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

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078
FXUS64 KEWX 171523
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1023 AM CDT Thu Aug 17 2017

.UPDATE...
Low temperatures and overnight dewpoints were higher than expected
which should lead to slightly higher values of moisture this
afternoon. In addition, southwest winds at the surface may lead to
higher high temperatures as well today. Upped high temps and
dewpoints across the eastern counties and this should lead to heat
advisory criteria being exceeded in the far eastern counties. Went
ahead and issued a heat advisory for Lee, Fayette, Lavaca, Gonzales,
and DeWitt County.

&&

.PREV DISCUSSION... /ISSUED 617 AM CDT Thu Aug 17 2017/

UPDATE...
12Z Aviation Update Below.

AVIATION...
MVFR ceilings in stratus continues early this morning along the
escarpment edge into portions of the Hill Country. This includes
SAT/SSF and at times AUS. There is a chance the stratus may develop
into DRT around or after 12Z but lower confidence in this. VFR
conditions redevelop 15Z-17Z as stratus mixes out. Generally SHRAs
and TSTMs should stay north and west of the area and TAF sites today.

PREV DISCUSSION... /ISSUED 350 AM CDT Thu Aug 17 2017/

SHORT TERM (Today through Friday)...
High pressure aloft remains dominant over most of South Central TX,
with some subtle instability hanging on over parts of the Edwards
Plateau into Nrn Mexico early this morning. A tropical moisture axis
and weak mid-level cyclonic shear over this region has allowed for a
few elevated overnight showers generally north of Rocksprings and
West of Del Rio. Satellite imagery loops depicting mid level clouds
and rapid refresh models would suggest some cyclonic shear rooted
over the Serranias del Burros will extend into swrn parts of Val
Verde county to allow for continued weak elevated convection into the
daytime.

The mid level ridge axis over Central Tx will both strengthen and
build northward today and tonight, leading to reduced surface winds
in the late afternoon and evening over the next couple days. While
the trend for very gradual lowering of day-to-day heat indices should
continue through afternoon mixing and lack of rainfall, triple digit
heat index values remain in the picture. Those spending several
hours outdoors in the afternoon and evening along and east of I-35
may not sense the slight improvement due to the lighter winds from
the seabreeze. Will continue show the elevated heat indices in the
SPS/HWO.

LONG TERM (Friday Night through Wednesday)...
The mid-level ridge axis remains entrenched over Central TX over the
weekend, with the orientation tilting as heights build nwd over the
Srn Rockies Saturday. By Monday, a Pacific upper low erodes this wrn
build-up again and a TUTT low over the Gulf pinches off the eastern
edge of the ridge axis. In the center, the upper ridge remains
dominant over TX with only subtle changes in the surface winds
expected early next week versus this weekend.

Some deterministic data anticipates a surge in pwat values to
generate rain chances, but knowing subsidence enhancement precedes
the approach of a TUTT from the east, the drier GFS looks most
probable. None of the above mentioned mid/upper level pattern shifts
should put a significant dent in the triple digit heat and/or heat
indices through at least Tuesday.

Model diverge a bit on the broader mid-level pattern on Wednesday
with the GFS showing an inverted trough over S Tx left from the
weakening TUTT and the ECM depicting potential instability arriving
from the north after a westward shift of the ridge aloft. Will give
in to a generic isolated rain chance for Wednesday for all areas as
either of these solutions seem plausible.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              77 100  77 100  76 /  10   0   0  -   10
Austin Bergstrom Intl Airport  76  99  76 100  75 /  10   0   0  -   10
New Braunfels Muni Airport     75  99  75 100  75 /  -    0   0  -   10
Burnet Muni Airport            74  98  74  98  74 /  10   0   0  -   -
Del Rio Intl Airport           78  99  78 101  78 /   0  -    0  -    0
Georgetown Muni Airport        76  98  75  98  75 /  10   0   0  -   10
Hondo Muni Airport             75 100  74 101  74 /   0   0   0   0  -
San Marcos Muni Airport        76  99  75 100  75 /  10   0   0  -   10
La Grange - Fayette Regional   77  99  76 100  76 /  10  -    0  10  10
San Antonio Intl Airport       76  99  76  99  76 /   0   0   0   0  10
Stinson Muni Airport           76 101  76 101  76 /   0   0   0   0  10

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from 2 PM this afternoon to 7 PM CDT this evening for
the following counties: De Witt...Fayette...Gonzales...Lavaca...Lee.

&&

$$

Mesoscale/Aviation...LH
Synoptic/Grids...Hampshire



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