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

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FXUS64 KEWX 230000
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
700 PM CDT Tue Aug 22 2017

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

&&

.AVIATION...
VFR conditions are expected at area TAF sites through the current
forecast period. A few showers will linger over the coastal plains
and for areas north of a Del Rio to Rocksprings line through 02Z.
This activity will not impact our TAF sites. Otherwise, we should see
clouds dissipate through this evening with the loss of daytime
heating. Some low cloud development is expected overnight, but with
low-level winds trending to a more south-southwesterly direction
along I-35, we will not mention any MVFR cigs at this time. Winds
should then trend to a more southerly direction by early afternoon
along I-35. We also expect VFR conditions out west at DRT as winds
back to a more easterly direction this evening, then veer
southeasterly by late morning.

&&

.PREV DISCUSSION... /ISSUED 259 PM CDT Tue Aug 22 2017/

SHORT TERM (Tonight through Wednesday Night)...

Main weather highlight will be the continued hot temperatures and
elevated heat index values today and again tomorrow. Heat index
values will reach upwards of 100-105F during the afternoons. In
addition, a weak front will likely reach far northern areas of
south-central Texas that will help scattered showers and
thunderstorms develop. Attention will also turn to the tropics in the
long-term section below with heavy rainfall and flooding potential
increasing.

A retreating H5 ridge over west Texas is shifting farther westward
today with a TUTT low analyzed over the western Gulf of Mexico today.
A few coastal plains showers will occur this afternoon and early
evening but will weaken quickly post-sunset as stabilization occurs.

For Wednesday, a weak convectively-forced boundary will shift south
towards the region and help provide more low-level convergence to aid
in scattered thunderstorm growth over the Hill Country and portions
of the Edwards Plateau. CAPE values are advertised to remain near
1000 J/Kg and this should keep storms from being too strong. Most of
the activity, again, should decrease after sunset. Heat index values
Wednesday afternoon will be in the 100-106F range, minus the Hill
Country area.

LONG TERM (Thursday through Tuesday)...

Concern for heavy rain and flooding from a likely reengergized
landfalling tropical cyclone Harvey continues to increase for late
Friday and through the entire weekend for much of the region. While
models still need to resolve the tropical system as it is in the
midst of forming, and also its future track, the guidance envelope
suggests the possibility for moderate to high impacts in the form of
heavy rain leading to river and flash flooding.

Thursday will be will be a dynamic day as two systems driving the
weather will be at play. The aforementioned boundary will linger
across the region and likely support a weak convergence zone to
support diurnal showers and thunderstorms for a 20-40% chance of
scattered activity. While most of this activity should remain
scattered enough not to cause any flooding impacts itself, it will
act to help saturate some soils ahead of the possible tropical
cyclone rainfall totals over the weekend. This preceding rain could
exacerbate possible flooding impacts Friday afternoon and into the
weekend depending on evolution/track of Harvey.

Greater concern for heavy rain and flooding impacts across at least
the I-35 corridor and eastward are increasing per latest model
envelope guidance. The NHC is current giving the remnants of Harvey
a 100% chance of redevelopment into a tropical depression and storm
over the next couple days. This development could occur as early as
tomorrow morning with possible advisories along the Texas coast in
the next 24-48 hours, if not sooner. Both the GFS and EC are in
decent agreement of a strengthening tropical system nearing the
southern to middle Texas coast Friday morning. Anywhere from
Brownsville to Houston could be a possibility as the storm is in its
formation stages currently. Many models are depicting high rainfall
totals across portions of the region. Based on the EC/GFS at least
2-6" is expected along and east of I-35 corridor Friday afternoon
through Sunday afternoon. This total will change and depending on the
track could go significantly higher if the track is farther south
and west or if the EC/GFS solutions pan out. Confidence in these
solutions is slowly growing. Of note, there will be a tight western
rainfall amount gradient somewhere. It is too far out at this time to
speculate where that will be. The rainfall amounts will be refined
and updated as new model guidance and upper-air balloon data are
collected. In addition to heavy rainfall leading to river and flash
flooding, some areas of the eastern Coastal Plains could see tropical
storm force winds Friday into early Saturday. Will not get into
specifics as of yet until the NHC begins advisories on the system.

The longevity of the rainfall this weekend will be due to the system
being advertised to stall near or over the region as it is blocked
by an H5 ridge over the four-corners and attempts to be picked up by
only weak troughing across the mid-Mississippi River Valley. With
tropical downpours and the potential for the system to move slowly,
excessive rainfall amounts are appearing to look possible. Again,
stay tuned through the next 24-48 hours as totals and the track are
refined with extra data being collected.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              77  98  77  97  77 /  -   20  20  40  30
Austin Bergstrom Intl Airport  75  97  76  97  76 /  -   20  20  40  30
New Braunfels Muni Airport     74  98  75  98  75 /  -   20  20  30  20
Burnet Muni Airport            73  94  73  92  72 /  -   30  30  40  40
Del Rio Intl Airport           78 100  78 100  78 /  -   20  20  30  20
Georgetown Muni Airport        75  95  74  94  74 /  -   20  30  40  40
Hondo Muni Airport             73 100  74  98  74 /  -   10  10  30  20
San Marcos Muni Airport        74  99  75  99  75 /  -   20  20  40  20
La Grange - Fayette Regional   75  99  76  99  76 /  -   20  20  30  30
San Antonio Intl Airport       76  97  76  97  76 /  -   10  20  30  20
Stinson Muni Airport           75  97  76  97  76 /  -   10  10  30  20

&&

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

$$

Mesoscale/Aviation...24
Synoptic/Grids...09
Public Service/Data Collection...30



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