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

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FXUS64 KEWX 200026

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
726 PM CDT Thu Oct 19 2017

Shower activity remains east of our CWA where the best low-level lift
is confined. Should be after midnight before rain chances pick up in
our CWA associated with weak isentropic lift and have opted to break
apart the PoP grid for tonight to show increasing chances after 3 AM.
Model and aircraft soundings show a strong CAP around 750 mb which
will limit thunder chances and have tapered back the mention of
thunder to just isolated within areas of chance showers. Only other
adjustments were to hourly grids based on current trends.


.PREV DISCUSSION... /ISSUED 659 PM CDT Thu Oct 19 2017/

VFR conditions will continue this evening as southeasterly 10-15 knot
flow gradually decreases to 5-10 knots later tonight. MVFR ceilings
are expected by 6Z at SAT/SSF and 8Z at DRT and AUS before lowering
further to IFR with patchy advection fog by 10Z at all sites except
AUS where IFR ceilings should be more delayed if they develop. Model
streamlines suggest that AUS may not have a strong onshore component
to their low-level flow tonight relative to SAT/SSF and DRT where
LIFR conditions will be possible closer to sunrise. However, for now
we have IFR ceilings at all sites with just a shorter duration at
AUS. Streamer showers and 5-10 knot winds expected tomorrow morning
at the I-35 TAF sites should help prevent dense fog from developing.
Convective activity tomorrow afternoon should generally remain east
of the I-35 TAF sites, so we have kept this out of the TAFs for now.

PREV DISCUSSION... /ISSUED 252 PM CDT Thu Oct 19 2017/

SHORT TERM (Tonight through Friday Night)...
A low level trough moving onshore from the Gulf will bring a surge of
tropical moisture into the Coastal Prairies this afternoon and
evening with scattered showers and isolated storms expected to spread
inland to near the I-35 corridor by early this evening. The coastal
troughing feature is expected to dissolve gradually but continue
funneling a moisture axis generally up the I-37 corridor into the
Hill Country later tonight.

Chance category PoPs concentrate well to the SE of San Antonio
through sunset as the cyclonic surface shear zone is shown by higher
res models move generally toward VCT through this time. Afterwards
qpf fields from mesoscale model runs vary wildly as the moisture
surge broadens and begins to interact with a positively tilted upper
troughing feature and nocturnal low level wind increases in the
boundary layer. Typical streamer shower activity is expected in the
overnight hours, but upper level instability present aloft, an
isolated overnight storm can`t be ruled out.

The initial perturbation associated with the upper trough moving
into TX lifts northeast and pulls the deeper moisture axis into SE TX
by late Friday morning. Model consensus indicates that pwat values
over 1.5 inches are generally held south/east of the Balcones
Escarpment, but values between 1 and 1.5 inches could still be enough
for the upper troughing feature to generate either elevated showers
moving in from the west or some some streamer showers along and
north of the escarpment. In any event, rainfall amounts should be
mostly less than 1/10 inch over the central and eastern two-thirds of
the forecast area.

LONG TERM (Saturday through Thursday)...
The core portion of the approaching upper trough should become the
dominant feature by early Saturday and maintain a near static
moisture profile over South Central TX with the deepest moisture
pinned east of I-35. While some model data hinted at deeper low level
mixing to set up a dry-line environment, the more accepted scenario
such as the 12Z gfs keeps a fairly deep low level moisture layer with
the pattern favoring more of a capped environment. Thus PoPs over
the western two-thirds of the forecast area remain limited, and this
doesn`t chance with the approach of the cold front late Saturday
night. Some strong storms are forecast by SPC for North Central TX,
but the timing of the front should reflect mostly weakening cells in
the northern counties over the late night hours as the front arrives.
The front should generally arrive into the I-35 corridor by around
12z and quickly dry out the area by midday. Some western counties may
need to consider elevated fire weather conditions, but presence of
frontal rain chances and good cold air advection accompanying the
dry-out leaves confidence poor.

A broader trough over the Nrn US deepens and sharpens in the wake of
the departing low latitude low Monday with another front to follow on
Tuesday morning. Thus the first 3 or 4 days of the work week next
week should remain cool and dry.


Austin Camp Mabry              67  81  71  85  67 /  20  40  30  30  70
Austin Bergstrom Intl Airport  67  81  70  85  66 /  20  40  30  30  70
New Braunfels Muni Airport     67  82  70  86  68 /  30  30  20  30  50
Burnet Muni Airport            65  78  68  83  62 /  20  30  20  20  70
Del Rio Intl Airport           67  81  68  86  65 /  10  20  10  10  30
Georgetown Muni Airport        66  79  70  84  64 /  20  40  30  30  70
Hondo Muni Airport             67  84  69  88  67 /  30  30  10  10  50
San Marcos Muni Airport        67  81  70  86  67 /  30  40  30  30  60
La Grange - Fayette Regional   68  82  70  86  69 /  30  60  30  40  60
San Antonio Intl Airport       70  83  70  87  68 /  40  30  20  20  50
Stinson Muni Airport           70  85  71  88  69 /  30  30  20  20  50




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