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

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FXUS64 KEWX 151750
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1150 AM CST Fri Dec 15 2017

.AVIATION.../18Z_TAFS/
VFR conditions will continue through the early morning hours at all
TAF sites as broken to overcast high cloud decks associated with an
active subtropical jet generally remain above 20 kft. Northerly winds
around 10 knots this afternoon become light and variable tonight
before veering to south-southeast at 5-10 knots tomorrow morning.

Rain chances will increase late tomorrow morning between 15-18Z at
all TAF sites as a cutoff low currently over the Gulf of California
ejects northeast across the region. Widespread moderate rain showers
should develop by early tomorrow afternoon at the I-35 TAF sites as
isentropic ascent increases dramatically over the region, causing
visibilities and ceilings to persist at IFR if not even LIFR. Very
isolated thunderstorms will be possible due to elevated instability,
but chances are too low to include in the TAFs at this time.

&&

.PREV DISCUSSION... /ISSUED 844 AM CST Fri Dec 15 2017/

UPDATE... /TRENDED DRIER TODAY/

Dry air continues to push southward with radar echoes retreating
southward perhaps a bit faster than shown in the earlier model runs.
Will show no mention of rain along and N of Hwy 90 and mainly
sprinkles for those areas under the light echoes over the southern
counties. Temperatures were retrended for the morning hours with
otherwise only cosmetic changes.

PREV DISCUSSION... /ISSUED 515 AM CST Fri Dec 15 2017/

UPDATE...
12Z Aviation update below.

AVIATION...
VFR conditions today and tonight. A BKN-OVC 10-12K foot deck will
shift south later this morning leaving behind BKN high cirrus.
Lowering ceilings to around 6K feet will take place late morning on
Saturday as -RA develops SW to NE.

Winds generally NW to NE 6-11 kts through the morning and early
afternoon hours, decreasing to less than 5 kts at SAT/AUS this
evening and turning S to SE at DRT around 5 kts.

PREV DISCUSSION... /ISSUED 414 AM CST Fri Dec 15 2017/

SHORT TERM (Today through Saturday)...
The short term forecast focused heavily on Saturday`s rainfall
potential associated with the impressive dynamical system progged to
move through Saturday. While the overall forecast has not changed
significantly with this package compared to the previous forecast,
confidence continues to increase on PoP potential and areas of higher
QPF potential.

For today, the colder air is knocking on the door of the northern CWA
border as of 330AM. While north winds have been noted across the CWA
already, the colder air is lagging behind significantly, and the temp
gradient is finally beginning to enter the northern tier of the CWA.
This should slowly continue across the southern Plateau today and
combined with mostly cloudy skies across much of the area, should
prevent highs from reaching the 60s today. The light but widespread
shower activity seen along I-10 this morning should continue to
slowly push southeast as drier low level air filters into the CWA.
Thus have lowered PoPs over the course of the day and favoring the
southern third of the CWA.

A brief lull in activity is expected overnight tonight before the
main rain event Saturday. The weak closed low over Mexico responsible
for this event will begin to interact with the quickly approaching
shortwave diving south from the Pacific Northwest. This will force
the Mexican low to swing northeast, undergo a shift to a negative
tilt, eject PVA across Texas, and put the region in a southwest flow
regime and increasing upper level winds/divergence.

While the atmospheric column will begin initially dry below 500 mb
Saturday morning, the column is progged to quickly moisten as low
level flow becomes southerly and surface low pressure develops over
South Texas and the southern Texas coast. This will likely result in
elevated shower activity in the morning hours approaching from the
south, but increasing in coverage and intensity throughout the course
of the day as lifting becomes more focused due to focused isentropic
lift, PWATs increasing to over an inch, moisture flux convergence
and UL divergence all phase, according to the 00Z and 06Z GFS and 00Z
ECMWF, by 18Z Saturday afternoon across the I-35 corridor. Around
this time, expectations are for precip intensity to increase to more
moderate, with some locations seeing some periodic, locally heavy
rainfall, especially east of I-35. Forecast soundings continue to
show little to no instability, and while that should serve as the
main reason for limiting this activity from becoming impactful, there
is more than enough isentropic and dynamical lift to produce
widespread rain activity with some embedded isolated thunderstorms.

As far as QPF potential goes, there is a strong favoring in various
ensemble members for the eastern CWA to see around a half inch to an
inch of rain Saturday when all is said and done. The outlier seems to
be the deterministic GFS, which produces closer to 2 inches Saturday
night along the extreme eastern CWA, but ensemble member spread
varies considerably, and the 06Z run puts this QPF max farther into
east Texas than the 00Z run. Would not be surprised to see further
acceleration of this system as we approach Saturday as this has been
the trend throughout the last several days. And if so, storm total
QPF would likely decrease due to the faster passage of the upper low.

LONG TERM (Saturday Night through Thursday)...
As the aforementioned system continues to accelerate with each
successive run, the cessation of rainfall for the CWA becomes earlier
and earlier, now fully east of the I-35 corridor by 12Z Sunday, if
not completely east of the CWA. Thus, PoP chances have been removed
after 12Z Sunday completely and highs increased into the mid to upper
60s.

After Sunday, the forecast confidence decreases as consistency has
been lacking from run to run and model to model. Models seem to be
having trouble progressing the secondary low center after Sunday.
While there is consensus for this feature to become a closed system
south of the Four Corners late Sunday, the speed of the eastward
progression of this low is still unclear. The 06Z GFS has trended
more in line with the 00Z ECMWF but this has not been the case in the
last few runs. What it seems to boil down to is a prolonged
possibility for PoPs across the eastern zones as continued low level
moisture pumps into the eastern third of Texas. The weaker, but still
adequate lift from this next shortwave should maintain rain chances
through the mid week across southeast Texas.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              59  41  56  44  67 /  -    0  80  70  -
Austin Bergstrom Intl Airport  58  41  55  42  67 /  -    0  80  80  -
New Braunfels Muni Airport     59  43  54  43  67 /  -    0  90  70  -
Burnet Muni Airport            56  37  54  42  68 /  -    0  70  60  -
Del Rio Intl Airport           59  42  52  41  63 /  -   10  40  10   0
Georgetown Muni Airport        57  38  56  42  68 /  -    0  70  70  -
Hondo Muni Airport             60  43  54  40  66 /  -   10  80  40  -
San Marcos Muni Airport        58  42  55  43  67 /  -    0  90  70  -
La Grange - Fayette Regional   58  41  57  44  67 /  -    0  80  90  -
San Antonio Intl Airport       59  44  54  44  67 /  -   -   90  60  -
Stinson Muni Airport           60  44  54  43  67 /  -   -   90  60  -

&&

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

$$

Mesoscale/Aviation...LH
Synoptic/Grids...Oaks
Public Service/Data Collection...30



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