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

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FXUS64 KEWX 060505

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1105 PM CST Tue Dec 5 2017

Please see the 06Z aviation forecast discussion below.


Overall, no significant changes have been made to the 06Z TAFs. We
still expect rain to gradually expand in coverage overnight into
Wednesday morning across the region. There is still enough dry air,
even with intermittent rainfall, to keep cigs VFR for at least the
next several hours. With rain set to slowly expand and increase
overnight, cigs should gradually lower. We did opt to delay the onset
of MVFR cigs by 2-3 hours based on the latest guidance. Otherwise,
expect persistent, light rain through most of the day on Wednesday.


.PREV DISCUSSION... /ISSUED 810 PM CST Tue Dec 5 2017/

A significant amount of elevated instability was present on the 00Z
DRT sounding (MUCAPE of 254 J/kg) and aircraft soundings out of AUS
and SAT also confirm this. This instability is primarily concentrated
in the 575-700 mb layer where steep lapse rates are present between a
stout frontal inversion around 850 mb and another inversion above 575
mb. We have seen a couple of lightning strikes with the elevated
convective cells out in Llano and Gillespie Counties and model
soundings indicate this instability will continue through at least
Midnight. Therefore, we have added isolated thunderstorms to the
grids for the Hill Country into the I-35 corridor north of Austin. We
will monitor radar and model trends to decide if this should be
extended past Midnight in a few hours. Rainfall accumulations through
Midnight will generally average below a quarter of an inch, but we
may see some isolated totals closer to an inch over the Hill Country
if multiple elevated convective cores move over the same location.

For now, we have not made any updates to the temperature or dew point
grids aside from trending them based on observations. Significant
decreases in temperature due to evaporational cooling and increases
in dew point temperatures are occurring with the heaviest rainfall,
with dew points increasing into the upper 30s at a number of sites in
the Hill Country. Considering temperatures and dew points are in the
lower 50s at the top of the inversion just a mile up at 850 mb, we
certainly have the potential to increase dew points into the 40s with
the heaviest rainfall. However, areas that have received minimal or
no rainfall remain much drier and warmer, with teens and 20s located
on both sides of the precipitation shield. Thus, for now we will
keep dew points on the lower end to avoid chasing the local increases
due to convection as dew points should fall to some extent once the
rainfall passes through (as seen in Kerrville). Some models are
suggesting temperatures will be a bit warmer tonight than we
currently are forecasting due in part to this effect, but are holding
off on making any changes to the temperature forecast for now until
we get a better idea of how much moistening will occur and how much
temperatures are able to cool aloft.


Please see the 00Z aviation forecast discussion below.

Current radar data indicates light showers ongoing across the Rio
Grande plains and especially the southern Edwards Plateau. Radar
trends along with hi-res guidance shows the onset of persistent,
light rainfall will be during the early morning hours on Wednesday.
We will show light rain and MVFR cigs beginning around 08Z along
I-35 to around 11Z at DRT. Until then, deepening dry air in the low-
levels will bring a slow rise in cigs to area TAF sites and we will
keep skies VFR through the evening into early Wednesday. Light rain
and MVFR cigs and vsbys continue through the day tomorrow.


SHORT TERM (Tonight through Thursday)...

A wintry mix is expected across portions of the Texas Hill Country
and the Southern Edwards Plateau starting as early as Wednesday
afternoon and lingering through early Thursday morning. No to low
impacts are expected at this time due to very limited to no snow/ice
accumulations in higher elevation areas given the recent warm
temperatures. Outside of the wintry mix areas where temperatures will
be warmer, widespread light to moderate rain showers are expected
overnight tonight and occur through all of Wednesday into Thursday
morning. Rainfall accumulations will range from 1 to 2 inches total
starting late tonight to mid-day Thursday. See more details below on
forecast breakdown.

The well advertised cold front has now pushed through the entire
region with current temperatures in the mid 50s to the low 60s
areawide along with gusty north winds upwards of 25 mph. A closed H5
low is currently across southern California along with a longwave
trough axis pivoting across the southern Plains. In response, a H5-H3
strengthening jet will occur over central Texas through the overnight
and through the day Wednesday with greater DPVA. This combination
will enhance mid/upper level ascent and divergence overtop the
shallow surface cold airmass. In response, isentropic upglide and
deeper level ascent will work in concert to develop elevated light
rain showers by this evening and through all of Wednesday into at
least Thursday morning. High confidence exists in precipitation
falling, the bigger question will be the dynamic low-level vertical
temperature profiles causing varying precipitation types to be
possible Wednesday afternoon through Thursday morning.

Across the model suite, the GFS remains the warmer solution with
ECMWF in the middle and the NAM/CMC on the cooler side. All of this
combined, have followed a middle road temperature guidance given
prior model trends and forecast consistency. Ultimately precipitation
type will all come down to 2 factors: 1) degree of wet-bulbing from
evaporative cooling and 2) amount of dynamic cooling from adiabatic
ascent. Sounding analysis near Waco overnight tonight into early
Wednesday morning suggests some sleet could mix in with rain but even
the coolest guidance at Austin remains too warm for this likelihood.
Have keep precipitation type all rain overnight tonight into early
Wednesday afternoon.

Wednesday will be a rainy, raw, cold day as temperatures struggle to
warm from lows in the morning. While dry-bulb temperatures are
expected to remain above freezing, wet-bulb temperatures will begin
to fall in the mid to low 30s across the Hill Country and Southern
Edwards Plateau in the afternoon. Vertical temperature profiles will
support good dendritic growth in the -10C to -20C level and freezing
levels above ground level should fall from near 1500ft to near or
below 1000ft. This should allow for some snow to survive down with
the newly formed rain (due to some melting). By midnight Wednesday
to 6am Thursday, wet-bulb freezing levels could fall to near 500ft or
lower and possibly support a full snow conversion in heavier
precipitation bursts. The greatest area per models for the lowest
wet-bulb freezing heights appears to be in northern Edwards/Val Verde
and Kerr Counties. However, given the warm ground of late and the
already wet surface from the previous rainfall, accumulations of any
sort will be difficult to come by. As such, impacts should be

Temperatures will slowly warm mid-Thursday morning with full snow
conversion (if it occurs) going back to a rain/snow mix as the above
freezing wet-bulb near surface layer thickens. Further conversion
back to all rain should occur by early Thursday afternoon while
precipitation tappers off from north to south as mid and upper-level
dry air filters in as downglide and a stronger trough passage occurs
across the southern Plains.

LONG TERM (Friday through Tuesday)...

A reinforcing cold front will occur late Thursday and shift in low
PWAT air into the region. A cold morning will occur Friday with a
decent widespread freeze. Freeze advisories may be needed for this
period. Warmer and near normal temperatures with dry conditions
expected over the weekend into Monday with lots of sun. A dry front
will pass through the region Monday and keep temperatures near normal
given its Pacific origins.


Austin Camp Mabry              39  40  39  51  32 /  90 100  70  20  10
Austin Bergstrom Intl Airport  38  41  38  50  30 /  90 100  70  30  10
New Braunfels Muni Airport     38  41  38  49  32 /  80 100  80  40  10
Burnet Muni Airport            38  39  38  50  30 /  90 100  60  20  10
Del Rio Intl Airport           43  44  38  51  32 /  50  90  80  60  20
Georgetown Muni Airport        39  41  40  51  31 /  90 100  60  20  10
Hondo Muni Airport             40  43  36  52  28 /  60  90  80  60  20
San Marcos Muni Airport        38  40  38  51  31 /  80 100  80  40  10
La Grange - Fayette Regional   41  41  39  48  33 /  80 100  70  40  10
San Antonio Intl Airport       41  43  40  49  34 /  70  90  80  50  10
Stinson Muni Airport           42  45  40  49  34 /  60  90  80  60  20




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