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

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FXUS64 KEWX 141145

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
545 AM CST Thu Dec 14 2017

VFR flying conditions will prevail today through Friday with clouds
mainly 10K FT and above. However, as the upward forcing mentioned
below increases, CIGs may fall to 5K FT as RA develops this evening
into the overnight. RA will be mostly south of the TAF sites with the
best chances of RA and the lowering CIGs being at KSAT/KSSF. Will
monitor model and radar trends for possible later mention to add RA.
A cold front will sag south across our area with a somewhat tighter
surface pressure gradient lagging behind. Winds become Nly 5 to 7 KTs
later this morning into afternoon and then increase to 10 to 15 KTs
with occasional gusts to 22 KTs this evening into overnight.


.PREV DISCUSSION... /ISSUED 441 AM CST Thu Dec 14 2017/

SHORT TERM (Today through Friday)...
A weak cold front will continue to advance towards the CWA today,
resulting in little more than a wind shift of winds from light and
variable this morning to northerly late this morning. A sub-tropical
jet is evident on WV/IR satellite this morning and this jet will
continue to pump elevated moisture into the region and keep high
clouds through most of the day. This jet is a result of trough over
Baja California which will continue to move southward, while another
secondary shortwave quickly dives south across the Rockies.

What this will do in the short term, is provide ample dynamical lift
in an elevated moisture regime late this afternoon and into the
evening hours. Omega fields increase above 500 mb in response to the
increasing jet magnitude which should be positioned just south of the
I-10 corridor. While today`s cold front passage will continue to
keep the low level atmosphere dry, and additionally an isentropic
ascent regime will exist through much of the daytime hours, when this
better dynamical lift arrives later tonight, PoPs will increase
across the southern half of the CWA along this area of lifting.

Tomorrow will start off chilly and only warm into the 50s for much of
the area being in the post-frontal air mass and also with cloudy
skies across the area. PoPs will again be confined to southern areas
as the jet streak continues to pump pieces of energy across the
south. The aforementioned Baja California low gets cut off and
temporarily wobbles over west-central Mexico, setting the stage for
our upcoming rain possibilities Saturday.

LONG TERM (Friday Night through Wednesday)...
This Mexican low will be forced northeast as yet another shortwave
quickly dives south across the western seaboard. This will force the
low to become negatively tilted an eject PVA across the region
Saturday. This low is progged to quickly push northeast Saturday,
passing directly over the CWA by late Saturday.

While there will be plenty of UL lifting, strong isentropic ascent
and deep layer shear present, very little instability will be
present, nor a focused surface lifting mechanism to allow this system
to have much surface support. Forecast soundings continue to show
the atmosphere moistening from the top down Saturday, but a strong
low level inversion, weak lapse rates, and again no CAPE means that
UL divergence will be responsible for the "heavy lifting" of air
parcels. And while there is plenty of energy to expect widespread
rainfall Saturday for almost all of the CWA, the potential for heavy
rain and thunder is rather low. The 00Z deterministic GFS showed
omega fields increasing dramatically across the eastern half of the
CWA after 18Z Saturday so one would expect shower activity to
increase from south to north, initially with light returns scattered
broadly and becoming more focused across the I-35 corridor and east
moving into the overnight hours. Rainfall totals through Sunday
morning should be generally less than a half inch, but cannot rule
out a few isolated locations seeing up to an inch or so, again most
likely east of the I-35 corridor.

The shortwave quickly exits the area before dawn Sunday and should
take all the rain with it as dry air quickly moves in with
southwesterly flow. Models continue to show pretty significant
disagreement beyond Sunday. Earlier runs showed another cold front
passage Sunday as the Rocky Mountain low pushed east across the
state, and while the GFS continues to show that, the 00Z ECMWF does
not. This unfortunately decreases confidence in the forecast for next
week, especially with PoP potential for the early to middle part of
the week.

It should be noted that long range models are continuing to show
signals for a pattern which could be rather active synoptically for
the next weekend and following week. This pattern of 2-3 cold front
passages per week will likely continue, some more significant than
others, but details beyond large scale pattern changes cannot be
determined at this stage with any reasonable confidence. Especially
if those details (prospects for winter weather) hinge on a
temperature forecast of 32 vs 33+ degrees, as is often the case for
this part of the country. So use long range models with extreme
caution. More details will be discussed as our confidence increases.


Austin Camp Mabry              64  43  58  40  56 /   0  20  -   -   50
Austin Bergstrom Intl Airport  64  43  57  38  55 /   0  20  -   -   60
New Braunfels Muni Airport     66  44  57  40  54 /   0  30  10  -   60
Burnet Muni Airport            63  38  56  36  56 /   0  10  -   -   50
Del Rio Intl Airport           64  45  60  40  55 /   0  20  -   -   30
Georgetown Muni Airport        63  41  57  38  56 /   0  10  -   -   50
Hondo Muni Airport             66  45  59  41  54 /   0  30  10  -   50
San Marcos Muni Airport        64  44  57  39  54 /   0  20  -   -   60
La Grange - Fayette Regional   64  45  57  40  56 /   0  20  -   -   60
San Antonio Intl Airport       67  45  59  42  54 /   0  30  10  -   50
Stinson Muni Airport           67  46  58  42  55 /   0  30  10  -   60




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