Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 180536 AAB

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1236 AM CDT Fri Aug 18 2017

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

.AVIATION...Satellite images and surface observations indicate a
few low clouds across portions of the CWA late tonight. Expect VFR
conditions with brief MVFR ceilings to prevail the rest of
tonight into early Fri morning even as a weak inverted 500mb
trough moves across the lower Texas coast. 500mb subtropical ridge
across the northwest Gulf of Mexico will provide subsidence across
the Rio Grande valley and northern ranchlands Friday.

.PREV DISCUSSION... /issued 709 PM CDT Thu Aug 17 2017/
DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Mostly clear skies and gusty southeast winds prevail
across deep south Texas this evening. Winds will gradually
diminish this evening and become light overnight. More moderate
southeast winds will develop around mid-morning on Friday and
continue through the afternoon due to a relaxed pressure gradient
with surface high pressure extending over much of the Gulf of
Mexico. VFR conditions expected over the next 24 hours. However,
can`t rule out a brief period of MVFR ceilings overnight.

PREV DISCUSSION... /issued 227 PM CDT Thu Aug 17 2017/

SHORT TERM (Tonight through Friday Night):  Slightly above-normal
temperatures and mainly rain-free conditions will persist through
Friday night.  This is despite the mid-tropospheric inverted trough
moving slowly westward into the area from the western Gulf between
now and Friday afternoon.  Lots of dry air evident on the western
(leading) side of this feature, per GOES-16 mid-level water vapor
imagery, which is just now approaching the coastline.  As the trough
axis moves inland during the early part of the day on Friday, a
rather modest increase in RH around 500mb is progged (per NAM and
GFS time-height sections) to move through.  Combined with a
potential sea-breeze front moving inland around the same time (due
to weaker background flow with high pressure overspreading the
western Gulf), can`t completely rule out a sprinkle or light shower
during the early afternoon between Hwy. 77/I-69E and Hwy. 281/I-69C.
 Will keep PoP`s silent, though, as MOS guidance mostly in the
single digits.  Land areas should again stay dry Friday night, but
another area of deeper moisture moving into the offshore waters
could produce isolated showers.

Min temps both tonight and tomorrow night continue in the mid-70s to
near 80F.  Guidance pushing for temps a couple degrees hotter on
Friday vs. today, but will limit the increase to a degree or so over
today as thicknesses are essentially the same and H85 temps actually
nudge down a bit.  Still expecting 96/99/101 for BRO/HRL/MFE,
respectively, with afternoon heat indices again in the 105-110
degree range over much of the area.

LONG TERM (Saturday through Thursday): In the big picture, broad
500 mb continues to dominate most of the southern U.S. while
energetic (for mid to late August) short waves scoot from the
northern Plains eastward, tending to deepen as they hit what`s
been a persistent eastern North American trough that has extended
south from Hudson Bay and kept the fronts trucking through
locations from Michigan to Maine. This pattern ensures the 500 mb
ridge remains anchored far enough south to keep tropical cyclones
or deep tropical waves well south of the Valley, ensuring
temperatures remain somewhat above normal.

As for rainfall...the question heading into next week will be how
individual quick moving but weak easterly waves can "produce" as
they undercut the ridge and sneak across the western/southwestern
Gulf. In aggregate, this sets up as a slow moving inverted 500 mb
trough that eases through the western Gulf and south/southeast
Texas and eventually into northern Mexico between Monday and
Wednesday before the ridge rebuilds back in from east to west by
next Thursday. The feeling here is that these waves are not truly
tapping deep tropical moisture necessary for widespread and
locally heavy rainfall; in fact, northerly flow aloft ahead of
the broader trough would tend to favor little more than isolated
activity on the sea breeze, initially favoring coastal counties
through the mid Valley before the better opportunity for more
coverage arrives in briefly deeper southeast flow behind the
inverted trough Wednesday into Thursday.

For the sensible weather: Prior to the arrival of moisture bursts
associated with the trough, the ridge dominates for the weekend
with plenty of deep dry air and little more than few to scattered
fair weather cumulus. With the surface ridge nosing west across
the Gulf and no upper level systems in the Plains to generate
surface lows and increasing southerly gradient, surface winds will
go nearly calm toward daybreak and only pick up to 10 to 15 mph
with higher gusts as the sea breeze advances inland. With the
thermal parameters still on the warm side, have continued a hot
surface forecast of 96 Brownsville/99 Harlingen/101-102 McAllen.
Enough mixing of the drier air should keep surface dewpoints in
the upper 60s to lower 70s in the Valley, and mid to upper 60s
farther west...continuing the trend of 105-109 heat index values
each afternoon. Lesser wind, however, may increase the discomfort.
Lowering waves/seas/winds will bring rip current threat down to
low in time for the weekend, so a perfect setup for some end of
summer fun prior to the start of school in some areas next week.

As for next week: On Monday, the first moisture plume arrives but
as mentioned above there is not a lot of energy to hang ones hat
on. Just enough moisture should be available to aid isolated sea
breeze showers and perhaps a storm, but moisture field falls apart
inland and with most of Valley still on front side of the inverted
trough have continued a silent 5/10 percent forecast farther west.
Cloud cover should be mixed but overall partly cloudy...which
should allow for partial eclipse viewing in most areas during the
peak midday/early afternoon hours. That plume is replaced by drier
air Monday night into Tuesday on most land areas, though the next
plume may arrive toward the coast late and could produce increased
coverage over the outer Gulf waters as well.

That moisture plume washes out over land during Tuesday evening,
with the final and perhaps more active plume arriving late
Wednesday and continuing into Thursday on the backside of the
inverted trough. By active, we`re not talking about a widespread
rain event as the inverted trough is actually embedded underneath
the sprawling 500 mb ridge and there remains uncertainty as to how
much deep tropical moisture that typically resides over the Bay of
Campeche can be tapped. For now, punted to a area-wide slight
chance (20 percent); did not adjust temperatures much and still
expect triple digits before any rain for the mid/upper Valley and
Rio Grande Plains ranchlands through the period.

MARINE: Tonight through Friday Night:  Latest report from Buoy
42020 indicating SSE winds of 14G18 knots with seas of 4-5 ft.
Small Craft should Exercise Caution (SCEC) conditions continue on
the Laguna Madre this afternoon, with winds in the 15-20 kt. range
with a few higher gusts. Winds and seas will generally be on a
downward trend through Friday night, as the Gulf surface high
builds further westward over the coastal waters. Could see a
period of SCEC on portions of the Gulf waters this evening, but
otherwise no headlines should be needed. By Friday, winds of 10-15
knots will be the rule, with seas subsiding to 2-3 ft.

Saturday through Tuesday Night: Compared with the nuisance wind
waves and gusty south/southeast winds for much of this week, the
weekend into early next week will be a "breeze", no pun intended.
Light winds and slight seas...mainly less than 3 feet...will
dominate the period with east to southeast winds over the Gulf
around 10 knots in most cases, and briefly 15 knots during the
afternoon over Laguna Madre. About the only fly in the ointment is
hit or miss showers/thunderstorms, but the better potential for
coverage should hold off until after the weekend with perhaps the
best potential Tuesday afternoon/evening.



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