable features supercells even if they
  have no significant rotation
• A mesocyclone requires rotation that is
  persistent and of sufficient strength and depth
• Rotation may not always be visible to the naked
  eye, especially at low-levels

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Supercell Threats

• Rotation within the updraft results in a much
  strong updraft, perhaps as much as 2 times more
  than instability alone
• Storm tops can reach upwards of 60,000,
  resulting in a great amount of precipitation,
  large hail and damaging winds
• Rotation increases the threat of tornadoes in the
  storm as well, although only about 20 of
  supercells result in tornadoes

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Classic Supercell

• Usually, fairly easy to identify storm structures
• Flanking line towers may or may not be present,
  usually dependent on the strength and age of the
  supercell
• Most severe weather events occur at the
  updraft-downdraft interface
• The tornado is an updraft feature while large
  hail and damaging winds are downdraft features

Spot from Here
Visual Vault
Tail Cloud
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Classic Supercell
Striations
Visual Vault
Rain Free Base
Rear Flank Gust Front
Wall Cloud
Forward Flank
RFD
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Supercell

• Supercell echoes are likely to have a hook and a
  strong reflectivity gradient on its inflow side
• The inflow notch is the part of the storm where
  the updraft-downdraft interface can be identified
• A V-notch does not guarantee a storm is a
  supercell, but it does give added indications
  that the updraft is intense

V-notch
Hook Echo
Inflow Notch
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V-Notch
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Supercell and TBSS
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Springfield Supercell 03-13-06
Strong Shear
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Supercell Sizes
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Greensburg, KS May 4, 2007
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Tornadoes within Lines
Difficult to identify features with lines of
storms and even more difficult to anticipate
very difficult storms to chase

• Frustrates chasers with reports of tornadoes
• Encourages dangerous driving always trying to
  catch up to the tornado

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Velocity Datas Utility
Which are supercells?
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What is This?
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Use Velocity Data to Detect the Mesocyclone
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Overshooting Top
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Rain Free Base (RFB)
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RFB and Striated Tower
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Mesocyclone from the East
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Mesocyclone from the West
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NW side of Mesocyclone
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Wall and Tail Cloud
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Wall Clouds
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Scud Clouds Scattered Cumulus Under Deck
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Scud Sucking
Movie

• Cooler air from the forward flank is drawn into
  the storm and reaches saturation below the
  clouds LCL
• Often the wall will lean toward the precipitation
• Strong motions are often apparent to the spotter

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Characteristics of a Tornadic Wall Cloud

• Surface-based inflow
• Rapid vertical motion (scud-sucking)
• Persistent
• Persistent rotation
• The key, however, is the development of a clear
  slot

Clear Slot
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Visual Vault
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Visual Vault and Wall Cloud
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Mammatus
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Mammatus
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Feeder Bands
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Hail Shaft
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Supercell Threats

• Supercells almost always result in severe weather
  in the form of damaging winds, large hail and
  tornadoes
• Most threats are found close to the updraft
  downdraft interface, although the width and path
  length of severe weather can be much larger
• Supercells can be very long-lasting with some
  storms lasting upwards of 6 hours

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Supercells Types

• Even harder to understand is where the tornado
  would form. Supercell varieties were difficult to
  spot and offered additional challenges to our
  understanding
• This storm, which corresponds nicely to the
  supercell schematic is called a classic supercell

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High Precipitation (HP) Supercells

• These storms are difficult to spot for two
  primary reasons obscuration and misplacement of
  important features and safety. The best place to
  spot the tornado is usually in its path
• Originally, HP supercells were considered rare.
  In reality, perhaps half of all supercells are HP
• HP supercells are also prolific tornado
  producers, much more than originally thought.
• These are usually big and scary storms.

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HP Supercells
HP supercells have a great deal more
precipitation in the RFD with rain actually
falling through the rain free base!
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HP Supercell
Visual Vault
Base of Mesocyclone
Beavers Tail
Inflow Notch
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HP Supercell
Large Rear Flank
Inflow Notch
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Visual Ease of HP Identification
Find the updraft-downdraft interface by following
the beavers tail into the storm underneath the
visual vault
Visual Vault
Beavers Tail
Look Here
Heavy Rain in the RFD
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Beavers Tail
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Beavers Tail Inflow along the Forward Flank
Inflow into the Storm
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Beavers Tail
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Difficulties of HP Spotting

• Area of interest is further north than
  anticipated
• Great deal of rain falling through the RFB
  obscures features and reduces visual contrast
• Tornado may become rain-wrapped
• Best place to look is in direction of storms
  motion

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Rain Wrapped Tornado
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Shapes in the Sky

• Often referred to as a mothership
• At this point HPs are probably not tornadic
  because they are outflow dominant as indicated by
  the shelf cloud along the entire base of the
  mesocyclone

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HP Supercells
Movies
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Low Precipitation (LP) Supercells

• These storms are easy to identify marked by light
  precipitation in the main downdraft
• LPs tornadic potential is somewhat limited
• They do produce damaging hail and can change
  modes throughout their lifetime

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LP Supercells
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LP Supercells
Often have a barber pole, or cork screw
structure, especially when decaying
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LP - Almost No Precip at All
Movie
This storm had a tornado warning issued because
of strong mid-level rotation on radar
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LP Threats

• LP supercells do not often produce tornadoes
  because of the lack of a good RFD
• If tornadoes do form the storm may be morphing
  into a more classic-type supercell
• The are prolific producers of large hail even
  when the look is benign
• On radar, although they may be small, LP
  supercells tend to have high reflectivity echoes

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LP Storms Radar Challenges
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Hybrid Storms
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Hybrid HP or Classic?