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

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FXUS64 KEWX 170550
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1250 AM CDT Wed May 17 2017

.AVIATION...
SHRA/TSRA along a KOZA to KSPS line will move east overnight while
propagating to the south across the Hill Country and weaken as it
approaches the I-35 corridor around sunrise on Wednesday due to
stronger capping there. Have removed VCSH from KDRT as drier mid
levels already filtering into that area. Have upgraded KAUS from
VCSH to -SHRA and added VCTS 12Z-16Z as better PROBs of maintaining
SHRA and ISOLD TSRA there while keeping VCSH at KSAT/KSSF. Any
SHRA/TSRA will be east of I-35 by midday, then dissipate early
evening. MVFR CIGs will continue, though some lowering to IFR is
expected especially in the higher terrain of the Hill Country as well
as in SHRA/TSRA. CIGs will rise to VFR by early afternoon with
mostly SKC Hill Country and Rio Grande Plains west of the dryline.
MVFR CIGs will return Wednesday evening and spread to the west
Wednesday night. S to SE winds 10 to 15 KTs will decrease to less
than 7 KTs as the dryline approaches, then increase to 7 to 11 KTs as
the dryline shifts back to the west Wednesday night. N to NW winds 5
to 10 KTs are expected west of the dryline.

&&

.PREV DISCUSSION... /ISSUED 1008 PM CDT Tue May 16 2017/

UPDATE...
While the isolated thunderstorms over the Rio Grande never
materialized due to the strong cap the dryline convection has
finally initiated from northeast of Fort Stockton to Big Spring and
northward. This is the advertised line of storms that will continue
to push eastward overnight. High resolution models continue to show
the line slowly weakening as it approaches the I-35 corridor around
sunrise on Wednesday. The best chances of heavier rain and
thunderstorms will be across the northern Hill Country along and
north of a Kerrville to Austin to Lexington line. Chances for rain
are lesser south of that line. I have adjusted PoPs to reflect this
from midnight through daybreak. For the next few hours any chances
for precipitation will be confined to the Rio Grande Plains. Water
Vapor imagery shows some forcing approaching over northern Mexico
which may help the southern part of the dryline overcome the cap and
develop southward into the Edwards Plateau. Storms over the next few
hours continue to be very conditional, with the main show being
during the early morning hours as the line of convection approaches.

In addition to adjusting the overnight chances for rain I retrended
the temperatures which were a degree or two cooler due to the ongoing
cloud cover.

PREV DISCUSSION... /ISSUED 745 PM CDT Tue May 16 2017/

UPDATE...
The Storm Prediction Center in Norman has issued a Severe
Thunderstorm Watch for Val Verde County through 3 am. High resolution
models (HRRR and Texas Tech WRF) have been hinting at the possibility
of an isolated thunderstorm or two across the northern Rio Grande
Plains this evening kicked off by the southern portion of the dry
line. Looking at the 00z DRT sounding that just came in the severe
threat is very conditional at this time. There is plenty of CAPE and
shear to promote severe weather, BUT there is a stout capping
inversion still in place over DRT. If this cap could be overcome any
storm that does form could easily become severe. The question is
whether that cap can be overcome through the evening. The other
limiting factor so far this evening has been the upper level
shear...there is almost too much of it. Satellite data has shown
three different updrafts that have attempted to go up in Terrell
county and northern Mexico through the evening, but these have gotten
quickly sheared apart. Due to the conditional risk the Severe
Thunderstorm Watch is in place.

The rest of the forecast is still on track. Storms should continue to
develop through the rest of the evening and overnight along the
dryline over West Texas. These will form into a line overnight and
move towards the Edwards Plateau and Hill Country through the early
morning hours. Models are still showing the line progressively
weakening as it approaches the I-35 corridor around daybreak
Wednesday.

PREV DISCUSSION... /ISSUED 701 PM CDT Tue May 16 2017/

AVIATION.../00Z TAF CYCLE/

Main aviation concern through the next 12-16 hours will center around
developing showers and storms across West Texas and their progression
east towards the south-central TAF sites. Strong storms are expected
to develop across San Angelo area and shift east after 03-06Z.
Storms will weaken prior to arriving at KAUS/KSAT/KSSF near 11-12Z
where SHRA may be all that is left with the main line. KDRT could be
near some stronger activity near 03-07Z but it could miss just to the
north. Will monitor trends closely for direct weather inclusion if
needed.

MVFR ceiling heights are ongoing at KSSF/KSAT and this will persist
through the overnight hours and will further decrease to IFR ceiling
heights 11-14Z. KAUS remains low-end VFR currently but will fall to
low-end MVFR overnight with brief IFR possible. KDRT is VFR as of 00z
and will remain so through the evening but then fall to MVFR before
coming back up to VFR with the passage of a dryline through the early
morning hours. KSAT/KSSF/KAUS will remain MVFR through 16Z then
scatter out to VFR through Wednesday afternoon.

PREV DISCUSSION... /ISSUED 340 PM CDT Tue May 16 2017/

SHORT TERM (Tonight through Wednesday Night)...
The latest water vapor satellite loop shows an upper level low
pressure system over the desert southwest. In advance of this low,
southerly flow in the low-levels continues with afternoon surface
dewpoint temperatures currently in the mid 60s to lower 70s F. For
this evening, the bulk of the upper lift is expected to remain north
of our region. However, we can`t completely rule out an isolated
storm or two moving into Val Verde county early this evening. The
better chance for showers and thunderstorms will arrive after
midnight across the southern Edwards Plateau and Rio Grande plains
as a line of convection approaches from the northwest. Given the
stronger lift is expected to remain north of our region, the best
chance for severe weather appears to remain west of the I-35 corridor
for the overnight hours. Damaging winds and large hail appear to be
the main severe weather concerns. As the line of showers and storms
approaches the I-35 corridor, a gradual weakening is expected due to
strengthening of the mid-level capping inversion and the trend for
this line to move away from the mid and upper level support. We will
continue a chance for showers and storms Wednesday morning generally
along and east of I-35, then shift the chance of rainfall east of
I-35 in the afternoon. Quiet weather is in store Wednesday night and
we will keep the forecast dry at this time.

LONG TERM (Thursday through Tuesday)...
The next chance for active weather across south central Texas will
arrive on Thursday as another upper level trough approaches from the
west. At this time it appears the best chance for severe weather will
remain along and west of the I-35 corridor. Large hail and damaging
winds will be the main severe weather threats. Shower and thunderstorm
chances continue on Friday as active southwest flow aloft remains
intact across south central Texas. The next item of concern will be
during the upcoming weekend as a cold front moves in from the
northwest sometime Saturday evening into Sunday morning. Moisture
pooling ahead of this front will send precipitable water values into
the 1.5-1.75" range. Concerns are increasing for locally heavy
rainfall across south central Texas this weekend given the cold
front, increasing moisture levels and a fairly active flow aloft.

FIRE WEATHER...
On Wednesday, dry air will move into the southern Edwards Plateau and
Rio Grande Plains. Afternoon humidities across these areas will drop
into the 10 to 20 percent range. However, 20 foot winds speeds will
generally remain at or less than 10 mph.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              74  89  73  89  72 /  10  30  30  20  50
Austin Bergstrom Intl Airport  73  89  73  89  72 /  10  30  30  20  40
New Braunfels Muni Airport     73  89  73  88  72 /  10  30  30  20  40
Burnet Muni Airport            71  86  71  85  69 /  10  40  40  30  50
Del Rio Intl Airport           70  94  74  90  72 /  -   30  20  30  40
Georgetown Muni Airport        73  87  72  87  71 /  10  30  30  20  50
Hondo Muni Airport             72  91  73  89  72 /  10  30  30  30  30
San Marcos Muni Airport        74  89  73  89  72 /  10  30  30  20  40
La Grange - Fayette Regional   75  90  74  90  73 /  10  20  10  10  20
San Antonio Intl Airport       74  89  74  89  73 /  10  30  30  20  40
Stinson Muni Airport           74  90  74  90  73 /  10  30  30  20  30

&&

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

$$

Mesoscale/Aviation...04
Synoptic/Grids...TB3



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