Flash Flood Guidance
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AWUS01 KWNH 182017

Mesoscale Precipitation Discussion 0027
NWS Weather Prediction Center College Park MD
313 PM EST Sat Jan 18 2020

Areas affected...Middle to Upper TX Coast...South-Central to Far
South TX

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

Valid 182012Z - 190100Z

SUMMARY...Very slow-moving thunderstorms with very heavy rainfall
rates to near 2 inches/hr will pose at least a localized flash
flood threat through the remainder of the afternoon.

DISCUSSION...The latest GOES-16 IR satellite imagery shows rapidly
cooling convective tops over areas of the middle to upper TX coast
this afternoon as a cold front gradually settles southeast across
southern and eastern TX. Very heavy showers and thunderstorms are
seen expanding in coverage in radar imagery with dual-pol QPE
suggesting hourly rainfall amounts reaching up near 2 inches/hr
within the strongest convective cores where a few overshooting
tops are also noted.

A moderately unstable airmass is pooled up across the immediate
southeast TX coastal plain ahead of the aforementioned front, with
MLCAPE values now in the 1000 to 1500 j/kg range. Meanwhile, PWs
are generally around 1.6 to 1.7 inches, with low-level moisture
characterized by surface dewpoints in the upper 60s to low 70s.
This is supporting a favorable thermodynamic environment for
convective initiation considering the proximity of low-level
frontal convergence and deeper layer ascent associated with
right-entrance region upper-level jet dynamics which is strongly
suggested by the convective-V notch signature in the IR satellite

The last few runs of the HRRR guidance has been suggesting
stronger convective trends, with heavier rainfall rate potential
given the uptick in boundary layer instability. Some expansion of
the convection farther south down the coast and into portions of
south-central and far south Texas will be possible over the next
few hours as the arrival of the front interacts with the favorable
pool of aforementioned moisture and instability across the region.

Expect rainfall rates again to be near 2 inches/hr with the
additional convective coverage this afternoon, with some storm
totals in the 3 to 5 inch range. The slow cell-motions seen in
radar imagery will facilitate these heavier storm totals.

Given the ongoing convective trends/organization and rainfall
rates, some localized flash flooding will be possible through the
balance of the afternoon.


...Please see www.wpc.ncep.noaa.gov for graphic product...



LAT...LON   29489512 29349481 28949511 28699563 28309633
            27579704 26839725 26409723 26039715 25879748
            25939792 26369826 27279812 27979773 28669704
            28889663 29389566

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