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

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FXUS64 KEWX 290916

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
416 AM CDT Sat Apr 29 2017

.SHORT TERM (Today through Sunday)...
Low stratus has spread to the north and west overnight, now covering
the entire region except the Rio Grande Plains which should become
overcast prior to sunrise. These clouds combined with Gulf moisture
and southeast 10-15 knot winds have kept temperatures up in the mid
to upper 70s across the entire region, more reminiscent of summer
than late April. Isolated streamer showers are possible this morning
given these relatively strong winds and saturated boundary layer.
Although these low clouds should persist a bit longer this afternoon
than they did yesterday, they should begin to lift and break up a bit
from southwest to northeast this afternoon to allow temperatures to
climb back up into the upper 80s to lower 90s across most of the
region with mid 90s to lower 100s in the Rio Grande Plains. Southerly
winds will generally be around 10-15 mph, but gusts to 25 mph will
be possible out west behind the dryline and in the Coastal Plains
where a 25-30 knot low-level jet may mix down to the surface.

Most hi-res and global models show that the strong capping inversion
at 775-850 mb will hold for this afternoon across the entire region
except perhaps along the dryline/surface trough ahead of the cold
front in the northern Hill Country. The GFS remains a notable outlier
in that it erodes the cap by 4 pm for areas along and east of I-35
north of San Antonio. However, this solution may have some merit to
it if a differential heating boundary sets up ahead of the dryline
due to greater cloud cover over the Coastal Plains relative to areas
further west that should warm up nicely. This will have to be watched
closely, but for now have kept mostly 20 POPs for the afternoon
increasing slightly further north along the I-35 corridor towards
Austin and Georgetown to account for this conditional threat. Given
that MUCAPE values will likely exceed 4000 J/kg (with mid-level lapse
rates in the 8 deg C/km range) and shear values should exceed 40
knots, any isolated thunderstorms that develop should be supercells
capable of producing at least large hail and damaging winds. The
tornado threat should be relatively low with any storm that develops
given relatively LCLs above 2000 feet and weakly curved hodographs
supporting SRH values below 100 m^2/s^2, but one cannot be ruled out
over our northern and northeast CWA with these storms.

The greatest threat of severe storms continues to be this evening as
a trough currently digging south of the Four Corners region becomes
negatively tilted as it ejects northeast into the Texas panhandle.
This will help push a relatively strong cold front into the region
that should overtake the dryline/surface trough in the Hill Country
early this evening before pushing into the I-35 corridor between 8-11
pm and exiting the Coastal Plains by 1-2 am. Hi-res models suggest
that storms will initially be supercellular which makes sense given
the strong shear and instability mentioned above before becoming
overtaken by the cold front. Although hi-res and global models agree
that these cells should transition into at least a broken line of
storms once the front pushes into the region, they remain split on
whether this front will undercut the cells to reduce their severity
and coverage or provide additional lift to produce a squall line
with bow echo segments capable of producing widespread straight-line
wind damage. Models are also split on how far southwest storms will
extend, with a Kerrville to San Antonio line appearing to be the line
between separating lower than 50 POPs further west from more likely
chances further east. SPC has expanded their enhanced and slight
risks slightly further south to include our eastern two-thirds of the
CWA primarily for large hail, damaging winds, and a very conditional
tornado threat, with a marginal risk for our western third.

Rain amounts should generally average below an inch with isolated
pockets of 1-2 inches. The strong forcing and fast progression of
this system should keep the flash flood threat fairly low across the
region. Additional elevated showers and thunderstorms may continue
behind the front overnight with rain chances ending from west to east
and clearing the CWA by sunrise. Clouds will also clear from west to
east tomorrow morning to set up a sunny day with highs in the 70s.
Northwest 10-20 mph winds with gusts to 25-30 knots will be possible
on Sunday before they eventually subside by the evening. However,
these winds combined with a much drier air mass will lead to elevated
to near critical fire weather conditions in the Rio Grande Plains
and Edwards Plateau which should not receive wetting rains tonight.


.LONG TERM (Sunday Night through Friday)...
Winds will become relatively calm by Sunday night once the surface
pressure gradient weakens and high pressure moves into the region,
leading to a cool Monday morning with lows in the mid 40s to mid 50s.
Temperatures should warm back up to normal on Monday and above normal
for Tuesday and Wednesday before another round of showers and
thunderstorms occurs Wednesday afternoon and evening with our next
cold front. Considering the positive tilt of the upper level trough
digging from the Rockies and fairly modest shear values, these
storms do not look to have a very high severe potential. After a
brief cool down, temperatures should warm back up into the 80s for
the weekend as an upper level ridge moves over the region.


Austin Camp Mabry              89  57  77  52  86 /  30  60  -    0   0
Austin Bergstrom Intl Airport  88  57  76  48  86 /  30  70  -    0   0
New Braunfels Muni Airport     89  57  78  48  86 /  20  60  -    0   0
Burnet Muni Airport            86  51  73  47  84 /  30  50  -    0   0
Del Rio Intl Airport           92  55  81  53  92 /  10  20  -    0   0
Georgetown Muni Airport        87  55  74  49  84 /  40  60  -    0   0
Hondo Muni Airport             93  55  80  47  89 /  10  50  -    0   0
San Marcos Muni Airport        89  56  77  48  86 /  30  60  -    0   0
La Grange - Fayette Regional   87  59  76  50  85 /  30  60  10   0   0
San Antonio Intl Airport       91  56  79  51  87 /  20  60  -    0   0
Stinson Muni Airport           92  58  80  51  88 /  20  60  -    0   0




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