Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 210658 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
158 AM CDT SUN AUG 21 2016

.DISCUSSION...Updated for latest aviation discussion below.

&&

.AVIATION...Mid and High level cld cover associated with the
convection firing to our north will move southwards over the RGV
through the night and early morning hours. The convection to our
north has not shown much progress pushing southwards. Daytime
heating later today after sunrise may destabilize the atms enough
to allow some isolated convection to form up across the RGV.
Expect any convection that forms to likely miss the RGV airports
during the afternoon. So will not include any vcts or tempo
groups for convection later in the current TAF period.

&&

.PREV DISCUSSION... /issued 649 PM CDT SAT AUG 20 2016/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...VFR is anticipated through the next 24 hours. Winds
will diminish tonight with a few clouds in place. Convection will
likely develop over the western portion of Deep South Texas and
the Rio Grande Valley but should remain away from all of the
aerodromes. Tomorrow, breezy winds are expected with scattered
clouds overhead.

PREV DISCUSSION... /issued 248 PM CDT SAT AUG 20 2016/

SHORT TERM:(Now through Sunday Night): For some areas, a
somewhat difficult forecast to close the weekend while others, as
mentioned earlier today, should be able ride fairly easy based on
persistence and analog cases. The main issue remains the 500 mb
shear axis/weak trough that passes through central/south central
texas and possibly leaves a "tail" behind it, with a lifting zone
including the aforementioned area of concern of the far Upper
Valley and Rio Grande Plains. The 12Z ECMWF came in a bit more
robust on this system, and even brings a surface wind shift into
the Rio Grande Plains ranchlands from a little after sunset
Saturday through Sunday morning. Such a wind shift was a notable
helper for the welcome but fairly "big" rains out there a week
ago. The GFS is not as bullish but has the same configuration. And
therein lies the good news: No matter which model or trend you
choose, the dividing line between potential heavy rain, a little
rain, and no rain is fairly clear.

For the sensible weather, then: This afternoon shaping up just as
expected: breezy, hot, and humid enough to push heat index in the
105-110 range for most areas east of U.S. 281.  Current forecast
covers this well,only introducing a mention of isolated
thunderstorms across Zapata/western Starr toward sunset.

This evening should play out with activation of thunderstorm
clusters or even some type of band across Zapata.  Exactly how much
rain and whether training gets involved is a difficult question, as
the ECMWF and the latest WPC QPF data suggest the possibility; later
shifts will need to monitor potential for nuisance road flooding
from Zapata to San Ygancio on US 83 and perhaps along SR 16 from
Zapata to Hebbronville.

Outflows should be able to kick out some of the convection farther
east to include western Starr/Jim Hogg, probably between 9-10 PM and
1 AM, before fading away in the much deeper/drier air farther
east and southeast, particularly the heavily populated RGV. Depicted
a sharp cutoff from the higher chances to non-mentionable, which
seems appropriate based on both ECMWF/GFS/NAM consensus and
climatology with similar events.

For Sunday, with deeper moisture remaining out west and north of
the RGV as well as uncertainty with regard to where the shear axis
lies (GFS wipes it out across the mid Valley and King Ranch while
ECMWF keeps it across the lee of the Sierra Madre), maintained low
end rain chances across the northern tier, shading back to isolated
across the agricultural areas of Hidalgo/Willacy and basically
little more than isolated light showers for all but Brownsville,
mainly in the morning. With whatever MCS/energy to our north fading
away during the afternoon, expect any mid/high clouds to thin over
the Valley, and with a still decent southeast flow, temperatures
should be able to rise well into the 90s for the Valley and could
even touch 100 again in McAllen, but held just short with more
clouds to begin the day. Farther north and west, temperatures may
struggle into the low 90s - and the current forecast from
Hebbronville to Zapata is an uncertain lower 90s; should light rain
prevail into early afternoon there, mid to high 80s will be more
likely.

By Sunday night, both the GFS and ECMWF recede what`s left of the
shear axis back toward the Big Bend region as the upper ridge
starts nosing back in from the east. Deeper dry air, particularly
on the GFS over land, should kill off mentionable rain chances for
all but the northern tier. Did add in some isolated streamer
potential right at the coast (and Gulf) after midnight as layers
from 700 to the surface temporarily fill with 60-80 percent RH.

LONG TERM (Monday through Saturday):GFS/ECMWF are in very good
agreement building the Bermuda Ridge west across the Gulf of
Mexico the first half of next week. The center of the ridge works
to its farthest point west, Louisiana, Wednesday before retreating
east by the end of the week. Once the ridge begins to move east
Gulf moisture will be on the increase with the chance of rain,
albeit low, returning to the CWA. Atmospheric moisture begins to
lower Monday with limited values to support convection. South
winds may also be to strong and not in the most favored direction
for sea breeze. Mean RH values drop to there lowest levels and
PWATS falling well below seasonal averages Tuesday and Wednesday.
Models are similar in retreating the ridge back to the east as
broad upper trough tracks across the Northern Plains. Moisture
begins its return but does not become deep or sufficient enough to
support convection until Friday and Saturday. Winds become lighter
and back more easterly favoring conditions for possible sea breeze
activity gearing up for late next week. With the dry conditions
through midweek temperatures are likely to be well above normal
and will go above guidance. Then once the rain chances return
temperatures to trend towards normal.

MARINE:(Now through Sunday Night): Not a great day for fishing
anywhere, as winds over the thinner Laguna Madre north of
Mansfield are flirting with 20+ knots while the Bay between Port
Isabel and SPI is running a few knots lower, in caution level but
with choppy waters. The Gulf isn`t much better; winds are fresh
but tolerable while seas are dominated by wind waves vs. swell. At
least it`s sunny.

For the overnight...continued southeast gradient will keep
winds in caution levels over the Gulf while the Laguna will
settle down, and while Sunday morning should be rain-free south of
Mansfield, waves/chop will remain a nuisance issue especially for
offshore excursions. By Sunday night, conditions should wind down
some over the Gulf, making Monday morning fishing a better bet -
though there will be a few dodge `em showers in all waters by
then.

(Monday through Thursday)...the pressure gradient across the
Western Gulf to strengthen through Wednesday as building high
pressure over the Gulf combines with lower pressure over the Bay
of Campeche and the Sierra Madre to produce a moderate south to
southeast wind. Seas to builds also but winds and sea are not
anticipated to reach small craft advisory levels but could at
times attain marginal SCA levels. The gradient begins to weaken
Thursday and into the weekend as the high pressure ridge moves
east.

HYDROLOGY...The big rains along the Rio Grande near Del Rio today,
estimated between 6 and 8 inches (at least), are producing a flood
"wave" that will reach Laredo and could produce minor flooding in
some low lying unprotected locations along the river on Sunday. Why
does this matter here?  According to the Texas Water Development
Board, the Texas share of reservoir capacity at Falcon had fallen to
32 percent recently - a clear response to our near record hot summer
and "La Canicula" pattern. The flood "wave" will give a temporary
boost to Falcon - nothing to get hopes too high, but at least put
the lowering values in reverse (back up) next week to the tune of a
few percentage points.

&&

.BRO Watches/Warnings/Advisories...
TX...None.
GM...None.
&&

$$

This product is also available on the web at:
HTTP://WEATHER.GOV/RGV

Short Term...60
Long Term...69



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