Areal Flood Advisory 221, NWS Austin/San Antonio, TX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
630 AM CDT Mon Aug 2 2021

.AVIATION.../12Z TAFS/
A few thunderstorms are producing very heavy rainfall rates with
VSBYS below 1 mile and frequent lightning early this morning. The
storms are firing as discrete cells and moving slowly, which means
there could be several hours of thunder in the VCNTY at all 4 TAF
locations. The radar trends over the past hour suggest that the
activity may try to congeal into a solid area of organized storms. If
this happens there should be a more progressive push southward and
improved conditions by midday. Since this organizing process is slow
to occur, will leave mention of nearby storms for much of today.
Chances are trending down but 20-40 percent type coverage will remain
in play for the remainder of the TAF periods. Model timings are all
over the map, so will leave the later periods as benign until the
impacts of the current convection become more apparent. Assuming
nearly all areas expected to see some rain sometime today or tonight,
we should expect to see some pockets of low clouds and perhaps a few
patches of fog around daybreak Tuesday.

&&

.PREV DISCUSSION... /ISSUED 333 AM CDT Mon Aug 2 2021/

SHORT TERM (Today through Tuesday)...
Most portions of South Central Texas should experience at least one
round of rain and storms today. An amplifying northerly flow aloft
pattern has brought a weak frontal zone into South Central Texas
with a broad area of low to mid level moisture pooling and a broad
synoptic scale cyclonic pattern over TX with embedded smaller scale
perturbations scattering showers and storms over several discrete
locations. Synoptic scale models have performed well with this
northerly flow pattern, seemingly outperforming the finer resolution
models that were more aggressive with the first round of storms.
Highest PoPs of the week have zeroed in on the first period today,
although many areas may wind up seeing the bulk of the rains falling
in the predawn hours before the valid time of the first period. In
shifting more emphasis for today's rain chances, model trends are
pulling back on continued rain chances in later periods.

Slow storm motions this morning could lead to some isolated pockets
of 3 to 4 inches which would be sufficient for localized flooding.
However, we don't expect a significant portion of the area to see
these types of rains and the low rainfall amounts tallied up over
the past 10 days or so would not suggest a threat for high impact
events. Thus will continue to message the locally heavy rainfall
threat and not issue a watch at this time. High temperature trends
for today have come down significantly from earlier forecasts and
the ongoing convection should justify that trend. Northerly wind
directions today and tonight should also lend toward a cooler min
temp tonight, although the moist dewpoints will limit the cooling
potential. Areas that see significant rain may need to be considered
as areas for potential fog in later forecast updates. Scattered
convection and mild high temperatures should continue into Tuesday,
but there could be some more stability moving into parts of the Hill
Country and Central TX with the deepest moisture pooled over our
southern CWA.

LONG TERM (Tuesday Night through Sunday)...
A decent number of question remain in the long term about the
strength of the upper ridge and the impact on precipitation chances.
A general look at the models show the weakness in the ridge still
over the area Tuesday night, but a small ridge of high pressure
trying to move across the state on Wednesday. By Thursday, with the
large upper ridge still across the Southwestern US and the large
trough across the East, a shortwave digs across the Central Plains
weakening the ridge over Texas for Thursday into Friday. By the
weekend the Southwestern ridge reestablishes itself across Texas
returning the state to an early August weather pattern.

At the surface weak northerly or easterly flow continues across much
of the area with the weakening front situated somewhere along the
Rio Grande and Coastal Plains. The front will be the focus of
precipitation for the Wednesday and Thursday time periods with the
convergence along the front focusing precipitable water values around
2 inches. As we have seen Sunday evening into early this morning the
moisture rich atmosphere can produce locally heavy rainfall of 2 to
5 inches in a hurry. A few pockets of these higher totals will
continue to be possible through mid week. Areas across the Hill
Country and Austin Metro area may not see much precipitation at all
in the long term if the drier trend in the synoptic models prevails.
This would mean that even the current forecast of slight to chance
PoPs for these areas on Wednesday and Thursday is overdone as the
likely focus for precipitation will be to the south and west of these
areas.

By Friday and for sure into the weekend a drier trend prevails with
only a few seabreeze showers and thunderstorms possible each
afternoon east of Interstate 35. Temperatures will run below average
through Thursday, with 'cool' mornings in the upper 60s across the
Hill Country and afternoon highs only topping out in the low 90s once
again. By the weekend though with the ridge building normal highs in
the mid to upper 90s return.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              89  74  91  72  91 /  70  40  30  10  20
Austin Bergstrom Intl Airport  89  73  91  71  91 /  70  50  30  10  20
New Braunfels Muni Airport     90  73  92  73  92 /  60  40  40  10  20
Burnet Muni Airport            86  71  90  70  89 /  70  30  20  -   20
Del Rio Intl Airport           92  76  93  75  92 /  60  50  50  30  40
Georgetown Muni Airport        87  73  90  71  90 /  70  30  20  10  20
Hondo Muni Airport             90  73  91  74  90 /  50  50  40  20  30
San Marcos Muni Airport        90  73  92  71  92 /  60  50  30  10  20
La Grange - Fayette Regional   90  76  92  74  93 /  60  50  40  10  20
San Antonio Intl Airport       89  75  90  74  90 /  50  40  40  20  30
Stinson Muni Airport           90  76  92  77  92 /  50  50  40  20  30

&&

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

Short-Term/Aviation...Oaks
Long-Term...Treadway

Hazardous Weather Outlook
National Weather Service Austin/San Antonio TX
458 AM CDT Mon Aug 2 2021

TXZ171>173-183>194-202>209-217>225-228-031000-
Llano-Burnet-Williamson-Val Verde-Edwards-Real-Kerr-Bandera-
Gillespie-Kendall-Blanco-Hays-Travis-Bastrop-Lee-Kinney-Uvalde-
Medina-Bexar-Comal-Guadalupe-Caldwell-Fayette-Maverick-Zavala-
Frio-Atascosa-Wilson-Karnes-Gonzales-De Witt-Lavaca-Dimmit-
458 AM CDT Mon Aug 2 2021

This Hazardous Weather Outlook is for South-Central Texas..

.DAY ONE...Today And Tonight.

Numerous showers and thunderstorms are expected to continue today
through tonight. Some storms will move very slowly, resulting in
an isolated potential of 3 to 4 inch rains. Localized flooding
will be possible today and tonight. The stronger storms may also
contain gusty winds of around 40 mph.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

Scattered thunderstorms are expected Tuesday and Wednesday.
Isolated pockets of heavy rainfall will be possible each day.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated.

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