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

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FXUS64 KEWX 031758
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1158 AM CST Sat Dec 3 2016

.AVIATION.../18Z TAFS/
Widespread showers with very isolated thunderstorms that combined
are producing only a few lightning strikes per hour continue to
blanket the region. IFR ceilings generally prevail across the
region, but all sites continue to experience brief MVFR or VFR
ceilings and reduced visibilities with the heavier showers. This
shower activity will begin to decrease in coverage by the middle
of the afternoon as isentropic ascent gradually weakens to allow
for a relative lull. Although a strike or two is possible at our
TAF sites due to elevated instability aloft, the chances are too
low at all sites to include in the TAFs and will amend as needed.

Hi-res model guidance consistently increases shower coverage with
a few isolated thunderstorms overnight as SW-NE oriented
isentropic ascent strengthens and possibly a weak shortwave
disturbance moves through the region. Coverage will begin to
increase between 3-6Z in the Rio Grande Plains and possibly DRT
before expanding northeast into the I-35 TAF sites by 6-9Z.
Although thunderstorms will be possible, chances are still too low
to prevail in the TAF and have chosen for now just to increase
rainfall intensities and decrease visibilities overnight. Rain
showers should decrease in coverage and intensity by late tomorrow
morning before the final wave of increased shower and thunderstorm
activity occurs later on Sunday just beyond the TAF period.


&&

.PREV DISCUSSION... /ISSUED 556 AM CST Sat Dec 3 2016/

AVIATION... /12Z TAFS/
Convective focus along I-35 should gradually lift NE this morning with
some occasional thunder to impact SAT/SSF/AUS terminals through
15z. Lighter overrunning rains should continue to produce
intermittent variations of predominant IFR conditions for the rest
of the TAF periods with some heavier showers possibly bringing
additional brief LIFR cigs and vsbys. Some late afternoon
improvements to MVFR cigs are possible, but will not show this at
SAT/SSF given the amount of rain and air saturation in that area.

PREV DISCUSSION... /ISSUED 330 AM CST Sat Dec 3 2016/

SHORT TERM (Today through Sunday)...
00z upper air analysis showed the large slow moving upper level
low just south of Arizona with 45 to 55 knots of southwest flow
ahead of the trough over Texas. With northerly flow and cold air
at the surface there is strong isentropic upslope on the 295K-310K
theta-e levels. The effects of this strong and deep lift can be
seen on radar at the present time with widespread rain across much
of the CWA. SPC mesoanalysis shows about 750 J/kg of MUCAPE across
the southern half of the area. This instability is enough for
continued chances of embedded thunderstorms within the activity
overnight and into the morning. Could even see some small hail
out of the stronger updrafts given the colder thermal profile and
the moderate levels of instability.

Isentropic ascent will remain strong on the 295/300k planes but
will be weaker above that level later today. With lift remaining
should continue to see a high coverage of activity this afternoon,
but with the lift being more shallow, intensity of the rainfall
could slacken a bit. High temperatures today will be in the 50s
for most locations and some places, especially in the Hill
Country, could struggle to get out of the 40s.

By tonight, the slow-moving upper low will be pushing southeast
and be over northern Mexico. Another impulse of energy ahead of
the upper low should increase the isentropic ascent once again
with northerly flow continuing at the surface. As the disturbance
arrives, rainfall activity should begin to increase across the
western counties and translate to the east throughout the night
bringing additional widespread rain to the area. There will be
little diurnal cooling with lows Sunday in the middle 40s to the
lower 50s.

The upper low is not expected to move much as we get into the day
on Sunday and will remain over northern Mexico. Scattered to
numerous showers and isolated thunderstorms will continue across
the area. By late Sunday afternoon, the bulk of the rainfall
should be across the western counties as the large scale dynamical
forcing associated with the slow-moving trough arrives to the
region. High temperatures on Sunday will be in the 50s once again.

LONG TERM (Sunday Night through Friday)...
The upper level low will finally begin to move east at the start
of the long-term forecast. As it does so, another large batch of
rainfall will move across the region from west to east Sunday
night and into Monday morning. The large scale forcing will keep
categorical PoPs in the forecast for Sunday night and will
continue likely PoPs for the eastern counties for Monday morning.
Rainfall should end by late Monday afternoon as the upper low
exits the region and subsidence commences on the backside of
the system. Storm total rainfall amounts of 1-3" for the west and
2-5" for the eastern counties still looks reasonable.

Will see a bit of a break on Tuesday in between systems with high
temperatures topping out in the upper 60s to lower 70s. The next
major system will be taking shape across the mountainous west
with very cold arctic air moving into western Canada and the
northwestern CONUS. As the trough moves east it will open up the
gates for this cold air to surge south and into the CWA Wednesday
afternoon. There will be a slight chance of a shower across the
eastern counties with this front. The bigger story will be the
cold air as this front will be by far the strongest of the season
Lows behind the front Thursday morning will be near freezing for
the northern CWA with breezy northerly winds. Highs Thursday will
likely remain in the 40s and with calmer winds Friday morning,
most locations throughout the CWA could see freezing temperatures
if this solution pans out.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              53  48  55  47  59 / 100  80  80  80  60
Austin Bergstrom Intl Airport  51  48  54  48  59 / 100  90  80  80  60
New Braunfels Muni Airport     53  48  54  48  61 / 100  80  80  80  50
Burnet Muni Airport            50  45  51  44  55 / 100  80  70  80  60
Del Rio Intl Airport           57  50  55  47  63 /  70  80  70  80  20
Georgetown Muni Airport        50  47  52  47  56 / 100  80  80  80  60
Hondo Muni Airport             57  50  57  47  64 /  80  80  80  80  30
San Marcos Muni Airport        52  48  54  48  60 / 100  80  80  80  50
La Grange - Fayette Regional   55  50  55  49  60 / 100  90  80  80  60
San Antonio Intl Airport       55  50  55  49  62 / 100  80  80  80  40
Stinson Muni Airport           55  51  56  50  63 / 100  80  80  80  40

&&

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

$$

Mesoscale/Aviation...LH
Synoptic/Grids...24
Public Service/Data Collection...33



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