Transcript 2.1 - DeepConvection.com

A Case Study of Two Left-Moving
Mesoanticyclonic Supercells on 24 April 2006
Chris Bowman
National Weather Service – Wichita, KS
The Situation…
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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.
RUC Model 22Z Forecast Sounding
22Z RUC Forecast Hodograph
KICT Reflectivity and SRM 0.5° Evolution
21:38Z – 22:13Z Animation
KICT Reflectivity and SRM All-Tilts
22:13Z 0.5° -19.5° Animation
KVNX Reflectivity and SRM 0.5° Evolution
21:37Z – 22:11Z Animation
KVNX Reflectivity and SRM 0.5° Evolution
22:16Z – 22:35Z Animation
KICT and KVNX Summary
Observed Items of Interest
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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).
What’s the decision?
Tornado? Severe? No warning?
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You’re 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.
What Actually Happened
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Tornado warning issued at 22:15Z 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.
KICT and KVNX Summary
The Unexplained
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Although the environment favored left-moving,
mesoanticyclonic storms (more on this later), there is
no clear explanation for KICT’s 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.
Storm #2
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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.
KVNX Storm #2 0.5° Reflectivity and SRM
21:27Z – 21:51Z Animation
Storms #1 and #2 Summary
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Two storms in the same county within several
volume scans of each other both develop lowlevel 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.
Environmental Data
Environmental Data
Environmental Data
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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.
Environmental Data
Environmental Data
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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.
Environmental Data
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Unmodified hodograph
with derived parameters.
V(obs) SRH values are
positive.
Storm motion = 255 @ 21
kts.
Environmental Data
Modified hodograph
with derived parameters
using an elevated depth of
1 km and actual storm
motion of 200 @ 29 kts.
 V(obs) SRH values have
become negative.
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Case Summary
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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.
Case Summary
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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.
Acknowledgements
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Matt Bunkers, SOO, WFO Rapid City
For his help and expert opinions as well as his
hodograph spreadsheet.
Ken Cook, SOO, WFO Wichita