Tropical Storm Warning 1009, NWS Austin/San Antonio, TX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
112 AM CDT Thu Aug 24 2017

.AVIATION.../06Z_TAFS/
Isolated to scattered showers and thunderstorms continue to move
southwest just east of SAT/SSF with a stationary front parked near
the I-10/Hwy 90 corridor. These storms should stay out of SAT/SSF as
they continue to dissipate, but we added VCSH at SSF for the next
hour and will amend both TAFs if needed. Convective activity should
end within the next few hours before redeveloping early tomorrow
afternoon. We have included VCSH at the I-35 TAF sites from 19-00Z as
coverage should remain scattered and confidence in thunderstorms is
too low to mention at this time. Convective coverage will probably be
more isolated at DRT, so VCSH has been removed for that site for
now. Winds should generally remain east-northeast below 10 knots
with broken to overcast ceilings remaining high enough to allow VFR
conditions to persist at all sites through Thursday evening.

&&

.PREV DISCUSSION... /ISSUED 1043 PM CDT Wed Aug 23 2017/

UPDATE... /TRACK ADJUSTMENT OF HARVEY AND COASTAL PRAIRIE POPS/
Minor adjustments to the forecast track of T.D. Harvey will
potentially add to the forecast rainfall potential over the I-35
corridor and max totals over the southeast counties. Only minor
adjustments made to the tropical cyclone related forecast products
and the Messaging Key Points which are revised below.

Another evening change was made to reflect the ongoing convection
associated from the weak cold front that interacted with the sea-
breeze. Despite good pwat values in the area, expect to see surface
winds to become less convergent overnight with gradually decreasing
coverage.

Messaging Key Points

1) Heavy rainfall possible east of I-35 Friday
through Monday, possibly leading to flash flooding and river
flooding.
2) Average rainfall amounts of 8 to 12 inches possible east
of I-35 with isolated higher amounts in excess of 15 inches in the
Tropical Storm Watch area. Areas along and west of I-35 could see 2
to 4 inches with isolated higher amounts.
3) There remains uncertainty in the forecast track of Harvey across
Texas. Small changes to the track or forward speed of Harvey will
result in large changes to impacts across South Central Texas. If the
the track of Harvey shifts to the west or the system stalls, then
much higher amounts will be possible.

PREV DISCUSSION... /ISSUED 706 PM CDT Wed Aug 23 2017/

UPDATE...
Please see the 00Z aviation forecast discussion below.

AVIATION...
Convection from earlier has decreased significantly and should not
impact the TAF sites through this evening. Can't completely rule out
a few showers overnight as depicted by some of the hi-res models.
However, confidence is low and we will not mention in the TAFs at
this time. We do expect a better chance for some scattered convection
after 24/18Z and will mention VCSH for all sites. Otherwise, we will
continue to mention VFR cigs through the period, but monitor for some
slightly lower cloud bases near DRT after 24/08Z.

PREV DISCUSSION... /ISSUED 408 PM CDT Wed Aug 23 2017/

Harvey could bring heavy rains to South Central Texas
Friday into the weekend...

SHORT TERM (Tonight through Thursday Night)...
Top story first...

Tropical depression Harvey is expected to approach the middle Texas
coast Friday and move inland late Friday into Saturday. This tropical
system could bring heavy rain late Friday into the weekend across
South Central Texas. Word of caution, there remains a lot of
uncertainty about the track and intensity of Harvey as it moves to
the northwest through Friday. Below are some of the key messaging
points that we are advertising for situational awareness purposes.

Messaging Key Points
1) Heavy rainfall possible east of I-35 Friday through the weekend, possibly
leading to flash flooding and river flooding.
2) Average rainfall amounts of 6 to 10 inches possible east of I-35
with isolated higher amounts in excess of 12 inches in the Tropical
Storm Watch area. Areas along and west of I-35 could see 2 to 4
inches.
3) There remains uncertainty in the forecast track of Harvey
across Texas. Small changes to the track or forward speed of Harvey
will result in large changes to impacts across South Central Texas.
If the the track of Harvey shifts to the west or the system stalls,
then much higher amounts will be possible.

Now, the short term forecast period. Scattered showers and isolated
thunderstorms are possible the rest of this afternoon and evening as
a weak frontal boundary stalls across the area. Some of the HiRes
models suggest for the boundary to push to the north and generates
isolated to scattered showers and isolated thunderstorms across the
Hill Country overnight into Thursday.

LONG TERM (Friday through Wednesday)...
More on Harvey and expected weather conditions through the extended
forecast period.
A weakening upper level ridge across the desert southwest will pull
to the northwest and allows Harvey to approach the middle Texas coast
and make landfall late Friday into early Saturday. The system is
expected to slowly move to the north and into the Hill Country and
possibly stalls as an upper level trough digs down across the Central
Plains Saturday into Sunday. With the operational GFS, GFSensemble
and ECMWF models agree on this solution this far out, we urge you to
closely monitor the weather conditions for the next few days as
changes will be made to the forecast and updated information will be
shared for situational awareness and preparedness before, during and
after the event. There is a good chance for a Flash Flood Watch to go
out later tonight as we continue to adjust upward rainfall amounts,
mainly across the Tropical Storm Watch area.

The upper level ridge above mentioned is expected to rebuild across
the Intermountain West late Sunday into Monday and then push to the
east. Also, an upper level trough is expected to swing across the
Ohio Valley and help Harvey to move to the northeast. This
combination will allow Harvey to curve to the northeast and away from
the area with rain chances ending Tuesday. The rest of the week
looks dry and cooler with highs in the lower 90s....

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              93  75  93  74  84 /  50  30  40  40  80
Austin Bergstrom Intl Airport  93  75  93  74  83 /  50  30  40  50  80
New Braunfels Muni Airport     94  74  92  74  84 /  40  30  40  50  80
Burnet Muni Airport            88  72  89  71  82 /  50  30  30  40  60
Del Rio Intl Airport           97  76  95  75  92 /  30  20  30  20  30
Georgetown Muni Airport        90  74  91  72  82 /  50  30  40  40  70
Hondo Muni Airport             95  74  93  74  88 /  40  20  40  30  60
San Marcos Muni Airport        94  74  93  73  83 /  40  30  40  50  80
La Grange - Fayette Regional   95  75  92  75  84 /  40  30  50  80  90
San Antonio Intl Airport       93  75  91  74  85 /  50  20  40  50  70
Stinson Muni Airport           93  75  92  75  84 /  40  20  40  50  70

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
Tropical Storm Watch for the following counties: De Witt...
Fayette...Gonzales...Karnes...Lavaca...Wilson.

&&

Mesoscale/Aviation...LH
Synoptic/Grids...Hampshire
Public Service/Data Collection...30

Hazardous Weather Outlook...UPDATED
National Weather Service Austin/San Antonio TX
446 PM CDT Wed Aug 23 2017

TXZ171>173-183>194-202>209-217>225-228-242200-
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-
446 PM CDT Wed Aug 23 2017

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

.DAY ONE...Tonight.

Evening scattered thunderstorms are expected across the Hill
Country and Edwards Plateau, where isolated heavy rain and gusty
winds of 30 to 40 mph are possible as a frontal boundary stalls
over the region.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Harvey is expected to move to the northwest and approach the
Texas coast Friday. The tropical system will bring tropical storm
winds and heavy rain across South Central Texas beginning Friday
and continuing into the weekend. There is a Tropical Storm Watch
for counties east of Interstate 35. Flash flooding and river
flooding is possible, mainly across the Tropical Storm Watch area.
Please refer to the NHC website (www.nhc.noaa.gov) and our local
office website (www.weather.gov/sanantonio) for latest information
and to follow the evolution of Harvey as it gets closer to the
Texas coast.

As of now, the storm total rainfall amounts of 6 to 10 inches are
expected east of Interstate 35 with isolated 12 inches across
counties in the Tropical Storm Watch from Friday afternoon through
Monday afternoon. It should be stressed that there is the
potential for much higher amounts across South Central Texas as
they will be highly dependent on the track and intensity of
Harvey.

There is a good chance for the issuance of a Flash Flood Watch
later tonight (overnight weather package) as the result of
rainfallamounts increasing. The FFW is likely for the areas
under the Tropical Storm Watch.

There is a low risk of brief tornadoes east of Interstate 35 Friday
evening into the weekend associated with tropical rain bands.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated at this time. However,
weather reports are welcome during the event on Friday into the
weekend.

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