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

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FXUS64 KEWX 200546

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1246 AM CDT Sun May 20 2018

All terminals are currently VFR. We expect ceilings in Austin and
San Antonio to drop to MVFR within the next hour or two. There is a
weakening line of showers and thunderstorms approaching the San
Antonio area from the west. We`re not sure if these storms will hold
together long enough to move across SAT and SSF. Maybe a better
chance for SSF to see TSRA between now and 09Z. A stronger showers
will drop visibility to MVFR. These storms should be either
dissipated or moved to the east by 09Z. Ceilings will be MVFR
overnight and then improve to VFR by late morning. There will be
another chance for convection during the late afternoon in Austin and
San Antonio. Storms will drop the visibility to MVFR.

At DRT, the ceiling will drop to MVFR by around 08Z and rebound to
VFR by early afternoon. The chance for convection at DRT will be
lower than farther east, so we have not included any mention in the
TAF. There will be a slight chance for showers or thunderstorms to
lower the visibility to MVFR during the late afternoon.


.PREV DISCUSSION... /ISSUED 836 PM CDT Sat May 19 2018/

Repeated runs of the HRRR are falling in alignment with some of the
earlier HREF runs suggesting a small complex holding together through
the late evening before weakening. PoPs were raised to likely over a
small area of the southern Edwards Plateau, and nudged up slightly
for areas just west of San Antonio. Overnight PoPs were adjusted up
farther east to follow the trend of the weakening storm complex.
The HRRR trends indicate chances may need to increase over the
Coastal Prairies toward daybreak perhaps as streamer showers, but
without much support from other models and lack of an expected
surface or mid-level focus, will disregard that feature for now.

PREV DISCUSSION... /ISSUED 255 PM CDT Sat May 19 2018/

SHORT TERM (Tonight through Sunday Night)...
For the immediate short term, the focus is mostly out west where
isolated convection is expected to develop along another dry line
positioned west of Val Verde county. CAMs are having a hard time
pinpointing CI this afternoon, with many like the RAP/TTU-WRF/HRRR
keeping CI along the dry line farther west, but other CAMs including
the ARW/NMM initiate discrete cells near Del Rio after 21Z. Looking
at latest GOES VIS imagery, you can clearly see an inversion holding
steady over that region, and RAP forecast soundings show that won`t
change. However, NAM/GFS soundings DO erode the inversion.
Nevertheless, very strong mid level lapse rates, abundant MUCAPE
availability, and 30+ knots of 0-6 km effective shear check the boxes
for the potential for any CI to occur to be severe with large hail
potential and damaging straight line winds, especially given the very
dry low level atmosphere hydrometeors would fall into (inverted V
sounding). Slightly increased wording in the HWO to reflect these
concerns and went with a 40 PoP max out west with decreasing PoPs
farther east. It is worth noting that no CAM maintained convection to
the I-35 corridor overnight so this threat looks like it should be
confined to areas west.

The next question is the potential for convection farther north to
push southward and impact the area towards tomorrow morning. There
seems to be pretty good agreement that the front will stall well to
the north of the CWA, more along I-20. However, the concern would be
organized convection developing into an MCS and pushing southward.
There are indications in both the HRRR and WRFs that this would be
the case, but losing it`s parent lifting mechanism and being
primarily outflow driven, it should be sub-severe by the time it
reaches the Plateau, which is progged at roughly 15-18Z tomorrow.
This system will complicate pinpointing the best location for
potential re-development tomorrow afternoon and will likely rest on
the positioning of the remnant outflow from this system. Thus, PoPs
exist just about CWA-wide tomorrow, with a slight preference to the
north and west.

LONG TERM (Monday through Saturday)...
Scattered convection will be in the forecast through the weekend
before PoP chances decreases by Tuesday. Between 00Z tonight and 12Z
Tuesday, we are calling for about a quarter and a half inch of rain
across the area which will be quite nice for the region which is
mostly behind the monthly average to date.

Synoptically, a stubborn ridge axis over Mexico will hold off the
broad low out west from bringing its shortwaves close enough to our
area to bring meaningful rain to the area through the week next week.
Looks like eastern New Mexico and West Texas will stand to benefit
the most as a result. That low lifts north and rides along the
strengthening ridge axis by late week and should keep the region dry
through the weekend. Long range models bring a weak cold front
through next Saturday but moisture availability will be in question.
If a dry passage, fire weather may be the biggest concern next


Austin Camp Mabry              88  71  89  70  91 /  40  30  30  10  10
Austin Bergstrom Intl Airport  87  70  89  68  90 /  40  30  30  10  10
New Braunfels Muni Airport     88  71  89  69  90 /  40  20  30  10  10
Burnet Muni Airport            85  68  86  67  88 /  50  40  30  10  10
Del Rio Intl Airport           91  71  88  71  89 /  30  40  40  20  20
Georgetown Muni Airport        87  69  88  69  90 /  40  30  30  10  10
Hondo Muni Airport             90  71  89  69  90 /  30  40  30  10  10
San Marcos Muni Airport        89  70  88  69  90 /  40  20  30  10  10
La Grange - Fayette Regional   89  72  89  71  90 /  30  20  30  10  20
San Antonio Intl Airport       88  72  88  71  89 /  40  30  30  10  10
Stinson Muni Airport           89  73  89  71  91 /  30  20  30  10  10




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