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

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FXUS64 KEWX 241116
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
516 AM CST Tue Nov 24 2020

.AVIATION...
IFR ceilings currently impacting the I-35 sites and an MVFR deck out
at DRT. Seeing some light returns on radar over the I-35 corridor.
High res models continue this streamer shower activity for the next
few hours and for this reason have kept in vicinity showers in the
TAFs. Ceilings will struggle to lift this morning as the boundary
layer remains saturated. Fog development has been limited by
southeasterly winds remaining above 5 kts. Should see ceilings
improve later this morning as well as a dissipation of shower
activity. Winds will be gusting up to 25 knots today across all sites
before relaxing this evening. Ceilings won`t quite return in the
fashion they did this morning for Wednesday morning as a cold front
makes it`s way over the region just before daybreak tomorrow. Have
annotated the initial wind shift in the 36 hour TAFs.

&&

.PREV DISCUSSION... /ISSUED 330 AM CST Tue Nov 24 2020/

SHORT TERM (Today through Wednesday)...
Return of low level moisture is well underway this morning. As
southeasterly flow continues, could see some spotty streamer shower
activity throughout the early morning hours. Slight chances for rain
exist for areas along and east of the I-35 corridor where the
moisture return is strongest. Patches to areas of fog are possible
this morning, mainly over the interior Hill Country and open low
lying areas along I-35. Persistent southeast winds remaining above 5
mph will help prevent widespread fog development.

The synoptic pattern this morning shows upper level subtropical
ridging over the south-central GOM and an approaching shortwave
trough axis that is currently moving over the four corners region
and northern Mexico. This feature will be responsible for our next
front that is expected Wednesday. Today however, temperatures will
climb 10+ degrees above climo norms along with dewpoints nearing the
mid to upper 60s. Dewpoints will gradually fall for our western
zones as the dryline approaches from the west in preparation of the
cold front. Models are in good agreement with the handling of the
frontal boundary, with it reaching our northwestern CWA by 03Z
Wednesday and clearing the region around 15Z. Texas Tech wrf, NAM
NEST, and HRRR all depict a thin band of thunderstorms developing
along this boundary. Unfortunately, it seems that the best chances
for precip will come for areas just northeast of the CWA with only
slight to chance POPs coming for our northeastern zones. Also, given
that this boundary will be progressive, this likely thin band of
precip will not drop much in the way of measurable rainfall. Perhaps
a few lucky locations will pick up a couple tenths of an inch.

Post-frontal airmass will filter in behind the boundary with
Wednesday highs much closer to normal for this time of year. High
temperature map for Wednesday shows how the timing of the front can
affect a day`s maximum temperature. The southern half of the CWA
will be able to reach the upper 70s to low 80s but the northern half
is unable to warm efficiently before the front moves through before
sunrise. Drier air, clearing skies, and seasonable temperatures will
be the name of the game for the end of the short term period.

LONG TERM (Wednesday Night through Monday)...
Surface high pressure shifts off to the east as pressures fall in the
Plains ahead of an upper level trough (currently approaching British
Columbia) nearing the Four Corners region. Southerly flow returns
leading to increasing moisture/clouds and warmer, well above normal
high temperatures for Thanksgiving Day that will be more typical of
Columbus Day. The low level jet and a weak mid level impulse will
generate streamer showers east of US 281 Thursday night. A
thunderstorm or two is possible due to weak instability.

For Friday into the weekend, models still disagree on the passage of
a cold front and the above mentioned upper level trough/closed low.
Upward forcing by these features on a moist airmass will result in
showers with weak instability and mid level lapse rates allowing for
mainly isolated thunderstorms. Below normal temperatures are expected
in the wake of the frontal passage.

The GFS remains the fastest with a cold frontal passage on Friday
and the upper level trough moving through on Saturday. This would
bring a faster end to the showers and thunderstorms and faster surge
of much cooler temperatures across all areas. The ECMWF is slower
with the frontal passage being Friday night into Saturday and the
upper level trough/closed low on Sunday. The GEM shows a faster
frontal passage slightly slower than the GFS, but similar to the
ECMWF on the upper level trough/closed low. These two models end the
showers and thunderstorms late Saturday night into Sunday with a
slower progression of the much cooler temperatures. For now, will
maintain POPs for Friday through Sunday per blended guidances,
however, later forecasts may end the POPs earlier should ECMWF/GEM
come into agreement with the GFS.

A surge of even colder air is expected early next week followed by
additional surges through next week into the following weekend
maintaining well below normal temperatures similar to mid winter
normals. Freezes are possible in low lying spots in the Hill Country
and along the Escarpment late night and early morning hours each day.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              79  55  72  48  79 /  20  30   0   0  -
Austin Bergstrom Intl Airport  80  55  73  45  80 /  20  30   0   0  -
New Braunfels Muni Airport     83  58  78  50  82 /  20  10   0   0  -
Burnet Muni Airport            77  50  70  46  78 /  10  20   0   0   0
Del Rio Intl Airport           85  53  78  50  81 /  -    0   0   0   0
Georgetown Muni Airport        78  51  70  45  77 /  20  30   0   0   0
Hondo Muni Airport             83  55  80  49  82 /  10  -    0   0   0
San Marcos Muni Airport        80  56  75  46  80 /  20  20   0   0  -
La Grange - Fayette Regional   81  60  74  49  80 /  20  20   0   0  -
San Antonio Intl Airport       80  58  77  51  80 /  20  10   0   0  -
Stinson Muni Airport           81  58  78  51  81 /  20  -    0   0  -

&&

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

$$

Short-Term/Aviation...EWilliams
Long-Term...04


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