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

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FXUS64 KEWX 181658

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1158 AM CDT Thu May 18 2017

The strong capping inversion around 800 mb that has inhibited
convection from developing over the I-35 corridor the past few weeks
is still in place based on recent aircraft soundings at SAT and AUS.
However, the cap at DRT was weaker and higher at 700 mb on this
morning`s 12Z sounding, so we will be launching an intermediate 18Z
sounding to get a better sense of how conditions out west have
evolved just ahead of the dryline which is pushing into western Val
Verde County. Abundant instability (over 4000 J/kg) existed at DRT
this morning despite the weak cap (-16 J/kg) due to adiabatic lapse
rates at midlevels, so any erosion of the cap would support very
strong to severe thunderstorms capable of producing large hail and
damaging winds. Skies have cleared over portions of the Edwards
Plateau and western Hill Country, so these spots and areas north of
our CWA appear to be the most likely area where storms will develop
once the dryline bulges further east later this afternoon. Hi-res
models have generally trended towards very low coverage of convection
over our area due to the cap with most storms staying north, but the
latest two HRRR runs do show isolated cells developing north of
Uvalde and Hondo before quickly dissipating. This seems reasonable
given current cloud cover, with the main uncertainty being whether
the cell will move past the inversion or not.

Should any cells make it above the inversion out west, severe storms
would be likely with significant hail above 2 inches in diameter
quite possible given the steep lapse rates aloft. Although sfc-3 km
shear was below 30 knots this morning, we expect this to gradually
increase above 40 knots by this afternoon as additional shortwave
forcing passes to our northwest, with SPC Mesoanalysis already
suggesting this is occurring. Thus, supercells with rotating updrafts
would be the most likely initial mode, with a tornado being possible
but not expected. Further east towards the Interstate 35 corridor,
the cap is projected by most hi-res models to hold and we currently
do not see a reason to believe that it will not hold. However, we
will continue to monitor any cap erosion that occurs once this
morning`s low clouds lift further. Thus, for now SPC has generally
pulled the slight risk of severe storms over our area further north
and west to only include the western Hill Country and Edwards
Plateau, with the Rio Grande Plains and I-35 corridor including
Austin and San Antonio in a marginal risk. Please continue to monitor
this low probability, but high impact threat over the next several
hours. We may issue another intermediate AFD after the 18Z DRT
sounding comes in if it warrants; otherwise that information will be
in the normal afternoon AFD.


.PREV DISCUSSION... /ISSUED 634 AM CDT Thu May 18 2017/

ISOLD/SCT SHRA will develop by midday with ISOLD/SCT TSRA during the
afternoon into evening. ISOLD/SCT SHRA/TSRA will develop over the
Edwards Plateau early Friday, then move east across the Hill Country
on Friday. For now, left mention out of the TAFs as the best chances
will be in the Hill Country along with some uncertainty in timing. A
mix of IFR/MVFR CIGs and patchy VSBYs will lift to VFR midday into
early afternoon. However, brief IFR/MVFR CIGs/VSBYs are expected in
SHRA/TSRA. IFR/MVFR CIGs return this evening into overnight. S to SE
winds 5 to 12 KTs increase to 12 to 20 KTs with gusts to 30 KTs late
this morning into afternoon, then decrease to 5 to 12s KTs tonight.
ISOLD wind gusts to 50 KTs are possible in/near the stronger TSRA.

PREV DISCUSSION... /ISSUED 449 AM CDT Thu May 18 2017/

SHORT TERM (Today through Friday)...
There will be a slight risk for severe weather today for areas along
and west of the I-35 corridor. The severe threat decreases east of
the I-35 corridor.

A dry line is retreating back to the west this morning and allowing
moisture to return across the CWA. This morning, low level stratus is
developing across the majority of the CWA illustrating the rich
moisture advection taking place this morning. VAD wind profile from
KDFX shows a 40 kt LLJ out of the south this morning. Through the
morning, continued moisture advection will filter into the area and
pool on the east side of the westward moving dry line. Later in the
day as the dry line begins to shift back to the east and daytime
heating increases, convective initiation along the dry line is
expected. Analysis of RAP proximity soundings show a very unstable
atmosphere above a strong morning capping inversion at 850mb. This
cap will weaken while daytime heating increases and should be able to
hold on through 17-18Z, however, a mid level shortwave disturbance is
progged to approach from the Big Bend region and move across the dry
line shortly after 18Z.

Between 20-23Z, the mid level disturbance, coupled with strong
moisture flux convergence along the dry line located just west of
Ozona and San Angelo should begin CI of discrete supercells with a
significant hail threat associated with them. Most of the hi-
resolution guidance initiates these storms well to the north of the
CWA but there are some indications of isolated discrete convection
farther east and south in some of the guidance outcomes.
Interrogating soundings over the Hill Country and Rio Grande shows
weaker cap strength compared to locations along the I-35 corridor and
by 21Z, would be susceptible to breaking...either by the passing mid
level shortwave or outflow from nearby convection farther north or
both. Should that occur, convection would be in a highly unstable
(4000+ J/KG CAPE), with 40+ kts of effective bulk shear and surface
to 6 km lapse rates over 8 deg C per km. Very large hail will be the
main concern should isolated supercells develop. Again, the majority
of the hi-res outcomes do not produce this and keep the majority of
the convection within the threat areas farther north, but wanted to
acknowledge the possibility of isolated severe storms in our CWA that
some some guidance is suggesting. Regardless, will need to pay close
attention to satellite/radar trends this afternoon.

Early tomorrow morning, additional development along the dry line is
expected again to the northwest of the CWA as yet another embedded
disturbance impacts the dry line. TTU-WRF as well as synoptic
solutions keep this activity confined mainly to the plateau areas but
could produce a line of storms which pushes into the I-35 corridor by
early afternoon. More realistically, the line would weaken but eject
outflow that would progress eastward and act as the focus for
additional storm development tomorrow afternoon in the Hill Country
and eastward. For now, we are slight risked for tomorrow as well for
the same general area as today, but storm chances will be highly
dependent on the evolution of morning activity.

LONG TERM (Friday Night through Wednesday)...
The active pattern will continue into the weekend with perhaps our
best shot at widespread heavy rainfall. Models are continuing to
increase QPF totals produced by a cold front progged to reach the
CWA Saturday morning. Very favorable convergence along the cold front
will efficiently produce showers and storms will heavy rainfall
potential beginning Saturday late morning and last through Sunday.
PWAT values are progged above 1.5" and as high as 2" according to the
GFS late Saturday. Impressive 1000-500 mb column moisture, over 80%
RH throughout the layer, continued convergence along the front, and a
relatively slow passage of the front, could result in a prolonged
rain event this weekend which would have flash flood implications
should model QPF trends continue to increase. SREF QPF plumes show a
mean of around 1-2 inches next 3 days across the Plateau but about a
third of the ensembles members are higher, with an upper bound of 3-4
inches through 12Z Sunday. WPC guidance currently suggests the 1-2
inch and believe that is an appropriate range for now, but isolated
amounts of 4-5 inches may be possible, especially if the frontal
system progresses slower than anticipated Saturday.

The disturbed pattern persists through at least the early part of
next week as yet another H5 shortwave moves over the southern half of
the state but model solutions begin to diverge after Monday. By mid
week, ECMWF is the drier of the long range models while the GFS and
Canadian bring yet another round of showers and storms through with
another cold front. More details on this with later model runs.


Austin Camp Mabry              89  73  89  73  86 /  30  30  30  40  60
Austin Bergstrom Intl Airport  89  73  89  73  86 /  30  30  30  30  60
New Braunfels Muni Airport     89  73  90  73  86 /  30  30  20  30  50
Burnet Muni Airport            86  71  85  71  82 /  40  40  40  40  60
Del Rio Intl Airport           92  75  91  72  86 /  20  20  30  40  50
Georgetown Muni Airport        89  73  88  72  84 /  30  30  40  40  60
Hondo Muni Airport             92  74  90  73  86 /  30  30  40  40  50
San Marcos Muni Airport        90  73  89  73  86 /  30  30  20  30  60
La Grange - Fayette Regional   89  75  89  74  87 /  20  20  10  20  60
San Antonio Intl Airport       90  74  90  74  86 /  30  30  30  40  50
Stinson Muni Airport           90  75  90  74  86 /  20  20  20  40  40




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