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

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FXUS64 KEWX 201814
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1214 PM CST Sat Jan 20 2018

.AVIATION.../18Z_TAFS/
Ceilings and visibilities are in the process of rapidly lifting at
all TAF sites from the persistent LIFR conditions we had most of this
morning, with all sites currently having IFR ceilings. DRT should
quickly transition to VFR by 19Z with AUS becoming MVFR between
18-19Z and most likely VFR between 19-20Z. Ceilings will take a bit
longer to recover at SAT and SSF as their ceilings are currently
lower due to southerly low-level flow being more perpendicular to the
Escarpment with greater coverage of low clouds to their south. Model
guidance has a wide spread of scenarios for SAT/SSF, so for now their
TAF splits these differences down the middle with IFR conditions by
19Z and MVFR conditions between 20-21Z. Although they may see VFR
conditions later this afternoon, for now we have a more pessimistic
forecast with broken ceilings above 2000 feet. All sites may see
brief S-SW wind gusts of 15-20 knots over the next few hours as the
boundary layer mixes out and cloud decks lift.

Another round of advection fog with patchy dense fog, drizzle, and
LIFR ceilings is expected once again tonight at all sites by 7Z with
IFR conditions returning by 2-3Z at the I-35 sites. Winds should
stay up around 5 knots and become more southeasterly at all sites
except maybe AUS. Model guidance is generally forecasting dense fog
tonight at all sites except AUS, but for now only have 1/2 SM at
SAT/SSF because advection fog with drizzle makes persistent dense fog
unlikely even though it did occur for about an hour at SSF this
morning. Confidence in dense fog occurring is lowest at AUS as
streamlines will be a bit more southerly there and it may take more
time for the boundary layer to decouple considering they will have
greater clearing this afternoon. Visibilities and ceilings should
lift to VFR by 17Z at DRT and IFR around 17-18Z at the I-35 sites as
a pre-frontal trough and wind shift pushes into the region with some
light shower activity. VFR conditions are expected behind this line
of showers by mid-afternoon with thunderstorms most likely holding
off until the line is east of the I-35 sites late in the afternoon.


&&

.PREV DISCUSSION... /ISSUED 905 AM CST Sat Jan 20 2018/

UPDATE... /DENSE FOG ADVY CLEARED HC & EDWARDS PLATEAU/

Areas north of a DRT to SAT to AUS line have seen steady improvements
to vsbys while southern parts of San Antonio remain in dense fog.
Will expect to see fog to begin lifting over the Coastal Prairies and
Rio Grande Plains in the next 1-2 hours. A slight adjustment also was
made to weather grids to shift the placement of patchy drizzle this
morning. Rest of the forecast is on track.

PREV DISCUSSION... /ISSUED 542 AM CST Sat Jan 20 2018/

UPDATE...
12Z Aviation forecast below.

AVIATION...
Widespread LIFR conditions are ongoing across South Central Texas
early this morning. Ceilings and visibility will gradually rise to
IFR/MVFR between 16Z-18Z, with DRT going VFR around 19Z and SAT/AUS
potentially going VFR around 20Z-21Z. IFR ceilings and MVFR
visibility in low stratus and fog will then re-develop back over
SAT/AUS 01Z-03Z and then become LIFR again overnight, along with DRT.
Winds S to SW 5-10 kts at SAT/AUS and SE 5-10 kts at DRT.

PREV DISCUSSION... /ISSUED 334 AM CST Sat Jan 20 2018/

SHORT TERM (Today through Sunday)...
Upper air analysis last evening showed a trough along the Texas coast
and a ridge extending from northwestern Mexico into Oklahoma. At the
surface, winds across our CWA were generally light from the south to
southwest. The moist flow has reduced dew point depressions to zero
over much of the area and fog is widespread. Some places will have
visibility reduced to a quarter of a mile or less and a dense fog
advisory is in effect until 11 AM. The upper ridge will move across
Texas today. The low level flow will continue from the south and
temperatures will warm significantly with highs around 70 over most
of the area. Tonight an upper level trough will move across the
Rockies and bring a Pacific cold front into West Texas. There will be
a slight chance for showers ahead of this front mainly across the
east where moisture will be deepest. Widespread fog is also likely
with the strong moist flow. Sunday the front will move across our CWA
bringing a better chance for showers. There will also be isolated
thunderstorms over the eastern half of the area during the
afternoon.

LONG TERM (Sunday Night through Friday)...
Showers and isolated thunderstorms will continue Sunday evening as
the front moves away to the east and the upper trough axis moves
across Texas. Low level flow behind the front will be northwesterly
bringing drier air. This will produce low relative humidity and
elevated fire weather conditions Monday afternoon (see Fire Weather
discussion below). Monday the upper pattern will become nearly zonal
leading to a dry period. Dry weather will continue through the
middle of the week. The low level flow will be mainly easterly and
temperatures will be near normal through Thursday. Thursday a shallow
upper level trough will move over Texas bringing a slight chance for
showers Friday.

FIRE WEATHER...
A cold front on Sunday will bring drier air with northwesterly winds
Monday. Temperatures will only be around five degrees cooler and this
will allow relative humidity to drop below 20% across the western
half of the CWA Monday afternoon. 20ft winds will be 10 to 15 mph
during this time. This will mean elevated fire weather conditions.
Low relative humidity will retreat northward Tuesday afternoon and
winds will decrease across the region for better fire weather
conditions.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              70  58  72  45  64 /  10  20  60  20   0
Austin Bergstrom Intl Airport  70  58  73  45  64 /  10  30  60  30   0
New Braunfels Muni Airport     69  57  72  45  65 /  10  30  50  20   0
Burnet Muni Airport            72  53  71  40  62 /  -   20  30  10   0
Del Rio Intl Airport           67  48  72  42  66 /   0  20  10   0   0
Georgetown Muni Airport        71  56  71  42  63 /  10  20  60  20   0
Hondo Muni Airport             69  53  73  41  68 /  -   30  40  -    0
San Marcos Muni Airport        70  58  72  45  65 /  10  30  50  20   0
La Grange - Fayette Regional   71  60  74  47  66 /  20  20  50  50   0
San Antonio Intl Airport       70  57  72  46  66 /  10  30  40  10   0
Stinson Muni Airport           69  57  73  45  67 /  10  30  40  10   0

&&

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

$$

Mesoscale/Aviation...LH
Synoptic/Grids...Oaks
Public Service/Data Collection...Treadway



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