1. Introduction
During the afternoon and evening of 10 May 2002 three supercell
thunderstorms produced destructive hail across Pecos County,
Texas (figure
#1). Through the event’s duration (6 hours) frequent
observations of potential tornadic activity were inferred from
local warning radar imagery, however, only one brief F0 tornado
was observed.
The advantages gained through maintaining situational
awareness (SA), defined as "the ability to perceive data,
comprehend its meaning, and project relevant outcomes"
(Endsley 1995), during severe weather warning operations have
recently been emphasized in National Weather Service (NWS) training
(WDTB 2000-2003). High levels of SA enable forecasters to make
enhanced warning decisions.
This paper examines the Pecos County hail storms
from a warning decision perspective. Radar animations and corresponding
video from storm spotters will be utilized to present four critical
warning decisions which were enhanced through factors contributing
to high levels of SA.
2. Event Overview and Public Impact
The 10 May 2002 hail storms had a significant economic impact.
Despite the sparsely populated nature of the region, monetary
losses exceeded three million dollars (NOAA 2002). Most of the
damage was agricultural. Coyanosa, a small community in northern
Pecos County, received 7.0 cm (baseball) hail during a 40 minute
period. Melon, onion, and pecan crops were destroyed.
Property damage also occurred in the city of Fort Stockton.
The most significant damage was across the city’s north
side, where 11.0 cm (softball) hail was reported. Damage consisted
of broken windows, damaged roofs and vinyl siding, and damaged
vehicles (figure
#2). Similar losses occurred east of the city along Interstate
10.
3. Pre-Storm Environment
Bunting and Vescio (2002) suggest warning decisions are enhanced
through knowledge of the large scale environment. On 10 May
2002, the meso and synoptic scale environments were dominated
by a moist and unstable boundary layer over the Southern Plains,
and moderate southwest flow aloft. A dryline was situated along
the lee of the mountains in eastern New Mexico and West Texas.
Surface conditions were uniform across most of West Texas with
temperatures ranging from 29°C to 32°C, and dewpoints
were generally near 18°C. Southeast surface winds of 10
to 20 kt with higher gusts aided to provide ample deep layer
shear for storm organization (figure
#3).
The integration of near storm environment (NSE) data is also
an important aspect of effective warning decision making (WDM)
(Wolf and Grant 2001). The recognition of patterns known to
produce specific types of severe weather can heighten forecasters’
awareness, helping them evaluate potential threats thereby making
the best possible warning decisions. Analysis of surface observations
and LAPS data revealed the complexity of the NSE across Pecos
County. Channeled low level flow advected higher theta-e air
northwest through the Pecos River Valley. A narrow axis of higher
instability resulted across the eastern parts of the county
(figure
#4). In addition, these locally backed winds combined with
veered flow over the higher terrain to the west to produce an
area of persistent convergence near the Reeves/Pecos County
border, where the three supercells discussed in this paper initiated.
Given the proximity of the localized theta-e maximum to the
zone of low level convergence, the Pecos County area appeared
to be the most favored area across the NWSFO Midland County
Warning Area (CWA) for significant severe weather.
Recent studies identify multiple low level thermodynamic parameters
as important discriminators between tornadic and nontornadic
supercell environments (Rasmussen and Blanchard 1998, Edwards
and Thompson 2000, Craven et al. 2002, Davies 2002). Lower lifting
condensation level heights (LCL’s) and surface dewpoint
depressions correspond to increased low level relative humidity
and buoyancy, increasing tornado potential (Markowski et al.
2002). In addition, level of free convection height (LFC), convective
inhibition (CIN), and 0-3 km CAPE also have significant relevance.
Although forecast soundings were used during the
event, the observed 0000 11 May (all times UTC) sounding from
Midland is utilized as a proximity sounding in this study (figure
#5). Due to the similarity of surface conditions at Fort
Stockton and Midland, no modifications to the sounding were
performed.
Recognizing adequate shear and instability for supercells, forecasters
assessed the tornado threat using techniques referenced above.
The sounding depicted surface dewpoint depressions of 16°C,
along with a LCL height of 1,652 m and a LFC height of 2,887
m. The values of these low level thermodynamic parameters fell
in ranges characteristic of non-tornadic supercell environments.
Specifically, the LCL height (1,652 m) was in excess of the
third quartile for non-tornadic supercells as determined by
Edwards and Thompson (2000). Despite the uniformity of surface
conditions, no assumptions were made concerning thermodynamic
properties above the LCL. The examination of these parameters
helped forecasters build a conceptual model of anticipated storm
type and threats based on these sounding characteristics. This
included high based and nontornadic supercells with large hail
and damaging winds the anticipated threats.
Of equal importance to the WDM process is the
knowledge of storm scale environmental changes which may alter
storm type and threats (Cunningham and Wolf 1998). On this day,
forecasters were able to anticipate modifications to the NSE
through analysis of MesoEta forecast soundings and LCL heights.
The model suggested a trend of decreased LCL’s (-640 m)
between 2100 and 0000 (figure
#6). This trend signaled a potentially increased risk of
isolated weak tornadoes if supercellular convection could be
sustained.
4. Sectorization and Summary of Warnings
The consideration of both meteorological and non-meteorological
factors comprise WDM methodology (Quoetone and Huckabee 1995).
Although staffing is generally a human factors issue (Quoetone
and LaDue 2003), decisions regarding the necessary staffing levels
are highly dependent on the given meteorological situation. Staffing
and sectorization of warning responsibilities are critical aspects
of successful warning operations which improve efficiency and
warning strategies (Andra et al. 2002).
Based on radar trends and knowledge of the NSE, sectorization
of warning operations by both geographical area and threat type
was delegated as depicted in figure
#7. This strategy was found to be effective, and distributed
workloads in a manner which helped optimize SA.
A total of nine severe local storm warnings were issued for Pecos
County spanning 5 hours and 58 minutes. Warnings and statements
also were issued by NWSFO Midland for severe storms elsewhere
in the CWA (figure
#8). Due to the societal impacts and the remarkably high quality
of real-time spotter reports, the emphasis of this paper is restricted
to warning decisions regarding the storms in Pecos County. Pecos
County warning and statement distribution, along with verifying
reports are shown in figure
#9.
5. Warning Operations
The first of the three supercells (supercell #1) developed near
the Reeves/Pecos County border around 2100, and propagated northeast
toward Coyanosa (figure
#10). Large hail measuring 3.8 cm
(ping-pong ball) was first observed at 2150, 6.4 km south of Coyanosa.
As the storm approached Coyanosa, an increase in storm intensity
was noted in base reflectivity along the southwest flank, thus
forward propagation decreased. As a result, observed hail size
increased to 7.0 cm (baseball) near Coyanosa, and persisted from
2155 to 2230. With large hail producing the most immediate threat
to life and property, observations of supercell #1 supported the
forecasters’ conceptual model.
Supercell #2 developed near the same area as supercell #1 around
2210 (figure
#11). The storm propagated northeast and eventually merged
with supercell #1 near Coyanosa. Supercell #2 presented forecasters
with two instances which challenged their conceptual model, leading
to critical warning decisions. Golfball size hail (4.4 cm) also
was reported with this storm, but despite reflectivity and storm
relative motion (SRM) data to the contrary, no tornadoes were
observed by storm spotters.
Supercell #3 developed in western Pecos County around 0000 11
May. This storm moved east along and just north of Interstate
10 across central Pecos County through 0400 (figure
#12), and challenged warning forecasters with two additional
critical warning decisions. Modifications to the NSE throughout
the storm’s life span resulted in low level thermodynamics
which were becoming increasingly favorable for potential weak
tornado development. Despite near continuous radar indications
of tornado potential, only one very brief F0 tornado was observed.
a. Critical Warning Decision #1
At 2300, a store manager reported a funnel cloud south of Coyanosa.
The report correlated with strong rotational velocities (rotational
velocity is used instead of delta-v due to storm distance from
the radar) in 0.5° SRM data (2259 scan) (figure
#13). As with each of the four instances examined in this
study, the lowest elevation sampled by radar was well above the
expected LCL heights. In this case, continuous communication was
maintained with the store manager and enabled forecasters to confirm
the small and high based nature of the funnel.
Through knowledge of the pre-storm environment, including the
previous discussion on low level thermodynamic parameters, and
the manager’s reports that indicated a small and high based
funnel cloud, warning forecasters determined the likelihood of
tornadoes remained low. As a result the previously issued severe
thunderstorm warning (SVR) was not upgraded to a tornado warning
(TOR).
b. Critical Warning Decision #2
Between 0000 and 0025 radar imagery of supercell #2 depicted reflectivity
geometry which became increasingly suggestive of tornadic activity
(figure
#14). By 0015 the storm exhibited well defined hook echo and
inflow notch structures. The corresponding SRM data depicted moderate
but broad rotational velocities.
At that time, a storm spotter (this paper’s second author)
was traveling south through the storm’s core on Farm to
Market Road 1053 from Imperial to Fort Stockton, and reported
4.4 cm (golfball) hail 30 km north of Fort Stockton at 0018 (figure
#15). The spotter soon acquired a view of the updraft region,
and relayed to warning forecasters that the storm’s base
visually appeared high and somewhat disorganized. If warning decisions
were based on radar interpretation only, forecasters would have
been inclined and fully justified to issue a TOR. The introduction
of reliable spotter reports, and knowledge of the environment,
led to no upgrade to the ongoing SVR.
c. Critical Warning Decision #3
Radar interpretation continued to indicate the potential of tornadoes
as supercell #3 approached Fort Stockton between 0050 and 0120
(figure
#16). Supercell #3 also displayed multiple three body scatter
spikes (TBSS) (Lemon 1994) as it approached the city. The well
defined and pronounced nature of the TBSS prompted the issuance
of warnings and statements which highlighted the enhanced hail
threat. It is likely, based on spotter observations of large hail,
that TBSS was present with the previous storms, but was masked
by intervening reflectivity echoes. This was not confirmed with
the examination of spectrum width data.
Warning forecasters suspected modifications to the NSE were occurring
as supercell #3 moved east, close to the theta-e ridge. In addition,
the effects of previous storms likely contributed to these modifications
on scales not detectable through the use of radar or the area's
sparse surface observation network. Forecasters became concerned
that the previously developed conceptual model of storm type and
threats was becoming compromised as the low level thermodynamics
modified. It was believed this modification could slightly increase
the threat of weak tornadoes.
The aforementioned spotter obtained a vantage point on the northwest
side of Fort Stockton by 0100, and provided continuous ground
truth information as the storm approached the city (figure
#17). These reports indicated a well defined wall cloud, with
slow rotation. The spotter also indicated lower cloud bases relative
to the previous storm, but as the storm approached Fort Stockton,
a broadening cloud base circulation was observed. Based on the
spotter reports, and the relatively broad nature of the rotation
detected in SRM data, a TOR was not issued as the mesocyclone
passed over the city.
d. Critical Warning Decision #4
The spotter provided near continuous information as the storm
progressed east of Fort Stockton. At 0141, a funnel cloud was
observed just south of Interstate 10. The 0142 SRM data showed
the low level circulation had rapidly increased and tightened,
with 45 kt of gate-to-gate convergent maximum rotational velocities
(figure
#18). Based on the spotter report, increasingly favorable
signatures in SRM data, and modifications to the NSE, a TOR was
issued at 0145.
The 0147 0.5° SRM depicted greater than 45 kt maximum gate-to-gate
pure rotational velocities. Inbound velocities greater than 50
kt were observed. The spotter observed a dust whirl beneath a
well developed funnel cloud at 0149. The tornado persisted 10
seconds before it dissipated 18 km southeast of Fort Stockton.
The following scan depicted a divergent rotation associated with
the circulations occlusion, and the immediate tornado threat was
decreasing.
6. Conclusion
It is interesting to note that no reports of damaging thunderstorm
winds were received during the event or inferred from post storms
surveys. With a deep inverted-v boundary layer structure in observed
and forecasted soundings, the threat of damaging winds was included
in the forecasters’ conceptual model. Lemon (1994) noted
that observations of TBSS often correlate with damaging winds
and large hail. The authors offer no explanation for the lack
of observed severe winds associated with this case. In addition,
TBSS-induced contamination may have influenced the apparent mesocyclone
strength in a few of the images shown. These contributions, however,
were likely weak or slightly negative and the data is considered
representative.
Continuous ground truth from storm spotters, knowledge of the
pre-storm environment, and sectorized warning operations enabled
forecasters to maintain high levels of SA and enhanced the WDM
process during the 10 May 2002 hail event in Pecos County, Texas.
This SA enabled forecasters to recognize modifications to the
NSE prior to the occurrence of a brief tornado, and alter their
conceptual model which initially diagnosed the limited tornado
potential.
The timeliness, accuracy, and credibility of spotter reports
which generally agreed with forecasters’ pre-event conceptual
models and knowledge of the NSE allowed greater emphasis to be
placed on ground
truth information over other data inputs in the integrated WDM
process. Continuous communication with reliable storm spotters
complimented radar imagery and was essential to the WDM process.
In the absence of such high quality spotter reports and observations,
radar representation of the storms would have likely superceded
the forecasters' confidence in conceptual models of anticipated
storm structure and threats. This would have lead to the issuance
of multiple TOR's despite a less than conducive NSE for tornado
development.
It is believed that population density did not contribute to the
lack of observed tornadoes. This conclusion is supported by the
lack of tornado reports, despite excellent spotter coverage, and
by the confirmation of no resulting damage through post storm
surveys conducted by NWS staff and local authorities in both rural
areas and the city of Fort Stockton proper.
Acknowledgments:
The authors thank Steve Cobb of the NWSFO Lubbock, Jeffrey Cupo,
Eric Platt, Greg Jackson, and Pat Vesper all of the NWSFO Midland
for providing technical and editorial assistance. The authors
also thank Elizabeth Quoetone of the NWS WDTB for her input. Sharon
Tarbet also assisted in the editing process.
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