"Jarrell" Reprise
A Fresh Look at the Unusual Central
Texas Tornado Outbreak of May 27, 1997
Lon Curtis
KWTX-TV
Waco, Texas
and
Alan R. Moller
NOAA/NWS
Fort Worth, Texas
May, 1997 Southern
Plains Overview
>Only 48 tornadoes in the first 24 days of May… then … 98
tornadoes in three days, including:
>25th: 48 tornadoes, mainly in Kansas, southern Oklahoma, TX Red River
valley…
>26th: 26 tornadoes, mostly in eastern Oklahoma…
>27th: 24 tornadoes, central Texas…
1200UTC (7am CDT)
Synoptic Situation
>250mb low centered
near Scottbluff, NE
>Upper tropospheric
jet southwest to northeast over northern Oklahoma; Central Texas in right
entrance region …
>500mb low centered
in central Nebraska with trough axis southward to western Texas …
>850mb low over
southeast Kansas and 850mb cold front along TUL-ADM-ABI line
>Significant
cold advection at 850mb occurring over Texas Panhandle and western Oklahoma
north of the front …
Synoptic situation
... an atypical pattern
The NOAA Service
Assessment for this event called the synoptic situation "atypical" because
(1) tt occurred well south of stronger mid-level flow, (2) a strong low-level
jet (usually a feature in violent tornado events) was absent, and (3) there
was 'apparently' weak low- and mid-level winds and weak vertical wind shears
(an uncommon scenario for violent tornadoes).
Service Assessment
The Central Texas Tornadoes
of May 27, 1997
N.O.A.A. 1998
... but this "atypical
pattern" produced:
34 reports
of severe weather
12 reports of
large hail
24 reports of
tornadoes, including several significant tornadoes:
One F5, one
F4 & three F3 tornadoes
Why a "Jarrell"
Reprise?
Five years have passed since this unusual event. In the interim, there
have been many other strong/violent tornadoes and, unfortunately, many
lives have been lost as a result. Although there was an initial burst of
interest in examining this event, the insuing events (in particular, the
Oklahoma outbreak on May 3, 1999) resulted in redirection of interest away
from this event, and a tendency to view this case as an unusual, perhaps
inexplicable outlier. We think the case may not be such an outlier (i.e.
that there may be other cases where significant tornadoes have occurred
given similar atmospheric parameters). In a nutshell, our reasons for re-examining
the case are these:
>Unusually strong
tornado for apparently weak low- and mid-level upper flow
>Unusually deviant
storm and tornado motion
>Question: What
was the role of gravity wave in storm evolution and motion?
>Question: How
much of an outlier was this event?
The Gravity Wave:
Was it a necessary component?
Strong to severe thunderstorms developed over eastern Oklahoma on the evening
of May 26th. Those storms evolved into a large convective complex that
moved into western Arkansas during the overnight hours. Before sunrise,
the storm complex began to collapse, generating one or more gravity waves.
One wave propagated southwestward across northeastern Texas, reaching central
Texas before midday. More than one meteorologist investigating this
event proposed that the gravity wave was involved in either storm initiation
or storm motion. Among other issues, it was suggested that convective initiation
occurred very early in the day considering the capping inversion found
in the morning soundings. Corfidi (Preprints, 19th SLS, 1998) suggested
that the motion of the low-level pressure perturbation field associated
with the gravity wave favored new updraft development on the southwest
side of the cell … permitting the original updraft to replicate itself
southwestward in nearly continuous fashion. On the other hand, Purdom &
Motta (Appendix B, NOAA Service Assessment, 1997) examined satellite imagery
and concluded that the gravity wave may have played a role in the initiation
of the storm complex … (but) that gravity wave moved south of the storm
complex before the beginning of tornado activity. We will return to this
issue later.
It may be useful to briefly review some of the previous research by others
with respect to this event. One of the troubling issues arising from the
events of May 27th, 1997 was the origin of low- and mid-level vorticity
and subsequent rotation of the storm updrafts in what was perceived to
be an environment characterized by weak low-level flow and meager mid-level
flow.
Previous Research:
Sources of Vorticity
Magsig et al (16th W.A.F., 1998 and 19th S.L.S. 1998) examined
the various tornadoes generated by the storm complex and found that all
significant tornado damage occurred with three consecutive storms ... Lake
Belton, Jarrell and Pedernales Valley. (This nomenclature is based on the
location of the most significant tornado damage; there were other tornadoes.)
Their examination of the data resulted in a judgment that the origins of
mesocyclone and tornado rotation associated with the six well-sampled tornadoes
did not come directly from stretching of vertical velocity on the pre-existing
windshift … but from storm generated sources (tilting of baroclinically
generated horizontal vorticity and/or stretching of vertical vorticity
on storm generated gust fronts). (The pre-existing windshift boundary was
a north-northeast - south-southwest boundary that bisected the area from
near Waco to between Temple and Killeen, to west of Austin. It was readily
identifiable on high-resolution visible satellite imagery, in WSR-88D base
reflectivity data, and in surface observations.)
Not directly related to this event but helpful in understanding the question
of how the low-level vorticity may have been generated is research by Lee
et al (19th S.L.S., 1998) into what they referred
to as Non-Supercell Tornadoes*. Their conclusions were:
>Strong relationship
exists between environmental CAPE and misocyclone/NST evolution and intensity.
>Boundary layer
vertical shear was shown to be a critical factor in NST evolution …
>Simulations
indicate that ambient vertical shear within a range of ~80 to ~120 percent
of the optimal boundary layer vertical shear (of opposite sign to the cold
pool circulation) produced well defined families of tornadic strength vortices
...
_____________________________
*[We prefer the term non-mesocyclone tornado. The object of drawing the
distinction in the first place is to ascertain whether the tornado was
directly related to the mesocyclonic process or resulted from other processes,
including those described by Lee et al. Our “quarrel” is with the semantics,
not with the science presented by Lee et al.]
Previous Research:
Model Simulations
Wicker et al (e-published, 1998 TAMU Dept. of Meteorology &
CIAMS), utilizing the COMMAS model, simulated this event
by running two simulations using the observed surface conditions at Temple
and a modified composite sounding based on an M-CLASS sounding that was
launched near Calvert, about 55 miles east-southeast of Temple (TPL). In
one simulation, the conditions did not include the NNE-SSW boundary.
That simulation produced a single, intense updraft that evolved into a
storm that died within a hour as the cold pool generated by precipitation
choked the updraft. A second simulation with the NNE-SSW boundary produced
a persistent supercell-like "storm" that developed intense low-level
cyclonic circulations & repeatedly developed new updrafts on the southwest
side of the main complex with new updrafts being drawn into and merging
with the main complex.
[Later we will briefly discuss renewed efforts to simulate this event using
I-COMMAS, the successor program to COMMAS.]
Objective Analysis-1200UTC
To begin our reprise of this event, we used GEMPAK to objectively examine
a number of atmospheric fields beginning 36 hours prior to the event (i.e.
at 1200UTC on May 26th). The images below represent a small fraction of
those we have examined.
(Click
individual image for full-size)
The top left panel is a plot of temperature advection at 850mb, which shows
significant cold advection over Oklahoma and northwest Texas. This pattern
was supporting a surface cold front that was along a Fort Worth-Abilene-Big
Springs line at 7am CDT. The top right panel is a plot of 700mb omega.
It reflects an area of upward vertical motion over western and central
Texas. The center left panel is a plot of 500mb heights and isotachs. A
band of 35 to 40-knot flow is depicted across the Texas Panhandle and much
of Oklahoma to the south of a vort center in Nebraska. The right center
panel is a plot of 300-700mb Q-vectors, and the lower left panel is a plot
of Q-vector convergence for the same tropospheric section. Upward vertical
motion is indicated by the area of Q-vector convergence that stretches
from southwestern Oklahoma into central Texas. Not shown here, the Q-vector
convergence analysis for 700-850mb also depicted an area of upward motion
over central Texas.1
The right lower panel is a plot of 250mb heights and isotachs, which depicts
an upper tropospheric jet across the eastern Texas Panhandle, Oklahoma,
and Kansas, eastward into Illinois. The location of this jet feature placed
central Texas in the right entrance region of the jet max. All of these
analyses taken together suggest that central Texas was in an area of persistent
upward motion as early as 7am CDT on May 27th.
========================
1 What's a Q-vector?
A Q-vector contains the effects of geostrophic deformation on the horizontal
temperature gradient vector in both magnitude and direction. If geostrophic
deformation and temperature gradients exist (i.e., a disturbance exists),
then thermal wind balance is being destroyed. The Q-vector describes this
geostrophic "disturbance", which is forcing the atmosphere away from hydrostatic
and geostrophic balance. For more information on Q-vectors, see http://meted.ucar.edu/awips/validate/qvector.htm
.
1200UTC Mesoscale
Analysis
At
7am CDT, the mesoscale set-up reflected the following features: (a) a weak
sub-synoptic low near Dallas; (b) dryline from the previous day (May 26th)
along a Dallas - Junction line; (c) the cold front along a Fort Worth-Abilene-Big
Springs line; (d) pooling of surface dewpoints greater than 74 deg F inland
from the Gulf of Mexico to the old dryline; and (e) one or more gravity
waves over eastern Texas propagating to the south and southwest. The first
image below is the 7am CDT hand-drawn analysis created by Lon Curtis on
the morning of May 27th. The set of images below that is a time-series
of surface moisture convergence analyses utilizing data from the 0-hour
output of the RUC model at 7pm on May 26th, and 1am, 7am and 1pm (all CDT)
on May 27th. These plots depict a persistent moisture convergence maximum,
probably associated with the sub-synoptic low, as it drifted southeastward
during the overnight hours from southwestern Oklahoma to near Dallas, then
to near Waco by 1pm CDT (Corfidi, private correspondence).
Surface Analysis-1100UTC
This hand-analyzed
regional map was prepared from one of the co-authors just prior to 1200UTC
on the morning of 27 May 1997.
Surface Moisture
Convergence
(Click
individual image to enlarge to full screen)
Hi-res Visible Image
1645UTC
This high-resolution visible image is from 1645UTC. Note the rope cloud
structure that is oriented generally NE-SW from northeast of Dallas to
near Waco to west of Austin to north of Del Rio. This feature had persisted
most of the morning and coincides with the location of the day-old dryline.
The gravity wave was just passing Waco at this time, as may be seen in
a loop of visible imagery (below).
Visible Satellite
Loop
KEWX
Nexrad Radar Loop
This is a loop of the base reflectivity images from the nexrad site at
New Braunfels, TX (KEWX) covering the entire event. At the first frame
(1735UTC), the initial thunderstorm is evolving in McLennan County near
Waco. The storm(s) propagate southwestward along the NE-SW boundary while
(beginning about 1935UTC) additional storms develop west of San Antonio
and north of Del Rio. In addition to the storms, several mesoscale boundaries
are detected. One of these (seen moving past and the radar and southward
about 1900UTC) may be the gravity wave.
1800UTC POINT SOUNDING FOR TEMPLE
(TPL)
Updated with TPL
1800UTC Surface Observation
Jon Davies (a private meteorologist in Wichita, KS) has been conducting
research into issues relating to severe storms and tornadogenesis. In this
thermodynamic diagram (used with permission), Davies used the ETA point
sounding for Temple, TX (TPL) at 1800UTC and adjusted the lowest portion
of the diagram for the observed temperature and dewpoint at Temple at that
hour. As shown, this adjustment produced an extremely large CAPE value,
with a significant portion of that CAPE in the 0-3km layer. Also note that
the lifted condensation level (LCL) and level of free convection (LFC)
were quite low and quite close together. This is consistent with some of
Davies' recent work which suggests that tornadogenesis is enhanced by low
LCLs and LFCs.
KGRK VAD Wind Profile
1729UTC
VAD Wind Profiles are generated automatically by the WSR-88d (nexrad) radars.
This image is from the Central Texas-Fort Hood nexrad, located near Granger,
TX, located about 25 miles northeast of Austin and about 60 miles south
of Waco. The image is from 1729UTC, less than an hour before the first
tornado formed at Spring Valley, southwest of Waco near Hewitt and Lorena.
An interesting aspect of this image is the weak southerly flow in the lowest
gates, which turns to the southwest around 3000 feet AGL and to northwesterly
between 4000 and 6000 feet, then back to a southerly flow around 8-9 thousand
feet, then back to a moderate northwesterly flow in the layer from 11 to
16 thousand feet.
Hourly Surface
Analyses
1700-2000UTC
(Click
individual image for full-size)
20-minute Surface
Theta-e
1700-2000UTC
This is a composite plot of surface observations (across the bottom) at
Temple (TPL), Killeen (ILE) and Georgetown (GTU), along with a graph showing
variation in surface Theta-e between 1700 and 2100 UTC. In addition, the
time of the most significant tornadoes is indicated. Theta-e is plotted
in degrees K and each station is denoted by a different color. Temple and
Georgetown were both east of the NNE-SSW surface boundary while Killeen
was to the west of the boundary. Surface Theta-e reached a peak of
more than 367K at Temple prior to the development of the nearby Lake Belton
tornado at Morgans Point Resort. Note that there was little variation in
the Theta-e at Killeen until outflow from the supercell storm to the east
reached Killeen around 1955UTC. There also was little variation at Georgetown,
but this plot ends just prior to the onset of precipitation there.
Nexrad Radar &
Tornado Photos
(Paired
by time)
Spring Valley-Lorena
Tornado
Damage rated
F2, 2.0mile path, maximum width 75yds
Radar image
during Moody Tornado
Damage rated
F3, 3.7mile path, maximum width 150yds
(Sorry, no
photo available)
Lake Belton-Morgans
Point Resort Tornado
Damage rated
F3, 1.0mile path, maximum width 275yds
Prairie Dell
Tornado
Damage rated
F1, 2 mile path, maximum width 100yds
Prairie Dell
Tornado at 2026UTC
Beginning transition
that leads to Jarrell Tornado
Jarrell Tornado
in multi-vortex organizing phase at 2029UTC
Jarrell Tornado
intensifying at 2032UTC
Jarrell Tornado
intensifying and approaching Double Creek Estates at 2036UTC
Jarrell Tornado
entering Double Creek Estates at 2041UTC
Damage rated
F5, 5 mile path, maximum width 650yds
GRK base velocity
at 2039UTC
Cedar Park
Tornado
Damage rated
F3, 9.2 mile path, maximum width 250yds
(Photo (C) Paul Chamberlain
and used with his permission)
Pedernales
Falls Tornado
Damage rated
F4, ? mile path, maximum width ???yds
(Sorry, no
photo available)
Del Rio (DRT) RAOB
PLOTS
27/1200UTC
28/0000UTC
EDAS PLOTS
The ETA Data Assimilation System (EDAS) is an intermittent data assimilation
system that is generated every three hours by taking the most recent 3-hour
forecast from the ETA and updating it with high frequency observations,
such as wind profiler, NEXRAD, and aircraft data. The result is a gridded
output that often captures features on a smaller-than-synoptic scale. In
this study, we looked at the EDAS upper air data at 1800, 2100, and 0000
(all UTC) for indications of sub-synoptic accelerations in the flow that
may have had an impact on the evolution of this storm system. The
charts below are from the 2100UTC (4pm CDT) EDAS output.
These products, reflecting conditions at 2100UTC (4pm CDT), show that the
mid-level flow over the area where the Jarrell tornado occurred was considerably
more energetic than had been forecast by the morning suite of models, with
the 700mb output showing a 25 knot west-southwesterly flow over the area,
the 500mb output showing a westerly flow of 40-45 knots impinging from
the west, and the 300mb output showing around 30 knots from the west-southwest.
Current Research:
Low Shear, High CAPE Tornadoes
As noted earlier, Jon Davies, has been conducting research into issues
relating to severe storms and tornadogenesis for many years. At the
A.M.S. 21st Conference on Severe Local Storms (2002) he detailed some of
his work dealing with low shear, high CAPE tornado cases. He found that
a number of these cases, including the Jarrell event, invol common factors:
a pre-existing wind shift boundary, along which storms develop rapidly;
large low-level CAPE (rapid positive buoyancy increase in low-levels),
and small near-surface CIN; and storm motion that remains on or close to
the boundary, well to the right of the mean environmental wind. A scattergram,
used with permission, shows the parameter space (decrease in lifted index
per kilometer versus the height (AGL) of the midpoint of maximum lifted
index decrease) for several of the low shear, high CAPE cases he has studied.
Davies writes: "Estimated environments for the "weaker shear" tornado cases
from this presentation are indicated by red squares, while environments
for several "stronger shear" supercell tornado cases from 1999-2001 are
indicated by black open squares. Notice that, compared to the black
"stronger shear" cases, all of the red "weaker shear" cases fall in the
upper left half of the diagram. *** "Although these are only a few cases,
the diagram hints that this stronger increase in low-level buoyancy may
be a notable feature of "weaker shear" environments producing significant
tornadoes, and certainly suggests further research in this area.
The diagram also suggests that a somewhat different combination of ingredients
comes together for more typical supercell tornadoes in stronger shear environments
where low-level buoyancy does not appear to be as crucial.
Current Research:
Model Simulation of "Jarrell" Storm
Adam Houston and Robert Wilhelmson are using a supercomputer at the University
of Illinois-Urbana/Champaign to simulate the "Jarrell" storm and to research
the role of pre-existing low-level boundaries on storm evolution in low-shear
environments. Their research uses the I-COMMAS model of localized deep
convection, a successor to the COMMAS model developed by Wicker, Wilhelmson
and Skamarock. This model permits the researcher to specify (and change)
environmental conditions between successive runs of the model. In this
instance, the researchers specified the atmospheric conditions that preceded
the evolution of the "Jarrell" storm, but have been investigating the role
of the pre-existing north-northeast to south-southwest boundary by changing
its depth and characteristics as well as by omitting it from the specified
conditions.
Image from
I-COMMAS model simulating one stage of the "Jarrell" storm
Their preliminary results (reported at the A.M.S. 21st Conference on Severe
Local Storms, 2002) are as follows:(a) cell initiation: location of the
pre-existing boundary may have fostered mid-level and low-level mesocyclone
formation and intensity by allowing cells to track along the gust front;
(b) not all tornadoes were preceded by or even associated with mergers
and not all mergers preceded or were associated with tornadoes; and (c)development
of mesocyclones was not directly dependent on the pre-existing boundary
but, the boundary likely resulted in intensification and protraction of
cells and the formation mechanism may have promoted cell mergers.
The use of very high resolution and high speed computer models to simulate
severe deep convective storms may offer some of the most important advances
in severe storm research in the future. If the I-COMMAS model can be tuned
to accurately simulate severe thunderstorms, then researchers will be able
to study storms in the laboratory without the need to intercept them in
the field.
Palestine Profiler
The National Weather Service operates a network of wind profilers that
continuously detect the wind speed and direction through a substantial
part of the troposphere. (Unfortunately, the latest proposed budget for
NOAA/NWS terminates funding for the profiler program.) In May of 1997,
the closest profiler to the event was at Palestine, about 80 miles east
of Waco. The profiler was probably too far east and north of the storm
system to provide data pertaining to the early stages of the system, but
it does appear that the profiler may have detected the mid-level acceleration
depicted by the EDAS plots shown above, as the higher speed winds reached
into the Palestine area around 4pm CDT. On the far left, thin red rectangles
indicate the accelerated flow in two segments, 2-3km (6500-9500 feet) above
ground and 5-7km (15,000-22,000 feet) above ground.
Palestine profiler
plot for 48-hour period ending at 7pm CDT on May 27, 1997 (time runs right
to left)
Our Findings
>Mid- and
upper-level flow strengthened during the day … as seen in EDAS data, the
Palestine profiler, and comparison of 12Z and 0Z UA data. This raises the
question: how much of an outlier was this event? Davies has accumulated
several other cases of strongly deviant motion in proximity to NNE-SSW
boundary; we believe that there may be others (possibly the F4 in Mason
County, TX 5-11-99). There is interesting research underway to model this
storm and the deviant storm and tornado motion mayyet be explained by the
work of Houston & Wilhelmson.
>The gravity
wave may have aided in preconditioning the incipient storm environment,
but was not a necessary component of initiation of this storm system. Moisture
convergence over the area had been persistent and pronounced for more than
24 hours prior to the event as evidenced by the mixing ratio at 925hPa:
~18g/Kg near FTW at 0Z on the 27th, ~16g/Kg near ACT at 12Z on the 27th,
and ~18g/Kg near DRT at 0Z on the 28th.
>The sub-synoptic
low, originating near Wichita Falls on the 26th, was near Dallas at 12Z
on the 27th, and moved south-southwest along the old dryline to near ACT
at 18Z, providing enhancement of surface moisture convergence. The sub-synoptic
low became the focus of the initial storm development and propagated to
the south-southwest with the storm system through the afternoon.
>Persistent
and widespread ascent was present over the area through a deep layer of
the troposphere as reflected in Q-vector analysis and div-Q plots from
both 0Z and 12Z on the 27th. The Q-vector convergence depicted appears
to be the result of the right-entrance region of the 250hPa jet which was
located primarily over Oklahoma, eastern Kansas and southwestern Missouri
between 0Z on the 27th and 0Z on the 28th.
What's Next?
We intend to take a closer look at mesoscale and sub-synoptic features
using satellite cloud motion and derived satellite winds. We also will
examine mesoscale winds using VAD wind profiles from the nexrad sites surrounding
the event. And we have begun examining other Texas cases that may involve
similar processes (F4 tornado in Mason Co., 1999; F3 tornado near Lake
Whitney, 2000). Click here
to
view some preliminary work on those cases.
