To cross or not to cross? That is the question!

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Severe MCS's and the Appalachians
"To cross or not to cross? That is the question!"
Steve KeightonScience and Operations OfficerNWS
Blacksburg, VA
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Forecast Considerations

• Recognize or anticipate upstream severe MCS (Ohio
  or Tennessee Valleys)?
• Will it reach western slopes of mountains,
  penetrate into mountains, reach Piedmont, or die
  in mountains but redevelop in Piedmont???
• What are favorable synoptic patterns instability
  and shear profiles in and on either side of mtns
  influence of cold pool/terrain interactions
  roles of low-level boundaries low-level winds
  and shear relative to ridge orientation diurnal
  trends???
• Will convection survive through mtns but weaken
  below severe thresholds?

RLX, MRX
RNK
GSP
RAH, CAE
LWX
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Forecast Considerations

• Fairly clear anecdotal evidence by forecasters
  that majority of severe MCSs approaching
  Appalachians from the W or NW weaken as they move
  into mountains, and sometimes even before
  reaching western slopes (nocturnal death?).

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July 10, 2003Not-crossing
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July 10, 2003Not-crossing
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Forecast Considerations

• A few do cross the Appalachiansand on rare
  occasion quite vigorously!

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August 9, 2000Crossing Duo of Derechos
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August 9, 2000Derecho 1
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August 9, 2000Derecho 2
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RNK/SPC Study Objectives

• Use severe wx report counts and radar-based
  definition of MCS to classify various scenarios,
  using sub-domains within area of study
• For each category, examine synoptic patterns,
  thermodynamic and kinematic profiles, and
  mesoscale features to look for similarities and
  develop understanding of important physical
  processes
• To limited extent, use numerical simulations to
  study a subset of the events
• Develop better forecast methodology based on
  findings

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RNK/SPC Study Domains
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RNK/SPC StudyProposed Classification Scheme

• Must begin with significant number of reports in
  NW or SW (gt15)
• Dissipating West - significant reduction in
  reports between NW1 and NW2 (or SW1 and SW2), and
  no reports in mountains
• Not-crossing- no significant reduction in
  reports between NW12 (or SW12), and 5 or less
  reports in either of the two mountain zones
• Crossing- same initial criteria for
  Not-crossing, and gt5 reports in either mountain
  zone, and at least 2 reports in NE or SE (may
  need outlier category called penetrating if any
  cases dont meet last part of this definition)
• Redeveloping- lt5 reports in either mtn zone, and
  gt5 reports in either NE or SE zone

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RNK/SPC StudyClassifying Challenges

• Will be a handful of hybrid events that dont
  quite meet any definition
• May need to fall back on radar definition to help
  with these
• Have to be careful not to count svr reports that
  are not a part of progression of MCS (i.e., pulse
  cells out ahead of squall line)
• Have to consider MCSs moving with more of a
  N-to-S component

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RNK/SPC StudyPotential Cases

• Period of study 2000-2005 (utilizing SPC archive
  event database)
• Have over 90 potential dates total (some with
  multiple events), but several will be weeded out
  due to not meeting MCS definition
• Small appear to fit crossing category, with
  not-crossing and re-developing much more
  common

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RNK/SPC StudyCollaboraters

• Steve Keighton and Jan Jackson (RNK)
• Jared Guyer and Jeff Peters (SPC)
• With some help from Jacob Carley under Doug
  Miller (UNC-A)
• Also VA Tech student volunteers helping with data
  collection

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RNK/SPC StudyLots left to do!

• Complete svr report counts and classification for
  several events remaining events on 2000-2004
  initial list, and start on 2005 potential list
• Finalize classification of all events, using
  radar data where necessary
• Finish gathering sfc data and RUC fcst soundings
  to create representative soundings for domains
  west, east, and in mtns
• Analyze all environmental data and get additional
  insight from handful of simulations

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CSTAR Role

• MCS crossing mountains is big issue, affects
  multiple offices, and plenty of room for CSTAR
  involvement
• Help analyzing data and determining important
  physical processes
• NWP simulations of key events in each category
  (can complement recent Frame and Markowski work
  using idealized simulations)
• Climatology for each category (favored times of
  day, year, map types)
• Other areas?

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References

• Coniglio and Stensrud, 2004 Interpreting the
  Climatology of Derechos, Wea Fcstg.
• Frame and Markowski, 2006 The Interaction of
  Simulated Squall Lines with Idealized Mountain
  Ridges, Mon. Wea. Rev.
• Johns and Hirt, 1987 Derechos Widespread
  Convectively Induced Windstorms, Wea Fcstg.
• Johns, Howard, and Maddox, 1990 Conditions
  Associated with Long-lived Derechos An
  Examination of the Large Scale Environment,
  Preprints, 16th Conf on Severe Local Storms.
• Skamarock, Weisman, and Klemp, 1994 3D Evolution
  of Simulated Long-lived Squall Lines, J. Atmos.
  Sci.
• Weisman, 1993 The Genesis of Severe, Long-lived
  Bow Echoes, J. Atmos. Sci.