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

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FXUS64 KEWX 270532
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1132 PM CST Sun Feb 26 2017

.AVIATION.../06Z_TAFS/
The showers and thunderstorms that formed east of the I-35 TAF sites
have dissipated and redevelopment is generally not expected tonight.
A few hi-res models are indicating that some storms may develop NW of
the area between SJT and JCT after 9Z as a weak disturbance pushes
through, but these storms should stay north of the TAF sites. There
is a slight chance of showers and thunderstorms through tomorrow
afternoon, but chances are too low to include in TAFs at this time.

MVFR ceilings are currently in place at all I-35 TAF sites and these
should become IFR between 7-8Z before potentially lowering to LIFR
between 11-17Z. Advection fog should develop at all sites given
increasing moisture with southerly 5 knot winds. Visibilities should
be lowest between 12-15Z, but models are generally trending away
from dense fog occurring as enough mixing should occur. All sites
should recover to IFR by late morning with SAT/SSF most likely to
become VFR by 19-20Z while AUS may only improve to MVFR. Ceilings at
DRT are currently VFR and should only decrease to IFR by 11Z before
improving to MVFR by late morning and VFR by the afternoon.


&&

.PREV DISCUSSION... /ISSUED 939 PM CST Sun Feb 26 2017/

UPDATE...
The main changes to the late night forecast is to update the PoP,
QPF, and Weather grids to reflect the current radar and high
resolution models for the rest of the night. Have confined the
highest chances for precipitation to Lee, Fayette, and Lavaca
counties where storms continue to move east. The last few radar scans
show a weakening trend to these storms.

For midnight to dawn, have maintained the threat for fog across all
of South Central Texas. PoPs for this period have been reduced,
except for areas across the far northern part of the County Warning
Area from Llano to Williamson to Lee Counties. The HRRR show showers
and storms developing around 7am in this area. Other high resolution
models show isolated showers across much of South Central Texas for
the rest of the night into the first part of tomorrow. Have
maintained slight chances of precipitation to cover this.

To summarize...once the storms across Fayette and Lavaca counties
weaken it should be a quiet night for the most part, other than a showers
and the fog developing. Then around sunrise we could see a few
thunderstorms develop across the Hill Country and trek east.

PREV DISCUSSION... /ISSUED 627 PM CST Sun Feb 26 2017/

UPDATE...
Quick update to the afternoon forecast package. The main change is to
increase QPF across Guadalupe, Caldwell, Gonzales, and Bastrop
counties where the heavier showers have been moving late this
afternoon and evening. The LCRA Hydromet Gauge near Dale, Texas in
Caldwell county picked up a quick 0.53 inches of rainfall as a shower
moved over. We ahead and re-trended the hourly weather grids as well
to match current observations.

PREV DISCUSSION... /ISSUED 600 PM CST Sun Feb 26 2017/

AVIATION.../00Z_TAFS/
Some impressive convective showers with a few IC strikes have
developed just northeast of SAT to southeast of AUS. However, CG
strikes have not been observed yet as these cells are struggling to
make it past an inversion from 16-18 kft. Thus, for now have only
included VCSH at SAT/SSF through 2Z, but left VCTS in at AUS through
1Z due to close proximity of current cells before ending VCSH by 4Z.
Although there is a slight chance of TSRA this evening (particularly
at AUS) as dynamic forcing increases as the upper level disturbance
approaches, chances are too low to include at this time.

MVFR-VFR ceilings above 2000 feet persist at the I-35 TAF sites, but
these will gradually lower to IFR around 4-5Z. Patchy fog should
begin to develop towards 9Z as LIFR ceilings and visibilities occur
at some if not all of the I-35 TAF sites, with the best chances at
SAT/SSF. These LIFR-IFR conditions should persist through most of the
morning before rapidly improving to VFR between 18-20Z. DRT will
remain VFR this evening until MVFR ceilings develop after 8Z and IFR-
LIFR ceilings and visibilities move in between 10-12Z. Similar to
today, DRT should recover to VFR more quickly between 16-17Z.

PREV DISCUSSION... /ISSUED 314 PM CST Sun Feb 26 2017/

SHORT TERM (Tonight through Monday Night)...

Main highlight will be light to moderate showers that are ongoing
this afternoon and the likely development of a few thunderstorms
through the next 2-6 hours. Temperatures are expected to be well-
above normal to start off the work week tomorrow.

A shortwave trough and associated downstream geostrophic ascent is
evident in water vapor and visible satellite images with developing
upper level cirrus over Texas. Latest SPC mesoanalysis suggests
building instability across the south half of the region as cloud
breaks have allowed for greater insolation and diurnal heating. The
low-level convergence zone near I-35 is aiding light to moderate
showers as of 3pm but some stronger updraft growth could occur
through the late afternoon and evening as stability decreases
farther north in conjuction with the deeper ascent shifting across
the region. One or two strong thunderstorms can`t be ruled out with
the most likely location along and north of a Johnson City to La
Grange line. However, further destabilization of the mixed layer
still needs to take place still given evidence of the horizontal
convective rolls seen that suggest the inversion remains in-tact.

The briskly moving shortwave trough will pivot away from south-
central Texas by Monday morning but deep southwest flow will remain
over the area. Areas of widespread fog appear likely Monday morning
and will need to watch for dense fog possibilities. Once the low
clouds/fog mix out, the southwest flow will allow for well-above
normal temperatures as many areas reach into the 80s. With weak
forcing and weak instability, only a few showers are expected
tomorrow with very limited coverage.

LONG TERM (Tuesday through Sunday)...

No significant highlights in the long term impact wise outside of a
cold frontal passage on Wednesday and then increasing rain chances
once again over next weekend. Depending on evolution of next weekend
weather system, pockets of heavy rain will need to be monitored for.

A stronger longwave trough will shift across the central CONUS late
Tuesday into Wednesday that will shift a front through early
Wednesday morning. Little rain is expected with the frontal passage
at this stage given stronger dynamics remain north of the region with
only limited instability given the early morning passage. Much drier
air will shift into the region by late Wednesday and much cooler
temperatures into the 30s for lows are expected Thursday morning.
Will need to watch cirrus clouds closely Thursday morning. If they
clear out sooner, a light patchy freeze could be possible across the
Hill Country.

Expecting dry conditions Friday with generally zonal flow persisting
over the region as a deep cut-off low develops off the coast of west
Mexico. Model depictions are not in very good agreement with this
system as ECMWF is much more progressive with GFS slower. Have backed
off on precipitation as both models slowed this model cycle run into
Saturday. Will need to watch closely as the cut-off low shifts slowly
this direction by late weekend as PWATs could increase ahead of its
influencing lift and pockets of heavy rain could potentially be
possible Sunday. Low confidence exists currently in this possibility.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              61  81  65  85  62 /  20  20  10  20  20
Austin Bergstrom Intl Airport  61  80  64  86  63 /  20  20  10  20  20
New Braunfels Muni Airport     61  81  64  85  62 /  20  20  10  20  20
Burnet Muni Airport            57  79  61  81  58 /  30  20  10  20  20
Del Rio Intl Airport           53  87  57  89  56 /  10  -   -   -   -
Georgetown Muni Airport        59  79  63  83  60 /  30  20  10  20  20
Hondo Muni Airport             58  85  61  88  59 /  10  10  10  10  10
San Marcos Muni Airport        61  81  64  86  62 /  20  20  10  20  20
La Grange - Fayette Regional   64  81  67  86  65 /  40  20  10  20  20
San Antonio Intl Airport       60  82  65  86  62 /  20  20  10  20  20
Stinson Muni Airport           61  82  65  87  63 /  20  10  10  20  10

&&

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

$$

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



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