Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 052349 AAA

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
549 PM CST Mon Dec 5 2016

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


.AVIATION...Clear skies and light winds prevail across Deep South
Texas this evening. VFR conditions expected across the region for
the next 24 hours as surface high pressure settles across the
area tonight then moves east on Tuesday. However, due to moist
soils and light winds, some MIFG (field fog) is possible,
especially at HRL, after midnight/early Tuesday morning. Mostly clear
skies and south to southwest winds will prevail across the area
after 15Z Tuesday.


.PREV DISCUSSION... /issued 341 PM CST Mon Dec 5 2016/

..Roller Coaster Weather Week Underway: Keep Clothes for All
Seasons Nearby...

SHORT TERM (Tonight through Tuesday Night): Clouds are finally
beginning to break across the region as dry air and some surface
heating allow for mixing. Conditions will continue to improve
through the evening with mostly clear skies overnight. The H5 trough
across south central TX will continue to weaken and fill as it moves
north and east across TX and into LA by Tuesday morning. Do expect
there to be some patchy fog given the moist ground conditions and
lighter winds overnight.

Quiet and sunny weather will return on Tuesday to the Valley as
weak...but brief...high pressure takes control and winds become
southerly. Temperatures will be a warm mid 70s to mid 80s. Tuesday night...a shallow cool air mass will slide
south into Deep South Texas and winds will shift from south to
north. This will start ushering in some cooler readings and have
adjusted the Tuesday overnight lows down. There may also be some
patchy fog potential as well overnight...however drier air from the
north may inhibit develop across the Northern Ranchlands.

LONG TERM (Wednesday through Monday): Little change to the going
forecast and the "big picture" event which is the season`s first
true big `norther still on track for late Wednesday night (Valley)
and a little earlier across the ranchlands. One subtle difference
today is the potential for an initial wind shift as early as late
Tuesday night/early Wednesday morning associated with a weak upper
level "dip" and associated bubble high behind it. Both GFS and NAM
show this feature, while the ECMWF is a little less definitive.
Given what occurred last Saturday along with the time of year that
would tend to favor such things without a strong low and mid level
southerly well as coordination with NWS Corpus
Christi...have gone with this idea and kept the Upper Valley
through ranchlands a shade cooler than previously forecast. How
far south the cooling gets remains to be seen; for now, cut the
line just west and north of McAllen through Raymondville and kept
the Lower Valley near 80, with McAllen in the mid/upper 70s while
the ranchlands hold in the 60s.

Thereafter, the front with the modified arctic air mass arrives
pretty much on schedule, oozing south at first before rapidly
clearing the lower Valley by or just before daybreak Thursday.
Guidance has been trending a smidge upward for the early morning
high temperatures especially for the Valley, so did likewise
particularly in the southeast corner (Brownsville-Port Isabel-SPI)
before steady or slowly falling temperatures take over by mid
morning there. Farther north, values in the 40s will budge little
through the day. Accompanying the chill will be a steel gray sky
and occasional light rain. The rain will be enough to measure and
wet the ground but not much more; for chances, upped in all areas
and now have likely for the Valley and mid to high chance (30-50
percent) elsewhere Thursday into Friday morning.

Friday morning will be the chilliest morning when combining
temperatures (upper 35 north and 40 southeast) with wind (still 10
to 15 mph except 15 to 20 mph Lower Valley) with a few higher
gusts. Wind chill values will range from the upper 20s to
lower/mid 30s, above advisory criteria (25 ranches and 30 Valley)
but plenty miserable when comparing with the near record heat that
domninated July through November and returned for some last
Saturday. Continued near 100 percent RH below 700 mb Friday for
the Valley through the southern ranchlands ensures solid clouds
and still more light rain/drizzle. The colder starting points will
ensure afternoon temperatures struggle to reach 50, so wind chill
will hold in the upper 30s to mid 40s. Quite fitting for those
looking for chilly weather during the winter holidays!

The chill relaxes its grip on Saturday, but how much is a bit
uncertain. ECMWF holds onto a weak inverted trough along the coast
and north to northwest surface winds on land through at least
midday. This would tend to lock in clouds and perhaps keep some
drizzle going, and if lasting long enough could keep highs from
reaching 60 for a third consecutive day. Have seen this often in
December so something to keep in mind. For now, split the
difference and forecast an uncertain low to upper 60s with light
winds becoming south through the day.

Winds should finally turn more southerly by Sunday as flat upper
ridge is replaced by a flat trough aloft, which induces a Great
Plains surface low that should pull the southerly winds back in.
ECMWF hints at a minor "Valley Wind Machine" situation while GFS
is less emphatic; for now split the difference and brought
sustained southerly winds of 15 knots by late afternoon.
Temperatures will respond accordingly, heading back above normal
by a few degrees Sunday afternoon (mid to upper 70s) which will
continue into Monday along with a much milder night in between.

MARINE: (Tonight through Tuesday Night): Mariners can expect
Caution conditions to linger as some moderate waves or gusty NW
winds conditions continue through the late afternoon.
However...conditions will improve through tonight as the surface
low pressure system over the North Central Gulf continues to move
off to the northeast. Weak high pressure will settle over the area
tonight and through much of the day Tuesday with light to
moderate southerly winds and low seas expected. By Tuesday night a
winds will shift back to the north and east.

Wednesday through Saturday: As mentioned earlier, get those
boating/fishing trips in Wednesday before the hazardous conditions
arrive with a vengeance late Wednesday night and beyond. Winds
will tend to be light on Wednesday though will be watching the
possibility of an initial northerly push during the day which
could elevate seas a hair but still slight to moderate at this
point. All systems "go" for a rapid onset Gale event between late
Wednesday night continuing through Thursday evening; seas will
build rapidly from 3 to 10 feet or more in just a few hours and
this is most likely at the typical early morning fishing outing.
Seas will build on average to 10 to 12 feet by Thursday afternoon
and continue right into Friday morning; some 14+ foot waves are
possible during the period. Occasional light rain and blowing
spray will make things miserable right into Friday. Conditions
begin to improve Friday night though seas will still remain
hazardous...with winds and seas subsiding a bit more on Saturday,
which should be a better day for the Laguna Madre.




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