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

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FXUS64 KEWX 281137
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
637 AM CDT Fri Jul 28 2017

.UPDATE...
12Z Aviation Update Below.

&&

.AVIATION...
Some patchy IFR-MVFR stratus has formed early this morning just south
of SAT. Low level winds should keep the stratus from developing into
SAT area, but will watch closely. The stratus to the south should
scatter out between 14Z-15Z. Otherwise VFR conditions. Winds SW to
SE 5-10 kts increasing to around 10-15 kts this afternoon. A few
occasional gusts 15-20 kts possible this afternoon at SAT and DRT.

&&

.PREV DISCUSSION... /ISSUED 449 AM CDT Fri Jul 28 2017/

SHORT TERM (Today through Saturday)...
The short term forecast continues to focus on heat and the prospect
of heat advisories. The last several days, Superblend and similar
dewpoint guidance has handled daytime atmospheric mixing quite
poorly, resulting in advertised dewpoints in the mid 70s in the
southeast and heat indices in the 108-110 range. As a result, opted
with the more conservative NationalBlend as its performance has been
significantly better.

Using this as a baseline for dewpoints and continuing the exceptional
performance of the bias corrected Max T guidance over the last couple
weeks, forecast heat indices this afternoon appear to be more in line
with the last few days, as they should given no significant change in
air mass (other than incremental increase in air T). Thus, believe
the 103-107 heat indices everywhere but the Edwards Plateau are quite
appropriate for today and thus a heat advisory (with local criteria
of 108 heat index) will not be needed.

The most significant change in the forecast for Saturday will be the
increase in Max T of 1-2 degrees compared to today which will result
in near record or record breaking Max T. The heat advisory criteria
for air temperature is 103 degrees for all counties in the CWA that
are not near the Rio Grande. While the heat index criteria should
be safely met, it may not matter as 103 is forecast for many
locations from Austin to San Antonio and elsewhere not on the
Plateau. As a result, unless a significant change in the models
occurs between now and 24 hours from now, would anticipate a heat
advisory issuance for much of the CWA Saturday.

LONG TERM (Saturday Night through Thursday)...
A very welcome change in the overall pattern looks to begin Sunday as
a backdoor front continues to be advertised in both the GFS and
ECMWF. Again, there are still plenty of differences in timing and PoP
potential with this frontal passage, but confidence is increasing on
at least low end chance PoPs Sunday, likely favored along and east of
the I-35 corridor, and 5-7 degrees of relief in high temperatures.
Moisture availability and wind fields at all levels are meager at
best, so not expecting much in the way of strong storms or even heavy
rain prospects, but any rain will be welcome rain given the overall
dry pattern over the last few weeks.

This backdoor front will also be made possible by the larger scale
change in H5 pattern which is the shifting of the infamous ridge to
the west. This will put the region in a weak northwest flow regime or
the right side of the anti-cyclonic wind flow around the high. This
will enable a shortwave trough to travel south along the periphery of
the high, paralleling the lee of the Rockies, and reach the Red River
valley by Tuesday evening.

Both the ECMWF and GFS deterministic model solutions depict a fair
amount of QPF generation associated with this shortwave beginning
Wednesday. However, it does appear at this time that the majority of
the lifting needed for widespread PoPs will be confined to dynamical
lifting vs low level forcing as only a very weak low level boundary
is evident in the ECMWF, or in the case of the GFS, no boundary at
all. Thus, it doesn`t appear that this precip will be very impactful
(heavy rain/flooding threat), and could serve to be quite beneficial
for areas needing rainfall across the area.

After Wednesday, there remains some considerable differences in the
progression of this shortwave, but there is agreement in this
feature phasing with a deepening low over the Great Lakes and
providing a couple days of more than slight chance PoPs, depending on
how fast that progression occurs. Have trended more towards the more
progressive ECMWF for the late week.

And last but not least, temperature relief! MOS guidance shows highs
in the low 90s and even upper 80s for late week in the northernmost
zones. Would not hold my breath for 80s just yet as this is the
lowest of the ensemble solutions, but it is fair to say that max
temps are trending down for the late week from run to run. Thus, a
late period forecast of possible rain and 10+ degree cooler
temperatures will be quite welcome to many fatigued by the hot and
dry pattern as of late...including the author of this AFD.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry             103  77 104  79  98 /   0   0  -   20  20
Austin Bergstrom Intl Airport 100  75 103  76  99 /   0   0   0  20  20
New Braunfels Muni Airport    100  74 103  76  99 /   0   0   0  20  30
Burnet Muni Airport            98  75 100  76  97 /   0   0  -   20  20
Del Rio Intl Airport          102  77 106  79 104 /   0   0   0  -   10
Georgetown Muni Airport        99  77 101  77  97 /   0   0  10  20  20
Hondo Muni Airport            102  73 106  75 100 /   0   0   0  10  20
San Marcos Muni Airport       100  75 103  76  98 /   0   0  -   20  30
La Grange - Fayette Regional   99  77 103  77  97 /   0   0  -   20  30
San Antonio Intl Airport      100  76 104  78 100 /   0   0   0  10  20
Stinson Muni Airport          103  75 103  77 101 /   0   0   0  10  20

&&

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

&&

$$

Mesoscale/Aviation...Runyen
Synoptic/Grids...TB3


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