Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 182328 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
628 PM CDT Fri Aug 18 2017

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


.AVIATION...Broad surface high pressure will sustain continued
light to moderate southeast winds with afternoon gusts. May see a
few overnight low clouds, as well as day time low cumulus, with
VFR conditions the rule.


.PREV DISCUSSION... /issued 401 PM CDT Fri Aug 18 2017/

SHORT TERM (Tonight through Saturday Night): The 500 mb subtropical
ridge situated across the northern Gulf of Mexico today slowly
shifts westward through the period, but will remain the dominant
feature in the short term. Associated subsidence will maintain
generally rainfree conditions across the inland areas, as moisture
remains generally confined to 850 mb and below. Maintained silent
pops across inland areas, for any very isolated activity that
develops along the potential seabreeze; a few streamer showers
remain possible offshore timed with the typical diurnal pattern,
where moisture is a little deeper. Persistence remains the name of
the game, with hot and humid summer conditions continuing and
have made only minor tweaks to the temperature grids. Highs
expected to top out past the century mark generally west of 77,
with only slightly cooler temperatures closer to the coast.
Overnights to remain warm and muggy with lows in the mid to upper
70s and lower 80s.

LONG TERM (Sunday through Friday): A few changes to the forecast
based on the big picture, which is slowly coming into focus.
Sunday ends where Saturday leaves off...a perfect weekend for a
dip in the pool or the beach with plenty of August heat and
sunshine along with light winds and minimal surf waves/currents.
For Monday through Wednesday, minor deeper moisture impulses
appear ahead of a very slow moving inverted 500 mb trough embedded
underneath the sprawling southern U.S. ridge, which shows up
nicely on traditional water vapor centered over western Cuba and
the northern Caribbean. This trough gets a nudge from the
southward displaced Bermuda High (east of Florida) and eventually
crosses into Texas by late week. Both the GFS and ECMWF slowed
down the movement of this system as it approaches Texas and now
appear to hang it up across south Texas before washing it out at
some point next weekend.

Ahead of the trough axis, north to northeast flow above 700 mb
keeps deep dry air more in place than not, and this is the story
through Wednesday, at least. Assuming the model trends are
correct, the deeper moisture arrives by late week - if not
Thursday, then on Friday. All this said, those are Day 6/7
forecast and a lot can change between now and then given the
difficulty of forecasting TUTT-type systems this far out.

For the sensible weather (beyond Sunday), have continued
mentionable rain across mainly the eastern third of the Valley
and the King Ranch for Eclipse Day (Monday) but even then isolated
should be the coverage. Outside of any showers/storms skies will
be a mix of clouds and sunshine, and if the sea breeze activates
the precipitation the beach may be the best place to see the
Eclipse (generally from 1145 AM to 245 PM, though only partial) as
northeast winds aloft would push any anvils away from the coast.
Any anvil or debris clouds could thin the sun across the Mid
Valley, but exactly where that would be is anyone`s guess now.
Even with a few more clouds, temperatures will only be shaved a
degree or two from Sunday`s highs...still a couple degrees above
average overall.

Deep dry air looks to be the rule Tuesday and Wednesday in
northerly flow aloft ahead of the trough. However, still enough
low level moisture and with winds light/calm to start each day
can`t rule out more isolated late morning through early to mid
afternoon sea breeze activity, which would start near the coast
and redevelop inland, clearing out the beaches. Model sky cover
blends pick up on this idea and have reduced rain chances to
"slight" (20 percent) with any morning cells reforming over the
lower/mid Valley away from the beach. Raised afternoon
temperatures by a degree or two and lowered sunrise temperatures
for both more clear/partly cloudy skies and light to near calm
winds at sunrise each day. Could see some fuzzy haze/fog each
morning but not expecting enough visibility restrictions to

By Thursday the trough gradually passes the longitude of the Lower
Valley and deeper moisture appears to follow. The morning should
still be dry across the upper Valley/Rio Grande Plains but all
areas have a shot at isolated showers/storms during the afternoon,
out west due to westward progress of the sea breeze and toward
the coast due to increasing deep layer moisture. Due to
uncertainty with the timing of the trough elected to stay at 20
percent (slight) chances for now. By Friday, with increasing model
consensus on deeper moisture, raised chances a bit to 30 percent
(scattered) category. Could easily see higher probabilities and
potential for locally heavier rainfall but far too soon to
speculate given it`s a Day 7 forecast and uncertainty described
above. Temperatures especially Friday should dip below triple
digits in locations like McAllen due to the clouds and potential
precipitation. The chance value is a split between the
deterministic GFS and ECMWF.

MARINE: (Today through Saturday Night): At 2 pm, seas were 3 feet
at Buoy020 with southeast winds around 6 knots. Pressure gradient
remains weak through the period with surface high pressure draped
across the Gulf of Mexico. Light to moderate southeast winds and
low seas will prevail, with isolated showers not out of the
question tonight into Saturday morning.

Sunday through Wednesday Night: Forecast rather simple with broad
but weak surface ridge covering most of the Gulf and lee side of
Sierra Madre trough much weaker than earlier this week when upper
level trough helped enhance the pressure gradient. Bottom line?
Light east to southeast winds and slight seas with a general
southeast swell of no consequence through the period. The only
concern would be isolated to perhaps scattered showers and
thunderstorms mainly Monday through Wednesday night, tending to
favor the waters between 20 to 60 nm at most times, with
nearshore/Laguna activity mainly during the early morning through
early afternoon, at most. Overall, a good spell for boating and
fishing compared with many stretches of nuisance or worse wind
waves this summer. Those will be 2 feet or less through the




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