Severe and Unusual Weather ESAS 1115

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Severe and Unusual Weather ESAS 1115

• Spotter Training and Radar Meteorology
• Part 4 Multicell Thunderstorms

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Multicell Thunderstorms

• Several distinct updraft/downdraft couplets exist
• Can form as a cluster or a squall line
• A vertically sheared environment results in
  tilting of the updrafts and a longer lasting
  system

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Multicell StormsHow Many Cells are There?
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Multicell Cluster

• Series of evolving cells where new updraft growth
  is on a preferred flank
• Child cells kill their parents with cold outflow
  air
• New updraft growth is generated along outflow
  from the parent cells preferentially based upon
  low-level convergence and in conjunction with
  low-level wind shear

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Multicell Evolution
Southwest side of multicell cluster (looking to
the northeast)
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Overshooting Tops
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Multiple Updraft Growth in Close Proximity to
Each Other
In moderate wind shear environments, multicell
storms may take a few hours to evolve into a
supercell storm
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Multicell (Squall) Line

• A squall line is a line of thunderstorms with a
  continuous, well developed gust front at its
  leading edge
• Discrete cells are difficult to distinguish
• New cell growth is generally along the whole line
  rather than on just one preferred flank
• Large hail is possible in the highest
  reflectivity cores
• Damaging winds are possible at the leading edge
  but also further behind the front edge

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Line Echo Wave Pattern (LEWP)
Refers to a series of bow echo segments a wavy
appearing squall line can result in an enhanced
severe threat at the leading edge of the bow or
in the crest of the waves
Crest
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Squall Line Cross Section

• Compare with supercell schematic
• Updrafts are on the leading edge of the cold
  pool, hence tornado threat is small after passage
  of gust front although hail will now occur
• Squall lines have well-developed cold pools

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Cross Section of Multicell
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Outflow Boundary (OFB)
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OFB on Composite Radar
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Shelf Cloud Along the OFB
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Shelf Cloud
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Underneath the Shelf

• The texture underneath the shelf cloud is
  considered boiling
• Although turbulent and swirling, the underside of
  a shelf cloud is a region of warm and cold air
  mixing and thus not tornadic

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Boiling Texture
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Typical Look
Movie

• Look for ragged appearance, even if it is not
  extremely turbulent
• Think about where the rain is falling and whether
  or not cool air is pushing away from the storm
• Be aware that sheared motion along the outflow is
  often mistaken for rotation
• Sometimes, the only significant feature readily
  identifiable is the shelf cloud

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Intense Skies
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On Top of the Shelf

• The top of a shelf is usually quite laminar
  (smooth)
• Laminar cloud texture is usually the result of
  forced ascent, not convective in nature
• Laminar banding is not uncommon in a stacked
  shelf cloud

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Laminar Bandings in a Stacked Shelf Cloud
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Forced Ascent along Squall Line Cold Pool
Definitely dark and scary!
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Outflow Dominant Line of Storms
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Outflow Features of a Squall Line
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Haboob (Dust Storm)
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Gustnadoes
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Gustnadoes

• Gustnadoes are not really tornadoes as they are
  not pendant to a Cb
• Although these vortices are relatively weak, they
  still pose some threat to those in their paths
• Note the boiling texture underneath the shelf
  cloud

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Broken Squall Line Threats

• Tornadoes are most likely within supercells and
  can form where storms are more isolated
• These storms are most likely at the southern most
  end (Tail End Charlie) or just north of a break
• Isolated storms ahead of a squall line also pose
  significant risk for severe events and/or
  tornadoes

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Bow Echoes
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Development of a Bow Echo
Movie
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Bow Echo with Trailing Stratiform Rain
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Underneath a Bow
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MCS and MCC

• A large organization of thunderstorms is called
  an MCS (Mesoscale Convective System)
• A squall line is considered a linear MCS
• When the IR satellite presents a more circular
  perspective of a large cluster of thunderstorms
  and it persists for more than 6 hours, we refer
  to this MCS as an MCC (Mesoscale Convective
  Complex)

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Linear vs. Circular
Eccentricity Ratio of the minor axis to the
major axis
Eccentricity close to 1
Eccentricity much less than 1
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Mesoscale Convective Vortex (MCV)
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Multicell Thunderstorms

• Gust front processes dominate multicell processes
• The addition of vertical wind shear allows for
  new updrafts to reach maturity in some organized
  fashion
• Even though each cell moves with the mean wind of
  the cloud-bearing layer, new updrafts form where
  low-level convergence is strongest and is
  enhanced by the vertical wind shear
• When updrafts initiate at a preferred location,
  multicell clusters form
• Updrafts that initiate along the leading edge of
  the cold pool will result in a squall line
• The severe threat increases as instability and
  shear increase and where strong cold pools do not
  overwhelm new storm growth
• Severe weather events are more predictable ahead
  of strong cells and with features that are easily
  identifiable on radar (e.g. bow echoes)
• Tornadoes are possible with storms that are
  supercellular or isolated (such as a head of a
  line) and with storms north of a bow echo