Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 250140 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
840 PM CDT Sat Sep 24 2016

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

&&

.AVIATION...Evening convection to the west of the RGV is leaving
behind plenty of high level debris cld cover. The RGV will remain
under the west side of the 500 mb ridge axis centered over the
Gulf Coast region. This will maintain a fairly stable atms in
place throughout the bulk of the upcoming TAF period. However the
latest NAM and ECMWF guidance indicates that enough daytime
heating tomorrow afternoon will combine with better moisture
advection to allow for some isold/sct afternoon conv. The GFS
continues to be the outlier maintaining single digit pops through
tomorrow. Will include a prob30 group for convection during the
latter hours of the current TAF package. Otherwise VFR conditions
to continue through the upcoming 24 hours.

&&

.PREV DISCUSSION... /issued 256 PM CDT Sat Sep 24 2016/

SHORT TERM (Now through Sunday night): For the near-term,
elected to drop PoP`s to 10% for the 3 western counties for the
remainder of the afternoon per satellite and radar trends.

Overall, rain chances are expected to gradually increase during
the short-term as a cold front currently draped across NW TX
approaches...moving roughly to a position across central TX by
early Monday morning. Gulf moisture remains steady or slowly
increases...though it seems the higher rain amounts for the CWA
will hold off until the daytime hours Monday and beyond.

After a mostly dry period with only isolated showers /
thunderstorms for the land areas through tomorrow morning, 12Z
runs of GFS and NAM differ on how quickly to ramp up rain chances
Sunday afternoon and night...especially in their guidance PoP`s.
GFS has been keeping a "tongue" of slightly drier air aloft over
the CWA until late Sunday night, while NAM is indicating 40-50%
12-hr PoP`s on Sunday vs. 15-20% from the GFS. NAM also indicating
a mid-level disturbance rotating through the northern tier of the
CWA Sunday night with some decent QPF. ECS guidance seems to be
more in line with higher NAM numbers, so after considering all
this and coordinating with CRP... will lean toward Superblend
PoP`s in this timeframe.

As far as temperatures, once again expecting mid-upper 70`s mins
across the area tonight. Hot once again on Sunday afternoon
(depending somewhat on cloud/precip coverage), with max temps
running 5-7 degrees above normal in the mid-Lower Rio Grande
Valley. Heat indices will top out near 105.

LONG TERM (Monday through Saturday): A deep mid level low over
the upper Midwest will extend a weakness southwest back to a cut
off low over Baja. At the surface, a cold front will push into
Central Texas by Mon morn with 1028 mb high pressure over the
central to southern high plains. The current concern will be
rainfall potential as the front moves toward and through deep
South Texas early in the week. Precip chances will remain elevated
Monday through Wednesday before beginning to taper off. There was
little change in that department from the previous forecast. QPF
amounts for all of Monday should be below an inch, though a half
inch to an inch of rain is indicated by gridded QPF guidance for
the Upper Valley Mon afternoon as the front begins to affect the
ranchlands and brush country. The possibility is supported by
forecast precipitable water values near two inches even that far
inland. Could also have some input still from Eastern Pac upper
moisture moving across Mexico from the southwest. High temps
Monday will moderate closer to normal with mainly mid to upper 80s
and some 90s for highs, mainly due to elevated cloudiness.
Overnight lows will still be in the 70s, as the main new air
behind the front will not yet be in the area.

The front will continue to edge through the CWA on Tuesday, with
another half inch to three quarters of an inch of QPF possible
based just on gridded guidance. Forecast lapse rates don`t look
excessive, and should see a good deal of garden variety showers
with embedded thunderstorms around the area. High temperatures
will be capped in the mid 80s, close to normal, with some cooler
air making it into the area through the day. Drier air will still
be to the north, so not looking for the end of rain chances this
early. Overnight low temps will be in the lower 70s with some
upper 60s possible across the ranchlands and brush country Tuesday
night. Weak north winds will also not help the front push farther
very quickly, as the time for the tightest pressure gradient will
already be past, and upstream ridging will already be building
over West Texas, nudging high pressure east.

Rainfall on Wednesday will be focused more on the Lower Valley,
where another half inch of QPF is indicated. The Mid and Upper
Valley will see much lower QPF as rain chances begin to fizzle.
The front will linger over the Lower Valley as mid level ridging
builds over Texas and drier air pushes into East Texas from high
pressure beginning to retreat to the East. High temperatures will
be in the 80s across the board, with overnight lows in the lower
to mid 70s.

By Thursday only isolated showers will remain near Brownsville.
Drier air will be into the Upper Valley, but may have a hard time
clearing out the Lower Valley since light east winds will develop,
trying to push moist air back onshore. High temperatures will be
trending up at this point with upper 80s to a few lower 90s across
the area, with Thursday night low temperatures in the lower to mid
70s. The drying and warming trend will continue on Friday and
Saturday.

MARINE:
Now through Sunday night...SSE winds will gradually back to more
ESE through the period as the front advances across TX. Winds may
flirt with SCEC on the offshore waters tonight and the Laguna
Madre during the daytime on Sunday before decreasing to more
moderate levels Sunday night. Seas will generally remain moderate
in the 3-4 ft. range. Current isolated showers and thunderstorms
become more scattered on Sunday and perhaps even numerous by late
Sunday night. Mariners should be aware of lightning strikes and
locally elevated seas and wind gusts in the vicinity of showers
and thunderstorms.

Monday through Thursday...Ridging over the North Gulf will break
down in the short term, so that a deep mid level low will be
moving across the upper Midwest on Monday, trailing a surface cold
front southwest into Central Tex. Light east winds will back to
northeast and become moderate late Monday as the front and
associated high pressure approach from upstream. The strongest
northeast winds will develop by Tuesday morning as the front
remains across the Lower Texas Coast and tries to push offshore,
with winds possibly reaching exercise caution criteria. By Tuesday
night the front should be just offshore, nearly stationary. Light
to moderate northeast winds will prevail in the wake of the front.
Wave heights offshore will build a couple of feet to four or five
feet through the period, possibly briefly reaching exercise
caution conditions. Light to moderate northeast winds will prevail
through Thursday, when they will slowly veer again to east and
southeast.

&&

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

$$

This product is also available on the web at:
HTTP://WEATHER.GOV/RGV

Short Term...60
Long Term...63



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