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

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000
FXUS64 KEWX 220548
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1248 AM CDT Sat Sep 22 2018

.AVIATION...
Convection to the west of SAT/SSF/AUS will be impacting the airports
over the next several hours. Looks like DRT has likely seen the worse
of it and will keep the NW wind through the period. Have a mention
of VCSH for DRT first half of the forecast but shouldn`t see much
additional strong convection according to the mesoscale models. The
slow moving front will keep the convection along I-35 over the next
several hours. The HRRR and TTWRF show the line of convection to be
mainly east of AUS/SAT/SSF by late morning. Have dropped the tempo
groups of TS and kept VCSH through the early afternoon Saturday with
wrap around moisture. MVFR decks should hold strong most if not all
day. Don`t look for much improvement except maybe at DRT late in the
day when they might scatter out. Humidity cross sections through the
area have it socked in with very high RH despite the northerly
winds.

&&

.PREV DISCUSSION... /ISSUED 938 PM CDT Fri Sep 21 2018/

UPDATE...
Scattered showers and thunderstorms will continue overnight as a cold
front moves into our CWA. The extent of the convection has been less
than we previously forecast and we have lowered the POPs over the
northern and western parts of the area. The front is currently
approaching Val Verde County from the northwest. It should continue
to move to the southeast through the night and produce showers and
thunderstorms. The 00Z sounding from the DRT showed 1.92" PW, so
there is still excessive moisture in the atmosphere. Locally heavy
rain is possible and some this could lead to flash flooding. The
Flash Flood Watch will continue into Saturday.

PREV DISCUSSION... /ISSUED 649 PM CDT Fri Sep 21 2018/

AVIATION...
Timing of convection will be tough the next 24 hours. Until frontal
convection develops closer to the predawn hours, VCSH will be listed
in many of the time periods as a very moist atmosphere will allow
for some pop up convection after the sun goes down and the low level
jet strengthens. Cluster of storms out by DRT has weakened with
several outflows in that region. Another cluster of storms to the
east of AUS is pushing an outflow westward and may be a trigger for
later convection this evening. Will keep the NW wind out in DRT for
the duration of the TAF forecast...I-35 sites getting the wind shift
likely late morning. Prior to that, have a prevailing group of
showers predawn through late morning at AUS/SAT/SSF. No mention of TS
right now, but later shifts will likely have to add in for some
smaller time period. Cloud decks to should lower to MVFR all areas
after midnight...likely sooner at SAT/SSF. Not out of the question
that we could touch IFR before convection hits in the early morning.

PREV DISCUSSION... /ISSUED 314 PM CDT Fri Sep 21 2018/

SHORT TERM (Tonight through Sunday)...
Late afternoon surface observations show a cold front located across
the lower Trans Pecos region of west Texas, with plenty of moist,
southerly flow in place across south central Texas. The 21/12Z upper
air observations also show very high moisture in place with
precipitable water values around 2.2" at both CRP and DRT.

We are beginning to see some clearing as of late this afternoon
across portions of Val Verde county. As the above mentioned cold
front moves southward, we expect a line of showers and storms to
develop across the southern Edwards Plateau and Rio Grande plains
late this afternoon and evening. This line of convection is then
expected to move eastward through the remainder of this evening into
the overnight hours. Given good agreement among the hi-res models in
showing a line of convection moving southward overnight, we will
issue a Flash Flood Watch for portions of the Rio Grande plains,
southern Edwards Plateau and Hill Country. This will include counties
along and north of a Del Rio to Hondo to Burnet line. We contemplated
adding Maverick county into the watch, as the models do show a weak
signal for some heavier amounts this far south along the Rio Grande.
However, our confidence is a little low as of now and we will leave
Maverick county out of the Watch. With deep moisture, efficient
precipitation producing processes (tall, skinny CAPEs) and possible
training of cells along the front and wet ground, some flash flooding
is certainly possible. For now, the watch area will not cover the
I-35 corridor. However, we still can`t rule out some pockets of heavy
rainfall late tonight into Saturday morning along the I-35 corridor.
We will continue to monitor radar trends and model data and adjust
the Flash Flood Watch as necessary.

For late Saturday afternoon into Saturday evening, the cold front
should slide east and focus the better rain chances into areas east
of the I-35 corridor. Deep moisture will remain in place and with
frontal lift, we will continue to show a very good chance for
rainfall. On Sunday, the coastal plains and Highway 77 corridor will
continue to be favored for additional rainfall, with lesser chances
westward across the I-35 corridor and Hill Country. As for expected
rainfall amounts, the models have trended a little lower along the
I-35 corridor and a little higher across the southern Edwards
Plateau. For this event (tonight through Sunday), we will forecast
widespread 1-3 inches, with isolated higher amounts of 5 to 6 inches.
The focus for the higher totals is expected to remain in the Flash
Flood Watch Area.

LONG TERM (Sunday night through Friday)...
For Sunday night, the better rain chances will be located across the
coastal plains and Highway 77 corridor. Farther west, it appears
some drier air moving in from the west will greatly limit rainfall
chances. On Monday, the cold front will weaken as return southerly
flow spreads back across all of south central Texas. Rain chances
will again be favored across the coastal plains and Highway 77
corridor where deeper moisture will be found.

For the middle portion of the upcoming work week, a broad upper
trough will linger over the central and southern U.S. plains. A cold
front tries to make some southward progress into central Texas on
Wednesday, but may very well slow down and not move through or stall
over the region. Given a potential slow-moving or stalled front, the
weak upper trough and plenty of moisture, we will keep rain chances
in the forecast through the middle and later portion of the upcoming
work week.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              69  84  71  87  74 /  30  30  20  20  10
Austin Bergstrom Intl Airport  69  84  70  87  74 /  40  30  20  20  10
New Braunfels Muni Airport     69  85  70  88  73 /  30  20  10  20  10
Burnet Muni Airport            66  80  68  85  71 /  20  30  20  20  10
Del Rio Intl Airport           68  84  68  88  72 /  10  10  -   -   -
Georgetown Muni Airport        68  82  69  86  73 /  40  30  20  20  10
Hondo Muni Airport             68  83  69  88  72 /  20  20  10  10  10
San Marcos Muni Airport        68  85  70  88  73 /  40  20  20  20  10
La Grange - Fayette Regional   72  86  72  88  74 /  60  40  30  40  20
San Antonio Intl Airport       68  84  71  88  73 /  30  20  10  10  10
Stinson Muni Airport           70  85  72  88  74 /  30  20  10  10  10

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Bandera-Blanco-Burnet-
Edwards-Gillespie-Kendall-Kerr-Kinney-Llano-Medina-Real-Uvalde-Val
Verde.

&&

$$

Mesoscale/Aviation...09
Synoptic/Grids...04


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