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

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FXUS64 KEWX 182111

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
411 PM CDT Thu May 18 2017

.SHORT TERM (Tonight through Friday Night)...
Severe thunderstorms have developed along the dryline in northwest
Texas, but continue to struggle to fire much further southwest due
to a capping inversion. The best signs of convection struggling to
fire were with a convective cell just south of Abilene and some
shallow cumulus that went up in western Val Verde County along the
dryline. A special 18Z sounding at DRT showed a slight increase in
CAPE from 12Z to 4367 J/kg despite a stout cap (with -38 J/kg CIN)
at 850 mb that was previously higher up closer to 700 mb at 12Z.
Nevertheless, temperatures are continuing to increase along and just
ahead of the dryline that may allow convection to get going if
convective temperatures in the mid 90s can be reached. In addition,
the cap along the I-35 corridor has weakened somewhat over the past
few hours as temperatures have climbed into the lower 90s. However,
this cap is at 775 mb above the top of the boundary layer, so
additional warming may not help erode this cap. A few hi-res models
show some cells developing this afternoon along the I-35 corridor
along and north of San Antonio, but any cells that develop appear to
be short-lived most likely due to the cap.

Given the difficulty the cap is presenting to convective development
again today, we continue to have a very conditional threat of strong
to severe thunderstorms along and west of the I-35 corridor. Any
cells that make it past the aforementioned inversions would most
likely become supercellular initially with the potential to produce
very large hail greater than 2 inches in diameter and damaging winds
due to ample shear and instability aloft. The best chances of severe
storms continue to be over the Edwards Plateau and western Hill
Country where SPC has maintained a slight risk, with even better
chances further north and west of our CWA. However, if any storms do
develop in San Angelo`s CWA just west and north of our area, outflow
boundaries from these cells could trigger additional development
further east over mainly Llano, Burnet, and Williamson Counties very
late this afternoon or (more likely) this evening. Most hi-res models
keep this activity just north of our area, but we monitor convective
trends as these outflows would help provide additional lift needed
to overcome the cap. It does not appear that the weak isentropic
ascent and shortwave forcing associated with the ejecting wave over
the Texas panhandle will be enough to do so, but a weak wave over Big
Bend may be able to trigger convection later this evening.

Low clouds are expected to return very quickly this evening,
persisting in the early afternoon hours across most of the region
ahead of the dryline that should set up somewhere in Val Verde
County. The setup for tomorrow is very similar to today`s as another
shortwave rotating around the cutoff low over Colorado moves through.
This wave may be able to provide a bit more forcing further south
than today`s to help force cell past the cap, but the cap will
continue to be an issue to contend with as has been the case for the
last few weeks. For what it is worth, hi-res models are currently
more bullish on convection developing tomorrow across the area
relative to today, but for now tomorrow afternoon`s forecast is very
similar to today with SPC having a marginal risk along and west of
I-35 with a slight risk just a row of counties west of that. Once
again, it is a low probability but high impact forecast with large
hail and damaging winds being the main threats.


.LONG TERM (Saturday through Thursday)...
The higher confidence forecast is actually for this weekend as a cold
front approaches the region by Saturday morning and slowly pushes
through the region into Sunday as the closed low over Colorado
finally ejects into the Northern Plains. Although we will only have a
relatively weak shortwave moving through the region, abundant low-
level moisture and increasing upper-level moisture ahead of the front
will increase the potential for heavy rainfall while the severe
threat decreases given reduced shear and instability. The frontal
forcing coupled with large-scale isentropic ascent should overcome
whatever is left of the cap aloft by Saturday afternoon, with a weak
southerly low-level jet allowing convection to continue into the
evening hours along the front. GEFS ensembles are hitting up on the
Saturday evening period having the greatest potential for locally
heavy rainfall that could produce flash flooding, particularly if
rains occur over any areas that are saturated from previous rains.
WPC has already issued a marginal to slight risk of excessive
rainfall for Saturday into Sunday morning for convection developing
along this stalled or slow-moving front with 1-2 inches expected
with isolated 4-5 inch totals still possible.

Shower and thunderstorm activity will continue into Sunday with
storms becoming more elevated behind the front to reduce the heavy
rainfall threat. Storms may become more surface-based in the Rio
Grande Plains by Monday to bring back the severe and heavy rainfall
threats, continuing into Tuesday before a relatively strong front for
the second half of May moves through the region. Although this is
climatologically well past the median date of May 11th for the last
strong front of spring, both the ECMWF and GFS show a pretty
significant trough digging into the Southern Plains that suggest this
front might actually materialize. If so, that would end our rainfall
chances by Wednesday and give us some cooler and drier than normal
weather going into the second half of next week.


Austin Camp Mabry              74  90  72  86  67 /  20  30  30  60  60
Austin Bergstrom Intl Airport  74  90  72  86  67 /  20  20  30  60  60
New Braunfels Muni Airport     74  90  72  86  68 /  20  20  30  50  60
Burnet Muni Airport            71  86  70  81  63 /  30  40  40  60  60
Del Rio Intl Airport           76  92  72  85  69 /  20  20  30  50  60
Georgetown Muni Airport        73  89  72  84  65 /  30  40  30  60  60
Hondo Muni Airport             74  91  72  87  69 /  20  20  30  50  60
San Marcos Muni Airport        74  89  72  86  68 /  20  20  30  50  60
La Grange - Fayette Regional   75  90  74  88  71 /  20  10  20  50  60
San Antonio Intl Airport       74  90  74  86  70 /  20  20  30  50  60
Stinson Muni Airport           75  91  74  86  71 /  20  20  20  40  60




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