Area Forecast Discussion
Issued by NWS Austin/San Antonio, TX
FXUS64 KEWX 260011
Area Forecast Discussion
National Weather Service Austin/San Antonio TX
711 PM CDT SUN SEP 25 2016
Although showers are ongoing at the I-35 TAF sites, none are
currently experiencing thunderstorms as one line of thunderstorms
is moving north of GTU while another more intense line develops
along an outflow boundary or cold front SE of PEZ. This line
extends southwest towards LRD and APY and is slowly propagating to
the SE with some elements overrunning the boundary to the north.
Showers with very isolated embedded thunderstorms are expected at
the I-35 TAF sites, but lightning potential remains too low to
mention for the next six hours at any site. Ceilings for the I-35
TAF sites should gradually lower but generally remain MVFR for the
next six hours, while DRT should main VFR for the most part.
The larger- scale cold front currently south of the I-20 corridor
to our northwest will continue to push southeast. A midlevel
shortwave moving north across the Rio Grande in Mexico will help
provide some additional upper level support to allow additional
showers with embedded thunderstorms to develop as this front
approaches before convective potential decreases behind the front.
Thus, have included TEMPO groups for thunderstorms between 6-9Z
for SAT/SSF and a PROB30 group for DRT between 9-12Z. The 21Z and
22Z HRRR model runs that are best handling the current scenario
both keep thunderstorms out of AUS, so have done the same. Patchy
fog may end up being a greater concern at AUS due to saturated
soils and greater cold air advection, so currently have LIFR
ceilings and visibilities there between 9-15Z.
.PREV DISCUSSION... /ISSUED 422 PM CDT SUN SEP 25 2016/
SHORT TERM (Tonight through Monday Night)...
The complex of showers and thunderstorms that continues to track
E-NE along the Escarpment on the cool side an outflow boundary
continues to produce up to 2 inches of rain per hour. The storms
are becoming less organized as the cold pool has propagated away
from better dynamic support aloft associated with a cutoff mid-to-
upper level low retrograding in the Mexican state of Sonora.
Although an MCV that briefly developed over the western Hill
Country has dissipated, there still is plenty of instability
south of the surface boundary where temperatures have warmed to
around 90 degrees. In addition, differential heating on both
sides of this boundary may also be generating frontogenetical
forcing that combined with southerly flow up the Escarpment, will
continue to encourage additional scattered to widespread showers
and thunderstorms through early evening. Thus, the Flash Flood
Watch has been extended east by one row of counties in the Hill
Country to account for heavier rainfall totals. Along and east of
I-35, lower 1-3 inch totals are expected through this evening as
storms should move fast enough to prevent higher totals.
Heavier rainfall totals did occur over the western Hill Country
and Edwards Plateau this morning where widespread 2-4 inch totals
fell with isolated areas receiving over 6 inches. The hardest hit
area appears to be in eastern Edwards County where up to 8 inches
of rain has fallen causing the closure of all roads into and out
of Rocksprings. Heavy rain has also prompted closures in parts of
Real and Kinney Counties. These areas have stabilized in the wake
of the complex that passed through as stratiform rain falls, so
additional convective development is not expected there for the
next few hours. However, stronger isentropic ascent will arrive
this evening as the synoptic cold front along the TX-OK border
arrives and clearing prior to sunset may allow destabilization to
occur. Any additional rainfall would accentuate ongoing flooding
in these areas, so the Flash Flood Watch remains in effect there.
The greatest potential for locally heavy rainfall through Monday
morning is in the Rio Grande Plains, particularly along and south
of the weak surface boundary oriented near US Highway 57. Although
the cutoff low is retrograding further to the west, it has kicked
off a shortwave that is helping generate showers and thunderstorms
in Coahuila. All models continue to lag behind the actual
progression of this system, but a few hi-res models continue to
suggest that some training of storms could occur along the surface
boundary with additional dynamic support aloft from the shortwave.
Thus, we have included Maverick, Zavala, Dimmit, and Frio Counties,
but the threat should remain low in over the next few hours if
the surface boundary continue to propagate south and east. As the
storms over Mexico expand and move north as they deepen by this
evening, heavier convective rainfall may occur over the Rio Grande
Plains and move into Val Verde County and the Edwards Plateau as
rain becomes more stratiform in nature. However, confidence is low
in this solution given all models continue to be less progressive
with the overall system and front than has occurred today, and the
passage of the front would reduce the heavy rainfall threat.
Scattered to widespread rain showers with isolated thunderstorms
will be possible across the region tomorrow due to isentropic
ascent allowing for moisture to overrun the front. However, rain
totals should remain below a quarter inch for most areas except
closer to the Rio Grande Plains where heavier rainfall closer to
an inch will be possible. Cloud cover and N-NE winds will cause
much cooler high temperatures in the 70s and lower 80s on Monday
with perhaps upper 60s in the Edwards Plateau.
LONG TERM (Tuesday through Sunday)...
More tranquil and pleasant fall weather is on the way once rain
chances end from northeast to southwest Monday evening through
Tuesday. An upper level ridge will materialize between the
retrograding closed low in the Pacific and another closed low
digging from the Midwest into the Southeast U.S. by Wednesday.
This digging low will allow reinforcing shots of cooler air to
keep temperatures slightly below normal in the 80s with clear
skies and dry conditions through next weekend. Low temperatures
will be in the mid 50s to mid 60s for much of next week due to
clear skies and continued weak northerly winds overnight. The best
time of year weather-wise in South Central Texas has arrived!
.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry 85 69 77 63 79 / 60 70 50 20 10
Austin Bergstrom Intl Airport 82 69 78 63 80 / 60 70 50 20 10
New Braunfels Muni Airport 90 69 78 63 81 / 60 70 60 30 20
Burnet Muni Airport 83 65 72 60 77 / 80 80 50 20 10
Del Rio Intl Airport 74 68 72 60 72 / 90 70 70 60 30
Georgetown Muni Airport 82 67 74 62 78 / 60 80 40 20 10
Hondo Muni Airport 89 70 78 63 80 / 60 80 60 40 20
San Marcos Muni Airport 89 69 78 63 80 / 60 70 50 30 20
La Grange - Fayette Regional 91 71 82 67 82 / 60 70 50 20 10
San Antonio Intl Airport 84 70 78 64 80 / 60 80 60 40 20
Stinson Muni Airport 92 72 80 66 82 / 60 70 60 40 20
Flash Flood Watch through Monday morning for the following
Public Service/Data Collection...30