AWUS01 KWNH 030118
FFGMPD
TXZ000-030705-

Mesoscale Precipitation Discussion 0965
NWS Weather Prediction Center College Park MD
917 PM EDT Mon Sep 02 2024

Areas affected...portions of west-central and northwest TX

Concerning...Heavy rainfall...Flash flooding likely

Valid 030105Z - 030705Z

Summary...Localized hourly rainfall totals of up to 1-2" to
continue overnight, occasionally repeating/training and resulting
in additional localized totals of 3-5"+. Isolated to widely
scattered intances of flash flooding are considered likely, and a
significant instance or two of flash flooding are possible
(especially given the sensitivity of hilly terrain).

Discussion...Areas of stratiform rainfall with disorganized,
embedded shower and thunderstorm activity continue across much of
west-central and northwest TX (including much of TX Hill Country
and Big Country), in the vicinity of a quasi-stationary surface
front (and just downstream of a shortwave/closed-low over West TX
and adjacent portions of Mexico). While much of the earlier
activity over West TX has diminished (as the front has slowly
shifted south and east with overturning and destabilization), at
least widely scattered convection is expected to persist into the
early overnight hours, likely shifting a bit more south and east
into the TX Hill Country (towards higher instability, as less
overturning has occurred to the southeast with ML CAPE of
1000-1500 J/kg, limited to 100-500 J/kg currently farther north
and west). With ample total tropospheric moisture content (1.8-2.2
inch PWATs, between the 90th percentile and max moving average,
per DRT sounding climatology) and continued easterly low-level
flow/moisture transport overnight, the 20-30 kts of effective bulk
shear should continue to organize individual and multi-cell
clusters, producing localized hourly totals of up to 1-2" (per
MRMS estimates throughout the afternoon).

Hi-res CAMs (12z and 18z runs) are a bit of a mess when comparing
individual model QPF depictions through 07z, as there is
relatively large spatial spread in where 1"+ amounts occur (per
the 18z HREF Ensemble Agreement Scale, EAS, which is maximized at
only 20% for the 1" threshold to the northwest of San Antonio,
smack dab in the middle of the TX Hill Country). Despite this poor
spatial agreement for 1"+ amounts, those totals are quite common
throughout the various members of the ensemble, as indicated by
the 40-km neighborhood probabilities for 2" exceedance (indidacted
to be as high as 50-70% in that same area). Even subsequent runs
of the HRRR (since 18z) have been depicting these localized high
totals, despite being one of the driest members of the ensemble,
as a few runs depict isolated totals of 5"+ (and which the HREF
40-km neighborhood probabilities for 5" exceedance even range from
10-30%).

Given the current environment, observational trends, and the
available model guidance, isolated to widely scattered instances
of flash flooding are considered to be likely. This is largely
because of the hydrologic sensitivity of the region, given that
6-hr Flash Flood Guidance (FFG) generally ranges from 2.0-3.0".
But 1-hr FFGs are often as low as 1.0-1.5" (or even less) over the
hilly terrain (with particular concerns for low water crossings).
Given the potential for localized totals of 3-5"+, some
significant instances of flash flooding cannot be ruled out.

Churchill

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!7JRVjwGI68xkzaShEh7Ck3YtJPm91rS87t7NHAbUon_SIFs5SwiELc4veprcAcFRduVE=
Wo_Icse7DFxPA5qhLWEwsQY$  for graphic product...

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

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

LAT...LON   32609932 32519817 31809754 30859759 30019799=20
            29259853 29109943 28990043 29500128 30450158=20
            31370159 32000127 32490036=20

=3D =3D =3D
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