Mesoscale Precipitation Discussion 0907
NWS Weather Prediction Center College Park MD
608 AM EDT Sat Sep 29 2018

Areas affected...Middle and Upper TX coast

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

Valid 291007Z - 291507Z

Summary...Thunderstorms are slowly increasing in coverage inland
of the Middle and Upper TX coasts.  Hourly rain totals to 2.5"
with local amounts to 4" are possible here.

Discussion...Showers and thunderstorms have been slowly expanding
in coverage with cooling cloud tops during the past hour northwest
of a boundary extending from south of Victoria TX to just offshore
the Upper TX and southwest LA coasts.  This is occurring east of a
slowly moving shortwave across western TX.  ML CAPE values have
been increasing overnight, with the storms developing within an
instability gradient within the coastal plain.  ML CAPE values of
1500-2500 J/kg exist here.  Precipitable water values of ~2.2" lie
across the region per GPS data.  Inflow at 850 hPa is southeast at
10 kts while it is south to southwest at 10-15 kts per VAD wind
profiles.

The mesoscale guidance appears too far east and too organized with
this activity as the effective bulk shear has trended away to the
east offshore LA during the past several hours.  While recent cell
motion has been west-northwest, believe that the strengthening
thunderstorms will begin to move parallel to the coast along with
the 850-400 hPa mean flow.  There may be cell collisions from time
to time between the less and more developed storms, but the main
concern is for short periods of southwest to northeast training
over urban areas and partially saturated ground which was received
100-400% of the average rainfall over the past couple weeks. 
Hourly rain totals to 2.5" with local amounts to 4" are possible
here over the next several hours.

Roth

ATTN...WFO...CRP...EWX...HGX...LCH...

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

LAT...LON   30009397 29659383 29439452 29099503 28689578
            27939676 28049786 28859730 29369650 29829533