Mesoscale Precipitation Discussion 0634
NWS Weather Prediction Center College Park MD
237 PM EDT Sat Aug 11 2018

Areas affected...Texas Hill Country and the Edwards Plateau

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

Valid 111835Z - 120035Z

Summary...A slow-moving cluster of thunderstorms was affecting the
Del Rio area and producing very heavy rainfall. The storms may
continue to persist and will likely produce flash flooding.
Additional storms may develop further north into Texas Hill
Country. Rain from these storms would be falling on areas that
have recently received moderate to heavy rainfall, and thus flash
flooding would also be likely in these areas.

Discussion...KDFX radar showed a prominent thunderstorm cluster
right along the Rio Grande River near Del Rio, Texas. A
right-moving supercell was likely embedded along the eastern edge
of this cluster, and was beginning to propagate into far western
Kinney County as of 1830Z. The associated mesocyclone did not
appear particularly strong at the time per KDFX velocity, but the
storm was following the Bunkers right-moving supercell vectors and
dropping more southeast along the river, as compared to northeast
like the Corfidi vectors and 850-300mb mean wind would suggest.
The result has been a slow-moving convective cluster moving right
across Del Rio, with KDFX dual pol estimates of 2+ in/hr rain
rates. This will likely lead to flash flooding in that immediate
area. There was also a larger convective cluster trailing the
supercell to the WNW, and some of these thunderstorms may begin to
propagate northeast -- back into the areas that have just received
heavy rainfall.

Further north, KSJT radar showed some weaker convection just
beginning to initiate near Sonora, Texas, along a surface
convergence zone evident in GOES-16 visible satellite (as a line
of enhanced cumulus development) and Texas mesonet wind fields.
The deep layer mean winds in this area were relatively weak, and
aligned with the orientation of the developing convection, and
thus the thunderstorms should have a proclivity to be slow-moving
and train. This convection is developing in an area of reduced FFG
due to recent heavy rainfall, and rain rates of around 1.5 in/hr
could be sufficient to produce flash flooding in the hilly terrain
of the region. This seems achievable given the expectations for
storm motion and the environment, with PWs approaching 2 inches.

Lamers

ATTN...WFO...EWX...FWD...SJT...

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

LAT...LON   31899896 30959846 30419904 29809976 28910002
            28420061 29110102 29580169 30450141 31690008