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

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FXUS64 KEWX 172327

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
627 PM CDT Sat Mar 17 2018

Of immediate concern is a cluster of thunderstorms around SSF and
SAT. Theses storms are moving off to the northeast and there will
likely be VCTS for the first hour or two of this period. They
shouldn`t cause any categorical change at either airport. We do not
expect any other storms to form in the San Antonio area this evening.
There is still a slight chance for thunderstorms in the Austin area
this evening, but the chances are too low to include in the TAF.

There will be a wind shift to the southwest at SAT and SSF within the
next hour and then a shift back to the southeast around 02Z.

CIGs will drop to MVFR and then IFR in Austin and San Antonio later
this evening. CIGs will be IFR overnight and then rebound to VFR by
early afternoon Sunday. There will be slight chances for TSRA again
Sunday afternoon in this area, but again probability is too low to
include in the TAFs.

DRT will remain VFR through this period. Winds will shift to the
north Sunday morning and then to the west during the afternoon.


.PREV DISCUSSION... /ISSUED 517 PM CDT Sat Mar 17 2018/

We have elected to issue a Fire Weather Watch for Monday across
portions of the Hill Country, Edwards Plateau, Rio Grande Plains,
and Winter Garden regions. See Fire Weather discussion below for
further information.

Another round of elevated to near critical fire weather will be
likely Sunday across Val Verde and portions of the southern
Edwards Plateau. A Red Flag Warning could be possible for Val
Verde Sunday but it will be close on wind criteria.

Dry, gusty, and warm conditions will prevail on Monday behind a
Pacific cold front. Relatively humidity values will fall to very
low values of 5 to 15 percent Monday afternoon. At the same time
northwest winds are forecast to be around 15 to 25 mph with
occasional gusts to 30 mph. This will likely result in critical
fire weather conditions, especially when combined with ongoing
drought conditions and fuels that are dry. The most likely areas
to experience potential Red Flag conditions will be near a Burnet
to Hondo to Carrizo Springs line and west to the Rio Grande.
Elevated to near critical fire weather conditions are also
expected into the I-35 corridor, and the Fire Weather Watch and
potential Red Flag Warning could be expanded farther east should
wind speeds trend higher.

PREV DISCUSSION... /ISSUED 358 PM CDT Sat Mar 17 2018/

SHORT TERM (Tonight through Sunday Night)...
Showers and thunderstorms have developed just northwest of our CWA
and Llano County an hour or so earlier than expected this afternoon.
This development is primarily along the dryline with the strongest
storms just south of the I-20 corridor closer to a stationary front.
A Severe Thunderstorm Watch is in effect for San Saba, Lampasas,
Bell, and Milam Counties on our northern CWA border until 11 PM CDT
tonight. We are currently not expecting that this watch will be
expanded further south tonight as nearly all hi-res models keep
strong to severe thunderstorms north of our area. Aircraft soundings
out of AUS (and SAT) show that the cap is gradually weakening at 850
mb, but it is still stronger than the GFS and other global models
show that have convection further south. The Texas Tech WRF has a
good handle on the cap, and it, like most hi-res models, keeps the
cap in place all afternoon before strengthening through the evening.
The two hi-res models that do bring storms into Williamson and Burnet
Counties do so after 6-7 pm as this activity is weakening along the
gust front that is expected to develop later this afternoon and move
S-SE as the cap is restrengthening. Thus, strong to severe storms are
unlikely tonight in our CWA, but the gust front may possibly move
into our northern counties by early evening to cause a brief period
of strong wind gusts. In the unlikely event that the Severe
Thunderstorm Watch is expanded southward, we will give an update.

After relatively uneventful weather overnight, isolated showers and
thunderstorms are expected along the dryline again tomorrow which
should push farther east with additional dynamic support aloft as a
negatively tilted trough digs into the Texas panhandle. The cap is
forecast to be stronger overnight and tomorrow morning than it was
tonight, and the dryline should move through the I-35 corridor early
in the afternoon before convective temperatures are reached. Thus,
our current forecast keeps 20 POPs along the I-35 corridor and 30
POPs in the Coastal Plains where convective initiation tomorrow
afternoon is more likely. However, there is considerable spread in
the hi-res models in where these storms may develop, and there is
some indication that supercells may develop just northeast of our
CWA which makes sense given moderate to strong shear parameters.
These storms may be develop over northeastern counties early
tomorrow afternoon, but they should not become strong to severe until
moving northeast of our CWA tomorrow.

The more significant concerns will revolve around fire weather as
elevated to near-critical fire weather conditions that are ongoing
this afternoon in Edwards and Val Verde Counties will expand a bit
further east tomorrow for areas west of Highway 281. West-southwest
winds at 10-15 mph and gusts to 20 mph are expected over these areas
tomorrow, with relative humidity values falling below 20 percent in
Val Verde County northwest of Del Rio and 25 percent along and west
of Highway 281. However, there is a decent amount of model spread on
how far east the dryline will progress and how fast it will do so,
which will play a major role in how low humidity values get. Thus, if
this dryline is more progressive, a Red Flag Warning may need to be
considered for Val Verde and Edwards Counties, with near critical
fire weather conditions needing to be expanded into the Highway 281
corridor and possibly further east. We will continue to monitor this
and handle with a Significant Weather Advisory as we have done today
if conditions still are below Red Flag Warning conditions tomorrow.

LONG TERM (Monday through Saturday)...
The most significant weather event of the forecast period appears to
be elevated to near critical fire weather conditions across all of
South Central Texas on Monday behind a cold front that is expected to
move through overnight Sunday into Monday. Although there are
uncertainties in how warm temperatures will get in terms of the
effects of downslope warming with cold air advection likely being
relatively weak, dew points will be very low in the teens out west
to the 30s in the Coastal Plains and winds will be out of the west-
northwest at 10-20 mph with gusts to 25 mph. Thus, it is beginning to
look more likely that a Red Flag Warning may be needed for some of
our western portions of South Central Texas on Monday with the worst
conditions during the afternoon. However, the uncertainty in the
temperatures and timing of maximum wind speeds just above the surface
at 925 mb (that global models indicate will peak at 30-35 knots by
mid-morning before rapidly weakening) make issuing a Red Flag Warning
for Monday difficult at this point. Greater confidence in the area of
where critical fire weather conditions will be expected on Monday
should occur over the next 12-24 hours as we move into the time frame
where hi-res models are able to depict this period.

Beyond Monday, uneventful weather is expected for the rest of next
week as we remain under northwest flow for the middle of the week and
a ridge for the end of the week. Moisture will begin to increase late
next Friday into the weekend as another trough approaches the region.
High temperatures behind the front on Tuesday and Wednesday will be
very pleasant in the 70s with lows in the 40s before gradually
warming up through the remainder of the week as southerly flow
returns on Wednesday and moisture returns on Thursday morning.


Austin Camp Mabry              66  89  57  81  49 /  20  20  10   0   0
Austin Bergstrom Intl Airport  67  88  56  80  47 /  20  20  10   0   0
New Braunfels Muni Airport     66  89  56  83  47 /  10  20  10   0   0
Burnet Muni Airport            63  86  50  77  44 /  20  20  -    0   0
Del Rio Intl Airport           63  90  54  82  48 /  10  10  -    0   0
Georgetown Muni Airport        66  86  54  80  46 /  20  20  -    0   0
Hondo Muni Airport             66  90  54  84  47 /  10  20  10   0   0
San Marcos Muni Airport        66  89  56  82  47 /  10  20  10   0   0
La Grange - Fayette Regional   66  85  59  80  49 /  20  30  20   0   0
San Antonio Intl Airport       67  90  57  83  50 /  10  20  10   0   0
Stinson Muni Airport           68  90  57  84  50 /  10  20  10   0   0




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