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

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FXUS64 KEWX 260515

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1215 AM CDT THU MAY 26 2016

Upper level lift is approaching the region from the west and is
causing convection to develop west of the Rio Grande. Will include
a mention of tempo -TSRA for the DRT TAF for the first few hours.
High-res model guidance has this activity in somewhat of a
weakened state as it nears the I-35 sites and will mention showers
with vicinity thunder. MVFR ceilings and perhaps IFR will continue
through the overnight period before lifting back to VFR. Another
round of showers and thunderstorms may approach from the west
later this evening and will mention VCTS to account for this.
Additional MVFR is possible overnight tomorrow.



A strong capping inversion over the Rio Grande Plains assisted in
part by a persistent cloud deck keeping surface high temperatures
down today really limited convective development and thus has
prevented the development of a SE-moving MCS tonight. Although
the capping inversion evident at DRT has weakened somewhat between
the 18Z and 00Z soundings, it will likely be stout enough to
prevent much convection from forming tonight across South Central
Texas. The latest aircraft soundings out of AUS and SAT also
support this, and SPC Mesoanalysis implies most of Texas east of
the dryline is capped except for the higher terrain of the Edwards
Plateau over Val Verde and Edwards Counties. The latest few runs
of the HRRR are finally beginning to initialize the general lack
of convection well and suggest that convection is not likely to
develop through at least 5-6Z, so POPs have been lowered to 20-30
through this time.

The latest HRRR runs continue to latch onto convection developing
after 5-6Z in the vicinity of Edwards County as an increasing
southerly 20-30 knot LLJ moves up the Escarpment to provide
additional lift in an environment that may be less capped to
generate convection. Thus, have maintained 40 POPs along and west
of Highway 281 and 30 POPs east out of respect for this solution
and any shortwave that may move in from Mexico to tap into an
increasingly moist atmosphere. Tonight`s slight risk from SPC is
very conditional given the chances for convective development
have decreased and SPC has taken out the mention for significant
hail. Main conditional threats for tonight with any convection
that forms will be severe hail up to golf ball size and locally
heavy rainfall over the Edwards Plateau and western Hill Country
should any develop. The 00Z and 06Z global and hi-res model runs
will hopefully initialize the relative lack of convection and
significant outflow boundaries well to help the next forecast
package reduce some uncertainities for the timing of the severe
and heavy rainfall threats for tomorrow. Capping should not be as
much of an issue tomorrow afternoon given the ongoing increase in
Pacific moisture aloft.


Please see the 00Z aviation forecast discussion below.

Cloud cover will remain thick through the forecast period as
plenty of moisture remains in place. This moisture along with
possible convection moving into the Rio Grande plains could lead
to -SHRA or -TSRA tonight and again tomorrow afternoon. For
tonight, we`ll go with VCSH at all TAF sites, but may adjust
pending how much convection develops over Mexico. We will also
include a PROB30 group for TSRA at the I-35 sites for late
afternoon and early evening. As for cigs, All TAF sites are
currently VFR, but will drop into MVFR by late evening. Cigs
continue to lower overnight with IFR expected to begin between
09Z-11Z along I-35, with continues MVFR at KDRT. Improving
conditions are in store by mid to late morning as cigs rise into
MVFR I-35 and VFR at KDRT.


SHORT TERM (Tonight through Thursday Night)...
This afternoon, cloud cover has resulted in 2PM temperatures
ranging from the mid 70s on the Plateau to low 90s in the
southernmost areas. This has also served to maintain cap strength
across the Rio Grande Plains as seen on the special 18Z sounding
done at KDRT. While instability above 850 mb is plentiful, surface
based parcels probably won`t be able to overcome the 4 deg C
inversion and 100+ J/KG of convective inhibition.

Out over the Mexican mountains however, cap strength is notably
weaker per RAP proximity soundings, and with a weak shortwave
strengthening and moving over the Rio Grande by 00Z this
evening, convective initiation looks much more likely over Mexico.
Now while surface based convection looks unlikely within the
CWA, there should be more than enough instability available to
maintain any updrafts we may inherit from Mexico. Latest hi-
resolution models initially develop discrete cells off the lee of
the Del Burro mountains and shift them to very near the Rio Grande
by around 00Z. The threats will initially be primarily very large
hail due to the steep lapse rates and substantial instability.
However...40+ kts of 0-6 km shear oriented from the due west and
PWATs approaching 2 inches (or over 2 inches if the GFS is
believed), heavy downpours will become an increasing concern
overnight as cells potentially organize into an MCS or multi-cell
cluster pushing east across the CWA.

Uncertainty still exists in the potential for these isolated cells
to impact the Rio Grande Plains and the evolution of the eventual
MCS as the hi-res models are advertising. As such, did not opt for
a Flash Flood Watch for tonight, although the potential for some
isolated flash flooding may be possible with some of these heavy
downpours the could develop overnight.

The system should exit the CWA to the southeast by Thursday
morning bringing a brief lull in rain coverage. However models are
recovering instability very quickly...developing another round of
thunderstorms around 18Z tomorrow along the I-35 corridor and
later in the evening back to the northwest with the next upper
level shortwave. The later activity looks to stay mostly north of
San Antonio`s latitude through Friday. Total QPF amounts through
Friday still look to be about 1-3 inches across the CWA with
locally up to 5 inches possible.

LONG TERM (Friday through Wednesday)...
The system should shift to the northeast areas as the shortwave
comes across the northern CWA Friday. Models are in pretty good
agreement of the precip being confined to the eastern half of the
CWA through 00Z Saturday and mostly over with by 06Z.

The main upper level pattern undergoes a transition period as
broad troughing develops through the weekend and shifts eastward
by mid week. With a continued tap of moisture from the gulf
through the long term, at least low end chance pops exist in the
forecast through day 7.


Austin Camp Mabry              71  84  72  88  72 /  60  70  30  30  20
Austin Bergstrom Intl Airport  72  84  71  88  72 /  50  70  30  30  20
New Braunfels Muni Airport     72  84  72  88  72 /  50  60  30  30  20
Burnet Muni Airport            69  83  69  87  70 /  70  70  30  30  20
Del Rio Intl Airport           70  93  70  89  73 /  50  20  10  30  30
Georgetown Muni Airport        70  82  71  87  70 /  60  70  30  30  20
Hondo Muni Airport             71  88  70  89  72 /  50  40  20  30  30
San Marcos Muni Airport        72  83  71  87  72 /  50  60  30  30  20
La Grange - Fayette Regional   75  84  73  88  72 /  40  70  30  30  10
San Antonio Intl Airport       72  86  72  87  72 /  50  40  30  30  30
Stinson Muni Airport           73  87  73  88  74 /  50  40  30  30  30




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