Mesoscale Precipitation Discussion 1042
NWS Weather Prediction Center College Park MD
1029 AM EST Fri Dec 07 2018

Areas affected...Central and Eastern Texas...

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

Valid 071530Z - 072100Z

SUMMARY...Prolonged moderate frontogenetic rain shield with
embedded convective elements increase threat for high rainfall
totals with possible initiation of flooding conditions toward
evening.

DISCUSSION...Very strong synoptic and mesoscale environment
beginning to set up across central to eastern Texas.  15z surface
analysis depicts a strong frontal zone extending northeast from a
weak surface wave near COT/PEZ to a secondary even weaker
inflection near LFK.  A dying frontal zone and inverted trof
extend NE and NNE from this low but are weakly defined with
minimal convergence along it at this point.  At this second
inflection, a subtle warm front extends south-southeast toward
TX/LA Gulf Coast differentiating higher theta-E air in the warm
sector depicted by upper 60s to mid-70s temps/Tds.

At the western edge of the Gulf mT air mass, increasing southerly
winds at 925-850mb turn southwesterly across the frontal boundary
to eventually parallel in far eastern TX into LA.  The southerly
component flow supports isentropic ascent along the frontal
boundary but is enhanced with greater orthogonality to the
boundary further west near the primary surface wave. Radar mosaic
depicts banded cumuliform showers beginning to increase in
intensity and coverage with the approach of better moisture flux
aloft (700-850mb) and upper level ascent/jet forcing approaching
from Coahuila/Chihuahua.

CIRA Layered PW analysis depicts a strong confined moisture plume
streaming over the Sierra Occidental and Mexican plateau,
coincident with the area of concern.  RAP analysis/forecasts
suggest little meandering of this plume, to maximized total
moisture in the column.   GOES-16/17 Meso sectors depict shortwave
energy upstream in Mexico with downstream ridge axis across
Southeast TX, supporting DPVA and modest divergence aloft at this
time.  Additional distant right entrance to 130 kt 3H jet over
OK/MO further evacuates the area over Eastern TX currently. 
However, a subtle 100kt jet streak is emerging from the Big Bend
of TX per GOES AMVs.  This jet max will strengthen and pass to the
north of the moisture confluence axis in the 17-19z time frame
rapidly enhancing UVVs from divergence aloft.  Response in the LLJ
will increas to 20-25kts at 85H advecting 1.75-1.9" Total PWs as
well as slanting 1000 J/kg SBCAPEs to allow MUCAPEs to remain over
500 J/kg. As such the broad area of shield precipitation with
.25-.5"/hr rates will have enhanced convection elements capable of
near or above 1"/hr rates.  As such an axis of 1-1.5" totals with
pockets of 2-2.5" totals through 21z will exist across the MPD
area.  This is not likely to produce flooding/flash flooding given
dry antecedent conditions, but will set the stage for prolonged
slow rise/inundation through the evening into the overnight hours.

Stronger more efficient thunderstorm activity is likely to exist
in the warm sector at/along Central TX coast and a separate MPD
may be issued for that area later if conditions warrant.

Gallina

ATTN...WFO...EWX...FWD...HGX...LCH...SHV...

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

LAT...LON   31919457 31839418 31309379 30869421 30169546
            29669633 29409677 29149756 29499805 30239794
            30629755 31189642 31579565 31779515