Mesoscale Precipitation Discussion 0647
NWS Weather Prediction Center College Park MD
534 AM EDT Thu Jul 18 2024
Areas affected...Hill Country of Texas east through the Piney
Woods
Concerning...Heavy rainfall...Flash flooding possible
Valid 180934Z - 181400Z
Summary...A corridor of thunderstorms with heavy rainfall will
continue to drop southward through the morning along a nearly
stationary front. These thunderstorms will contain rainfall rates
of 2-3"/hr, which could produce locally more than 4" of rain.
Flash flooding is possible.
Discussion...The regional radar mosaic early this morning
indicates an expanding line of thunderstorms from far western
Louisiana through central Texas. These storms are developing along
a stationary front which should sag slowly southward as a weak
cold front later this morning. North of this boundary, a potent
shortwave noted in WV imagery and in SPC RAP Differential
Vorticity fields is spinning southward, enhancing lift in a region
already favorable through isentropic upglide of a modest LLJ and
beneath a subtle mid-level deformation axis. Thermodynamics across
Texas remain supportive of heavy rainfall, with PWs measured by
GPS of 1.9 to 2.1 inches collocated with MUCAPE exceeding 2000
J/kg. The overlap of ascent into this airmass is providing the
favorable conditions for increasing thunderstorm development, and
recent radar-estimated rain rates from KGRK WSR-88D have been as
high as 2.5"/hr.
The CAMs this morning all offer differing solutions to the
evolution of this convection, but while spatial coverage and
footprint vary, the intensity is well aligned among the various
models which increases confidence in a heavy rain event. The 850mb
LLJ is already beginning to slowly veer as noted in regional VWPs,
and is expected to become westerly by late morning. This will
limit the isentropic ascent and slowly reduce moisture transport
northward, but will also then become more aligned to the advancing
front, helping to turn the mean cloud-layer 0-6km winds parallel
to the front as well. Additionally, the propagation vectors will
become increasingly anti-parallel to the mean flow as the LLJ
veers, suggesting continued backbuilding of echoes to the SW and
along the front into the greater instability. With both HREF and
REFS probabilities for 2"/1hr accumulations reaching 40%, this
could result in 2-3" of rain along the boundary, with locally 4+"
possible as noted by the neighborhood probabilities. The
discussion area was drawn to somewhat emulate the EAS
probabilities which are highest across the Hill Country, Balcones
Escarpment, and along portions of I-35.
Recent rainfall across this region has generally been below normal
the last 7 days according to AHPS, but locally, NASA SPoRT 0-10cm
RSM is above 70%. This indicates that some infiltration of heavy
rain is likely, which is reflected by the higher FFG and
corresponding 3-hr FFG exceedance probabilities peaking at just
10-20%. However, the favorable setup for training of 2-3"/hr rain
rates could still overwhelm soils, especially in urban areas or
the across any more sensitivesoils, leading to rapid runoff and
instances of flash flooding.
Weiss
ATTN...WFO...EWX...FWD...HGX...LCH...SHV...SJT...
ATTN...RFC...LMRFC...WGRFC...NWC...
LAT...LON 31969389 31959322 31719285 30559369 30129502
29899686 29939839 30429957 30880043 31290077
31570017 31709930 31719830 31789713 31819532
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