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

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FXUS64 KEWX 190226 AAA

Area Forecast Discussion...UPDATED
National Weather Service Austin/San Antonio TX
826 PM CST Sun Feb 18 2018

There is a pulse of energy moving across the northeast part of Mexico
this evening. IR satellite images show an increase on cloud top
temperatures as the cluster of thunderstorms move over the high
Mexican terrain. Both HRRR and the 3km WRF Texas Tech models pick up
on this feature and brings it across the Rio Grande Plains including
the Del Rio area around 11 PM late this evening through 1 AM Monday.
If the cluster of storms holds and move over Del Rio and vicinity,
expect heavy rain and thunderstorm wind gusts of 30 mph or so. As the
cluster of storms move into the Edwards Plateau, there will be less
instability in place and only showers are expected to continue moving
into the Hill Country overnight.


.PREV DISCUSSION... /ISSUED 537 PM CST Sun Feb 18 2018/

VFR conditions prevail at all sites but conditions will deteriorate
back to IFR/LIFR overnight. In addition, the latest runs of the HRRR
and TTU WRF show a complex of showers and thunderstorms developing
west of the Rio Grande late this evening and moving east into the CWA
around 5-6z. Will watch the trends of this closely and will mention
VCSH in the DRT TAF from 5-9z for now and amend as necessary. If this
complex were to develop, weakening showers could be possible for the
I-35 sites around daybreak. MVFR/VFR conditions will be slow to
arrive tomorrow afternoon and will likely only see a few hour
reprieve into VFR before IFR conditions return tomorrow evening.
Otherwise, southerly flow will be breezy tomorrow around 15 knots.

PREV DISCUSSION... /ISSUED 317 PM CST Sun Feb 18 2018/

SHORT TERM (Tonight through Monday Night)...
An area of scattered showers and isolated thunderstorms this morning
has moved off to the northeast as forecast leaving much of the CWA
dry today. There still exists a few weak echoes on radar as of 2PM
this afternoon associated with isentropic ascent in the lowest 100

After this moves out late this afternoon and evening, another
overnight of low cloud development looks likely. Latest runs of the
HRRR also indicate another weak disturbance generating some light
shower activity near Del Rio and shifting east overnight. This, plus
the potential for some light drizzle, warrants at least slight chance
PoPs in the forecast. The maintained isentropic ascent through the
day tomorrow will also keep skies cloudy and PoPs non-zero.

LONG TERM (Tuesday through Sunday)...
Models have continued to show a consistent signal for our first good
rainfall event beginning as early as late Monday evening and
persisting through at least Wednesday. WPC guidance for QPF continues
to highlight an area near TexArKana and by all indications at this
point, that still looks very good.

The thinking continues that initial convection could begin near Del
Rio and west as 925-850mb confluence aligns with the right entrance
region of a developing UL jet and quasi-dry line west of Val Verde
county. This alone will be enough to likely develop at least
elevated showers and storm potential, but the kicker will be
instability to allow these to become strong. Looking at forecast
soundings from the NAM/GFS for 00Z Tuesday, there is quite a stubborn
cap at 850 mb whereas the GFS is uncapped. This will obviously play
quite a role in whether hail or wind potential will be associated
with the storm development. It will likely also play a role in the
evolution of this system throughout the overnight period as these
features shift eastward. Currently, the thinking is this cap will
indeed hold with the extensive cloud cover likely around due to the
persistent isentropic ascent throughout the day Monday. Thus while
some storms could produce some small hail, the bigger focus will be
as this system progresses eastward and organizes.

As these features shift eastward overnight, a quasi-liner system will
likely progress across the Plateau towards the I-35 corridor. As this
occurs, PWATs will steadily increase to about 1.5+" by mid day
Tuesday according to the GFS. The dry line surges eastward in
response to a cold front diving south across the Red River Valley.
Additionally, another piece of UL energy comes across the Mexican
mountains and provides enhanced lifting parallel to the dry line and
advancing cold front. The more favorable convergence along the front,
additional UL support, and continued strong moisture flux Wednesday
morning should re-invigorate the line of rainfall and its at this
time that heavy rainfall potential will likely be maximized. QPF
values are still generally manageable, with even the highest
perturbations staying below 3 inches storm total. Thus, the previous
forecast`s mention of 1-2 isolated 3 to 4 inches still looks fair.
The variable with the least certainty is instability. There are some
indications that as much as 500-1000j/kg of SBCAPE may be present per
the KAUS proximity sounding (NAM/GFS). Not sure if this is
believable given the presence of shower activity, extensive cloud
cover, the potential for outflow should the linear system become more
organized, and any other unforeseeable mesoscale features, but IF
so, these QPF values would likely need to be raised. For now, there`s
just not enough certainty to up the forecast values beyond the
previous forecast.

After this system moves east and the cold front pushes through, this
should at least briefly cease rainfall potential overnight Wednesday
night, but overrunning appears agreed upon in the medium range models
and thus cloud cover and PoPs quickly return, much like the
conditions we have seen over the last several days.


Austin Camp Mabry              64  74  65  72  59 /  20  20  50  80  90
Austin Bergstrom Intl Airport  63  74  65  73  60 /  20  20  40  80  90
New Braunfels Muni Airport     63  75  65  73  61 /  10  20  40  80  90
Burnet Muni Airport            62  73  64  71  53 /  20  20  70  80  90
Del Rio Intl Airport           64  80  62  79  58 /  40  20  60  20  50
Georgetown Muni Airport        63  74  65  71  55 /  20  20  50  80  90
Hondo Muni Airport             64  80  66  77  61 /  20  20  50  60  80
San Marcos Muni Airport        63  75  65  73  60 /  20  20  40  80  90
La Grange - Fayette Regional   65  75  67  75  64 /  10  20  20  70  80
San Antonio Intl Airport       64  77  66  75  62 /  10  20  40  70  90
Stinson Muni Airport           64  78  66  76  63 /  10  20  40  70  80




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