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

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FXUS64 KEWX 181203

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
603 AM CST Mon Dec 18 2017

Low clouds and widespread advection fog (including patchy dense fog)
has allowed for LIFR or IFR visibilities and ceilings to settle in
over all of our TAF sites this morning. A strong frontal temperature
inversion from 1000-3500 feet will remain over the region today to
prevent conditions from improving much today. However, model guidance
has generally been a bit more pessimistic than the observations, so
our forecasts are a bit more optimistic than most model guidance.
MVFR conditions may occur today at AUS and DRT from around 18-03Z and
21-02Z at SAT/SSF before ceilings and visibilities rapidly go back
down to LIFR-IFR as widespread fog settles back over the region.

Scattered to widespread showers should develop to our southwest
tonight as an upper level disturbance approaches from the west and
isentropic ascent increases with a warm front pushing north. Hi-res
and global models both differ on the onset of rain showers at our
TAF sites, but for now we have leaned towards an earlier onset of 5Z
at DRT, 7Z at SAT/SSF, and 9Z at AUS. Showers should clear DRT around
sunrise tomorrow, but they may continue into the afternoon for the
I-35 TAF sites. Isolated thunderstorms are also expected due to the
presence of elevated instability aloft as lapse rates steepen ahead
of the approaching upper low, but confidence in thunderstorms at any
one terminal is too low to include them in the TAFs at this time.


.PREV DISCUSSION... /ISSUED 418 AM CST Mon Dec 18 2017/

SHORT TERM (Today through Tuesday)...
Widespread fog ongoing this morning with patchy drizzle. The fog is
dense in several locations and will maintain the Dense Fog Advisory
through 9 AM. There is the potential for the fog and low clouds to
persist longer in locations given the strong inversion. With this in
mind sided closer to cooler guidance for high temps today. Precip
chances today through early evening should be mainly isolated given
the inversion and awaiting on the main forcing arriving late evening
and overnight.

Late tonight, synoptic scale ascent will increase quickly from
southwest to northeast across the CWA in advance of the upper level
low elongating and moving into West Texas. Models are in good
agreement with widespread elevated showers developing across the
southwest CWA between 03Z-06Z and spreading quickly into the Hill
Country and I-35 corridor 06Z-09Z and persisting through Tuesday
morning. Given the strong forcing seen in forecast point time
series, and combined with deep layer shear values of 50-60 kts and
mid level lapse rates of 6-7 deg C/km, should see some embedded
elevated thunderstorms. Can`t completely rule out an isolated storm
or two producing small hail.

Tuesday afternoon a front works through western areas of the Hill
Country and Rio Grande and dry slotting should shut off precip
chances quickly across the western CWA. As the front moves east of
I-35 and I-37 a broken line of showers and storms is forecast to
develop and become surface based mid to late afternoon across the far
eastern CWA, where a pool of upper 60s dew points reside. MLCAPE
values around 1000 j/kg and deep layer shear values around 50 kts
will be sufficient to produce a risk for strong to possibly severe
storms during a small window around 21Z-00Z, mainly near and east of
U.S. 77, and conditional on morning and early afternoon precip
shield staying far enough north to allow for warm sector and
instability pool to develop. SPC Day 2 convection outlook clips
Fayette and Lavaca counties in a Marginal risk for severe storms.

Finally, while the overall nature of the system will be progressive,
can`t rule out some isolated locations seeing minor flooding issues
given how wet it`s been recently. The more favored locations for
this would be across Central Texas and near and east of I-35 and I-37
where PWAT values of 1.5-1.8" are pooled, near record high values for
this time of year.

LONG TERM (Tuesday Night through Sunday)...
Dry air will quickly filter into eastern areas of the CWA Tuesday
evening behind the front ending rain chances. Warmer and drier
conditions are then forecast Wednesday and Thursday.

A cold front is forecast to move through the area on Friday. Have
slided slightly closer to the ECMWF with frontal timing Friday
morning. Low chances for showers are forecast along the front on
Friday. Models continue to indicate the upper level pattern more
progressive than what was indicated a couple of days ago. As such the
forecast for Friday night and Saturday remains cool and dry.

A stronger cold front is forecast to move through the area late Saturday
night or Sunday. The ECMWF is slower and allows for a better return
flow across the east to set up in advance of the front, and as such
develops showers across the east on Sunday. However, given the degree
of troughing to the northwest and shallow, dense nature of the
airmass am siding slightly toward the faster and drier GFS, however
confidence in the precip forecast is low. It does appear at this time
Christmas Eve and Christmas day will be cool.


Austin Camp Mabry              60  57  66  50  73 /  20  80  90  10   0
Austin Bergstrom Intl Airport  60  58  67  49  73 /  20  80  90  10   0
New Braunfels Muni Airport     60  57  68  49  74 /  20  80  90  10   0
Burnet Muni Airport            61  56  63  46  71 /  20  70  90  -    0
Del Rio Intl Airport           59  53  68  44  73 /  -   40  20   0   0
Georgetown Muni Airport        59  56  64  48  71 /  20  70  90  10   0
Hondo Muni Airport             60  56  65  44  75 /  20  80  80   0   0
San Marcos Muni Airport        60  57  68  49  74 /  20  80  90  10   0
La Grange - Fayette Regional   62  60  72  52  71 /  50  70  80  30   0
San Antonio Intl Airport       61  58  66  50  75 /  20  80  80  -    0
Stinson Muni Airport           62  58  68  49  75 /  20  80  80  -    0


Dense Fog Advisory until 9 AM CST this morning for the following
counties: Atascosa...Bandera...Bastrop...Bexar...Blanco...Burnet...
Caldwell...Comal...De Witt...Dimmit...Edwards...Fayette...Frio...
Uvalde...Val Verde...Williamson...Wilson...Zavala.



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