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

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FXUS64 KEWX 181156

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
556 AM CST Sun Feb 18 2018

Patchy light rain and drizzle is helping to lower cigs and vsbys to
lifr values around AUS with mostly ifr or better vsbys farther south
at SAT/SSF. Cigs are forecast to dip into lifr for a few hours this
morning before returning to ifr at SAT/SSF by midday and perhaps a
few hours later at AUS. DRT is trending toward ifr and should get
there in the next hour or two and possibly last into the early
afternoon. The forecast shows slightly better conditions for the late
morning as areas of showers could generate better mixing. Light NEly
winds turn more southerly in the mid afternoon and this should allow
for most areas improving back to mvfr cigs. With winds remaining
southerly tonight, expect most areas to bottom out at ifr with cigs
dominating the category.


.PREV DISCUSSION... /ISSUED 408 AM CST Sun Feb 18 2018/

SHORT TERM (Today through Monday)...

No major impacts expected this period as we gear up for an active
period Tuesday into Wednesday. For today and tomorrow, expected
scattered light to moderate showers at times with warming conditions
as a warm front shifts north this evening.

Synoptic pattern this morning reveals a cut-off low pressure system
off the coast of Baja California with a developing longwave trough
diving south along the Pacific NW coast. This is the system and
resultant influences that will drive our weather over the next
several days. A stalled front is located just south of San Antonio
towards the coast where dewpoint differences are 10-15F degrees.
Surface winds remain north this morning but will become east and then
southeasterly through the day as the warm front shifts north. A
compact shortwave will traverse the Big Bend that could aid greater
shower coverage across the Escarpment and portions of the northern
Hill Country.

Expect clouds to remain in place through the overnight and into Monday
when scattered light drizzle occurs again tomorrow morning with lows
only falling into the low to mid 60s. Scattered to isolated light
showers will continue through most of the day as weak to little mid-
to upper-level forcing will be present along with a mid-level
capping inversion.

LONG TERM (Tuesday through Saturday)...

An elevated impact window for localized flooding and isolated
pockets of gusty winds associated with strong thunderstorms appears
possible Tuesday morning through Wednesday morning for portions of
South-Central Texas. One to two inches of rainfall is likely with
isolated pockets of 3 to 4 inches could be possible across portions
of the Escarpment, Hill Country and east along the I-35 corridor into
the Coast Plains. Some of the stronger thunderstorms could produce
gusty winds and small hail if they develop before the frontal passage
Wednesday afternoon. higher confidence exists on pockets of heavy
rain with low to moderate confidence on strong thunderstorms.

By late Monday night into Tuesday morning, ample and deep moisture
will be present over South-Central Texas as PWATs climb into the
1.3-1.6" range from west to east. These values are near the 99
percentile for moisture for this time of year with reflective surface
dewpoints in the low to mid 60s. The strong aforementioned trough
over the Pacific NW right now will pivot southeast into the
inter-mountain west and place Texas in stronger SW H5-H3 flow as
moderate strength embedded impulses eject across the region. ECMWF
and GFS suggest the first such impulse will shift across the region
early Tuesday morning as mid-level lapse rates steepen. With 0-6km
shear near 50 knots, an organized cluster of storms could develop
across the Escarpment/Hill Country west of I-35. This could be the
first round of storms with the second possibly occurring Tuesday
evening and into early Wednesday morning ahead of the frontal passage
Wednesday afternoon. The evolution of what occurs Tuesday morning
could impact convection possibilities that afternoon if a strong
enough cold pool develops or if it remains more isolated farther
west. The far reaches of NAM and hi-res model runs are not picking up
on this initial activity that is suggested by the global models and
will need to monitor evolution.

By Tuesday afternoon and evening, the consensus of models indicate a
weakening inversion and steepening lapse rates with 30-40 knots deep
layer shear. Worst case scenario would be strong training
thunderstorms that could produce localized heavy rainfall leading to
flash flooding concerns. Best case scenario is the cap remains too
strong or I-35 corridor is undercut by a cold pool from earlier
morning convection. Think the most likely solution falls in between
these possibilities as the question remains on when additional
impulses of enhanced dynamic lift stream across the region. Signals
are not there yet for a moderate to high impact event but localized
heavy rain resulting in some flooding seems plausible at this time.

Rain and thunderstorm chances will go up even more to essentially
100% by Wednesday morning for the eastern 2/3rds of south-central
Texas as a cold front approaches the region from the north. Once the
front passes Wednesday, rainfall rates will decrease quickly given
the removal of moisture and instability. The impact window will close
with the frontal passage.

By late week, low to moderate rain chances continue as over-running
pattern with southwest flow continues over the surface cold layer.
A warm front is expected to return by Friday as warmer temperatures
and moisture spread north. While light to scattered showers will be
possible each of these days, the greater coverage and intensity of
rain should remain north and east of this area of Texas.


Austin Camp Mabry              69  63  76  65  74 /  30  20  30  40  80
Austin Bergstrom Intl Airport  70  63  76  65  73 /  30  20  30  30  80
New Braunfels Muni Airport     71  62  77  64  74 /  30  20  30  30  80
Burnet Muni Airport            67  62  75  64  72 /  40  20  30  50  80
Del Rio Intl Airport           72  63  81  62  80 /  40  10  10  50  20
Georgetown Muni Airport        68  62  75  65  72 /  30  20  30  40  80
Hondo Muni Airport             73  63  81  65  78 /  30  20  20  40  60
San Marcos Muni Airport        70  62  77  64  74 /  30  20  30  30  80
La Grange - Fayette Regional   72  64  77  66  75 /  20  20  20  30  70
San Antonio Intl Airport       72  64  78  65  76 /  30  20  30  40  70
Stinson Muni Airport           74  64  80  65  77 /  20  20  20  40  70




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