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

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FXUS64 KEWX 231619 AAA
AFDEWX

Area Forecast Discussion...UPDATED
National Weather Service Austin/San Antonio TX
1119 AM CDT Wed Aug 23 2017

.UPDATE...
Tropical Storm Watch across the southeast counties in effect.
A quick update on the regeneration of Harvey into a tropical
depression late this morning. The tropical system now located across
the center of the Gulf of Mexico is forecast to move to the northwest
and should approach the Texas coast Friday. Harvey is expected to
produce heavy rain across South Central Texas and especially east of
Interstate 35. Storm total rainfall amounts will generally be around
the 3 to 6 inches range with isolated 10 inches across counties in
the Tropical Storm Watch area 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. Stay tuned and please
monitor closely 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.


&&

.PREV DISCUSSION... /ISSUED 621 AM CDT Wed Aug 23 2017/

AVIATION...
A fairly quiet morning is ongoing across all the airports. However, a
convective boundary should sag into the region today and this will
provide a focus for shower and possible storm development this
afternoon and evening. Will mention VCSH for now at all sites, but
AUS and DRT may have a better shot than the San Antonio sites.
Otherwise, VFR will prevail with light southeasterly flow continuing
through the period.

PREV DISCUSSION... /ISSUED 424 AM CDT Wed Aug 23 2017/

SHORT TERM (Today through Thursday)...
A weak front continues to slowly push southeast with the assistance
of outflows from scattered showers and thunderstorms to our north and
northwest that may push into northern Val Verde and Edwards Counties
in the next few hours before dissipating. This front that currently
stretches from Sonora to north of Hillsboro will push into northern
portions of South Central Texas later this morning and afternoon as
the mid-to-upper level subtropical ridge continues to retreat to the
west. Isolated to scattered showers and thunderstorms are expected
later this morning and afternoon  within this weak frontal zone
despite relatively weak dynamic forcing aloft as shortwave energy
should remain well northeast of the region towards the Ark-La-Tex.
Brief heavy downpours and strong wind gusts will be possible with
these storms as DCAPE values approach 1000 J/kg (supported by
inverted-V forecast soundings) as highs climb into the upper 90s
ahead of the front. Locations in the Hill Country and northern-most
counties should only see highs in the lower to mid 90s due to
increased cloud cover and rain chances closer to the front. Heat
index values should remain below 105 degrees similar to yesterday.

Convective activity should decrease in coverage by early evening with
just isolated to scattered showers overnight mainly over the northern
zones as the front continues to slowly push south. Thursday will
generally be a repeat of Wednesday except that with the front a bit
further south, isolated to scattered showers and thunderstorms will
be possible over the entire region with the best chances north of
Interstate 10 from the Hill Country to the Coastal Plains. Rainfall
amounts may increase a bit as the strong wind gust threat decreases
due to increasing moisture aloft decreasing CAPE and DCAPE ~200 J/kg
while PWATs increase ~0.2 inches. High temperatures should also
decrease a few degrees relative to today due to greater cloud cover,
but heat index values should remain around where they are today in
the 100-105 degree range outside of the Hill Country due to the
increased relative humidity.

LONG TERM (Thursday Night through Tuesday)...
The relatively uncertain long-term forecast continues to center
around what will become of the remnants of tropical cyclone Harvey
as it reemerges west of the Yucatan Peninsula over the southwestern
Gulf of Mexico and/or eastern Bay of Campeche today. Confidence
remains high that this system will re-develop into a tropical
depression or tropical storm later today or tonight. However, exactly
where the closed circulation will redevelop is fairly uncertain.
Thus, until we have a closed circulation to sample and simulate, the
forecast track will likewise remain relatively uncertain. Despite
these uncertainties, the GFS and its ensembles and the NAM have
remained fairly consistent with a track that would take Harvey on a
north-northwest track towards the middle or upper Texas Gulf Coast
late Friday evening into Saturday morning. The ECMWF has also merged
a bit further east towards the GFS with the track to help provide
some confidence in this solution. In addition, this feature would
also be tracking towards the weakness created by the upper-level
disturbance (TUTT low) in the Gulf that continues to very slowly
slide west off the coast of Louisiana. Thus, the eastward shift in
model guidance has some merit to it assuming this TUTT low continues
its slow movement, so for now we have forecast the greatest QPF and
wind speeds east of the Interstate 35 corridor for Friday evening
into Saturday. However, dynamics associated with the tropical cyclone
and remnant frontal boundary will cause a threat of locally heavy
rainfall across virtually the entire region.

Models continue to show confidence that Harvey`s circulation will
slow down and potentially stall somewhere over South Central or
southeast Texas this weekend due to the lack of a strong trough to
absorb it and take it into the westerlies. Therefore, there will be a
continued heavy rainfall threat that will likely cause flash and
river flooding somewhere near the Texas Gulf coast through the
entire weekend. We continue to forecast average rainfall amounts of
2-6 inches over our CWA with locally higher amounts expected. Average
amounts based on the current forecast will likely be higher east of
the Highway 77 corridor, but these higher rainfall amounts would
shift west if forecast tracks were to shift west due to the
likelihood that Harvey will stall somewhere this weekend. The only
somewhat confident part of the forecast is that Harvey should finally
become absorbed by a stronger trough early next week and be taken
east of the region as a trough from the upper Mississippi River
Valley digs south into the lower Mississippi River Valley. However,
copious amounts of rainfall will likely have fallen somewhere over
Texas by then. Therefore, please stay tuned for the next 24-48 hours
as the rainfall totals and track are refined once we have a closed
circulation to sample with additional data being taken across the
region.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              99  78  96  76  90 /  30  30  40  30  40
Austin Bergstrom Intl Airport  98  75  95  75  90 /  30  20  40  30  40
New Braunfels Muni Airport     96  75  97  75  90 /  30  20  40  30  40
Burnet Muni Airport            94  74  91  72  88 /  40  30  40  30  40
Del Rio Intl Airport           98  78  98  77  95 /  30  20  30  30  40
Georgetown Muni Airport        96  76  93  74  90 /  40  30  40  30  40
Hondo Muni Airport             98  75  96  75  93 /  20  20  30  30  40
San Marcos Muni Airport        97  76  96  75  89 /  30  20  40  30  40
La Grange - Fayette Regional   98  75  96  76  89 /  30  20  40  30  50
San Antonio Intl Airport       97  77  96  76  91 /  20  20  30  30  40
Stinson Muni Airport           98  77  96  76  91 /  20  20  30  30  40

&&

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

&&

$$

Mesoscale/Aviation...Treadway
Synoptic/Grids...17


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