Mesoscale Precipitation Discussion 0870
NWS Weather Prediction Center College Park MD
151 PM EDT Thu Sep 20 2018

Areas affected...Southern NM...Far West Texas...

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

Valid 201750Z - 202300Z

SUMMARY...Isolated flash flooding risk for quick burst of 1-2"
totals in 1-2 hours across low FFG with some initial potential
training. 

DISCUSSION...Some of the uncertainty with respect to heavy
rainfall has become lesser so, as mid-level cirrus has reduced in
the last few hours across southern NM and western portions of West
Texas.  Surface temps have risen into the mid-70s supporting
SBCAPEs to increase over 500 J/kg from modified 12z EPZ sounding. 
This sounding along with CIRA LPW suite depict very deep rich
tropical moisture from the remnants of TD-19E with nearly
saturated profiles that support Total PWATs over 1.5" in lower
terrain locations (like El Paso) but even 1.0-1.25" totals in the
higher terrain of the Sacramento/San Andres Ranges. 

MCV from 19E is lifting out of N Chihuahua toward West TX, with
numerous high reflectivity TCU breaking out...slow cell motions
near the center are expected and with deep moisture and weak
instability...inflow will likely not maintain stronger updrafts
for very long.  Still, downdraft/rainfall production will be
efficient with rates in excess of 1.75"/hr, leading to totals of
1-2" possible.

Further north, deep layered trof is oriented favorably for
potential training profiles particularly initially as convection
develops (as see in HDX radar N of ALM).  However, fairly deep
SSWly deep moist flow west of the trof is providing
moisture/instability but the forcing/convergence is driven from
westerly flow at 850-7H, limiting moisture flux with exceptions of
weak backbuilding from the SSW inflow near cloud base.   As such
mean cell motions will train, but propagation will be toward the
ESE, especially with recent backing and strengthening of surface
flow over SE NM.  Like near the MCV, cells will have rainfall
efficiency toward 1.5-1.75"/hr but duration to lead to flash
flooding remains the highest uncertainty at this point.  Low FFG
values through the Sacramento Range toward GDP and the Davis Range
lead to some neighborhood probabilities pushing 50% with rainfall
totals in the 1-2" range.

Gallina-

ATTN...WFO...ABQ...EPZ...MAF...

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

LAT...LON   34820460 34590402 34190405 33010453 31740436
            30760406 30530507 31190605 31560648 31680720
            31920745 32520712 34060591 34610517