Area Forecast Discussion
Issued by NWS Brownsville, TX

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106
FXUS64 KBRO 040544 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
1144 PM CST Sat Dec 3 2016

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

&&

.AVIATION...A shallow cold front moved through this evening, leaving
MVFR to high-end IFR ceilings and OVC skies. Overrunning and the
approach of an upper low from upstream should keep light rain and
low ceilings, along with light north winds, in the forecast. May
see ceilings degrade to IFR Sunday, with some visibility reduction
in mist as well. Confidence is high in these relatively poor
flying conditions continuing through the TAF period.

&&

.PREV DISCUSSION... /issued 539 PM CST Sat Dec 3 2016/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...The cold front is roughly at McAllen, with winds
northwest and with low VFR ceilings. Ceilings should lower to IFR
overnight, eventually at all sites, with a shallow wedge of cold
air setting up a low lvl inversion. Southeast winds at HRL and BRO
have eased up a bit and will shift to northwest tonight, though
winds aloft will remain out of the south. Not seeing a lot of
convection right now as the cold air puts the kibosh on that, so
have downplayed thunder in the TAFs for now.

PREV DISCUSSION... /issued 318 PM CST Sat Dec 3 2016/

.Summer Temperatures Set to Chill Down Later Tonight and Sunday as
Rain Arrives...

SHORT TERM (Tonight through Sunday Night): Lesson of the day - never
underestimate the "power" of a shallow cold front with a decent
chilly source air mass in early December. What had been a tricky
forecast in terms of timing the passage of said front has clarified
this afternoon, and the winning model overall was the ECMWF which
stuck to its guns with the faster progression of the front and much
cooler temperatures across nearly the entire area on Sunday. The
NAM, despite some flaky mesoscale wiggles, and now the GFS have come
around to this solution. The bad news on the speedier movement of
the surface boundary is a rather sharp change in "feel" to the air
for evening activities along and west of US 281, so folks headed to
holiday festivities outside should have layers of clothing to add as
temperatures will shift markedly from late summer swelter this
afternoon (upper 80s) to refreshingly cool readings by mid evening
(mid 60s).  The good news is that Valley precipitation should hold
off until late evening/overnight.  More on the details shortly.

For the near term, have already overhauled temperature forecast
behind the front, which is currently oozing slowly southeast and was
located from just south of Falfurrias through central Starr County
to near Roma. Low clouds, pockets of drizzle, and temperatures in
the upper 50s with a stiff north/northwest breeze dominate there and
will not change through sunset.  Ahead of the front, sultry soup
with gusty south winds and peeks of sunshine and temperatures
approaching 90 in some spots.  As day fades to evening by 5 PM, the
front should resume its march across Deep S. Texas and reach the mid
Valley between 6 and 8 pm and much of the Lower Valley by midnight.
Brownsville/SPI will be last to fall, likely toward daybreak.
Temperatures will initially fall some 10 to 15 degrees with passage,
then ease down into the 60 to 65 range in most Valley locations by
late night and upper 50s to lower 60s by sunrise.

Behind the surface front, excellent upper level divergence (right
rear quad of 110+ knot jet) arrives across the Upper Valley and
ranchlands, as does deep moisture and attendant UVV. This will bring
steady rain and embedded rumbles to these areas by mid evening,
continuing through the pre-dawn hours; heavy rainfall/training
should be enough to pump out 1+ inches, perhaps locally 2 inches -
generally good news for areas still in abnormally dry conditions.
Farther east/southeast, forcing not quite as impressive but
available moisture and proximity to the upper jet should bring rains
into the mid Valley after midnight, and the Brownsville/SPI region
around or just after daybreak Sunday.

Sunday looks like a good day to curl up with a book and a warm
beverage in most areas.  Steady rain will gradually shift toward the
coast, with larger totals remaining north of the Valley but still
enough rain across the Valley to make outdoor activities wet.  With
arrival of 12Z ECMWF which continues its cool trend but is now
joined by other models and has current conditions to add more heft,
have lowered forecast highs by some 10 to 15 degrees over what
was inherited. These closely align with the ECMWF, except a shade
milder near the coast where any northeast flow from the still warm
Gulf could keep highs closer to 70 from Brownsville/Port Isabel
to the beaches. Farther north and west, temperatures will range
from the low to mid 50s across the ranches along/west of US 281/IH
69C to around 60 elsewhere - and in all areas values will
generally remain steady or perhaps slowly fall from morning lows.

Another round of rain arrives Sunday night as the bulk of the 500 mb
cutoff low heads toward the Big Bend with the last piece of dynamic
energy rotating ahead of it.  This will allow low to jump to the
coast and strengthen, acting like a "Texas Nor`easter" briefly as
southerly flow above the backing surface winds to the northwest
bring another round of steady rain.  All areas should share in the
wealth of rain, which will fall without being too heavy to create
flood problems but enough to add another 0.25 to 1" (from south to
north) to the totals from late tonight and Sunday.  Overall, rain
totals from this event will generally range from 0.5 to 1" south and
1.5 to perhaps 3" across the northern tier from Hebbronville to
Falfurrias.

Sunday night will be downright blustery and chilly with temperatures
dipping into the 40s to mid 50s and "feels like" temperatures as low
as 40 for parts of the mid/upper Valley.  Get out the Valley winter
clothes...for a couple of days, anyway.

LONG TERM (Monday through Saturday): The 500 mb closed low that
is sweeping across northern Mexico and southern Texas will be
exiting the region on Monday allowing a drier and cooler airmass to
move into place. Accordingly pops will be on the decrease then.
The RGV comes under a more zonal 500 mb flow pattern for Tues and
Weds. This in combination with a low level flow returning from the
S-SE will allow the temps to warm up well above climo for
December through midweek. A series of short waves will rotate
around a large closed 500 mb low positioned over south central
Canada later next week. One of these short waves will bring a
strong cold front pushing down into the south central Plains
states on Wed. This front will eventually push through the RGV
late Wed night/early Thurs morning with a pretty significant
colder airmass moving in over the region starting Thurs and
continuing into the weekend. Some decent moisture levels pool
ahead of this front which will result in some isold/sct areas of
rainfall late in the forecast period.

The GFS and ECMWF show decent agreement in the synoptic pattern
through around Tues and Wed. The models then diverge somewhat in
the handling of the large 500 mb low sprawled across southern
Canada. The GFS is a bit deeper with digging a trough axis over
the eastern states versus the ECMWF. This is resulting in the GFS
coming in with a much cooler solution for the Thurs through Sat
for the RGV versus the ECMWF. Run to run consistency with the GFS
MEX MOS temps and pops is pretty stable and maintains the coldest
temps for Thurs and Fri with temps starting to warm up a bit into
Sat. So will lean a little more towards the cooler GFS temps after
the front with a general blend of the two models outside of these
days.

Overall longer range forecast confidence is a little above
average due to the better model to model and run to run
consistency. However the degree of the CAA in the wake of the
front for Thurs and Fri remains a bit of a question mark.

MARINE (Now through Sunday Night):  A lot going on through the
short term.  In the big picture, the next 48 hours are not desirable
to boat or fish; high seas will predominate with plenty of swell
continuing from the east and northeast as tight gradient continues
to push strong enough winds to maintain at least 6 foot seas.  Winds
become an issue at several iterations.  First, behind the initial
cool air surge, especially north of Mansfield, north/northeast winds
could increase to 20 knots and gusty as early as Sunday morning. GFS
showed this but elected to trim back just under 20 knots for the
balance of Sunday.  Said winds would likely stretch across the
Laguna Madre over the same areas.

By Sunday evening, the developing "Texas Nor`easter" could create
shifting winds and briefly allow them to dip, even north of Port
Mansfield.  As the low scoots northeast of the waters, Northwest
winds and atmospheric cool advection will surge in behind with windy
(20 knot) winds developing at some point after midnight across all
waters.

And, plenty of rain and thunderstorms, some likely to produce gusty
winds that could approach gale force, are on tap from late tonight
through Sunday night until the low passes northeast of the waters.
Various Small Craft Caution/Advisories are likely through the period
for wind and seas.

Monday through Thursday Night: The moderate to strong northwest
winds expected across the lower Texas coastline in the wake of the
500 mb closed low exiting the region on Monday will push the
conditions up to SCEC/borderline SCA territory through late
Monday. The marine conditions then improve Tues and Wed as the PGF
weakens somewhat. The Bay and Gulf conditions will then decay
pretty quickly late Wed into Thurs in the wake of the cold front
that moves through the region. The PGF will increase dramatically
in the wake of the fropa late Wed pushing into SCA/Gale conditions
on Thurs and Thurs night.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory for winds until midnight CST tonight for
     GMZ130-132-135.

     Small Craft Advisory until 3 AM CST Sunday for GMZ150-155-170-
     175.

&&

$$

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