Area Forecast Discussion
Issued by NWS Austin/San Antonio, TX

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023
FXUS64 KEWX 150022
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
722 PM CDT Sun Oct 14 2018

.AVIATION...
The main concerns for the 15/00Z TAFs center around the timing of a
cold front and associated northerly wind shift at the terminals,
potential for SHRA/TSRA along and behind the cold front, and a
prolonged period of MVFR and IFR ceilings behind FROPA.

Surface analysis as of 00Z places the cold front along a Pecos to San
Angelo to DFW line and based on current motion expect the front to
reach the I-35 terminals and DRT between 05-07Z. A fairly dramatic
northerly wind shift will occur with FROPA, with winds increasing
into the 15-20 knot range with stronger gusts 25-30 knots after 06Z
through at least 18Z Monday. Have concerns for a brief period of LLWS
with immediate FROPA, but with the strongest thermal gradient
lagging a bit behind the front it appears that the post-frontal layer
should deepen enough before onset of strongest winds.

High resolution guidance remains fairly consistent in breaking out a
line of SHRA/TSRA as the cold front encounters more moist/unstable
air located across South Central Texas tonight, but forecast
soundings appear to have dramatically under-initialized a capping
inversion evident on aircraft soundings out of both Austin and San
Antonio tonight. Expect enough forcing along the front to result in
SHRA but do not have high enough confidence in TSRA to carry more
than VCTS at this time. Will have to watch the higher terrain of
Mexico for any TSRA that are able to make a run for DRT before the
front, but the strength of the cap should result in decreasing
intensity should a storm attempt to propagate farther east. Upglide
may allow for a few lingering SHRA behind the front overnight, with a
secondary round closer to 12Z as the 850 MB front slides into the
region and stalls. Periods of rain may linger into the afternoon
hours.

Moisture trapped beneath the post-frontal inversion will result in
high-end IFR/low-end MVFR ceilings developing behind FROPA and
persisting through the end of the TAF period. Expect ceilings to
lower into IFR in heavier rain.

&&

.PREV DISCUSSION... /ISSUED 249 PM CDT Sun Oct 14 2018/

SHORT TERM (Tonight through Monday Night)...
At 2PM the strong cold front was approaching a Midland to Abilene
line and surging south. Models are in good agreement with the front
entering the Hill Country and south Edwards Plateau between 03Z-06Z
this evening and accelerating south through the U.S. 90 corridor
06Z-09Z, clearing the CWA before daybreak.

As the front approaches and enters the CWA convection should
initiate along and just behind it as it encounters a moderately
unstable airmass ahead of it. Also, ahead of the front, hi resolution
models have been consistent with indicating isolated storms
developing in a separate area early this evening across Coahuila and
moving eastward into Val Verde County, then increasing in coverage
through the southwest CWA overnight as the front arrives. Forecast
soundings indicate a pool of 2500-3000 J/kg MLCAPE through this
region and around 30 kts of deep layer shear. This will be adequate
to support a few strong to severe storms through this region.
Initially discrete supercellular structures will support large hail
and damaging winds, then transition late evening and overnight to
mainly a damaging wind threat as multicellular clusters possibly
develop. Farther northeast, the speed of the front undercutting the
convection should preclude organize severe weather threat. Nevertheless
a strong storm or two producing strong winds will be possible.

Of greater concern will be the potential for pockets of heavy
rainfall tonight through Monday morning through the Hill Country,
southern Edwards Plateau, and Rio Grande...over areas that are near
saturated from weeks of rainfall. Models are in good agreement with
1 to 3 inches of rainfall across these areas tonight as well as
setting up behind the front Monday, and some higher pockets of 4 to
6 inches could occur. We have issued a Flash Flood Watch for this
region. Farther east...along and east of I-35 we are generally
forecasting 1/2 to 1 1/2 inches of rainfall.

Much colder and windy conditions will occur Monday and Monday night.
Temperatures will gradually fall through the day Monday. We have
sided closer to the cooler raw model temps. Wind gusts of 25-40 mph
will produce wind chill readings in the mid 30s across the Hill
Country and upper 30s to mid 40s elsewhere Monday and Monday night.
Rain chances do continue into Monday night, however models are
indicating the heavier band of rainfall setting up just north of the
area.

LONG TERM (Tuesday through Sunday)...
An upper level low will remain cut off across the Southwest U.S.
through a good portion of the forecast. Isentropic ascent will lead
to continued chances for rain Tuesday. Heavier band just north of the
area Monday night could sag south into southern portions of the
Edwards Plateau and northern Hill Country Tuesday. Models are in
relatively good agreement of a temporary drying trend across the
eastern half of the CWA Wednesday.

A surge of moisture and disturbance aloft is indicated by GFS, ECMWF,
and Canadian to move north into the CWA on Thursday increasing rain
chances again. There are also some hints of a connection with energy
from the Pacific being ingested into the region Thursday through
Saturday, between the low cut off over the Southwest U.S. and ridge
across the northeast Gulf of Mexico. PoPs have been increased
Thursday through Saturday in this package.

Global models have come into slightly better agreement with a trough
across the Central and Northeast U.S. sending another front and drier
air into the region sometime late Saturday into Sunday.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              48  50  46  50  47 /  90  90  80  80  40
Austin Bergstrom Intl Airport  49  51  47  51  48 /  90  90  80  80  40
New Braunfels Muni Airport     51  54  47  52  48 /  90  90  80  80  40
Burnet Muni Airport            45  47  43  47  45 /  90  80  90  80  40
Del Rio Intl Airport           50  53  45  50  47 / 100  90  80  80  50
Georgetown Muni Airport        46  48  45  49  46 /  90  80  90  80  40
Hondo Muni Airport             52  55  46  52  49 / 100  90  80  80  40
San Marcos Muni Airport        50  53  47  51  48 /  90  90  80  80  40
La Grange - Fayette Regional   54  57  50  54  50 /  70  80  70  70  40
San Antonio Intl Airport       52  54  47  52  49 / 100  90  80  80  40
Stinson Muni Airport           54  57  48  53  50 / 100  90  80  80  40

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Monday evening for Bandera-Blanco-Burnet-
Edwards-Gillespie-Kendall-Kerr-Kinney-Llano-Maverick-Medina-Real-
Uvalde-Val Verde-Zavala.

&&

$$

Aviation...Huffman
Short-Term/Long-Term...05



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