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

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FXUS64 KEWX 161658 AAA

Area Forecast Discussion...UPDATED
National Weather Service Austin/San Antonio TX
1058 AM CST Sat Dec 16 2017

The upper level low pressure over the state of Chihuahua, Mexico is
forecast to move to the northeast into west Texas through this
afternoon and evening. As the afternoon progresses, the coverage and
intensity of showers are expected to increase as the best area for
lifting and instability moves over South Central Texas. As of this
update, there is a high confidence for the potential for moderate
to localized heavy showers mid to late afternoon and early evening
along and east of Interstate 35. There is even a slight change for
isolated thunderstorms to develop late this afternoon/early evening
along and east of Highway 77 where the best instability resides.
Area forecast soundings show pwats increasing up to 1.37 inches
across the far southeast part of South Central Texas. With that in
mind, an average of one half to one inch of rainfall is possible with
isolated spots getting up to inches or so before the rain comes to
an end.

Overall, forecast package is on track for the first and second
weather periods with no chances made for this update.


.PREV DISCUSSION... /ISSUED 551 AM CST Sat Dec 16 2017/

High confidence in widespread light to moderate RA today for mainly
the I-35 terminals. Radar analysis shows rain shield developing over
Laredo and moving northeast as expected. Should reach KDRT and
KSAT/KSSF by 14-15Z this morning, beginning light and becoming more
moderate by the afternoon hours. CIGs will be tough to time but are
expecting lowering throughout the day, becoming IFR by late in the
afternoon and after dark, falling to possibly LIFR CIGs. Fog will
also be a concern overnight after cessation of rain.

PREV DISCUSSION... /ISSUED 326 AM CST Sat Dec 16 2017/

SHORT TERM (Today through Sunday)...
Water vapor imagery early this morning indicates the upper level low
across northwest Mexico is beginning to lift northeast. As the upper
level low continues to approach today isentropic ascent will strengthen
over region and light rain is expected to develop from southwest to
northeast late morning through early afternoon. By 00Z a 130kt jet
streak will pivot into the region as the upper level low begins to
open. Point time series across the central and eastern CWA indicate
strong, deep omega developing late afternoon into the evening as the
dynamical forcing maximizes. The CWA should remain in the cool sector
given the coastal low lifting northeast. Nevertheless, a sharping
H85 axis and aforementioned forcing should yield pockets of moderate
to heavy rainfall in the widespread rain shield late afternoon and
evening near and especially east of I-35. Isolated, elevated
thunderstorms can`t be ruled out. A cool day given the rain, with
highs only in the mid 40s to low 50s.

The precip will quickly end west to east late this evening and
overnight as the mid and upper level trough axis ejects northeast of
the CWA. Low clouds and fog will linger behind into the morning hours
on Sunday. Highs Sunday afternoon rebound into the 60s for most

LONG TERM (Sunday Night through Friday)...
Another upper level low will dig into the Southwest U.S. Sunday and
Monday then advance east into West Texas Tuesday morning. Moisture
gets drawn back north into central and eastern areas of the CWA on
Monday leading to chances for showers. As the upper level low
advances east Monday night and Tuesday increasing synoptic scale
forcing will generate better chances for showers and isolated
thunderstorms. There are some differences in the placement of the
upper level low and developing surface low, with the ECMWF farther
north than the GFS. But generally PoPs have been increased across
central and eastern areas of the CWA for Monday Night and Tuesday.
Drier and warm conditions will develop over South Central Texas on
Wednesday and Thursday behind the departing upper level system.

A modified arctic cold front is forecast to move through the region
by both the GFS and ECMWF Thursday night and by the Canadian Friday
morning. At the same time another in a series of upper level lows is
forecast to cut off by all three models west of the region. This will
lead initially to the potential for cold rain over the area Friday.
There are some GFS ensemble members and the operational ECMWF that
are indicating the potential for a wintry mix of precipitation just
beyond the official forecast, heading into Friday night and Saturday.
It should be stressed that forecast confidence this far out in
winter precip type is very low. Future swings in the models will
likely occur leading to varying winter precip type forecasts.
Nevertheless it is something to pay attention over the upcoming week
given the holiday travel weekend.


Austin Camp Mabry              50  43  66  51  65 / 100 100   0  10  30
Austin Bergstrom Intl Airport  50  42  65  51  65 / 100 100   0  10  30
New Braunfels Muni Airport     49  42  65  52  67 / 100  90   0  10  30
Burnet Muni Airport            49  41  66  48  64 / 100  80   0  -   30
Del Rio Intl Airport           50  42  60  47  66 /  50  -    0  -   10
Georgetown Muni Airport        51  43  65  50  63 / 100 100   0  -   30
Hondo Muni Airport             49  41  64  50  69 / 100  40   0  10  30
San Marcos Muni Airport        50  42  65  52  66 / 100 100   0  10  30
La Grange - Fayette Regional   54  44  65  54  67 /  90 100   0  20  40
San Antonio Intl Airport       50  43  65  53  67 / 100  80   0  10  30
Stinson Muni Airport           50  43  63  54  68 / 100  80   0  10  30




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