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

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FXUS64 KEWX 181801
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1201 PM CST Sun Feb 18 2018

.UPDATE...
18Z Aviation forecast update.

&&

.AVIATION...
Widespread IFR to LIFR ceilings and IFR to MVFR visibilities in BR
and patchy -DZ continue at 18Z across South Central Texas. The
ceilings are forecast to gradually improve into the IFR to low end
MVFR category and visibilities to improve to MVFR to VFR between
18Z-21Z. Eventually VFR conditions across most of the area are
forecast to develop 22Z-01Z, with the exception of some areas of the
Hill Country where ceilings could remain VFR. E winds 3-7 kts at SAT
and AUS are forecast to gradually turn to the SE through the afternoon
and increasing to 8-12 kts. SE winds 10-14 kts will persist at DRT

For tonight, ceilings are forecast to re-develop and lower to IFR at
SAT and AUS between 03Z-06Z and potentially lower to LIFR again
08Z-12Z along with with IFR to MVFR visibility in BR. IFR ceilings
developing at DRT after 09Z and potentially LIFR after 12Z. Unlike
this morning, S to SE winds remaining up overnight and Monday morning
may prevent VLIFR from occurring Monday morning.

&&

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.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              63  76  65  74  56 /  20  30  40  80  90
Austin Bergstrom Intl Airport  63  76  65  73  57 /  20  30  30  80  90
New Braunfels Muni Airport     62  77  64  74  58 /  20  30  30  80  80
Burnet Muni Airport            62  75  64  72  51 /  20  30  50  80  80
Del Rio Intl Airport           63  81  62  80  56 /  10  10  50  20  50
Georgetown Muni Airport        62  75  65  72  54 /  20  30  40  80  90
Hondo Muni Airport             63  81  65  78  58 /  20  20  40  60  60
San Marcos Muni Airport        62  77  64  74  57 /  20  30  30  80  80
La Grange - Fayette Regional   64  77  66  75  61 /  20  20  30  70  80
San Antonio Intl Airport       64  78  65  76  58 /  20  30  40  70  70
Stinson Muni Airport           64  80  65  77  60 /  20  20  40  70  70

&&

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

&&

$$

Mesoscale/Aviation...Runyen
Synoptic/Grids...TB3
Public Service/Data Collection...Williams



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