A Tornadic Mini-Supercell With No

Cloud-To-Ground Lightning

Patrick D. MooreNational Weather Service Greer SC

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

19 April 2008 Tornado Event

• At 8:05 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

Synoptic Features 1200 UTC~12 hours to Impact

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

Synoptic Features 1800 UTC~6 hours to Impact

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

Convective Environment 1900 UTC

~5 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

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.

Convective Environment 2145 UTC

2 hours 20 minutes to Impact

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

Convective Environment 2300 UTC

~1 hour to Impact

• Effective bulk shear increased from ~ 40 kt to > 50 kt• 0-1 km SRH increased from ~ 100 m2s-2 to ~ 150 m2s-2

• Most Unstable CAPE increased from < 500 J kg-1 to > 500 J kg-1

2302 UTC - 2338 UTC 19 April63 – 27 minutes to Impact

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).

2343 UTC 19 April22 minutes to Impact

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

alert

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

2348 UTC 19 April17 minutes to Impact

• Supercell characteristics– Appendage/inflow notch– Weak Echo Region– Weak mesocyclone

• Freezing level ~ 9k ft

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

2358 UTC 19 April7 minutes to Impact

Funnel Cloud near Lawndale NC,Report and image not relayed to NWS until next day

0000 UTC 20 April5 minutes to Impact

KGSP

TCLT

KGSP

TCLT

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

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

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

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

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

The Swiss Cheese Model

• 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

Duke University Medical Center

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

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

Questions?