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Storm Development in an
Unfavorable Environment
SR/SSD 98-12
4-1-98
Technical Attachment
Mark Cunningham
National Weather Service
Jackson, Mississippi
Peter Wolf
National Weather Service
Wichita, Kansas
1. Introduction
Severe thunderstorms developed across northwest Mississippi
during the mid-afternoon hours of May 14, 1997. The thunderstorms
moved east-southeast across north-central Mississippi during the late
afternoon hours, while other strong storms developed over
western Mississippi. With the exception of one storm, the WSR-88D radar at
Jackson, Mississippi, showed no supercellular
characteristics with the thunderstorms that developed on this
day. The exception developed over Holmes county and tracked southeast
across Holmes, Attala and Leake counties in north-
central Mississippi. This storm produced golfball size hail,
damaging straight-line winds, and a short-lived F1 intensity tornado as it
moved across eastern Holmes county and western Attala county. The pre-
storm environment appeared to be
somewhat favorable for severe thunderstorms, but unfavorable
for supercellular convection. This paper will suggest, using WSR-88D
imagery, how storm-scale environmental changes in the
vicinity of an apparent outflow boundary led to unexpected supercell
formation.
2. Pre-storm environment
Then pre-storm airmass over Mississippi appeared to be only marginally
unstable. May 15, 1997, 0000 UTC Jackson sounding showed convective
available potential energy (CAPE) values below 1000 JKg-1 for surface-based
parcels. However, modifying the sounding (Fig. 1a) using slightly higher
surface dew point values of 60 to 65 degrees pooled just ahead of an
approaching surface pre-frontal trough showed CAPE values of 1300 to 1500
JKg-1 for surface-based parcels.
The Jackson hodograph (Fig. 1b) showed moderate speed shear but very
little directional shear in the lowest 0-3km layer. The 0-3km storm-
relative helicity (SRH) varied from near 60 m2s-2 using the average storm
motion vector observed on May 14, to around 110 m2s-2 (Fig. 2a) using the
motion vector for the lone
supercell detected. These values are below the typical threshold of 150
m2s-2 for mesocyclone formation (Davies-Jones and Burgess, 1990).
Furthermore, the 0-6km shear vector, considered a better indicator of
supercell potential than storm-relative helicity (Weisman 1996), was about
22 ms-1. This met the typical threshold of 20 ms-1 but was likely
insufficient given the low instability. An environment with insufficient
shear, when coupled with high instability can support supercells (Johns and
Doswell 1992). The environment in the May 14 case exhibited low values of
both shear and instability, suggesting weak updraft rotation would be much
more likely, then deep persistent mesocyclones.
The pre-storm environment showed no substantial synoptic or meso-scale
features over Mississippi to focus severe thunderstorm development, as
illustrated by the May 15, 1997 0000 UTC composite chart (Fig. 3). The
upper jet pattern, and the location of a upper-level trough well north of
Mississippi, suggested little in the way of synoptic-scale vertical motion
over the state during the afternoon of May 14. The only apparent focus for
thunderstorm development was a weak surface pre-frontal trough over
northwest Mississippi and northeast Louisiana during the afternoon.
3. Supercell development
Reflectivity products from the WSR-88D radar in Jackson, Mississippi,
showed the development of a multicellular band of convection over
Sunflower, Leflore, Carroll and Holmes counties through 2130 UTC (Fig. 4a).
The thunderstorms rapidly became severe within 30 minutes, producing large
hail and strong straight-line winds. Between 2130 and 2150 UTC, the
westernmost storm in the band became quite large, with maximum reflectivity
values greater than 65 dBZ existing through a depth of 5-6 km, and maximum
reflectivity values in the 72 to 74 dBZ range.
By 2155 UTC, the Jackson WSR-88D indicated supercell
characteristics associated with the large thunderstorm, including
a bounded weak echo region (Fig. 5) and the development of a moderate low-
level mesocyclone (Fig. 6) with rotational velocities around 15 ms-1 up to
a height of 4 km above ground level (AGL). Between 2155 and 2215 UTC, a
hook-like feature developed in the reflectivity pattern (Figs 4b-4c), and
the low-level mesocyclone extended upward to a height around 8 km AGL (Fig
6). The supercell produced hail up to 50 mm (2 inches) in diameter,
damaging straight-line winds, and a short-lived F1 intensity tornado as it
moved across eastern Holmes County and western Attala County. The
supercell gradually weakened after 2230 UTC, though it continued to produce
severe weather through 2330 UTC.
4. Role of storm-scale environmental changes
The pre-storm environment revealed CAPE values below 1500
J/Kg, and storm-relative helicity values well below 150 m2s-2,
characteristics not typically favorable for supercell thunderstorms.
Since, one of the thunderstorms that developed over north-central
Mississippi on May 14, 1997, did exhibit supercell characteristics,
including a bounded weak-echo region
and a deep, persistent mesocyclone. The modification to the pre-storm
environment likely resulted in the transition of a multicell storm to a
supercell. WSR-88D imagery suggested the formation of this supercell was
likely the result of storm-scale changes to the pre-storm environment in
the vicinity of this storm, not the result of the pre-storm environment
itself.
The supercell developed on the western edge of a band of
multicellular convection over north-central Mississippi (Figs
4a-4c). An apparent outflow boundary produced by this band of
thunderstorms may have played an important role in supercell development.
As the westernmost storm in the band moved southeast along this boundary
(Fig. 4b), strong horizontal streamwise vorticity produced by the boundary
was apparently ingested into the storm's updraft, leading to the
development of a strong low-level mesocyclone with rotational velocities of
15-20 ms-1. The mesocyclone developed below 3km AGL around 2150 UTC, and
extended upward into the middle portion of the storm (around 8 km AGL) by
2205 UTC (Fig. 6). As this occurred, a hook-like feature developed in the
low-level reflectivity pattern (Fig. 4c). Other studies (e.g. Moller et al
1990) have documented the important role of low-level boundaries in low-
level mesocyclone development. Such a boundary would have likely provided
the strong low-level horizontal streamwise vorticity necessary for low-
level mesocyclone generation in the case presented here.
Fig. 2a is the actual hodograph for Jackson, Mississippi, at May 15,
1997, 0000 UTC using the storm motion vector for the supercell. Fig. 2b is
the hodograph modified for estimated changes on the cool-side of the
apparent outflow boundary being intercepted by the intensifying cell.
Assuming low-level easterly inflow on the cool-side of the outflow
boundary, the modified hodograph reveals a storm-relative helicity value of
about 250 m2s-2. and a 0-6km shear vector of 30 ms-1. In addition, the
modified hodograph produces a 0-1 km horizontal streamwise vorticity value
of about 32 s-1, much greater in magnitude than the unmodified hodograph
value of about 5s-1. The storm-scale environmental changes in the vicinity
of the apparent outflow boundary seems to have made low-level mesocyclone
development much more likely.
The thunderstorm gradually lost its supercellular characteristics
after 2230 UTC, likely the result of the apparent outflow boundary moving
well to the south of the storm (Fig. 4d). As the apparent boundary was
moving southward, low-level rotation within the updraft of the supercell
weakened from nearly 20 ms-1 to around 10 ms-1 in the lowest 3 km by 2217
UTC (Fig. 6). At the same time, the mid-level rotation remained in the 15
to 20 ms-1 range. The entire mesocyclone quickly dissipated after 2230
UTC.
5. Conclusion
Supercell thunderstorm development is obviously not just a function of
the pre-storm environment given by local soundings. Changes to such
environments, which can be substantial especially on the storm-scale, can
alter the convective mode. On May 14, 1997, a non-supercell thunderstorm
became supercellular as it tracked southeast along an apparent outflow
boundary produced by
organized convection just to the east. As this cell tracked along the
boundary, strong horizontal streamwise vorticity was ingested into the
updraft, leading to the formation of a strong low-level mesocyclone.
Knowledge of the pre-storm environment, and severe weather threat, is
important for successful warning operations. Equally important is the
knowledge of environmental changes, even on the storm-scale level, that may
be occuring and how such changes may
alter the storm type and primary severe weather threat. Without detailed
data such as mesonet observations, radar operators must attempt to
determine such changes from other available data, including satellite, wind
profiler, and WSR-88D data.
6. References
- Davies-Jones, R. and D.W. Burgess, 1990: Test of helicity as
a tornado forecast parameter. Preprints, 16th Conf.
Severe Local Storms, Kananaskis Park, Alberta, Canada,
Amer. Meteor. Soc., 588-592.
- Johns, R.H. and C.A. Doswell III, 1992: Severe local storms
forecasting. Wea. Forecasting, 7, 588-612.
- Moller, A.R., C.A. Doswell III and R.W. Przbylinski, 1990:
High-precipitation supercells: a conceptual model and
documentation. Preprints, 16th Conf. Severe Local
Storms, Kananaskis Park, Alberta, Canada, Amer. Meteor.
Soc., 52-57.
- Weisman, M.L., 1996: On the use of vertical wind shear
versus helicity in interpreting supercell dynamics.
Preprints, 18th Conf. Severe Local Storms, San Francisco
California, Amer. Meteor. Soc., 200-204.
Figures
- The May 15, 1997, 0000 UTC Jackson, Mississippi, modified sounding (a) and
hodograph (b). The arrow points to the equilibrium level on the sounding
and to the storm motion vector on the hodograph.
- The May 15, 1997, 0000 UTC Jackson, hodograph modified using the observed
supercell motion (a), and modified for expected conditions on the
cool-side of an apparent outflow boundary (b). The arrow points to
the storm motion vector.
- The May 15, 1997, 0000 UTC synoptic composite chart. Large, thick arrows
represent upper-level jets; the large thin arrow represents the low-level
jet; small arrows indicate surface flow; surface fronts, 500 mb heights
(thin contours), and the 16 degree Celsius surface dew point contour (gray)
are also shown.
- The 0.5 degree base reflectivity product at 2131 UTC (a), 2152 UTC (b),
2217 UTC (c) and 2232 UTC (d). rear-flank downdrafts (RFD), hook echos
(HOOK), and apparent outflow boundaries are labelled.
- Reflectivity cross-section through the supercell at 2152 UTC showing
bounded weak echo region.
- Time-height series plot of rotational velocity (in ms-1) for the supercell.
A thin 14 ms-1 contour is drawn. Bold numbers indicate rotational velocities
greater than 15 ms-1, while "<" represents rotational velocities less than
9 ms-1.
- Acknowledgements
The authors would like to thank Russell Pfost, science and operation
officer and Alan Gerard for reviewing this paper.
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