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

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FXUS64 KEWX 081753 AAA
AFDEWX

Area Forecast Discussion...UPDATED
National Weather Service Austin/San Antonio TX
1253 PM CDT Sat May 8 2021

.UPDATE...
The forecast has been updated to add a low chance for some late
afternoon showers and thunderstorms across southern Val Verde county.
The latest round of hi-res models show some potential for an isolated
storm to develop over the Serranias Del Burro mountains, then move
eastward into the Rio Grande plains. Strong storms are possible with
hail and high winds the main severe weather concerns.

&&

.AVIATION...
Low clouds along the I-35 corridor continue to lift and scatter, with
VFR conditions ongoing or set to develop shortly. Gusty south to
southeast winds can also be expected across all areas this afternoon
and early evening. We added a mention of VCTS at DRT between 22-02Z
as some activity developing over the Serranias del Burro mountains
could move eastward toward DRT. Otherwise, winds will likely not
ease after sunset given a fairly tight surface pressure gradient and
warm temperatures helping to promote good mixing of higher momentum
winds downward to the surface. Low clouds will return to the I-35
corridor around 04Z, with MVFR cigs expected. At this time, we are
not as confident with low clouds returning to DRT and will only
mention SCT clouds at this time.

&&

.PREV DISCUSSION... /ISSUED 639 AM CDT Sat May 8 2021/

AVIATION...
MVFR CIGs will lift to SCT VFR late morning to midday as S to SE
winds increase to 12 to 20 KTs with gusts to 30 KTs due to mixing.
MVFR CIGs return late this evening at the I-35 sites and KDRT late
tonight as winds decrease to 10 to 15 KTs.

PREV DISCUSSION... /ISSUED 419 AM CDT Sat May 8 2021/

SHORT TERM (Today through Sunday)...
A moist southerly lower level flow has brought areas of stratus to
South Central Texas early this morning. The low level jet may
generate some streamer showers along the US 77 corridor before it
weakens by midday. Moist upslope flow on the Serrianas del Burro will
allow isolated thunderstorms to develop there this afternoon.
However, a weak steering flow and strong capping inversion indicates
they will remain fixed on that higher terrain or dissipate rapidly if
they come off of it and not make it to our Rio Grande areas. A
restrengthening low level jet may generate a few showers over our
Central Texas areas Sunday morning. The dryline will move across the
Rio Grande Plains and the western Hill Country while a cold front
remains just to our north over North Central Texas on Sunday.
Forecast soundings show a strong capping inversion. Other than the
dryline and front, there are no other mechanisms to favor convective
development. Have maintained no POPS for Sunday afternoon. Should the
cap be overcome, then there is a potential for rapidly developing
strong to severe storms due MUCAPE of 2500 J/Kg. Above normal
temperatures continue. Highs around 100 are expected along the Rio
Grande on Sunday. A tight surface pressure gradient brings a breezy
southerly flow most areas today and eastern areas on Sunday. In the
wake of the dryline passage, minimum humidities in the single digits
and teens are expected Sunday afternoon, however winds will be less
than 10 mph, except 10 to 15 mph over Val Verde county where there
may be a potential for elevated fire weather conditions.

LONG TERM (Sunday Night through Friday)...
While the model trends have remained conservative on the southward
push of the front Sunday night, by daybreak Monday, the trend of the
GFS runs show more agreement with the more progressive NAM while the
trend of the latest ECM runs show a weaker front. Usually by this
time of the season, we`d opt for the weaker frontal scenario, but
the past few weeks have been lacking in any significant warm periods;
thus will continue to work with the more progressive fronts as the
unstable zonal pattern developing aloft should offer low resistance.
Early in the day Monday, the pwat levels will remain partially
depleted from the dry-line intrusion the day prior, but by afternoon,
the low level moisture surges deeper and the mid level flow over the
Mexican plateau will tap into an unstable region of the Pacific where
a tropical disturbance is becoming more organized. This should erode
the cap at some point, but at which point it does along the front and
where the front will be located at the time of initiation remains
quite uncertain.

It is assumed a few strong to severe storms (CAPES 3-4K J/kg) will
be possible somewhere over the NW 2/3 of the forecast area for Monday
into Monday evening, with potentially slow storm motions that could
signal a threat for locally heavy rainfall. Highest model QPF values
among the deterministic models vary in location, but all seem to
signal a late afternoon to late evening timing for the heaviest
convective activity, as is normal for mid May.

The deterministic runs all show relatively less activity for early
Tuesday. The GFS is a temporary outlier in that it erases the front
location and repositions it farther north again, but the NAM/CMC/ECM
is preferred as it is assumed some strong outflows would have been
provided from the night before. Thus midday to late Tuesday has a
chance to be a good soaker for much of the area with 1.5 plus inch
pwat values over a shallow front while good mid-level dynamics roll
across the area. Again, there could be some elevated strong storms.
The SPC paints the whole forecast area with an elevated svr threat
for day 3, and that may also wind up being the case for day 4.

The potential for excessive rainfall Monday night through Wednesday
morning is probably not too alarming at this time, but 00Z runs of
the GFS/CMC/ECM all paint large areas of 1+ inch storm total amounts,
areas of 2+ inch totals, and the wetter GFS showing a possible 4 inch
max over the Hill Country. Should this be in the ballpark, there will
be a run-off concern over some areas, especially those areas hit hard
with heavy rains last week. There is still a fair amount of lead time
to monitor the situation, but any contributions anticipated from the
eastern Pacific could make this a more moderate impact event. As it
is, it`s a comfort to know a couple of our early spring hazards will
soon be escorted to the exits, those being drought and fire weather
conditions. Later in the week a trough moving through the Central
Plains will reinforce continental NEly low level winds while zonal
flow continues over TX. This should ensure mild temperatures, plenty
of broken ceilings and possible additional weak rain producing
disturbances moving through the area through the extended periods.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              89  73  90  72  86 /  -   10  10  20  40
Austin Bergstrom Intl Airport  88  72  90  72  87 /  -   10  10  10  40
New Braunfels Muni Airport     90  73  93  73  89 /  10  10  10  -   30
Burnet Muni Airport            87  71  90  69  80 /   0  20  20  20  50
Del Rio Intl Airport           95  73 101  72  96 /  20  20   0   0  20
Georgetown Muni Airport        88  72  90  70  83 /  -   10  20  20  50
Hondo Muni Airport             92  71  96  71  94 /  -   -   -   -   30
San Marcos Muni Airport        89  72  91  72  89 /  10  10  10  -   30
La Grange - Fayette Regional   87  74  89  74  89 /  10  10  10  10  30
San Antonio Intl Airport       90  73  93  72  90 /  10  10  -   -   30
Stinson Muni Airport           92  73  95  74  93 /  10  10  -   -   30

&&

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

$$

Short-Term/Aviation...Platt
Long-Term...KCW
Decision Support...Morris


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