Area Forecast Discussion
Issued by NWS Brownsville, TX

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104
FXUS64 KBRO 232343 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
543 PM CST Thu Nov 23 2017

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

&&

.AVIATION...Light and variable winds with mostly clear skies are
expected through the forecast period as high pressure dominates
the weather pattern.

&&

.PREV DISCUSSION... /issued 338 PM CST Thu Nov 23 2017/

..Glorious Weather to Continue through Holiday Weekend...

SHORT TERM (Tonight through Friday Night): No sense in complicating
an uncomplicated situation. Deep South Texas and the Rio Grande
Valley will remain under the influence of high pressure both at the
surface and in the middle layers of the atmosphere through the short
term forecast period. This synoptic pattern, coupled with a very dry
atmosphere, will produce dry weather with very limited cloud cover
across the BRO CWFA. Daytime high and overnight low temperatures
will moderate during the period, and will return to near normal
levels for this time of the year.

LONG TERM (Saturday through Thursday): The forecast is
straightforward to close the Holiday weekend and begin next week,
then confidence goes down the tubes by the middle of next week
with transition season uncertainty...and model handling of upper
level features...about as high as it can get. More on that in a
bit.

For Saturday and Sunday: About the only tweak is to nudge
afternoon temperatures up a degree or two based on current trends
(even today, with the "cool" airmass in place, we`re headed into
the mid to upper 70s). ECMWF guidance continues to indicate upper
80s for parts of the Lower/Mid Valley while GFS still a few
degrees lower, so split difference and leaned toward the warmer
temperatures with full sunshine and a nicely warming lower
atmosphere, along with favorable off-surface downslope flow. That
flow will keep relative humidity comfortable and return southerly
surface winds will also be light.

For Sunday, the ECMWF had not changed its tune one bit, with
decent 500 mb short wave diving down the backside of flat eastern
U.S. trough, along or just east of the Texas coast. The GFS has
come around to this solution somewhat with its wave now diving
into the western rather than eastern Gulf. For each case, a weak
boundary followed by a "bubble" high with some strength (1028 mb
over the ArkLaTex) turns winds to the northeast and this was
already in the forecast. Temperatures will be crimped just a bit,
but really not that noticeable for folks enjoying the wonderful
weather or traveling to or from the Valley. Northeast winds will
be a tad noticeable...10 mph or so with a few gusts...especially
near the coast.

The bubble ridge quickly scoots into the southeast U.S. Monday glancing
affects of the bubble ridge are quickly gone as the 500 mb ridge
eases into south Texas from the west. Surface winds veer back to
the southeast and with plenty of sunshine, afternoon temperatures
should begin rising again Monday and peak on Tuesday as southerly
surface flow picks up a bit...a minor version of the wind machine
with some afternoon gusts at or above 20 mph. Though 1000-500 mb
thicknesses support temperatures closing in on 90, lower level
temperatures are not equally as warm and given time of year have
capped highs in the mid 80s, in line with the warmer ECMWF
guidance.

Then the challenges begin. Because we`re dealing with Day 6 and
Day 7, when uncertainty can be great, especially during the
transition season when global models can struggle handling
individual perturbations or larger starting points of pattern
changes, the going forecast was largely left alone. The reason for
keeping a blended forecast are based on the divergence in handling
of a short wave running into the central Pacific coast late Sunday
or early Monday.

The ECMWF, as was the case on Wednesday, deepens
the wave along the California coast and digs it southward toward
the U.S. Four Corners region Monday night, before slowly lifting
the core east, then northeast, through the southern Rockies by
Tuesday night and into the mid-Mississippi Valley by Thursday
while eroding the core. All the while, the subtropical ridge is
basically flattened out and nothing more which keeps any fronts
well north of Deep S. Texas/RGV and ensures 5 to 10 degree above
average temperatures through the first couple days of December.

The GFS, on the other hand, doubles down on its solution by
progressively moving its short wave from the California/Oregon
coast east into southeast Oregon and Utah late Monday, then
sliding southeast into north Texas by early Tuesday while slightly
*deepening* the system which closes off briefly while moving
 through Oklahoma early Wednesday. Behind the digging trough,
 northwest flow at all levels pushes a breezy to windy (but
 rapidly drying) front through south Texas early Wednesday with
 very dry air to follow - with modified cooler temperatures a
 touch below seasonal norms Wednesday before a quick recovery
 Thursday, with chilly nights similar to what we`re experiencing
 now. The Canadian GGEM has a similar faster pattern but a weaker
 and slightly farther north system, and also holds up any true
 front for the middle of next week.

It`s very difficult to figure which solution will win the day.
Because the wavelength is short and all solutions are somewhat
progressive, leaning toward the GFS (which has handled the
November fronts in the post-Day 5 period better this month) is
still plausible move, but going full bore with near gale force
winds and cold mornings Wednesday/Thursday is a risky bet at this
point. Bottom line? Current forecast for Wednesday and Thursday
of highs in the mid to upper 70s and morning lows in the 50s to
lower 60s - all around or just a hair above normal - is the call.
As for rainfall? Inherited some streamer-type showers for the
Thursday forecast (and a broader low probability potential along
any boundary or moisture plume) so largely left alone. Either way,
not expecting anything of import in QPF with deep tropical
moisture and/or frontal forcing largely absent. Bottom line? Time
will tell, and we hope to hop on one of these "trains" to smooth
out next week`s trends.


MARINE...Slightly adverse seas are occurring on the Gulf of
Mexico waters adjacent to Deep South Texas and the Rio Grande
Valley as a remnant of the early Wednesday morning passage of a
cold front. Small Craft Exercise Caution is currently in effect
from 20 to 60 nautical miles offshore. These seas will continue to
subside tonight through Friday night as high pressure persists.
Generally light winds are also anticipated along the Lower Texas
Coast. Overall, favorable marine conditions are expected through
the period.

Saturday through Tuesday Night: As mentioned here yesterday,
largely uneventful with navigable wind and waves through the
period. For the most part, light (10 knots or less) wind and
slight (less than 3 feet) seas until Tuesday, when some increase
in southerly winds will be noted and we could edge close to
caution (15 to 20 knots) across Laguna Madre by afternoon. Sunday
may be a little more as well, with a 1028 mb high quickly scooting
by in northeast Texas. For now have capped northeast winds and 12
knots but seas may edge to 3 feet for a time which could make
small craft excursions a little more choppy. Something to keep in
mind for late weekend boating.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...None.
&&

$$

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