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

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681
FXUS64 KEWX 161737
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1237 PM CDT Wed Aug 16 2017

.AVIATION...
VFR conditions are prevailing at all TAF sites at the present time
and this should persist through the afternoon and evening hours.
Increasing low-level flow will allow the return of MVFR stratus to at
least the I-35 sites and perhaps DRT around sunrise tomorrow morning.
Otherwise, southerly winds will be around 10 to 20 knots for much of
the period.

&&

.PREV DISCUSSION... /ISSUED 1147 AM CDT Wed Aug 16 2017/

UPDATE...
A weak trough moving through the Texas panhandle is helping trigger
isolated showers mainly along and west of a line from Del Rio to
Kerrville to Llano. A few thunderstorms have briefly developed in
Mexico just southwest of Del Rio, but quickly dissipated due to a
fairly strong cap just above the boundary layer. Overcast skies
should continue for the next few hours for these areas due to weak
upper level divergence in the fringe of the right entrance region of
a jet streak moving through the Texas panhandle. However, skies
should eventually clear as we are already seeing some signs of the
cloud deck retreating north and west as this feature lifts to the
northeast. Thus, temperatures over the western Hill Country and
Edwards Plateau will be cooler in the lower to mid 90s, while most
other spots should still see upper 90s to lower 100s. Observed dew
points have been running about a degree cooler on average than what
was forecast for areas along and east of the I-35 corridor, so we
generally expect of these areas will have heat indices peaking in the
105-107 range with a few spots peaking in the 108-110 range mainly
in the Coastal Plains. Thus, for now we will stick with the current
SPS instead of issuing a Heat Advisory as confidence is low that the
criteria will be met over more than isolated locations.

PREV DISCUSSION... /ISSUED 626 AM CDT Wed Aug 16 2017/

UPDATE...
12Z Aviation Update Below.

AVIATION...
BKN MVFR stratus has developed along the edge of the Escarpment into
the Hill Country, including AUS and SAT. The stratus remains just off
to the south and east of DRT, but there is a slight chance it could
work into the DRT area over the next couple of hours. Due to confidence
have left a SCT group at DRT. The stratus should will mix out 15Z-17Z
with VFR conditions and southeast winds gusting at times to around
20 kts through the afternoon. Isolated SHRAs and TSTMs are possible
along the Rio Grande and western Hill Country today. Have left this
mention out of the DRT TAF due to expected coverage and lower
confidence in timing.

PREV DISCUSSION... /ISSUED 343 AM CDT Wed Aug 16 2017/

SHORT TERM (Today through Thursday)...
Another afternoon of near Heat Advisory criteria to consider. A
factor that could impact heat indices is a weak disturbance
currently producing shower activity over the Hill Country. This
disturbance is loosely tied to a larger/stronger disturbance over the
Central Plains which have collectively suppressed ridging aloft over
TX and enhanced the low level pressure gradients. Thus as this ill-
defined weakness lifts and shears apart northward today, low level
winds may ease up, leaving an air-mass that does not mix as it did on
Tuesday. Residual high clouds spilling south from this weakening
disturbance could also reduce mixing, but also could reduce ambient
temperatures. Given the expected borderline values, some model
tendencies to under-forecast the amount of mixing, and the
expectation of another gusty wind afternoon, will punt the idea of a
Heat Advy to the day shift and issue another SPS.

Will expect a near persistence day for most areas, but may need to
update and lower max temps over northern counties if the stratiform
precip area this morning doesn`t dissipate soon. Residual moisture
over the north, some early morning cloud streets showing up on
radar over the far SE, and isolated activity suggested by rapid
refresh models have us going slightly less stable with two areas of
isolated thunderstorm chances today.

Consensus of synoptic models show the mid to upper level ridge axis
sharpening and lifting northward today into tonight, so expect a
drier picture and possibly higher ambient temperatures with lower dew
points for Thursday.

LONG TERM (Thursday Night through Tuesday)...
The ridge axis aloft remains oriented mostly E-W across TX with near
persistence heat and dry weather to continue through the weekend. On
Monday, a TUTT low over the Gulf approaches the TX coast and could
send enough of a moisture surge for some isolated Coastal Prairie
convection Monday and Tuesday. The low appears to be driving west
into an upper ridge that is strengthening over the Swrn US over the
weekend, so will remain conservative this pattern and only consider
a slight chance of rain for the Sern counties.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              99  78  99  77 100 /  -   -   -   -   -
Austin Bergstrom Intl Airport  99  77  98  76 100 /  10  -   -    0  -
New Braunfels Muni Airport     99  76  99  75 100 /  -    0  -    0  -
Burnet Muni Airport            93  75  96  74  98 /  10  10  10  -   -
Del Rio Intl Airport           98  80 100  79 100 /  30  10  10   0  -
Georgetown Muni Airport        96  77  97  76  98 /  10  10  10  10  -
Hondo Muni Airport             99  76  99  75 101 /  10  -    0   0  -
San Marcos Muni Airport        99  77  98  76 100 /  -   -   -    0  -
La Grange - Fayette Regional   97  78  98  76  99 /  10  -   -   -   10
San Antonio Intl Airport       98  78  98  77 100 /  -   -   -    0  -
Stinson Muni Airport          100  77 100  77 100 /  -    0  -    0  10

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

Mesoscale/Aviation...Hampshire
Synoptic/Grids...LH
Public Service/Data Collection...33



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