Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 212354 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
654 PM CDT Mon Aug 21 2017

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


.AVIATION...The afternoon CU field will continue to dissipate
this evening as winds drop off, eventually becoming light and
VRB overnight. A few pockets of low clouds could develop again by
daybreak, but have held off on any MVFR cigs for now. Otherwise,
a similar day can be expected on Tuesday.


.PREV DISCUSSION... /issued 412 PM CDT Mon Aug 21 2017/

SHORT TERM (Now through Tuesday Night):  The moisture on the leading
edge of the central Gulf upper level gyre (spin) arrived today, but
just barely...and while the sea breeze produced the expected
"beefier" clouds actual showers have been few and far between. The
somewhat speedy easterly flow indeed pushed the sea breeze inland
pretty quickly, with nearly all of the coastal counties cleared
out this afternoon. For the rest of the afternoon and evening,
expect to see pretty offshore towers from the beach, and it`s
possible that one might reach the beach with a brief sprinkle and
rainbow, but most should fall apart given diurnally favored

Overnight, there are still hints of moisture through 700 mb which
may be enough to keep some action going, particularly toward or
after midnight but mainly in the Gulf. Elected for silent 10 percent
rain chances near the coast with nothing inland, and cloud cover
should be mostly clear overall. With light winds temperatures should
fall to same levels as this morning, including lower 70s over
preferred radiation areas to upper 70s in urban centers.

Tuesday looks like a repeat of today, with limited chances and
coverage of rainfall as Deep S. Texas/RGV remain on the west side of
the Gulf gyre and generally under the influence of the Canicular
ridge centered over west Texas/southeast New Mexico, with dry air
maintained above 700 mb. This layer actually dries out further
Tuesday night, which will allow similar lows - perhaps even a degree
lower with mainly clear skies and perhaps even lighter winds for a
longer time than tonight.

LONG TERM (Wednesday through Monday): Main question in the long-
term is obviously what ultimately becomes of Harvey`s remnants,
which currently appear fairly disorganized over the northwestern
Caribbean Sea. By early Wednesday, the remnant circulation should
have re-emerged into the Bay of Campeche, where conditions are
expected to be favorable for strengthening. Some uncertainty still
exists, of course, but warm SST`s of 85-86F over much of the
western Gulf are in place. Vertical wind shear is expected to be
favorable for development (i. e., low), though that depends on how
far west the upper gyre now over the central Gulf moves as well
as how much as it fills between now and then.

On Wednesday, GFS and ECMWF show an H25 trough axis along the
Texas Coast with H5 weakness over the western Gulf. Sensible
weather shouldn`t be greatly different from Tuesday, though
moisture may penetrate a bit further inland with the sea-breeze
vs. today and Tuesday. For most of the long-term, blended
inherited max/min temps with latest Superblend, which for
Wednesday again yields highs pretty close to normal.

Meanwhile, the re-energized circulation of Harvey moves slowly
toward the northwest, in the direction of Deep South Texas, on
Wednesday, per both the GFS/ECMWF and various other track
guidance. Strengthening is indicated through Thursday along with
somewhat of an increase in forward speed toward the area. For what
it`s worth, intensity guidance generally keeps the disturbance at
tropical storm-level, but this guidance is suspect until it
actually emerges into the Bay of Campeche. 12Z GFS brings the
center of circulation ashore just south of Brownsville around 06Z
Thursday, then basically up the Rio Grande Valley fairly quickly
through the remainder of the day on Friday. 12Z ECMWF a bit slower
and takes the track more northward across the eastern portion of
the CWA.

Shouldn`t focus too much exact details yet, but confidence is
increasing for some kind of impacts to the RGV/Deep S. TX later
this week. Main impact at this juncture appears to be rain, even
if only modest strengthening of the tropical system is realized.
This would be primarily from Thursday night through Friday. Model
QPF amounts of 2-3" in 6 hours are showing up, though amounts will
depend on "linger" time over the RGV. Raised PoP`s from those
inherited but capped off at 60-70% in the peak timeframe. Too soon
to get into specifics of wind, storm surge, etc.

Direct impact of the tropical system seems to be over by
Saturday. Moisture levels remain pretty decent, though, so have
kept chance PoP`s in play through the remainder of the period.
High temps Thursday and Friday likely to remain stuck in the low
to perhaps mid-90s with the increased cloud cover and rain. Into
the weekend, trended temps slowly upward.

Now is an ideal time to review preparedness plans and restock
emergency kits. Stay tuned!

MARINE (Now through Tuesday Night):  Light to briefly moderate winds
and slight seas make for pleasurable boating/fishing
weather...though the potential for pesky showers and a few storms
especially offshore could make for some need to go below deck at
times. Could see a few periods of 3 foot seas late at night with the
uptick in winds before dipping back from late morning through mid
evening each day. On the Laguna, calm winds/glassy waters each
morning will be ideal for fishing, and any sea breeze or onshore
moving showers should hold off until mid to late morning on Tuesday.
Afternoon will feature the usual fresh 10 to 15 knot wind in this
broad surface ridge pattern.

Wednesday through Saturday: Marine conditions expected to remain
copacetic (easterly winds around 10 knots with seas of 2-3 ft.) through
Wednesday night prior to impacts being felt from the possible
tropical disturbance in the western Gulf of Mexico. Thereafter,
increasing winds and building seas are expected, though
considerable uncertainty remains. Swell heights, of course, depend
on the intensity reached by the tropical disturbance, but could
reach 8-10 ft. (or even higher) on Friday. Small craft advisories
appear likely from Thursday night through much of Friday, with
confidence fairly high on improvement thereafter as the system`s
remnants trek well inland.

Mariners should be prepared to postpone trips Thursday and Friday
of this week.




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