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

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FXUS64 KEWX 220450

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1050 PM CST Sat Nov 21 2020

All terminals are VFR but low ceilings and visibility will develop
in the next few hours. Ceiling will drop to MVFR starting in the San
Antonio area around 07Z and then spread to Austin and finally DRT.
Ceilings will drop further and fog will develop overnight. Conditions
will be LIFR in Austin and San Antonio and IFR at DRT. Conditions
will gradually return to VFR by late morning or early afternoon. A
cold front will move into the region late in the day Sunday bringing
a wind shift to the north at AUS around 22Z while San Antonio and DRT
becoming easterly and northerly later in the evening.


.PREV DISCUSSION... /ISSUED 649 PM CST Sat Nov 21 2020/

Isolated showers have developed over the southeastern counties in the
moist, southeasterly flow. Short term models show this activity could
continue for a few hours this evening. We`ve updated the forecast to
include a slight chance for showers over the area southeast of I-35.

PREV DISCUSSION... /ISSUED 116 PM CST Sat Nov 21 2020/

SHORT TERM (Tonight through Sunday Night)...

The two main forecast challenges in the short term will be another
round of fog and low clouds for Sunday morning and then the approach
and passage of a weak cold front Sunday afternoon and evening. Model
soundings once again show a very saturated boundary layer tomorrow
across the interior of South Central Texas, so not surprisingly the
GFS, NAM, and other high resolution models are all indicating fog
formation for a few hours either side of sunrise. The main focus of
the fog for tomorrow looks to be around the San Antonio metro area,
with the approaching front keeping the gradient and winds slightly
higher for the Austin Metro area. Patchy to areas of fog are
forecast, with some dense fog not out of the question.

With respect to the cold front. All four Global Models
(GFS/NAM/ECMWF/Canadian) now show the front at least making it into
the Hill Country and Austin Metro areas by late afternoon on Sunday.
The GFS is the weakest with the front and washes it out along the I-
10 corridor, while the rest of the models push it all the way
through South Central Texas. The suite of high resolution models
from the Texas Tech WRF to ARW and NMM support this as well. The
late arrival in the day should mean that the front itself has little
impact on highs tomorrow other than knocking off a degree or two due
to the increased cloud cover with moisture pulling ahead of the
front. Models, both high resolution and global, show only a
sparseness of sprinkles, a peppering of precipitation, a rarity of
rain along the front. A few lucky folks could see a few hundredths
to a tenth of an inch, but most folks will stay dry. With the front
pushing through the area, tomorrow night will be a cooler night.
While overnight lows will still be about 10 degrees above normal,
they will be cool in the 50s rather than mild in the 60s like we
have seen. Highs though for Monday, will be right back into the low
to mid 70s under clearing skies. The northerly component to the wind
will be short lived with light east winds returning by mid-day
Monday, and southerly flow resuming Monday evening, further
highlighting how weak this front will be.

LONG TERM (Monday through Saturday)...

Monday morning, mid-level progs show a shortwave ridge over the
central US and troughing building over the west, with surface high
pressure centered over the Mississippi River Valley. Remnants of a
cold front may still be draped across south TX but will quickly wash
out through the day as southeasterly low level flow strengthens in
response to developing low pressure in the Lee of the Rockies. Highs
during the day should still be above normal, in the 70s to near 80.
If cloud cover holds on into the afternoon, a few spots north may
top out in the upper 60s.

The gulf warmth/moisture pump will continue into Tuesday with
southerly winds increasing to 10-20 mph with gusts as high as 30 mph
east and north. A very weak shortwave early in the day and
associated isentropic upglide could lead to a few isolated showers
Tuesday morning, however confidence is low, especially w/r/t
location, and so I`ve opted only a 10% shower chance at this time.
Regardless, morning cloud cover will gradually break up as usual
through midday and another day with well-above-normal temperatures
is expected as highs should reach the mid 70s to mid 80s.

The aforementioned western US trough will cross the southern Plains
Tuesday night into Wednesday, dragging a cold front across south-
central TX. The latest NAM data at 84 hours indicate a stark
displacement of the upper disturbance to the SW compared to the
other global guidance which will bear watching in terms of frontal
placement and timing. But the global models/ensembles all generally
agree than FROPA should come prior to sunrise Wednesday for most of
our area. Any precipitation chances along this front will likely be
relegated to our far eastern zones and amounts are likely to be
light. Expecting near-normal highs behind the front on Wednesday,
then as the surface high exits to the northeast on Thursday we`ll
again see warm, moist air return through the day on Thursday, though
not as earnestly as what is expected between Monday and Tuesday. It
should be a dry, warm Thanksgiving Day.

Some isentropic lift is again possible Thursday night into Friday
with shower activity returning over the east half of the region.
Some thunderstorms could also be possible for the first time in a
while Friday. However, model divergence grows for next weekend as
the next deep trough digs through the west. At this time, models are
relatively bullish on QPF through next weekend over eastern portions
of our area, but they`ve had a tendency to over-advertise in the
days 5-7 period and then pull back as events get closer, so I`m not
willing to bite on any significant rainfall at this time. But the
potential is at least there for a few spots to pick up a much needed
half inch or so east of I-35 over the next 7 days.


Austin Camp Mabry              63  75  53  73  61 /  -   20   0   0  10
Austin Bergstrom Intl Airport  61  76  52  75  59 /  -   20   0   0  10
New Braunfels Muni Airport     63  76  55  76  62 /  10  10  -    0  10
Burnet Muni Airport            60  72  51  69  59 /  -   20  -    0  10
Del Rio Intl Airport           61  77  60  76  63 /  -   10  30  10  -
Georgetown Muni Airport        60  74  50  71  58 /  -   20   0   0  10
Hondo Muni Airport             63  77  56  77  61 /  -   10  -    0  -
San Marcos Muni Airport        62  76  53  76  60 /  10  10  -    0  10
La Grange - Fayette Regional   61  78  54  77  61 /  20  10  -    0   0
San Antonio Intl Airport       64  77  56  76  62 /  -   10  -    0  10
Stinson Muni Airport           64  78  57  77  62 /  -   10  -    0  10




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