April 7, 2000 Supercell
A supercell storm formed at the intersection of a southward moving cold front and a subtle dryline over Central Texas Friday afternoon. The storm moved south-southeast (making it a classic right-mover) across Bell County, producing hail up to 1.75 inch in diameter.
The Storm Prediction Center outlooks for Friday, April 7th were for a moderate risk of severe storms in the Ohio Valley with a slight risk area into eastern Texas but well east of our area. Examining the morning computer (ETA and RUC) output, I was concerned about forecast instabilities for the area along and east of IH35 by late Friday afternoon. Around 10am CDT, I updated my forecast/outlook section, calling for a rish of severe storms along and east of a Waco-Austin line by 6pm CDT.
In addition to the forecast of enhanced instabilty into Central Texas, I based this outlook on the presence of a small jet maximum imbedded in the subtropical (S/T) jetstream, which was over the Tucson-El Paso area at 7am Friday and was forecast to be near Del Rio by 7pm Friday, at which time it would be in a favorable position to produce some vertical motion (upward) over Central Texas. I thought this feature was very important because the atmosphere over our area was expected to be strongly capped (warm air aloft, impeding rising air currents) all day. To break the cap, we would need either a strong forcing mechanism like a cold front, or an upper air mechanism to relieve the cap by causing upward vertical motion. Both forecast and observed upper air winds were adequate for the organization of severe storms if storm did develop.
There was a cold front moving south into Texas on Friday. However, the front was supported primarily by strong pressure rises in the central plains, and convergence along the front (which can be another source of upward forcing) didn't look all that primising. However, the 7am CDT RUC output did contour an area of enhanced surface moisture convergence from near Waco to east of Austin at 7pm CDT.
Temperatures were already quite warm in the area from
Del Rio to Waco and northwestward to the cold front, where mostly clear
skies were permitting maximum insolation. The airmass would be expected
to destabilize quite rapidly under these conditions given sufficient moisture
in the lower levels of the atmosphere but the capping inversion appeared
to be holding a lid on convective development. The
CAPS analysis of CAPE and convective inhibition (CINH) for the same time confirmed that instability was increasing but that the cap was holding firm. Here's that analysis:
CAPE, CINH and Helicity at 1pm CDT 4-7-2000.
The RUC model output from 1pm CDT continued to forecast
high values of CAPE and strong moisture convergence in the area from just
east of Waco southward to east of Austin at 7pm CDT Friday evening. Although
there was no immediately obvious reason for this moisture convergence maximum,
my experience with the RUC has been that this sort of feature deserves
attention. Click the links shown for the 6-hour RUC forecasts of CAPEand
surface pressure/boundary layer wind/moisture convergence (PMSL-mocon)
at 7pm CDT.
Multiple fineline boundaries are shown in the image above. The boundary northwest of Fort Worth (west of Sherman to near Mineral Wells) is probably the actual cold front. The boundary bisecting the radar site (McKinney to Glen Rose) is probably the prefrontal trough. Another boundary oriented NNE-SSW is seen from the radar site to eastern Hamilton County. A short time after this image was created, the radar switched to precipitation mode, which makes boundary identification more difficult. However, about the same time, the nexRAD at Granger (KGRK) was detecting a boundary, too. Here's that image:
The boundary in this image is from west of Waco across
the Killeen area to west of Austin. It is the linear area of red pixels.
Based on surface observations (described later), this boundary is a dryline.
To the west, the dewpoints are in the 50s and to the east the dewpoints
are in the upper 60s to near 70 (F). The intersection of boundaries
(such as where a dryline intersects a cold front) are areas that appear
to be favored for storm development, even in situations where some CINH
exists. This is probably due to enhanced vertical motion associated with
circulations along both the cold front and the dryline.
Mesoscale analysis of surface data for 5pm CDT indicated an area of lower surface pressure to the west and southwest of Waco. This area was probably associated with strong solar insolation in the drier air between the dryline and the cold front. Here's the 5pm CDT mesoscale analysis:
The cold front is oriented NE-SW along a Dallas-Hamilton-Mason line with a dryline (dashed with open half-circles) from the cold front over northern Bosque County south-southwest near Killeen, Burnet and Hondo. There is some suggestion of moisture pooling just ahead of the dryline from Waco to Austin, where dewpoints are near 70 deg F. This would be consistent with the moisture convergence pattern mentioned in connection with the first map (ROC) shown above. To take a closer look at this issue, I used Digital Atmosphere (a product of Weathergraphics [click here to go there!]) to analyze the 5pm CDT data. The parameter I wanted to examine was equivalent potential temperature (Theta-e) which is a meteorological veriable which is canbe computed from surface data and is a good proxy for CAPE (which requires either a vertical sounding or model data). Here's the 5pm CDT surface data analyzed for Theta-e:
Theta-e, as well as its related parameter, potential temperature
(Theta), is expressed in degrees Kelvin, which is another of the temperature
scales used scientifically. The countours on this chart are every two degrees
K, and the map depicts an axis of elevated Theta-e just ahead of the advancing
dryline from Waco to Austin. Storms often develop along the sharpest gradient
of Theta-e. When combined with the mesoscale surface analysis, this chart
points to the area west and northwest of Waco as a likely target.
NWS radar confirmed that, indeed, convective development was underway in this area, and by 5:30pm CDT, storms were occurring over western and northwestern McLennan County. Here's the 5:28pm CDT image from the nexRAD at Fort Worth:
Note in particular the radar thinlines extending westward
from each of the cells near Waco. Without additional elevation (higher)
images from the the radar, it isn't possible to be certain, but it looks
as if each cell is tilted sharply eastward with height. This would be consistent
with the wind profile in the atmosphere, which shifted to westerly within
one mile of the surface and to west-northwest within 2 miles of the surface.
By 6pm CDT, the storm southwest of Waco was clearly becoming the dominant storm. Here's the 5:58 pm CDT radar image:
The storms to the north of that cell appear to be weakening
as they move away from the thinline feature and as the anvil and precipitation
of the southwest McLennan County storm impede inflow farther north. We
can see a new reflectivity maximum on the western flank of the dominant
cell (east of Gatesville), and a new cell trying to form south of Gatesville
near North Fort Hood.
By 6:30pm CDT, the cell in southwest McLennan County was moving south-southeast through northern Bell County. Here's a clickable link to the related radar image. And here's an enlarged portion of the image:
Although the quality of this image is somewhat degraded,
the storm is showing a reflectivity pattern frequently associated with
supercells and sometimes referred to as the "flying eagle". This occurs
when strong upper levels winds are forced to flow around the strong updraft
associated with a mesocylone, sweeping precipitation downwind in a sidewise
"v" (<) pattern. It's too bad that we don't have any images from the
nexRAD at Granger. Much of the structure of this storm is below the radar
horizon of the Fort Worth nexRAD at this time.
By 7pm CDT, the storm was crossing Bell County near Nolanville. Hail and heavy rain were falling from Morgans Point Resort to Temple. Here's the radar image:
Note that all of the other storms in Central Texas have
dissipated, The storms in northeast Texas (north of Tyler) are increasing
and moving southeastward. Our storm still exhibits general supercell characteristics.
Visually at this time, the wall cloud was not as prominent.
Update in progress ...
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