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

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FXUS64 KEWX 251757
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1257 PM CDT Tue Apr 25 2017

.AVIATION.../18Z_TAFS/
MVFR ceilings along the I-35 corridor are in the process of lifting
to VFR which should be completed by 19Z. Southerly 10-15 knot winds
are expected this afternoon with gusts to 20-25 knots at AUS and
possibly SAT. Ceilings will rapidly lower to MVFR between 3-5Z and
IFR between 5-8Z at the I-35 sites, with AUS lowering more rapidly
than SAT/SSF as low-level moisture will recover more quickly further
east. DRT will remain VFR through the entire period. Winds tonight
will remain around 10-15 knots with gusts to 20-25 knots possible
after 5Z as the surface trough/dryline approaches ahead of a front
that will push through tomorrow morning. Models are trending towards
a faster frontal passage, with W-NW winds at 20 knots gusting to 30
knots possible later tomorrow morning and afternoon behind it.


&&

.PREV DISCUSSION... /ISSUED 609 AM CDT Tue Apr 25 2017/

AVIATION DISCUSSION...FOR 12Z TAF PERIOD...
Seeing MVFR CIGs building across the I-35 corridor as anticipated
this morning. Expect this deck to lower to IFR for the KSAT/KSSF
terminals in the first hour of the TAF. KAUS will come very close to
the 1kft level so will need to be closely monitored this morning for
a potential amendment to IFR. Improvement to VFR is expected to be
pretty rapid around the 15-16Z time frame this morning. Then breezy
winds will be the main impact to terminals. A return to MVFR CIGs
expected for the I-35 terminals late this evening is also expected.

PREV DISCUSSION... /ISSUED 422 AM CDT Tue Apr 25 2017/

SHORT TERM (Today through Wednesday)...

Big story today and tomorrow will center on the well-above normal
temperatures and the windy conditions. Readings will be 8-12F
degrees above normal as a dryline shifts through the region. This
dryline/front will shift through the entire area Wednesday, bringing
windy and critical fire weather conditions.

A broad longwave trough is currently deepening across the western
CONUS per water vapor satellite channels and RAP analysis. This
trough is aiding surface cyclogenesis across north Texas supporting
strong surface to H7 south to southwest winds. The tightening
pressure gradient in this layer is already being reflected in
elevated 15 mph surface winds this morning with KEWX radar VAD wind
profile indicating near 40 to 50 knots at 2 to 3 kft above the
ground. The question will be will these winds be able to translate
down through mixing late this morning/afternoon that result in
sustained winds greater than 25 mph? While wind gusts to 30 mph will
be likely, feel the sustained winds >25 mph could be difficult. Will
hold off on a wind advisory for now but if higher winds start
occurring by mid-morning, it is quite possible for a short duration
wind advisory to be issued.

A dryline situated across the Big Bend is expected to shift east
through the day today to at least the I-35 corridor. Much drier and
warmer 925-850mb air will move in behind this boundary and allow for
a rapid surface warm up. Much of the area will be above normal with
highs in the upper 80s east to mid to upper 90s west. No records are
expected. Thankfully, a strong enough cap, weak forcing, and limited
moisture along and east of the dryline will quell any shower or
storm potential today.

By Wednesday, the dryline retreats west initially overnight and then
shifts much farther eastward through the entire region by early
afternoon as the mid and upper trough sharpens over the south-
central Plains. Strong west winds of 20 to 30 mph will be likely
Wednesday. This wind combined with the very dry air (dewpoints in
the 20s west to 40s east) will result in critical fire weather
concerns along and west of the US HWY 281 corridor with elevated fire
conditions for the remaining areas. A red flag warning and a wind
advisory will need to be monitored for on Wednesday. Temperatures
will cool slightly out west but central and eastern areas will remain
hot before the slight cool down Thursday morning with the drier air
in place.

LONG TERM (Thursday through Monday)...

Despite the cooler morning Thursday, the heat will be back on late
week with increasing rain chances (20-40%) over the weekend as
another trough pivots across the central CONUS. Models are in
moderate agreement but finer scale details (see below) remain
incongruent on just how enhanced the rain/storm signals will be late
Saturday into Sunday when a stronger front is advertised to move
southward.

Thursday will start cool and finish warm with near normal readings.
However, a big warm-up occurs again Friday as south/southwest flow
develops in advance of a strengthening trough over the western US.
Low-level moisture is advertised to return in earnest Friday and
with temperatures expected in the upper 80s to low 90s, CAPE values
along with lapse rates are progged to be quite high. While shear
appears to be low, will need to watch closely for the potential of
isolated thunderstorms Friday afternoon. A weak cap appears to be in
place at this time (and this should win out) but if a storm was to
break through, it could go strong/severe quickly. Stronger forcing
and height falls will occur over north and west Texas Saturday
morning that should initiate convection and shift into the Red River
Valley in NE Texas. Much of this activity looks to remain north of
the area but as the trough axis shifts across north Texas Saturday
morning, it will also shift the dryline into the region. Isolated/
scattered showers trapped under the cap may be ongoing Saturday
morning but by the afternoon, the cap could erode just enough along
and east of I-35 to aid in thunderstorm development. Areas along and
east of this environment could be favorable for isolated
strong/severe storms. The location of the troughing and dryline
indicate that rainfall will not be widespread over the region but
confined to areas just along it and east.

By Sunday, the main trough axis will shift east and bring a secondary
cold front from the north. The big question at this stage will be
where is the dryline when the front moves south? EC has the dryline
farther west - allowing more moisture pooling ahead of the stronger
front. The GFS is farther east with the dryline and has moisture
confined to coastal plains with less rain coverage than EC depicts.
Have placed 30-40% chances for late Saturday to early Sunday
timeframe and will monitor for better agreement to refine timing and
chances. Both models do clear the stronger front south and east Sunday
afternoon with cooler and dry conditions likely early next week.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              71  90  54  86  68 /  10  10   0   0  10
Austin Bergstrom Intl Airport  71  90  52  85  67 /  10  10   0   0  10
New Braunfels Muni Airport     70  90  52  86  67 /  10  -    0   0  10
Burnet Muni Airport            67  84  49  84  66 /  10  -    0   0  -
Del Rio Intl Airport           68  89  57  92  68 /   0   0   0   0   0
Georgetown Muni Airport        70  87  52  84  67 /  10  10   0   0  10
Hondo Muni Airport             66  90  52  89  66 /   0   0   0   0   0
San Marcos Muni Airport        70  90  52  86  68 /  10  10   0   0  10
La Grange - Fayette Regional   72  90  52  86  68 /  10  10   0   0  10
San Antonio Intl Airport       70  90  54  87  68 /  -   -    0   0  -
Stinson Muni Airport           70  90  54  88  67 /  -   -    0   0  -

&&

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

$$

Mesoscale/Aviation...LH
Synoptic/Grids...04



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