Chapter 11: severe weather

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Chapter 11 severe weather!!

• thunderstorms (classification)
• tornadoes
• lightning, and
• hail

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Thunderstorm classificationa brief review
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Thunderstorms often cluster into systems 100
miles or more in size. These are called
mesoscale convective systems. MCSs come in many
shapes, including squall lines, bow echoes and
MCC (complexes).
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Fig. 11.2a
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Fig. 11.2b
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Airmass Thunderstorms
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Photo by NSSL
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Airmass thunderstorms have a life cycle with
three stages
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Mature airmass thunderstorms over the Pacific
seen by the Space Shuttle
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Airmass Thunderstorms

• These are scattered small thunderstorms that form
  in mainly summer.
• They typically develop in warm, moist air masses
  away far from any fronts.
• They occur when the winds aloft are weak (little
  wind shear)
• They are also typically short-lived and rarely
  produce extreme winds and/or hail.

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The reason why an airmass thunderstorms is so
shortlived is that there is little wind shear,
therefore the rainy downdraftquickly undercuts
and chokes off the updraft.
Photo by Moller
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Multicell Thunderstorms

• Life cycle of any one of the cells of a multicell
  thunderstorm is like any air-mass thunderstorm.
• The life cycle of the multicell is much different
  due to the interaction of the cells one with
  another.
• The key to the long life of the multicell is the
  development of the gust front.

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Shelf cloud some examples
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Fig. 11.11
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Multicell Thunderstorms

• Gust front appears like a mesoscale cold front.
• Same effects of pressure check, temperature drop,
  and wind shift upon passage.
• Outflow boundary is the remnant of a gust front.

• Shelf Cloud often indicates rising air over the
  gust front.
• New cells develop in front of the storm.
• Gust front maintained by the cool downdrafts.

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Multicell - cont'doutflow boundary as seen by
ground-based radar
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Photo by Moller
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Multicell movement
old cell
Multicell storms move slightly to the right of
the upper-level wind
young cell
Photo by Doswell
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north
south
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Why do new cells form mainly on the southern side
?
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Supercell Thunderstorms

• occur most frequently in the southern Great
  Plains in spring.
• compared to single cells, supercells are
• rare
• longer-lived
• larger
• organized with separate up- and downdrafts.

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weak echo region in a supercell storm

• As the storm intensifies, the updraft becomes
  stronger and more erect.
• The result are
• the development of mid-level echo overhang (WER)
• a tighter reflectivity gradient (hail is most
  common just north of the WER)
• a shift in cloud top position (right above the
  WER)
• these are strong indicators of a dangerously
  severe storm.

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How does the (bounded) weak echo region (WER)
form ?
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supercell - seen by ground-based radar
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Cross-section thru supercell - note BWER
Note lowest 7,000 ft cannot be seen by
radar because of the curvature of the earth.
54,000 ft tops
NW
SE
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Photo by Moller
This storm produced baseball hail east of
Carnegie, OK, as it was photographed looking east
from 30 miles. From right to left (south to
north), note the flanking line, the main storm
cell, and the downwind anvil above the
precipitation area.
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(above) a supercell with overshooting top, seen
from the SW (photo H. Bluestein) (right) a
Texas supercell seen from the NW note vertical
cloud wall and spreading anvil (photo by Moller)
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Supercell Thunderstorms

• Supercells only form when strong upper-level
  winds blow, and a significant change in wind
  direction occurs, generally southeasterly at the
  surface and WSW in the upper troposphere.
• Wall clouds may be found below the storms cloud
  base -- significant rotation may be seen (tornado
  spawning area).

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Thunderstorm evolution and shear

• no shear
• strong shear

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thunderstorm classificationa summary