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

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FXUS64 KEWX 100325
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1025 PM CDT Sun May 9 2021

.UPDATE...
And 25 minutes later the storm across Gillespie county decided to
weaken rather than tap into the 2000-3000 j/KG of CAPE that the HRRR
soundings and SPC mesoanalysis still showed. Based on that we have
let the last 5 remaining counties go in the Severe Thunderstorm
Watch. While an isolated shower or non-severe storm can`t be ruled
out early Monday morning as the front sags south chances are very
low. More severe weather and possibly heavy rain for Monday and
Tuesday though!

&&

.PREV DISCUSSION... /ISSUED 957 PM CDT Sun May 9 2021/

UPDATE...
Have dropped the Severe Thunderstorm Watch for the Austin Metro area
and Williamson County. Have retained it for some of the Hill Country
counties due to a pesky storm that likely formed when the outflow of
the earlier supercells interacted with the front. While the storm has
generally been struggling, there is still plenty of CAPE present for
the storm to tap into, so out of abundance of caution have extended
the watch for another 2 hours for Kerr, Kendall, Gillespie, Blanco,
and Llano Counties.

PREV DISCUSSION... /ISSUED 807 PM CDT Sun May 9 2021/

UPDATE...
Made some adjustments to the PoPs for the evening based on the
current radar trends. A single supercell is moving through Gillespie
and Blanco County. The storm has been moving to the southeast fairly
steadily which would take it into Southern Blanco, Northern Kendall,
and eventually Comal Counties. Think the storm should generally
weaken with the loss of daytime heating, but have raised PoPs
downstream of the storm to account for it. Have also lowered PoPs
across the northern Hill Country. Took the moment to retrend the
hourly grids as well.

PREV DISCUSSION... /ISSUED 628 PM CDT Sun May 9 2021/

AVIATION...
In terms of precipitation at AUS we will have to monitor the
supercell moving into Llano County. It it was going to impact AUS it
would be around 02z, but models are showing it generally weakening as
it moves east towards I-35.

Ceilings in general will drop at all sites tonight once more with
plenty of surface moisture in place. MVFR cigs will build in across
the I-35 corridor around 05z with IFR likely by 07z. While northerly
and northeasterly winds are likely to be breezy as a cold front sags
through the area preventing fog it will do little to help with
ceilings which will linger into Monday morning. IFR should stick
around through much of the morning hours across the I-35 terminals,
with ceilings rising to MVFR by early afternoon.

At DRT MVFR cigs will be possible by 10z and will stick around
through about 19z on Monday. Generally easterly winds around 10 knots
will be possible at DRT.

Scattered showers and thunderstorms will be possible late in the
period across the Rio Grande tomorrow evening, but will not include
those in this TAF package.

PREV DISCUSSION... /ISSUED 314 PM CDT Sun May 9 2021/

UPDATE...

A Severe Thunderstorm Watch is in effect until 10pm for Blanco,
Burnet, Comal, Gillespie, Hays, Kendall, Kerr, Llano, Travis, and
Williamson Counties. The primary hazards are large hail and damaging
wind gusts, with several very large hail reports of 2" or greater
possible.

-Morris

PREV DISCUSSION... /ISSUED 214 PM CDT Sun May 9 2021/

SHORT TERM (Today through Monday)...

A busy surface pattern is in place across portions of western and
central Texas this afternoon. As of 18Z, a dryline is noted from
near Sonora to near Del Rio with a cold front located from Brownwood
to just north of San Angelo into the central Permian Basin of west
Texas. Across most of south central Texas, a warm and moist air mass
is in place with current temperatures in the lower 80s to mid 90s
and dewpoints in the lower 60s to lower 70s.

The above mentioned cold front will continue to sag southward
through late this afternoon and early this evening. We are already
seeing some convection go up along the cold front across north Texas
and based on the latest hi-res model data, we should see additional
convection develop along the front over the far northern Hill
Country in the vicinity of Mills/San Saba/Lampasas counties where SPC
mesoanalysis places the highest CAPE values with little to no CIN.
Given plenty of deep layer shear and healthy mid-level lapse rates,
severe thunderstorm development appears likely. The latest SPC
outlook continues to highlight areas generally along and north of a
Comfort to Kyle to Giddings line in a Slight Risk (Level 2 out of 5).
This does include the Austin Metro area. Just south of this line,
including the cities of Kerrville, Boerne, San Marcos and La Grange,
the threat drops slightly into the Marginal (Level 1 out of 5).
Regardless of the threat level, strong to severe thunderstorms are
forecast to develop late this afternoon through early this evening.
The main severe weather concerns will be large hail and damaging
winds. Severe parameters favor some very large hail (>2" diameter)
for areas outlined in the Slight Risk category.

With the loss of daytime heating, we expect a decrease in the severe
weather threat along with an overall decrease in the coverage of
convection as we head into the very late evening hours. We will
maintain a low chance (20%) for some showers and thunderstorms after
midnight across the Hill Country and I-35 corridor north of San
Antonio. Overnight lows will be cooler behind the boundary in the
Hill Country with some lower to mid 60s, while it remains warm
elsewhere with lows generally in the mid 60s to mid 70s.

On Monday, another active weather day is shaping up during the late
afternoon and evening hours, especially for areas along and west of
a Llano to San Antonio to Dilley line. With a cold front in the area
and an approaching mid-level disturbance, we should see a round of
convection develop over the southern Edwards Plateau, western Hill
Country and portions of the Rio Grande plains. Large hail and
damaging thunderstorm winds will be the main severe weather
concerns. High temperatures will vary greatly depending on the
location of the front. For now, we will keep highs down into the
lower 70s across portions of Llano and Burnet counties. To the south
of the front, warm temperatures are in store with mid 80s to near
100 degrees expected. While the overall severe weather threat should
gradually ease as we head into early Monday morning, we will then
need to focus our attention on some pockets of heavy rainfall over
the southern Edwards Plateau and Hill country. Overnight lows will
again be tough to pinpoint with a front in the area. For now, we
will forecast lows as cool as the upper 50s in the Hill Country with
lower 70s for areas south of Highway 90.

LONG TERM (Monday Night through Saturday)...

Tuesday morning the surface cold front is likely to be draped near
our southern border and should continue sagging slowly southward.
However, flow at 850mb and 700mb will remain generally out of the
south, with rich moisture continuing to flow in aloft over the front.
Model PW values of 1.5-2.1" are forecast across all but far
northwestern portions of Val Verde and Edwards counties Tuesday into
Tuesday evening. Areas of showers and thunderstorms are likely
through the day thanks to the widespread isentropic ascent and a
potential mid- level perturbation as well.

We are currently under a broad-brushed marginal risk of severe
thunderstorms in the SPC Day 3 Outlook, with large hail likely to be
the primary threat from the elevated thunderstorm activity. SREF
mean MUCAPE hovers in the 1500-2500 J/kg range through the day,
night and into Wednesday, likely highest near the Rio Grande where
steeper mid-level lapse rates are anticipated in the EML. Even the
outlier NAM depicts ample elevated instability across much of the
area. Given the high moisture content, locally heavy rainfall could
also be in the cards, and we are under a marginal risk in the WPC Day
3 Excessive Rainfall Outlook for Tuesday to cover this risk. Nailing
down the timing and location of highest risk is impossible at this
moment. Suffice it to say the entire region should be on the lookout
for the potential for both large hail and localized flooding concerns
Tuesday and Tuesday night with more clarity hopefully to come after
the first couple rounds of storms which will have a large impact on
the environment.

I would anticipate a marginal risk of severe storms, again with hail
as the primary risk, may be needed for Wednesday too as overrunning
flow over the shallow cold layer will keep shower/storm chances in
the forecast through the day. Pwats should fall below 1.5" across
our northern counties during the day as the 850-700mb trough finally
pushes to the east and flow swings around to the north at mid-levels
Any heavy rainfall threat will hopefully remain south of the metros
by midday. The focus for storms should shift southwest to the Winter
Garden region by Wednesday night and at this time it looks like
we`ll finally dry out for Thursday and Friday.

In terms of temperatures, there`s still quite a bit of spread in the
models for Tuesday and beyond and the picture isn`t much clearer
than it was this time yesterday. The NAM remains a cold outlier, but
even in the GEFS the membership varies from highs in the upper 50s
to highs in the mid 80s on Wednesday at AUS. The SREF is much more
clustered around a somewhat warmer solution with only a couple of
cold outliers. This is likely lending some credence to the NAM being
out to lunch on this particular airmass, but it`s puzzling to see
such a stark difference between it and the SREF mean! Regardless,
cooler than normal conditions are rather likely in the Tue-Fri time
frame, coldest on Wednesday with a little bit of a warm up (and a
return of high dew points) possible through the weekend as mean sfc
ridging dominates the eastern CONUS. Could see rain chances return
on Saturday.

AVIATION... Cigs along the I-35 corridor have lifted back to VFR and
will generally remain VFR through this afternoon and early evening.
The exception will be near AUS where some convection is expected to
impact the terminal during the 23-03Z time frame. For now, we have
included a TEMPO group between 00-02Z, but this may need to be
altered depending on storm development to our north. Otherwise, look
for low clouds to develop and expand over the region late evening and
overnight with MVFR cigs in store for SAT and SSF. We did include a
period of IFR cigs at AUS as some slightly cooler air in the boundary
layer could promote lower cigs and some fog. MVFR cigs should also
make it to DRT and we will begin MVFR conditions at 10Z. All sites
should see some gradual improvement tomorrow, but AUS likely to
remain MVFR well into the afternoon hours.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              87  69  79  63  75 /  20  40  30  60  80
Austin Bergstrom Intl Airport  88  69  80  63  76 /  20  40  30  50  80
New Braunfels Muni Airport     88  71  84  65  79 /  10  50  30  50  70
Burnet Muni Airport            87  65  73  59  73 /  30  30  30  60  80
Del Rio Intl Airport          102  72  93  69  82 /   0  -   30  60  50
Georgetown Muni Airport        88  67  75  61  74 /  40  40  30  60  80
Hondo Muni Airport             93  72  91  67  82 /  -   -   30  50  70
San Marcos Muni Airport        88  69  82  63  77 /  20  40  30  50  70
La Grange - Fayette Regional   89  73  86  67  79 /  20  30  20  40  70
San Antonio Intl Airport       90  72  86  66  80 /  10  40  30  50  70
Stinson Muni Airport           93  74  88  67  82 /  10  10  20  50  60

&&

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

$$

Short-Term/Aviation...Treadway
Long-Term...05


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