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

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FXUS64 KEWX 191537

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1037 AM CDT Fri May 19 2017

Latest aircraft soundings continue to show a stout capping inversion
in place across South-Central Texas. The NAM which has been best with
the handling of the CAP also keeps it during the day and the latest
high-res model guidance shows little to no activity today. There are
some areas of light rain or drizzle on area radars which would be
rooted below the CAP but this should not amount to much. Trimmed up
PoPs a bit and will continue to show the higher PoPs across the Hill
Country in closer proximity to the upper low and where the CAP should
be weakest. Shear and instability are weaker than yesterday but
would still be sufficient for possible severe hail if anything were
to develop. Updated products have been sent.


.PREV DISCUSSION... /ISSUED 632 AM CDT Fri May 19 2017/

12Z Aviation Update Below.

MVFR stratus prevails across South Central Texas early this morning.
VFR conditions will re-develop areawide 17Z-19Z. SE winds generally
8-12 kts early this morning, increasing across the Hill Country and
along and east of I-35 late morning, with a few gusts 20-25 kts in
the afternoon. SHRAs and TSTMs across West Central Texas should stay
north of the TAF sites today. MVFR stratus will redevelop 03Z-07Z
and possibly become IFR early Saturday morning as low level flow

PREV DISCUSSION... /ISSUED 418 AM CDT Fri May 19 2017/

SHORT TERM (Today through Saturday)...
A similar atmospheric set up will occur today compared to yesterday
with a slight and marginal risk for severe weather positioned west of
the I-35 corridor. Also similar to yesterday, the million dollar
question will be whether or not capping can be overcome. Thus far
this season, models have been notoriously poor resolving cap strength
and seems to require more than broad or weak upper level lift to
overcome them. To make matters worse today, there seems to be very
little agreement in forecast soundings this afternoon with respect to
the cap. GFS proximity soundings along the SJT/EWX border north of
Fredericksburg (the southern edge of the slight risk) show a 00Z
profile uncapped, the RAP is essentially isothermic or weakly
inverted, and the NAM holds onto a strong inversion at the 700 mb
level...illustrating just how difficult it is to forecast these low
probability but high impact days as we`ve had.

The hi-res model suite this morning varies as well, with solutions
more or less in agreement on continuing to develop convection along
the dry line currently paralleling I-20 in SJT/MAFs neck of the
woods. Outflow from this activity should continue to push
southeastward through the day while daytime heating ensues. The main
question will be if storm outflow alone will be enough to overcome
our capping today. If the aforementioned NAM solution is to be
believed, likely things will stay quiet and the depth of the
inversion will be too much for outflows to make much of a difference.
Weaker strength capping could be influenced by this outflow however.
TTU-WRF is the most aggressive with CI this afternoon, but it has
whiffed on similar situations over the last week. Believe the most
reasonable situation is weak convection failing to overcome the cap
and resulting in not much more than showers with occasional lightning
this afternoon over the Plateau and Hill Country. However, IF an
updraft can get going this afternoon, very large hail will be the
biggest concern, with straight line winds a secondary threat.

Moving into the weekend, the overall pattern has not changed
substantially with the well advertised cold front expected to still
come through Saturday morning and bringing showers and thunderstorms
across the region. However, with Saturday now being within the realm
of the hi-res model suite, it is concerning how little QPF is being
advertised. GFS/ECMWF/Canadian all continue to advertise a widespread
QPF shield spanning from Del Rio through the ArkLaTex but this is
contrary to output from the 4km NAM, TTU-WRF, ARW/NMM and SREF. Thus,
did not opt to raise PoPs over the weekend any higher than they
already are, despite Superblend`s suggestion. Will continue to lean
towards the more consistent synoptic model trends of 1-3 inches for
now but if a consistently weaker QPF trend continues, rainfall totals
for Saturday/Sunday may need to be scaled back.

LONG TERM (Saturday Night through Thursday)...
Models continue to keep the latter half of the weekend wet and even
persist rainfall chances through at least Tuesday with a weak
Pacific shortwave moving across Mexico and entering the area Monday.
This may end up being the more significant rainfall event if the
cold front underperforms. GFS/ECMWF/Canadian all seem to be in good
agreement of this shortwave producing healthy QPF beginning along the
Rio Grande late Sunday night into Monday morning and spreading east
to the rest of the CWA over the course of the day Monday and
eventually exiting east Tuesday as our next cold front approaches
from the north. It is at this point that model outcomes diverge as
the GFS produces even further QPF associated with the cold front
passage Tuesday night but the ECMWF keeps things dry. Have trended
the forecast for mid week to lean more towards the drier ECWMF as the
main upper low should take much of the moisture along with it.


Austin Camp Mabry              89  73  83  66  81 /  10  40  50  60  30
Austin Bergstrom Intl Airport  89  73  84  66  82 /  10  40  50  60  30
New Braunfels Muni Airport     89  73  85  67  82 /  10  30  50  60  40
Burnet Muni Airport            85  69  79  63  79 /  30  50  50  60  30
Del Rio Intl Airport           91  71  82  68  83 /  10  30  50  60  40
Georgetown Muni Airport        87  71  81  64  80 /  20  40  50  60  30
Hondo Muni Airport             90  73  84  68  83 /  20  30  50  60  40
San Marcos Muni Airport        90  72  83  67  81 /  10  30  50  60  40
La Grange - Fayette Regional   90  74  86  69  83 /  10  20  50  60  40
San Antonio Intl Airport       90  74  85  68  82 /  10  30  50  60  40
Stinson Muni Airport           90  74  86  69  83 /  10  30  50  60  40




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