A Tornadic Mini-Supercell With No Cloud-To-Ground Lightning

1
A Tornadic Mini-Supercell With No
Cloud-To-Ground Lightning

• Patrick D. Moore
• National Weather Service Greer SC

2
Post-Mortem Analysis

• At NWS Greer, post-mortems are done for
  significant weather events
• Any tornado
• Any weather related fatality
• The goal is to increase our knowledge and
  identify systematic error, not to place blame

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19 April 2008 Tornado Event

• At 805 pm EDT (0005 UTC 20 Apr) a tornado
  touched down near the Double Shoals community in
  Cleveland County, NC
• No warning was issued
• Why?

National Geographic Television
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Synoptic Features 1200 UTC12 hours to Impact

• 500 mb height, temperature, and wind barbs at
  1200 UTC 19 April 2008

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Synoptic Features 1800 UTC6 hours to Impact

• Surface observations, fronts, and pressure at
  1800 UTC 19 April 2008

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Convective Environment 1900 UTC5 hours to Impact

• Supercells become more probable as effective bulk
  shear increases above 40 kt
• 0-1 km Storm Relative Helicity greater than 100
  m2s-2 suggests increased threat of tornadoes with
  supercells
• CAPE below 500 J kg-1 is a significant limitation
  to supercell development

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Convective Outlook 1934 UTC4 hours 29 minutes to
Impact
ALTHOUGH INSTABILITY REMAINS VERY WEAK ACROSS THE
REGION...SOME SFC HEATING HAS OCCURRED FROM CNTRL
SC NWD INTO CNTRL VA. LOW-LEVEL LAPSE RATES
SHOULD BE MAXIMIZED ALONG THIS CORRIDOR BY EARLY
AFTERNOON AND A MARGINAL THREAT FOR WIND DAMAGE
MAY DEVELOP.
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Convective Environment 2145 UTC2 hours 20
minutes to Impact

• GOES-12 visible imagery at 2145 UTC 19 April 2008

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Convective Environment 2300 UTC1 hour to Impact

• Effective bulk shear increased from 40 kt to gt
  50 kt
• 0-1 km SRH increased from 100 m2s-2 to 150
  m2s-2
• Most Unstable CAPE increased from lt 500 J kg-1 to
  gt 500 J kg-1

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2302 UTC - 2338 UTC 19 April63 27 minutes to
Impact
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2340 UTC 19 April25 minutes to Impact

• Bogus funnel cloud report from Caldwell County,
  NC
• Focused attention on rotation seen in new cell
  moving into Cleveland County, NC
• Cloud-to-ground lightning flash associated with
  old nearby cell (first flash since 2252 UTC).

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2343 UTC 19 April22 minutes to Impact

• No cloud-to-ground lightning
• No radar algorithm-identified meso or hail alert

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2348 UTC 19 April17 minutes to Impact

• Storm has acquired supercell characteristics
  which are more apparent in smoothed data viewed
  with GR2Analyst
• No cloud-to-ground lightning
• No radar algorithm defined meso or hail alerts

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2348 UTC 19 April17 minutes to Impact

• Supercell characteristics
• Appendage/inflow notch
• Weak Echo Region
• Weak mesocyclone
• Freezing level 9k ft

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2355 UTC 19 April10 minutes to Impact

• Skywarn report of rotation in cloud near Kingston
  (Cleveland County) NC
• Operator identified minimal mesocyclone
• VR 21 kt, shear 0.012 s-1 at 43 km (KGSP 2353 UTC
  scan)
• VR 19 kt, shear 0.008 s-1 at 33 km (TCLT 2354 UTC
  scan)
• No cloud-to-ground lightning since 2338 UTC
• No radar algorithm identified meso or hail alerts

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2358 UTC 19 April7 minutes to Impact
Funnel Cloud near Lawndale NC, Report and image
not relayed to NWS until next day
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0000 UTC 20 April5 minutes to Impact
KGSP
KGSP
TCLT
TCLT
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0003 UTC 20 April2 minutes to Impact

• Amateur cell phone video of wall cloud near
  Fallston, NC (Cleveland County). Report not
  relayed to NWS until next day
• TCLT storm relative motion contaminated with
  range folding, no 1.0 degree scan in this volume
• No cloud-to-ground lightning since 2338 UTC
• No radar algorithm identified meso or hail alerts

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0007 UTC 20 April2 minutes after Impact

• Tornado on the ground southwest of Fallston, NC.
  Secondhand report relayed to NWS at 0015 UTC
• No cloud-to-ground lightning since 2338 UTC
• No radar algorithm identified meso or hail alerts

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Radar Comparison 0007 UTC

• Operator identified mesocyclone remained in weak
  shear or minimal mesocyclone category before
  tornadogenesis
• KGSP radar algorithm did not identify
  mesocyclone, TCLT radar algorithm not available

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Radar Comparison 0007 UTC

• Rotational shear on both radars remained in
  minimal mesocyclone category prior to
  tornadogenesis
• Rotational shear on TCLT radar increased to
  tornado possible category on first volume scan
  after tornadogenesis

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Points of Failure on 19 April 2008

• Inadequate situation awareness of potential for
  supercells and tornadoes
• No prior experience with tornadoes generated by
  mini-supercells in the absence of cloud-to-ground
  lightning
• A mesocyclone too small to be properly sampled by
  radars?
• Range folded velocity data on TCLT radar, no 1.0
  degree scan prior to tornado formation
• Critical storm reports not relayed to the NWS

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The Swiss Cheese Model
Duke University Medical Center

• Every step in a process is a defensive layer, but
  has the potential for failure
• In rare cases, a series of unrelated failures
  will occur, resulting in a catastrophic event

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What Went Right?

• Tornado Warning issued for western Lincoln County
  NC at 0015 UTC based on significant increase in
  rotational velocity on TCLT radar data

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What Went Right?

• Tornado touched down in Lincoln County NC at 0020
  UTC, intermittent path through 0037 UTC
• Cloud-to-ground lightning began at 0040 UTC

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Questions?