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

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FXUS64 KEWX 180504 AAA
AFDEWX

Area Forecast Discussion...UPDATED
National Weather Service Austin/San Antonio TX
1104 PM CST Sat Feb 17 2018

.AVIATION...
/06Z TAFS/
MVFR/IFR conditions are expected across the I-35 sites for much of
the overnight hours. By day-break or close to 12Z Sunday, cigs and
vbsys are expected to lower to LIFR conditions with light drizzle and
patchy to areas of fog. Area forecast soundings using GFS data
suggest a very saturated surface layer with little improvement during
the morning period and cigs slightly improving to IFR for the
afternoon.

An mid to upper level pulse of energy is forecast to move from the
northeast of Mexico and push across the Rio Grande and maybe
affecting areas north of KDRT site. For now, will go with MVFR
conditions overnight becoming IFR around 12Z and back to MVFR late
Sunday afternoon.

Winds across the area will mainly be from the northeast across the
I-35 terminals around 4 to 6 knots overnight into Sunday morning
while wind remains from the southeast through the period AT KDRT.

&&

.PREV DISCUSSION... /ISSUED 919 PM CST Sat Feb 17 2018/

UPDATE...
Seeing some observations reporting reduced visibility across the
eastern areas and will include the mention of fog in the forecast to
account for this. Otherwise, light rain or drizzle is about all that
is expected for most of the night. Could see some additional activity
closer to 12z in the morning across the northwestern CWA as a weak
impulse moves into the area. The current forecast handles this well
as most of the activity will be north of our CWA. Only other changes
were to the hourly grids based on current trends.

PREV DISCUSSION... /ISSUED 619 PM CST Sat Feb 17 2018/

AVIATION...
/00Z TAFS/
There will be some light radar echos just south of KAUS and along the
San Antonio area airports this evening. So far, no reports of precip
observed at the airports. For the Austin terminals, cigs/vbsys are
forecast to stay at VFR category through about 04Z and then lower to
MVFR. Model guidance and HiRes models suggest for IFR conditions to
begin at 08Z through 12Z Saturday. Then GFS LAMP/GFS MOS and NAM MOS
guidance bring LIFR for KAUS for much of the day on Saturday. For
the San Antonio area sites, MVFR conditions are expected through 07Z
and lower to IFR afterward through 12Z or so. Then, LIFR conditions
are expected for much of the day. Across KDRT, VFR conditions are
expected through midnight tonight and then go to MVFR for an hour or
two before even lower IFR conditions take place and remain through
late in the afternoon.

Light northerly winds are expected to dominate the I-35 corridor
sites through midday Saturday while light to moderate southeast winds
prevail at KDRT.

PREV DISCUSSION... /ISSUED 310 PM CST Sat Feb 17 2018/

SHORT TERM (Tonight through Sunday Night)...
As of 230PM, satellite trends were indicating quite a decrease in
cloud cover over the western half of the CWA with an elevated front
pushing south still over the northern tier counties. This front has
resulted in only very minor shower activity and with subsidence
evident through much of the western CWA, opted to decrease PoPs
significantly throughout the rest of the afternoon.

For tonight, did opt to keep low end chance PoPs for another
overnight period of low clouds and drizzle/light rain. Again, much
like yesterday, our moisture tap will continue with isentropic lift
becoming stronger after 06Z along and east of the escarpment. Thus
this period has largely gone unchanged from previous forecasts.

Sunday, a weak disturbance may allow for some elevated shower
activity around the Big Bend region, possibly resulting in some
showers along the Rio Grande and Plateau areas, but by and large, not
much of a change either in the overall pattern compared to the last
several days.

LONG TERM (Monday through Saturday)...
The focus of the short term was on next Monday night through
Wednesday as an H5 trough digs over the Rockies and puts the region
under a southwest flow regime. As reference in the previous forecast,
confidence continues to increase on the potential for strong storms
with locally heavy rainfall early Tuesday morning through Wednesday
as multiple features come into phase. However, model disagreement
begins to become substantial from Wednesday night on, specifically on
how to resolve the frontal passage (stall/no stall) and whether or
not we will be dry or showery towards the tail end of the week.

Beginning possibly late Monday evening, the first piece of energy
from the aforementioned rough comes across the Mexican mountains and,
coupled with some decent surface based instability and 35-40 kt LLJ
and 50 kts of 0-6 km shear, could result in strong storms out west
near Del Rio, perhaps initially isolated. There is enough shearing
available to organize this into a linear feature and come across the
CWA from west to east overnight, and currently the models prog this
arriving around the I-35 corridor Tuesday morning. PWAT values
1.5-1.8" have been advertised in the deterministic NAM/GFS and as a
result, confidence in the possibility for rainfall with a well
organized system continues to rise. Deterministic models continue to
keep the highest QPF northeast of the CWA (centered over TexArKana)
which is where WPC is suggesting as well but models do range from
about a half inch to about 3 inches through the event. Due to the
orientation of the upper level features, and slow translation speed
of those features, some training of rainfall may indeed be possible
Tuesday into Wednesday which could bring those QPF estimates higher
but confidence on that is low still at the moment.

Towards the tail end of the event, one last piece of H5 energy sweeps
across the region and may provide another round of shower development
after the Wednesday front pushes through the CWA. This should be all
elevated however. Then, as mentioned before, model disagreement
begins with how to retrograde the cold front. Thus confidence
decreases substantially beyond 00Z Thursday.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              52  70  62  75  64 /  30  30  20  20  30
Austin Bergstrom Intl Airport  51  70  62  76  64 /  30  30  20  20  30
New Braunfels Muni Airport     54  72  61  76  64 /  30  30  20  20  40
Burnet Muni Airport            50  68  60  74  63 /  30  40  20  20  40
Del Rio Intl Airport           61  72  61  79  61 /  30  40  10  10  50
Georgetown Muni Airport        49  69  62  74  65 /  30  30  20  20  30
Hondo Muni Airport             57  74  62  80  65 /  30  40  20  20  50
San Marcos Muni Airport        53  71  61  76  64 /  30  30  20  20  30
La Grange - Fayette Regional   55  73  63  77  66 /  30  30  20  20  20
San Antonio Intl Airport       57  72  62  78  64 /  30  40  20  20  40
Stinson Muni Airport           58  74  63  80  65 /  30  30  20  20  40

&&

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

$$

Mesoscale/Aviation...17
Synoptic/Grids...Hampshire
Public Service/Data Collection...33


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