Mesoscale Precipitation Discussion 0955
NWS Weather Prediction Center College Park MD
105 PM EDT Tue Oct 09 2018

Areas affected...South-Central, Central, and North Texas

Concerning...Heavy rainfall...Flash flooding possible

Valid 091704Z - 092200Z

Summary...An extensive band of heavy rain and embedded
thunderstorms is expected to persist in the general vicinity of
the I-35 corridor in Texas into the early-mid afternoon. Localized
areas may see rain rates approach 2 in/hr, which could lead to
flash flooding.

Discussion...Regional radars showed a fairly extensive squall line
extending from the south side of DFW Metro, to near Killeen, to
near Uvalde. The squall line was making steady progress east, but
may begin to slow down as it moves further off a surface cold
front. Visible satellite imagery showed sufficient clearing ahead
of the squall line in central and south-central Texas to generate
strong instability. RAP analysis already indicated MLCAPE of
2000-3000 j/kg ahead of the convection. Hi-res models struggle to
maintain the southern end of this squall line, but given the
available instability and relatively strong convergence along the
associated outflow boundary, it should continue for at least
another few hours. There also appears to be large-scale support
for vertical motion as additional convection was developing in the
low-level inflow region from near Killeen and Austin south to near
Corpus Christi and Port Lavaca. The atmosphere was destabilizing,
but there was also strong upper level divergence noted on RAP
analysis as well. The scattered convection developing ahead of the
main squall line may eventually coalesce, leading to a larger band
of convection in some spots, and thus potentially longer duration
of heavy rain. Estimated rain rates in this region were already
peaking in the 1-2 in/hr range per WSR-88D dual pol and MRMS.
Therefore, rain rates peaking around 2 in/hr seems to be a
reasonable expectation as convection continues. This may lead to
flash flooding, particularly if it occurs over urban areas or in
the vicinity of the typically flash flood prone Balcones
Escarpment.

Further to the north, in North Texas around the DFW Metro Area to
near the Red River, instability was weaker, owing to more dense
cloud cover. This was even true ahead of an outflow boundary, with
some scattered mid-level clouds limiting destabilization in the
I-20 corridor between DFW and Tyler. Nevertheless, as the
afternoon progresses, the RAP shows the LLJ lifting north and the
nose of the LLJ becoming more focused in North Texas. Therefore,
it is possible that convection will become increasingly
concentrated further north and may pose more of a flash flood
threat with time. However, confidence is lower given how far east
the outflow boundary has pushed, and the ongoing cloud cover in
advance of that outflow boundary. Flash flooding will be possible
if convective bands can redevelop in areas that have already
received heavy rainfall this morning (including some parts of the
DFW Metro Area), or if destabilization can occur further north to
increase rain rates.

Lamers

ATTN...WFO...CRP...EWX...FWD...HGX...SHV...SJT...

ATTN...RFC...LMRFC...WGRFC...

LAT...LON   33369615 32609490 30319658 28819701 28119817
            28059945 28659992 29549944 30369879 31659820
            32949739