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

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FXUS64 KEWX 251556

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1056 AM CDT SUN SEP 25 2016

Widespread showers and thunderstorms are currently located along
and west of Highway 83 just north of Highway 57 over our five
western counties where our existing Flash Flood Watch is in place.
Most of the watch area has seen between 1-3 inches of rain with
some isolated spots up to 5 inches based on radar QPE. A weak
bowing segment with the leading line of convection is currently
moving into the Highway 57 corridor in Uvalde County where winds
up to 30 knots will be possible. Most models are lagging well
behind the progression of this complex except the hi-res WRF ARW
upon which this forecast is based. This model propagates this
complex of storms to the E-NE along the cool side of an outflow
boundary in the Hill Country that is moving towards the I-35
corridor. This relatively fast progression of the system should
help keep rainfall totals in the 1-2 inch range mainly along and
NW of the Escarpment to help keep the flooding threat down for
areas outside of the existing watch. However, if this system loses
its forward progress as an MCV develops, this may be revisited.
High temperatures for today have been decreased along and NW of
this boundary where temperatures should remain in the 70s.

Elsewhere, the forecast is on track with isolated to scattered
scattered showers and thunderstorms expected this afternoon with a
surge in Gulf moisture and greater daytime heating due to more
breaks in the clouds. We are still expecting more scattered to
widespread showers and thunderstorms later this evening as the
synoptic cold front moves down from central Oklahoma and west
Texas. The upper level trough to our north and west has split off
with a closed low forming over the Mexican state of Sonora. This
low will continue to retrograde further west today and tomorrow to
keep upper level dynamic support minimal over most of the region.
The one exception to this will be our western counties in the Rio
Grande Plains and Edwards Plateau where shortwave energy moving
north through the Coahuila may promote additional heavy rainfall
tonight. Model runs coming in over the next few hours will
hopefully be able to resolve this situation.




A shortwave disturbance lifting north across the Edwards Plateau
has enhanced numerous thunderstorms from Nrn Mexico into the Hill
Country while a packing of the surface pressure gradient over the
Coastal Prairies has enhanced scattered convection that extends SE
into the Gulf. The convection over the eastern Hill Country could
create timing problems for the cold front, and rapid refresh
models have struggled to maintain any form of consistency in
depicting enhanced periods of rain chances for TAF considerations.
The steady forecasts of VCTS and prevailing showers is probably
too pessimistic for the I-35 terminals, especially late periods
for SAT/SSF, whereas a constant influx of convection is more
agreed upon by models at DRT. Cold front timing is not well
illustrated in the TAF wind groups as there is some surface
troughing to the south that could start north winds ahead of time.
Given the fropa being subtle, the current thinking is that there
will be more opportunity for overrunning and possibly lower cigs.
Thus will go with steady IFR conditions in the wake of the front.


SHORT TERM (Today through Monday)...
Much of the overnight rain is staying west of Highway 83 within
the shear axis to the east of the upper level trough. Rainfall
amounts so far across Val Verde, Edwards, and Kinney County have
averaged 1 to 3 inches. This area of light to moderate showers and
thunderstorms will continue through the morning hours. There are
additional showers and storms developing over the Gulf of Mexico
and near the coast to our southeast. Could see some of this
activity develop or spread into our southeastern counties and
spread north throughout the day today.

By late this afternoon, the upper low will begin to retreat to
the west which will slightly weaken the upper support for
convection. At the same time, the main trough in the northern
plains will continue to move east. This feature will send a cold
front into Texas this afternoon and the front will approach the
northern CWA around midnight tonight. The boundary should serve
as a focus for the development of numerous showers and
thunderstorms for the overnight period. The center of the surface
trough will be near Val Verde County and this should help to
promote higher coverages and rain amounts in this region through
Monday morning. Rainfall amounts through Monday will average 2-5
inches west of Highway 83, 1-3 inches to the west of the I-35
corridor, and around an inch for the eastern counties.

Given the expected additional rainfall amounts and where the
heaviest rain has already fallen, have opted to issue a Flash
Flood Watch for Val Verde, Kinney, Edwards, Real, and Uvalde
County until 12z Monday. Temperatures behind the front Monday will
be in the lower 70s to middle 80s with lingering rain chances. The
threat of heavy rainfall will be less on Monday as the cold front
pushes south of the area by the afternoon hours and the best lift
will be weak isentropic lift on top of the frontal layer.

LONG TERM (Monday Night through Saturday)...
Rain chances will continue Monday night as weak post-frontal lift
continues across the area. Should finally start to see rain
chances end by Tuesday night as the lift shuts off. Additional
rainfall amounts Monday night through Tuesday night should average
less than a half of an inch. Temperatures on Tuesday and Wednesday
will be coolest of the fall season so far with lows in the 50s
across the Hill Country and 60s elsewhere. Highs will be in the
70s and 80s on Tuesday and will remain in the 80s for the
remainder of the forecast period. Beyond Tuesday night, the
forecast will remain rain free.


Austin Camp Mabry              85  71  80  63  81 /  60  60  60  20  20
Austin Bergstrom Intl Airport  86  70  80  64  81 /  60  60  60  20  20
New Braunfels Muni Airport     87  70  82  64  81 /  60  60  60  20  20
Burnet Muni Airport            79  67  74  60  77 /  80  70  60  20  10
Del Rio Intl Airport           79  70  76  62  73 /  90  90  80  60  30
Georgetown Muni Airport        85  68  77  62  79 /  60  70  60  20  10
Hondo Muni Airport             86  71  81  64  81 /  60  70  60  40  20
San Marcos Muni Airport        86  70  80  64  81 /  60  60  60  20  20
La Grange - Fayette Regional   88  72  85  67  83 /  60  60  50  30  20
San Antonio Intl Airport       86  72  82  65  81 /  60  60  60  30  20
Stinson Muni Airport           88  72  83  66  82 /  60  60  60  30  20


Flash Flood Watch through Monday morning for the following
counties: Edwards...Kinney...Real...Uvalde...Val Verde.



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