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

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238
FXUS64 KEWX 211732
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1232 PM CDT Sun May 21 2017

.AVIATION...
Showers and thunderstorms are on the increase from west to east
across the area with light rain already impacting KDRT. This activity
will spread into the I35 sites within the next 2-3 hours. Most of the
activity should be showers with embedded thunderstorms and will
prevail -SHRA with VCTS mentioned this afternoon and evening.
Ceilings will hang around MVFR for the remainder of the day. Showers
and thunderstorms will end from west to east late this afternoon and
evening with lingering activity expected through the remainder of
the TAF period. Will mention VCSH to cover that threat. Ceilings are
expected to lower into IFR overnight before lifting late tomorrow
morning or early afternoon.

&&

.PREV DISCUSSION... /ISSUED 1106 AM CDT Sun May 21 2017/

UPDATE...
An upper level short-wave trough moving across the Big Bend area is
triggering scattered thunderstorms over that region and northeast
Mexico. As this short-wave moves northeast throughout the day, a
surface boundary over the coastal Plains is forecast to push to the
north and into our southern counties. With plenty moisture and
instability in place, expect storms out west to enter the southern
Edwards Plateau and Rio Grande Plains late this morning and into the
afternoon period. Some storms could become strong to severe across
the the aforementioned areas and produce large hail.

12Z KDRT sounding shows a pwat value of 1.7 inches with area
forecast soundings showing an increase for this afternoon and values
as high as 1.9 inches. Therefore, heavy rain is likely to occur with
these storms and localized flooding is possible as storms could
produce 1 to 2 inches of rainfall in a short period of time.

Updated the first forecast period based on current observations,
radar trends and latest HRRR model guidance.

Scattered showers and isolated thunderstorms could begin to affect
the I-35 corridor by 3 PM and continuing through the evening hours.

PREV DISCUSSION... /ISSUED 621 AM CDT Sun May 21 2017/

AVIATION DISCUSSION...FOR 12Z TAF PERIOD...
SHRA has been developing over the last several hours around KDRT
resulting in some precip being recorded at the terminal. Expecting
coverage and intensity to increase over the course of the morning and
spread eastward to the I-35 terminals by late morning or mid day.
TSRA is expected to eventually impact all the terminals as continued
development continues through the afternoon hours. As a result,
fluctuations of abv and sub-2kft CIGs will likely occur requiring
some amendments throughout the first 12 hours of the TAF period.
Attempted to include prevailing lines for SHRA/TSRA based off model
guidance for general trends, but these times may need slight
adjusting as convection develops through the course of the first 6
hours of the TAF.

PREV DISCUSSION... /ISSUED 417 AM CDT Sun May 21 2017/

SHORT TERM (Today through Monday)...
The cold front early this morning was located south of the CWA,
across South Texas. Isentropic ascent was resulting in the
development of some weak showers across the western CWA. Coverage
should increase through the remainder of the early morning hours
closer to the Rio Grande. The potential will exist for isolated to
scattered elevated storms across the CWA during the day today as a
weak impulse in the flow aloft moves through the area. Surface based
convection will be located near the front later this afternoon which
may try to retreat slightly north closer to the CRP/EWX CWA line.
Can`t rule out a strong to severe storm this afternoon and evening
across the far southern and western CWA given forecast instability
and shear, with large hail and damaging straight-line wind gusts the
main threat. A few stronger storms with small hail can`t be ruled
out farther north into the CWA today given the steeper mid level
lapse rates.

A stronger mid and upper level disturbance will move out of Mexico
this evening and across the CWA tonight and Monday. Models over the
past few runs have trended farther south into the CRP CWA with heavier
QPF tonight, closer to the better instability axis and surface front.
Nevertheless, upper level forcing will produce good coverage of
showers and storms tonight and Monday across the EWX CWA. Precipitable
water values ranging from 1.8-2.0 inches across southern areas and
diffluent flow aloft with the approach of the left front quadrant of
an upper level jet streak could yield some heavier pockets of
rainfall across southwest areas of the CWA tonight and eastern areas
on Monday. Overall 1-2 inches of rain is expected through Monday with
isolated pockets up to 4 inches, primarily in the areas just
mentioned.

The disturbance should pass east by Monday evening, however the ECMWF
is a little slower and lingers precip across southeastern counties.

LONG TERM (Monday Night through Saturday)...
As the broad upper trough deepens across the central CONUS Monday
into Tuesday, a second upper level disturbance in the northwest flow
aloft will move out of West Texas and into Central Texas and the Hill
Country late Monday Night and through the remainder of the CWA early
Tuesday morning. There are some indications a complex of storms may
develop northwest of the CWA arriving into the Hill Country after
midnight and then weakening into the early morning hours Tuesday
across central and eastern areas of the CWA.

Drier air will filter into the area late Tuesday as a cold front
moves through the area. Clear and dry Wednesday. A return flow will
setup Thursday and strengthen through the end of the week, with
moisture and clouds returning back to the area Friday into the
holiday weekend.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              79  65  78  65  81 /  40  70  70  40  30
Austin Bergstrom Intl Airport  79  66  77  64  81 /  40  70  70  40  30
New Braunfels Muni Airport     80  67  79  65  82 /  30  70  70  40  30
Burnet Muni Airport            75  63  76  62  79 /  40  60  70  40  30
Del Rio Intl Airport           79  67  85  68  86 /  70  70  40  30  30
Georgetown Muni Airport        77  64  76  63  79 /  40  60  70  40  30
Hondo Muni Airport             82  67  82  66  85 /  30  80  50  30  30
San Marcos Muni Airport        80  66  79  65  82 /  40  70  70  40  30
La Grange - Fayette Regional   82  69  79  65  81 /  40  70  80  50  30
San Antonio Intl Airport       81  67  80  67  83 /  30  80  70  30  30
Stinson Muni Airport           82  69  80  68  84 /  40  80  70  30  30

&&

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

$$

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



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