Area Forecast Discussion
Issued by NWS Brownsville, TX

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000
FXUS64 KBRO 291733 AAC
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1233 PM CDT Sat Apr 29 2017

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

&&

.AVIATION...Strong and gusty south southeast winds now through
this evening. Winds will drop to moderate late this evening and
MVFR ceilings will develop by midnight, with brief excursions into
IFR possible overnight. A wind shift to strong northwest before
dawn as a cold front moves through will dash ceilings, with decks
quickly mixing out to mid level and higher by mid morning with the
arrival of drier air. A slow decrement of winds will evolve
through the day.

&&

.PREV DISCUSSION... /issued 1034 AM CDT Sat Apr 29 2017/

DISCUSSION...Updated for a high surf advisory through midnight
and a coastal county wind advisory through 8 pm. Winds remain
strong today due to the gradient between Gulf high pressure and a
deep surface low over West Texas. A persistent swell across the
Southwest Gulf has resulted in one of the largest southeast swells
in years.

PREV DISCUSSION... /issued 647 AM CDT Sat Apr 29 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...12Z BRO raob measured 45-knot winds at 2,000 ft. AGL,
which is about 15 knots less than indicated on BRO VWP. With
surface winds already just above 15 knots at both BRO and HRL,
will remove low-level wind shear mention. It will be another
breezy to windy day, though strongest speeds realized at the
surface will depend on if winds in the low-level jet can be mixed
down prior to the pressure gradient lessening a bit by mid-day. It
is possible that a quick burst of southeast winds gusting up to
35 knots (AWW criterion) could occur for an hour or two around
mid-morning, but confidence not high enough to include in TAF.

Somewhat patchy MVFR CIG`s indicated currently at the RGV airports
from METARs and GOES-16 fog product imagery. This should scatter
out for good after 14Z or so when winds increase further. MVFR
visibilities in haze/mist also reported at HRL and MFE which
should also lift back to VFR mid-morning. Winds fluctuate a bit
this afternoon, especially at MFE, as a surface trough approaches
(but likely does not push through) from the west. Some mist
expected overnight ahead of an approaching cold front, along with
redevelopment of MVFR ceilings. Guidance still mixed on
possibility of IFR CIG`s so will leave out for now. Front moves
through the aerodromes a few hours after midnight with gusty NNW
winds and continued MVFR ceilings. Slight chance of a shower at
BRO/HRL but certainly not prevailing.

PREV DISCUSSION... /issued 435 AM CDT Sat Apr 29 2017/

SHORT TERM (Now through Sunday):  Heat and wind continue to be
the main weather stories in the short-term. An upper-level low,
currently over the Four Corners, per water-vapor imagery and RAP
analysis, will swing across NM and into the Panhandle region today
before lifting slowly into KS tomorrow. This will maintain a
tight pressure gradient in the surface reflection, as well as
allow a pre- frontal trough/dryline to progress into the CWA today
(though its ultimate eastward progression a little tough to pin
down). Pretty quick onset of breezy conditions on tap again for
the eastern counties this morning, though guidance has been
consistent in keeping speeds down a touch from yesterday, so will
hold off on any Wind Advisories. Winds actually ease off some by
early afternoon, except right along the coast, as the trough
approaches.

Low-level westerly wind component will again allow for downslope
heating to occur.  H85 temps progged to run 25-27C this afternoon, a
degree or so (Celsius) higher than observed on 00Z BRO raob, so
temps should top out a couple deg F above those recorded yesterday.
This was well-reflected in the evening shift`s temp update so will
run with those.  Forecast highs for BRO/HRL/MFE are 97/100/104F,
respectively, vs. daily records of 99/101/102.  Heat indices of 105-
109F are likely across inland Cameron/Willacy and Kenedy counties,
where dewpoints remain in the upper 60s-low 70s.  A little trickier
for the McAllen metro, as dryline will be flirting with that area.
Not expecting Head Advisory criteria to be met, but an SPS will be
forthcoming due to the early season (still April!) oppressive
heat/humidity combo.

Tonight, a cold front moves into the area, reaching the northwestern
counties around 03-04Z (give or take, with GFS a little faster than
the NAM).  It moves through the area fairly quickly, probably
through BRO by sunrise.  A little moisture pools ahead of/along the
front, and there is some decent lifting, so will maintain isolated
PoP`s over eastern counties 06-12Z with more scattered coverage
offshore. Fairly stiff winds of 15-20 knots fill in behind the
front, with some gusts of 25-30KT in a narrow ribbon right along it.
 Drier air quickly pours in Sunday morning, ending any precip
threat. Dewpoints plunge into the 30s and 40s F by early afternoon.
With moderate north winds, some fire weather issues are possible;
see below.  High temps will actually be knocked down to the low-mid
80s, nearly 5 deg F below normal, so it will feel pleasant outside.
Elevated rip current risk will exist at area beaches, however.

LONG TERM (Sunday Night through Friday): The forecast period begins
with a 500mb low/trough over the Central Plains and surface high
pressure building across the northwest Gulf of Mexico. Pleasant
conditions will prevail Sunday night with low temperatures in the
upper 50s to lower 60s under clear skies. Weak mid level zonal flow
will develop over the region on Monday providing subsidence across
Deep South Texas through early next week. A 500mb shortwave trough
is expected to develop over northwest United States Tuesday and move
towards the Central Plains on Wednesday. Low to mid level moisture
will begin to increase across the region Tuesday into Wednesday
courtesy of the southeast flow. A slight chance of showers and
thunderstorms will be possible on Wednesday. Slightly better rain
chances develop Wednesday night into Thursday as a cold front moves
into Deep South Texas and the Rio Grande valley. Drier and slightly
cooler air will filter into the area in the wake of the front on
Thursday. Temperatures will generally remain above normal Monday
through Wednesday. Near normal temperatures and rain free conditions
arrive through the rest of the forecast period in the wake of the
front.

MARINE (Now through Sunday):  An extended period of unfavorable
marine conditions continues through the short-term.  Current
conditions at Buoy 020 (as of 2:50am CDT) show SE winds at 19G23KT
with seas of 6-7 ft.  An enhanced pressure gradient will cause these
winds to be maintained into this evening, with seas building as
well.  Winds on the Laguna Madre remained elevated much of the
night, so have issued a Small Craft Advisory for Winds there through
10pm for now. Winds quickly flip to the north and increase with a
cold frontal passage after midnight tonight, which will bring the
Laguna back to SCA conditions even if there is a brief lull late
this evening. A few gale-force gusts are possible in the Gulf with
the front, but window appears to be short (2-3 hours) so after
coordinating with neighbor office, will hold off on a Gale Watch.
SCA for the Gulf waters has been extended out until 1pm Sunday, due
to the aforementioned winds and seas building as high as 10-12 ft.
far offshore.  Winds subside Sunday afternoon, but seas likely to
remain elevated.

Sunday Night through Wednesday Night...Adverse marine conditions
may continue through the first half of Sunday night due to some
lingering elevated seas. Otherwise, marine conditions will improve
late Sunday night into Monday as surface high pressure builds
across the northwest Gulf. Light east winds Monday will increase
and become southeast Monday night high pressure moves east and low
pressure develops across west Texas. Light to moderate southeast
winds will prevail across the Lower Texas Coastal waters Tuesday
and Wednesday. A cold front will approach the Lower Texas Coast
late Wednesday night.

Fire weather:  RH values fall to 15-20% across Starr/Jim
Hogg/Zapata counties today, but afternoon wind speeds will drop to 5-
10mph, mitigating fire weather concerns.  Fuel moisture also still
currently rated "normal" across much of this area, though continued
drying will occur over the next two days.  For Sunday, elevated fire
weather conditions are likely (as outlined by SPC), as RH`s fall
into the 15-20% range (mainly west of Hwy. 281).  Latest guidance
has backed off slightly on 20-ft. winds from late morning onward.
Due to uncertainty, will opt not to issue a Fire Weather Watch at
this juncture but will let day shift take one more look.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...High Surf Advisory until midnight CDT tonight for TXZ251-256-257.

     Wind Advisory until 8 PM CDT this evening for TXZ251-254>257.

GM...Small Craft Advisory for winds until 10 PM CDT this evening for
     GMZ130-132-135.

     Small Craft Advisory for winds from 4 AM to noon CDT Sunday for
     GMZ130-132-135.

     Small Craft Advisory until 1 PM CDT Sunday for GMZ150-155-170-
     175.

&&

$$

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