Mesoscale Precipitation Discussion 1044
NWS Weather Prediction Center College Park MD
344 PM EST Fri Dec 07 2018

Areas affected...South Central to Eastern TX....West Central LA...

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

Valid 072045Z - 080245Z

SUMMARY...Continued prolonged frontogentically forced moderate
shield precipitation with embedded convective elements/waves
steadily saturate area with flooding possible. 

DISCUSSION...Initial mid-level shortwave continues to press
eastward in Eastern TX at this time, spurring surface cyclogenesis
near DWH.  A subtle warm front extends southeast toward BPT and
the LA coastline while a cold front extends southeast to VCT
Texas.  As the wave is passing into NW LA, the TROWAL at 850mb
remains fairly expansive across E TX into NW LA with 20-30 kts of
SWly flow overrunning the boundary supporting isentropic ascent
for a broad shield of moderate rainfall.  While the moisture is
increased with a broad wedge of total PWat values near 1.5" ahead
of the 850mb front in E TX/N LA, rain rates of .25-.5" have been
common.  The best theta-E air remains along the Western Gulf Coast
but some WAA is sufficient to support some 250 J/kg to 500 J/kg
aiding some weak convective elements that supported some .75"/hr
totals and up to 2" amounts over the last 6hrs, near UTS and
Nacogdoches to N Shelby counties in TX.

Clearly, rain rates have not been sufficient to produce flash
flooding even with these localized weak thunderstorms, but it is
the multiple bouts can lead to prolonged inundation an flooding
concerns through out the evening and early overnight hours.  This
will be driven by at least two additional waves that will approach
the area of concern.  The first wave can be seen east of the Big
Bend toward the Hill County providing enhanced ascent/confluence
of the mid-level moisture stream and southerly LLJ; the second is
the main trof axis crossing Northern Mexico and the Big Bend after
03z, likely to be discussed in future MPDs.

The focused DPVA from the first wave is enhancing broad scale
ascent and low level moisture transport with TPW values to rise
from 1.5 to greater than 1.75" into the 850 front.  Low level
veering is expected with the wave passage but slow
eastward/southward sagging of the boundary will remain fairly
stationary for continued isentropic ascent in veering low level
flow. The flux will provide similar but slightly increased
rainfall efficiency to the moderate shield precipitation across
the same axis as the wave entering LA attm. As such an additional
1-2" will fall, leading to totals of 3-4" across the MPD area
(especially further east near UTS due to longer duration) through
03z.  Inundation flooding is possible to occur and given modest
rates the inundation rises may be fast enough in localized areas
to  be considered flashy in nature.  

Gallina

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

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

LAT...LON   32589320 32429260 32059236 31729238 31419250
            31209271 30989321 30789423 30649471 30229571
            29569688 29609744 30069760 30489747 31089697
            31369650 31759572 32069503 32329446 32529380