Scientific Forecaster Discussion
fxus64 kewx 180447 aaa
Area forecast discussion...updated
National Weather Service Austin/San Antonio Texas
1047 PM CST Wed Jan 17 2018
Aviation.../06z taf cycle/
VFR continues to prevail this evening and is expected to persist
through all of Thursday with MVFR and IFR possible at the far end of
the 30 hour taf period for kaus/ksat early Friday morning.
Ovc060 stratus deck has shifted over all taf sites as anticipated and
should remain in place through Thursday with slightly lowering
ceilings to bkn/ovc040-050.
Surface winds will be light and variable overnight and become
southeasterly Thursday afternoon while remaining near or below 10
Previous discussion... /issued 830 PM CST Wed Jan 17 2018/
Update... /more clouds & warmer mins NE/
The interesting and nearly stationary cloud/clear air line from
this afternoon has since advanced NE, and is now forecast to overtake
all our south central Texas counties by daybreak. Light east/NE winds over
central Texas continues to maintain very dry surface air, making a hard
freeze still likely over the Austin Metro area and some of the
outlying areas around San Antonio. With full cloud cover expected
tonight over the still very dry air over San Antonio, much of the
city could wind up with a light freeze, or about 2-3 degrees warmer
than earlier assessed. A wind deformation zone NE of aus at the 5-6
kft layer of clouds could result in the cloud deck arriving into
parts of Burnet/Williamson/Lee counties well after midnight, and the
coldest min temps for tonight are shown here.
Regarding the upper level shortwave digging into nrn Mexico and just
upstream from south central TX, the forecast rain/snow mix remains in
play for mainly midnight to 14z Thursday. The only adjustment here
was to add more non-zero precip chances, or "silent 10's" to account
for a daytime sprinkle or two. Pending the arrival of the 00z models,
the forecast may need to be modified in the next several hours to
account for some mention of sprinkles or light showers along/S of
I-10 in the midday hours as forecast by around 1/3 of the finer res
Previous discussion... /issued 601 PM CST Wed Jan 17 2018/
Aviation.../00z taf cycle/
VFR conditions to prevail through the taf period as ovc060
altostratus deck shifts north through the next 6 hours. Kaus will be
under the deck near 03-04z with all other sites already under the
mid-level clouds. Cloud ceilings will slowly lower through Thursday
but remain VFR. Surface winds will be light and variable overnight
and then switch to the east and southeast Thursday afternoon while
remaining below 10 knots.
Previous discussion... /issued 241 PM CST Wed Jan 17 2018/
Short term (tonight through Thursday night)...
the focus of the short term period was on tonight's potential hard
freeze and prospects for any wintry precipitation associated with an
approaching upper low.
Over the course of the day, a stubborn cloud shield has been observed
across the southwest 2/3 of the County Warning Area which prevented low temperatures
from plummeting much like locations in our northern County Warning Area this morning.
This cloud shield, despite some model guidance indicating otherwise,
has not shown any signs of dissipating. As this upper low continues
to approach, GFS moisture fields bring in high clouds to boot in this
area. As a result, temperature guidance along the Rio Grande plains
is probably too cold again tonight and likely won't get as cold as
locations in the Austin area and Hill Country, where only slightly
higher lows from yesterday are forecast.
Additionally, some upper level energy is progged to wrap around the
low overnight and could produce some elevated precip over The Big
Bend and Rio Grande plains. Interrogation of forecast soundings do
show a weak warm nose (+1c) just below 700 mb but due to dry air
below that layer, will likely be able to dry-bulb down to below
freezing, especially if hydrometeors are seeded above 500 mb. But, if
surface temperatures aren't able to even reach freezing, then the end
result at the surface may end up being some very light rain and
snowflakes. Hi-res guidance has been inconsistent on resolving precip
development overnight, and synoptic models bring in this precip later
in the day, when most locations are above freezing. Either way, the
very dry low level atmosphere should prevent anything in the way of
significant quantitative precipitation forecast and thus no impacts are expected.
The low is expected to begin to push eastward from The Big Bend
region Thursday, tapping into low level moisture farther southeast
late Thursday and producing light shower activity into the overnight
hours. With mostly cloudy skies and warmer temperatures, feel pretty
confident this should all be rain in the southeast, although
temperatures will likely fall into the mid to upper 30s.
Long term (friday through wednesday)...
a warm weekend looks to be in store as ridging takes place over the
state after the aforementioned low pushes east. Model guidance is
showing highs in the 70s for both Saturday and Sunday and especially
Saturday, that looks certainly valid.
Maintained southerly low level flow should allow for moisture to feed
into the eastern third of the state over the weekend as well which
will be key in the development for more showers and possibly even
thunderstorms Sunday afternoon as another low and associated Pacific
front push east through the County Warning Area. An impressive llj, good lifting
along the front, and shear appears to be in phase along and east of
the I-35 corridor (according to the gfs) Sunday afternoon. Thus,
included isolated thunder mention in the wx grid for now, but
greatest uncertainty isnt so much thunder potential but timing as the
GFS seems to be quite fast in translation of the front which is often
overdone at the mid-range. So for now, included pops across eastern
2/3 of the County Warning Area to account for uncertainty with the higher chance pops
across the easternmost areas.
Preliminary point temps/pops...
Austin Camp Mabry 24 45 35 52 46 / 0 - 10 20 10
Austin Bergstrom Intl Airport 22 44 33 51 45 / 0 - 10 20 20
New Braunfels Muni Airport 26 43 34 52 43 / - 10 20 30 10
Burnet Muni Airport 22 44 32 53 43 / 0 0 - 10 10
del Rio Intl Airport 30 47 31 59 39 / 20 20 - - 0
Georgetown Muni Airport 21 44 32 51 45 / 0 - - 10 10
Hondo Muni Airport 28 45 36 55 40 / 10 10 20 20 -
San Marcos Muni Airport 24 42 34 51 44 / - 10 20 30 10
La Grange - Fayette regional 25 42 34 52 46 / - - 20 30 30
San Antonio Intl Airport 29 43 36 52 44 / 10 10 20 20 10
Stinson Muni Airport 29 43 36 53 43 / 10 10 20 20 10