A Case Study of Two Left-Moving Mesoanticyclonic Supercells on 24 April 2006 Chris Bowman National Weather Service

1
A Case Study of Two Left-Moving Mesoanticyclonic
Supercells on 24 April 2006Chris BowmanNational
Weather Service Wichita, KS
2
The Situation

• Synoptic environment favored supercells.
• Tornado Watch 244 already issued.
• As the event is unfolding, storms are not taking
  on classic supercell characteristics.
• Two cells and especially one in particular begin
  to exhibit unusual radar signatures.

3
RUC Model 22Z Forecast Sounding
4
22Z RUC Forecast Hodograph
5
KICT Reflectivity and SRM 0.5 Evolution
2138Z 2213Z Animation
6

KICT Reflectivity and SRM All-Tilts
2213Z 0.5 -19.5 Animation

7
KVNX Reflectivity and SRM 0.5 Evolution
2137Z 2211Z Animation
8
KVNX Reflectivity and SRM 0.5 Evolution
2216Z 2235Z Animation
9
KICT and KVNX SummaryObserved Items of Interest

• Boundary interaction or cell merger (BICM).
• Rapid strengthening of storm after BICM.
• Persistent and strengthening low-level (0.5)
  rotation.
• KICT indicates cyclonic (1,000 ft), KVNX
  indicates anticyclonic (4,750 ft).
• KICT indicates a transition from low-level
  cyclonic rotation (900 2,700 ft) to a
  convergent signature (3,000 10,000 ft) to a
  mesoanticyclone (12,000 ft 25,000 ft).

10
Whats the decision?Tornado? Severe? No warning?

• Youre in the warning chair under a tornado
  watch.
• Storms have not exhibited typical supercell
  characteristics.
• A storm develops rapidly with strengthening
  cyclonic rotation corroborated by another radar.
• Storm appears to be supercellular but opposite of
  what was anticipated.
• The storm is moving into the Wichita Metro.

11
What Actually Happened

• Tornado warning issued at 2215Z for SW Sedgwick
  and NW Sumner Counties but polygon kept Wichita
  out of the warning.
• Two off-duty NWS mets as well as private mets
  observe a wall cloud with the storm. Only live
  report came from an off-duty NWS met as the storm
  and ill-defined wall cloud moved over his
  house.
• No tornado was reported but several severe hail
  reports were received.
• The storm, as viewed from KVNX, maintained its
  supercellular characteristics as it moved over
  Wichita, producing only hail reports.

12
KICT and KVNX SummaryThe Unexplained

• Although the environment favored left-moving,
  mesoanticyclonic storms (more on this later),
  there is no clear explanation for KICTs vertical
  SRM transition from low-level cyclonic rotation
  to mid/upper-level anticyclonic rotation
• The low-level velocity signature from both KICT
  and KVNX are 3 miles north-northeast of the
  reflectivity signature.
• Reflectivity aloft is almost on top of low-level
  SRM signature.
• Possibly a boundary being ingested and stretched
  or even thunderstorm outflow.

13
Storm 2

• Actually occurred before the tornado warned
  storm.
• Exhibited similar characteristics (including
  velocity signature 3 miles north-northeast of
  reflectivity signature) but only observed from
  KVNX.
• Storm not as persistent and not as strong.

14
KVNX Storm 2 0.5 Reflectivity and SRM
2127Z 2151Z Animation
15
Storms 1 and 2 Summary

• Two storms in the same county within several
  volume scans of each other both develop low-level
  anticyclonic rotation displaced by about 3 miles
  to the north of the reflectivity signature.
• Both have a boundary interaction or cell merger
  and strengthen rapidly afterward.
• Both storms are in the same environment and this
  is the key.

16
Environmental Data
17
Environmental Data
18
Environmental Data

• Parameters derived from surface T/Td of 76/66.
• Near storm environment was actually 74/64.
• Not a huge difference right? In this case it
  actually makes a big difference.
• Need to modify sounding as a result.

19
Environmental Data
20
Environmental Data

• Modified sounding drops CAPE from 2,038 J/Kg to
  1,324 J/Kg.
• Increases CIN from -10 J/Kg to -68 J/Kg.
• Results in an environment that is elevated.
• As a result, a modified hodograph needs to be
  used.

21
Environmental Data

• Unmodified hodograph with derived parameters.
• V(obs) SRH values are positive.
• Storm motion 255 _at_ 21 kts.

22
Environmental Data

• Modified hodograph
• with derived parameters
• using an elevated depth of
• 1 km and actual storm
• motion of 200 _at_ 29 kts.
• V(obs) SRH values have
• become negative.

23
Case Summary

• Two storms occurred in the same county and within
  several volume scans of each other that exhibited
  unusual radar characteristics including
• Persistent and strengthening low-level (0.5)
  rotation. Cycolnic with KICT and anticyclonic
  with KVNX.
• The rotation (from KICT and KVNX) is displaced 3
  miles north-northeast of reflectivity.
• KICT indicates a transition from low-level
  cyclonic rotation to a mid/upper-level
  mesoanticyclone.

24
Case Summary

• Model data indicated a surface based environment
  when the near storm environment was actually
  elevated.
• Model forecast storm motion was not correct.
• Modified hodograph, taking into account the
  elevated nature and actual storm motion, reveals
  an environment favorable for left-moving,
  mesoanticyclonic supercells.

25
Acknowledgements

• Matt Bunkers, SOO, WFO Rapid City
• For his help and expert opinions as well as his
  hodograph spreadsheet.
• Ken Cook, SOO, WFO Wichita