Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 231654 AAA

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
1154 AM CDT Fri Sep 23 2016

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


.AVIATION...VFR TAFs now through 24 hours. Mid level ridging will
remain centered over the Mississippi Valley and East Texas,
supporting high pressure at the surface, and suppressing any
significant weather. Look for light south southeast winds tonight
through early Saturday morning...with the development of some
overnight low clouds, with winds becoming moderate and gusty from
the south by Saturday aftn. A limited amount of shower activity
will be focused mostly over the Gulf waters, though a few cells
could move ashore along the coast.


.PREV DISCUSSION... /issued 429 AM CDT Fri Sep 23 2016/

SHORT TERM (Today through Saturday): 500mb ridge across the south
central United States today will continue to provide subsidence
across most of the state of Texas this afternoon. Isolated to
scattered convection will continue to develop across portions of the
western Gulf of Mexico today with relatively high low to mid level
moisture across the area. Not much in the way of rain chances are
expected across the inland areas. Cannot rule out an isolated shower
or two along the seabreeze front this afternoon. The upper level
ridge across east TX and the northwest Gulf of Mexico will move
little tonight into Saturday. Some streamer showers may develop
across the coastal sections of northeast Mexico and deep south TX
late tonight into Saturday.

LONG TERM (Saturday Night through Thursday): The final weekend of
September 2016 fittingly closes with more oppressive heat for this
time of year and a struggle to bring any notable the
persistent 500 mb ridge hangs on for one more day. The GFS holds a
tad tougher than the ECMWF, but tough ridging has been the
dominant story of "endless summer" 2016 and with piece of 500
trough separating from the more pronounced autumnal version across
the Northern Plains and diving southwest, prefer to lean toward
the drier GFS which has been much better overall this summer
compared with the west-biased ECMWF. Combine the pattern with
increasing southeasterly flow below the relatively dry atmosphere
and the scene will be a short-lived version of the Valley Wind
Machine inland, with more modest winds on the beaches.
Temperatures will continue at 7 to 10 degrees above average for
one final day.

The forecast gets a bit more muddled Sunday night...but actually
sorts itself out more than in recent days as both the GFS and
ECMWF are on the same page with some slight differences discussed
here. For Sunday night, the GFS sends the diving 500 mb trough
along the southern Baja peninsula while ECMWF keeps it near
Sinaloa/Sonora. These are subtle differences at this point, but
both models are now in general agreement on the idea that the 500
mb ridge over south Texas will be able to win the war. How these
differences play out will determine if the Upper Valley/Rio Grande
Plains get soaked by late Sunday night or just see the start of
the rainfall. Uncertainty favors a blend that leans toward the GFS
based on persistence of the ridge, so a slow ramp up of rain
chances from west to east overnight. The Lower Valley may well
remain dry overnight with the exception of late night low-topped
streamers while increasing rain amounts are possible along the Rio
Grande in Zapata toward daybreak. For now have split the
difference and rolled ~0.25" average, and will have to keep a
close eye for any training activity especially if the farther east
ECMWF is the better solution.

For Monday and Tuesday, three things we`re confident in: 1)
Everyone gets some rain; 2) Plenty of cloud cover which 3) brings
temperatures to or even slightly below late September averages.
What we`re *not* confident in is exactly how much rain falls and
where. Favoring the first widespread rain in quite awhile (and the
first time in September) is the arrival of a wind shift/low level
lift associated with the fading front as well as the general air
mass clash. That alone should generate a period of convective
clusters favoring Monday and Monday night that eventually covers
all of Deep South Texas. Pointing against a "big" rain for the
region is the rebuilding of the 500 mb ridge over Deep South
Texas/northern Mexico; eventual deepening northwest to north flow
Tuesday into Wednesday should cut off the convective machine and
could kill of thunder altogether though that was not removed in
the forecast on Wednesday.

Thereafter, the models retreat to their usual biases: ECMWF
builds a stronger ridge with an axis extending from the Valley
into Louisiana Wednesday and Thursday, while the GFS strengthens
the ridge a bit farther west, with deepening northerly flow
bringing a slug of dry to potentially very dry air into east and
southeast Texas by late Thursday. With much uncertainty not to
mention time of year which still leans toward a less "comfortable"
dry air mass decided to blend once again especially on
dewpoints/RH but did lean toward the drier GFS, with only slight
chances of rain by Thursday and perhaps a cutoff by Thursday

As for temperatures, with the eventual wind shift to the north or
northeast and plenty of cloud cover and at least some rain Tuesday
will bring values down into the 80s, area-wide, for the first
time since the first week of while still humid, this
will be welcome relief from the searing heat that has dominated
September and is still likely headed for all-time records in a few
locations. Those 80s will continue into Wednesday but will begin
to recede as sunshine returns Thursday, but at least we can kiss
the triple digit values goodbye to close out September.

MARINE: (Today through Saturday): Seas were near 3 feet with south
to southeast winds near 16 knots at buoy020 early this morning.
Light southeast winds will prevail across the coastal waters today
with weak high pressure across the northern Gulf of Mexico and weak
low pressure across west Texas this afternoon. The pressure gradient
will increase slightly tonight as low pressure continues to develop
across west TX. Light to moderate southeast winds will prevail
across the western Gulf of Mexico tonight into Saturday.

Saturday Night through Tuesday Night: Changeable
forecast, not to mention increasing rainfall, makes boating a bit
more challenging then in recent weeks. Sunday should be a bit
more gusty on the Laguna by afternoon and 20 knot gusts and choppy
waters are not out of the question. Marine layer over the Gulf
should keep things in check...but still a bit rougher than in the
recent week or two with winds still reaching 15 knots over the
relatively warm waters.

Winds gradually slacken Sunday night and Monday as the front nears
and washes out, but bridging 1022+ surface ridge that noses all
the way down as far as Zapata County and on into Nuevo Leon
should have enough oomph to pick up northeast winds across the
Gulf by Tuesday, continuing into Tuesday night. Winds and seas
will become moderate and the threat of rain/thunderstorms will
persist, so the best fishing will be Sunday morning and perhaps a
squeeze in for Monday morning before things go downhill.




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