Preprints, 18th Conference on Severe Local Storms
San Francisco, CA
February 19-23, 1996
American Meteorological Society, Boston, Mass.
DETAILED SURFACE OBSERVATIONS DURING THE LAHOMA HAIL AND WINDSTORM
Dale A. Morris and Mark A. Shafer
Oklahoma Climatological Survey
University of Oklahoma
Norman, Oklahoma
1. INTRODUCTION
On August 17, 1994, extreme wind and hail
damage occurred in north-central Oklahoma from a
weakening, complex of thunderstorms that moved from
southern Kansas into a thermodynamically favorable
environment in western Oklahoma. The Oklahoma
Mesonetwork (Mesonet; Brock et al. 1995) is an
automated environmental monitoring network with at
least one station in each of Oklahoma's 77 counties. By
early afternoon on 17 August, it measured surface
temperatures between 35 ° C and 40 ° C and surface dew
points between 18 ° C and 21 ° C in western Oklahoma
ahead of the storm complex (Morris and Janish 1996; see
their Fig. 1). This rich environment allowed the storm to
reintensify to extraordinary limits as it passed directly
over the Mesonet site located 1 mile west-southwest of
Lahoma in Garfield County.
At the Lahoma site, the propeller-vane wind
monitor at 10 m above the surface recorded sustained
wind speeds of 34.6 m/s between 1945 and 1950 UTC,
and peak gusts exceeding 50 m/s between 1945 and 1950
and again between 1950 and 1955 (see Fig. 3). In
addition, a cup anemometer at 2 m measured sustained
winds over 25 m/s between 1945 and 1955 before wind-
driven golfball to tennis ball sized hail destroyed the wind
instruments. Eyewitnesses described some of the hail as
chunks of ice measuring 10 cm by 15 cm, and reported
hail depths of up to 15 cm. A Mesonet technician who
visited the Lahoma site after the storm described the
landscape as being "very winterlike because the hail
stripped every leaf off the trees in a 5 mile area." Most
structures had paint stripped off the north and east sides
of the buildings and many had broken windows. Two
people were treated for hypothermia after hail smashed
the windows and accumulated inside their car. Other
witnesses compared the approaching storm to the Dust
Bowl of the 1930's as they saw a large, dark cloud
approaching from the north (Les Lemon, personal
communication).
The original design of the Oklahoma
Mesonetwork (Elliott et al. 1992) called for
telecommunicating three sets of five-minute observations
every fifteen minutes from the remote sites to the base
station in Norman. However, to monitor an annular solar
eclipse that passed over Oklahoma on 10 May 1994,
modifications were made to the remote datalogger
programs to allow one-minute observations to be recorded
and transmitted (McPherson 1995). Because of the data
storage in the dataloggers, and because of the two-way
nature of the radio communications via the Oklahoma
Law Enforcement Telecommunications System (OLETS;
Crawford and Long 1993), the capability exists to retrieve
one-minute observations from a Mesonet site up to six
hours after an event. Mesonet staff exercised this
capability after the Lahoma storm and retrieved one-minute readings of air temperature at 1.5 m above the
surface (TAIR), solar radiation (SRAD), pressure (PRES),
and wind speed (WSPD) and direction (WDIR) at 10 m
from several sites in north central Oklahoma (Fig. 1).
Due to a power outage at the Kingfisher County Sheriff's
Office, one-minute data from the KING site were
unavailable; the five-minute observations were retrieved,
however. This paper presents a summary and an analysis
of this unique set of detailed surface observations.
Figure 1
Figure 1. Locations of the Mesonet sites used in this study.
Every site except KING reported one-minute data. Federal
highways are included for geographic reference. The Lamont ARM
site is also indicated.
2. LAHOMA TIME SERIES
Figure 2 presents one-minute average readings
of TAIR, PRES, WSPD, and WDIR at the Lahoma
Mesonet site (LAHO) from 1930 through 2030 UTC.
Five-minute averages of several parameters that were not
included in the one-minute datalogger program are
displayed in Figure 3. From these data a timeline of
events that occurred at and/or near the Lahoma site can be
constructed.
Figure 2
Figure 2. Time series plot of one-minute resolution data
from the Lahoma Mesonet site. Top: air temperature at 1.5 m (TAIR; heavy line)
in °C and station pressure (PRES;light line) in mb. Bottom: wind speed
at 10 m (WSPD; heavy line) in m/s and wind direction at 10 m (WDIR;diamonds).
The readings after 1954 occurred after the wind sensors were damaged.
Figure 3
Figure 3. Time series plot of five-minute resolution data from the
Lahoma Mesonet site. Top: dew point (TDEW; heavy line) in °C and
accumulated rainfall (RAIN; light line) in mm. Bottom: peak gust at 10 m (WMAX;
lightest line), average wind speed at 2 m(WS2M; medium line), average wind
speed at 10 m (WSPD; heavy line), and wind direction at 10 m (diamonds). All
wind speeds are in m/s.
Based upon the traces in Figure 2, the gust front
from the Lahoma storm passed the LAHO site at
approximately 1942 UTC. At this time, the pressure
began to fall rapidly, the wind shifted to the northeast, and
the temperature began to decrease. It should be noted that
the datalogger incorrectly reported several of the one-
minute average wind directions when the samples
included directions from both west of north and east of
north. The five-minute averages in Figure 3, however,
were correctly calculated. Additional evidence that
supports the gust frontal passage at 1942 includes the
facts that rain began to fall between 1945 and 1950
(Fig. 3) and that a decrease of 9° C 9-m air temperature
occurred between 1940 and 1945 (not shown).
Lemon and Parker (1996) documented a series of
mesocylones occurring in the Lahoma storm from an
analysis of Level II WSR-88D data from the Twin Lakes
radar. They identified a mesocyclone first appearing
southeast of Lahoma at 1944. This mesocyclone may be
partially responsible for the pressure fall ending at 1947.
The large wind speed maxima at 1946, 1949, and
1952 likely resulted from an intense downburst that may
have coincided with the onset of precipitation. The
heaviest precipitation occurred between 1950 and 1955.
Each of the three peaks in the one-minute average wind
speeds were followed by the expected pressure rises.
These three peaks may indicate that there were three
separate downbursts, or that one downburst actually thrust
three separate pulses of air toward the surface. At any
rate, eyewitness reports and the damage swath indicate
that the strong winds first came from the north and
subsequently from an easterly direction -- facts that are
corroborated by Mesonet observations. The direction
associated with the third downburst or downburst pulse
suggests that the downburst was located to the east or
northeast of the LAHO site. The large hail that
accompanied what may have been the final gust of wind
rendered both the 10-m wind monitor and 2-m wind
sentry useless. The large gust of wind also wrapped the
cables connecting the wind monitor and the 9-m
thermistor to the datalogger around the wind monitor.
In an investigation of the Plainfield, Illinois,
tornado, Dr. T. Theodore Fujita found the occurrence of
multiple microbursts prior to the formation of the tornado
(Storm Data 1990). Although the Lahoma storm did not
produce a significant tornado, the downbursts appeared
within minutes of the developing mesocyclone southeast
of Lahoma.
One of the more puzzling aspects of the one-minute averages is the sudden temperature decrease to
5.9 ° C at 1952. Several hypotheses to explain this
decrease have been investigated. The first hypothesis is
that the cool temperatures were associated with the
downburst. If this were the case, then a logical question
to ask is from what altitude did the cold air descend. To
answer this, two special soundings launched at nearby
Lamont by the ARM (Atmospheric Radiation
Measurement; Stokes and Schwartz 1994) program were
examined. The equivalent potential temperature of the
surface parcel was calculated, but no match could be
found in either of two soundings (one prior to and one just
after the Lahoma event). A portion of the 5.9 ° C reading
may be attributed to the downburst, but it is unlikely that
the downburst is the only mechanism responsible.
Another hypothesis is the "wetting effect" of
cold water on a radiation shield. Since there was
precipitation and large hail at the time of the 5.9 ° C
reading, the radiation shield that enclosed the temperature
and relative humidity (T&RH) sensor was drenched with
water having a temperature near freezing. Our theory is
that the 30 to 50 m/s wind aided the evaporation of the
cold water thereby biasing the temperature inside the
shield, making it cooler than ambient. The relative
humidity (a five-minute average) during this time period
ranged from 83% to 92%. An experiment was staged to
test the feasibility of this hypothesis. Two sets of sensors,
like those used at 1.5 m and 9 m, were paired and housed
in identical radiation shields. Ice water was poured over
one of the shields for 90 seconds during warm (26 ° C) and
relatively windy (10 m/s gusting to 14 m/s) but dry
(30% relative humidity) conditions. Figure 4 shows the
results from this simple experiment. It indicates that
the 5.9 ° C Lahoma temperature reading probably did not
result from this mechanism, because the 1.5 m sensor was
incapable of responding quickly to a step function input
of this magnitude. Additionally, the test was performed
in conditions more favorable for evaporation (hence more
cooling) than during the Lahoma event. Figure 4 also
indicates that significant temperature error can result
during precipitation if ambient conditions are favorable
for evaporation.
Figure 4
Figure 4. Results of cold-water test. The water was poured over
the test radiation shield for 90 seconds beginning at minute 15. The 1.5 m
T&RH sensor is larger than the 9 m sensor so it has a larger thermal mass and
a slower response.
The final theory is that the high winds at the
LAHO site may have caused a temporary loose
connection in the cabling of the T&RH sensor. When the
datalogger receives no input voltage from a thermistor, it
assumes the temperature to be -273.1 ° C. Even though the
technician who inspected the site the following day found
no problems with the T&RH sensor, one 3-second sample
of -273.1 ° C could transform a one-minute average
reading of 20 ° C to about 5 ° C. Thus a loose connection
is likely responsible not only for the 5.9 ° C temperature,
but also for the 5-minute average 10 ° C dewpoint at 1955
(Fig. 3) because the 5-minute average temperature from
1950 to 1955 was reported as 11.9 ° C. This point
underscores the importance and difficulty of quality
assurance routines for automated networks (Shafer and
Hughes 1996) and the repercussions of blindly accepting
a report from any meteorological instrument.
3. OBJECTIVE ANALYSIS
In order to place the events that occurred at the LAHO
site into context with conditions at neighboring sites, an
objective analysis of the one-minute data was performed. Since
one-minute observations from the KING site were unavailable,
its five-minute readings were interpolated to one-minute
intervals. A two-pass Barnes (1973) analysis using a time-to-space conversion
technique was performed on a grid having a 3 km mesh
(Fig. 5). The time-to-space conversion used a time
window of ± 5 minutes. The translation velocity of 19.5
m/s from 341° was determined subjectively using NIDS
(NEXRAD Information Dissemination Service) images
from the Twin Lakes (KTLX) and Vance AFB (KVNX)
radars.
Figure 5
Figure 5. Analysis grid for the objective analysis. Hash marks
indicate results of time-to-space conversion.
The Barnes technique with time-to-space conversion
is not necessarily well-suited for a quantitative analysis of
this particular situation. In this case, a quasi-steady state
assumption is not valid. Moreover, the observations are
clustered spatially. Barnes (1994) showed that the
scheme has better sampling properties when stations are
uniformly arrayed. Nevertheless, the technique is useful
for qualitative visualization.
An isochrone analysis of the 150 W/m² SRAD
isopleth is presented in Figure 6. The isochrones indicate
that the cloud shield and dust associated with the Lahoma
storm pushed westward until 2025 UTC when it began to
move toward the southeast. During this time, the storm
itself moved southeastward. The early westward motion
of the SRAD isopleths is supported by the eyewitness
reports and by the motion of the outflow boundary that
appeared on the KVNX imagery.
Figure 6
Figure 6. Five-minute ischrones of the 150 W/m² SRAD isopleth.
The pressure, temperature, and wind analyses are
given in Figures 7a-7f. Pre-storm conditions are
represented by the relatively low pressures over LAHO, a weak
temperature gradient in the southern two-thirds of the
domain, and predominately southerly flow at 1930 UTC (Fig,
7a).
Figure 7a
Isobars (heavy lines; 1 mb interval),
isotherms (light lines; 2 ° C interval) and wind vectors for 1930 UTC.
By 1942 (Fig. 7b), the time of the passage of the gust front at the
LAHO site, a mesohigh can be detected in the northeast portion
of the domain, and cooler temperatures and a predominate northeasterly flow are apparent in the northern half of the grid.
Figure 7b
Isobars (heavy lines; 1 mb interval),
isotherms (light lines; 2 ° C interval) and wind vectors for 1942 UTC.
At the time of the first downburst (1946; Fig. 7c), stronger winds
are located in the central portion of the grid, the
temperatures have continued to decrease, but the pressure
pattern remains similar to that observed four minutes
earlier.
Figure 7c
Isobars (heavy lines; 1 mb interval),
isotherms (light lines; 2 ° C interval) and wind vectors for 1946 UTC.
At 1955 (Fig. 7d), the time of the third downburst
pulse, the winds have an easterly component in Garfield,
western Alfalfa, and western Major counties. Surface
pressures continue to increase due to the downburst which
causes strengthening of the mesohigh.
Figure 7d
Isobars (heavy lines; 1 mb interval),
isotherms (light lines; 2 ° C interval) and wind vectors for 1955 UTC.
The spatial
patterns shown in Figure 7d remain relatively constant,
but varying in magnitude, until 2021 when pressures
begin to fall in Kingfisher County as the storm
approaches Kingfisher (Fig. 7e).
Figure 7e
Isobars (heavy lines; 1 mb interval),
isotherms (light lines; 2 ° C interval) and wind vectors for 2021 UTC.
Animated loops of the
analysis indicate that the interpolation of Kingfisher five-
minute resolution data to one-minute intervals seems
artificially to slow the storm's southward motion. The
spatial patterns likely changed before 2021, but without
the KING one-minute data, it is impossible to gauge when
this occurs. Figure 7f reveals that the storm created a
mesohigh which extended south to Kingfisher and
produced easterly flow throughout the domain. This
easterly flow regime agrees with the composite wind field
averaged over the storm's history as produced by Morris
and Janish (1996). Figure 7f also shows that the area
around Lahoma had a residual effect of lingering cool
temperatures. The time-series plot of the LAHO
temperatures (Fig. 2) indicates that temperatures remained
slightly above 15 ° C while other locations (e.g.,
Kingfisher and El Reno) affected by the same storm (also
with hail) had residual temperatures at least 5 to 10 ° C
warmer than the Lahoma temperatures. While it cannot
be proven with certainty, the latter locations likely were
not influenced by a downburst and the cool temperatures
it may have caused.
Figure 7f
Isobars (heavy lines; 1 mb interval),
isotherms (light lines; 2 ° C interval) and wind vectors for 2044 UTC.
4. SUMMARY
One-minute observations recorded by the Oklahoma
Mesonetwork during the Lahoma hail and wind storm
have been presented. The increased resolution of the
data, compared to the five-minute averages normally
obtained from the Mesonet, allowed the authors to
conclude at least three separate downbursts occurred near
the Lahoma site, each separated by a few minutes. The
downbursts caused abnormally high pressure readings,
and cool temperatures that lingered after the storm, than
observed at other nearby locations. The Mesonet
observations presented in this paper also are consistent
with eyewitness reports of the storm.
5. ACKNOWLEDGEMENTS
The authors would like to dedicate this paper to the memory of
David Shellberg, who was instrumental in collecting the one-minute
data. Bill Wyatt provided information about the Lahoma Mesonet site,
Fred Brock and Sherman Frederickson provided stimulating
conversation about the instrumentation, and Ken Crawford provided
helpful comments. Finally, David Grimsley and Jerry Brotzge assisted
with the radiation shield experiment.
6. REFERENCES
Barnes, S.L, 1973: Mesoscale objective map analysis using weighted
time-series observations. NOAA Tech. Memo. ERL NSSL-62,
National Severe Storms Laboratory, Norman, OK, 60pp.
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randomness. J. Atmospheric Oceanic Technology , 11, 1433-1448.
Brock, F.V., K.C. Crawford, R.L. Elliott, G.W. Cuperus, S.J. Stadler,
H.L. Johnson, and M.D. Eilts, 1995: The Oklahoma Mesonet: A
technical overview. J. Atmospheric Oceanic Technology, 12, 5-19.
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Department of Commerce. Storm Data. National Climatic Data Center,
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Elliott, R.L., F.V. Brock, M.L. Stone, and S.L. Harp, 1992: Configuring
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No. 922149, International Summer Meeting; Amer. Soc. of Agricultural
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Lemon, L.R. and S. Parker 1996: The Lahoma storm deep convergence
zone: its characteristics and role in storm dynamics and severity.
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eclipse. 4th Conf. on Education, Amer. Meteor. Soc., 105-110.
Morris, D.A., and P.R. Janish, 1996: The utility of mesoscale versus
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windstorm of 17 August 1994. 18th Conf. on Severe Local Storms,
Amer. Meteor. Soc.
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of data from the Oklahoma Mesonetwork. Preprints, 12th International
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design of the Cloud and Radiation Test Bed. Bull. Amer. Meteor.
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Corresponding Author's Address:
Dale A. Morris
Oklahoma Climatological Survey
University of Oklahoma
100 E. Boyd, Suite 1210
Norman, OK 73019
e-mail: dmorris@uoknor.edu