Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 192347 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
647 PM CDT Sat Aug 19 2017

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


.AVIATION...Surface high pressure is still in control with little
excitement expected overnight. Mostly clear skies will prevail
with very light winds. A few low clouds will be possible in the
early morning hours. Moderate southeast winds with some gustiness
will develop Sunday. The forecast sounding shows a shallow
elevated inversion setting up around 5 kft Sunday afternoon, which
may contribute to additional low clouds, but VFR conditions will
remain untarnished. Another weak sea breeze is likely, with a
signature early afternoon slight wind backing toward the east as
the breeze pushes inland.


.PREV DISCUSSION... /issued 317 PM CDT Sat Aug 19 2017/

SHORT TERM (Tonight through Sunday Night): The subtropical mid-level
high will continue to dominate the weather pattern during the short-
term, even as an upper-level low currently over the western Gulf
slowly moves in our direction.  Subsidence and a dry atmospheric
column (with precipitable water values running below the
climatological value of ~1.8") will keep land areas  precip-free. At
the surface, light SE winds, courtesy of weak high-pressure over the
central/western Gulf, will ensure continued humid conditions, with
only slight (perceived?) relief from the oppressive conditions
coming from the afternoon sea-breeze in the east and the mixing out
of dewpoints farther west.  By late on Sunday night, the western
edge of the aforementioned upper low (showing some definite TUTT
characteristics on current WV imagery) should be approaching the
coastal waters, with the NAM a bit faster with the westward drift
than the GFS. This will allow some mid-level moistening over the
coastal waters Sunday evening, then overspreading the
coastal/eastern sections by early Monday morning.

As far as the details go, really more of the same as we`ve seen. the
past few days. Did make a few minor adjustments to temps both
tonight and tomorrow. With light winds and mostly clear skies again
tonight, should see min temps fall pretty close to those observed
this morning, i. e. 76/73/78F at BRO/HRL/MFE, respectively.  Didn`t
go quite as hot for Sunday as the inherited forecast, either, as
today`s temps (especially inland) a couple of degrees off the
forecast pace. Still expecting heat indices to reach the 108-112F
over most all areas, though, so precautions will still be required,
especially with the persistence of the elevated heat and humidity.
Could see a shower or two approach the coast by late Sunday night,
but with confidence low, will hold PoP`s to silent levels.

LONG TERM (Monday through Saturday): Situation for the Monday-
Thursday time frame is coming into better focus this afternoon, with
the GFS/ECMWF solutions coming in line for the evolution and
movement of the upper level gyre now centered west of Key West but
whose circulation (and associated wind shear) covers much of the
eastern Gulf and western Caribbean. The gyre is forecast to edge
slooowly west through next week, more in line with the slower ECMWF
a couple days ago, and whose axis may pass the Texas coast Thursday
night which would open up our region to deep tropical moisture on
its back side next Friday and Saturday. Model agreement aside, there
is low confidence on exactly how this plays out; there remains an
outside chance that the back side of the gyre never gets here, and
what of Harvey`s remains or regeneration in the southwest Gulf/Bay
of Campeche which is highly uncertain this afternoon.

So, for the sensible weather, the big picture from Monday through at
least Wednesday is continued hot and humid with lighter winds
overall, allowing the sea breeze to activate isolated showers/storms
favoring the usual starting spots just west of Laguna Madre Bay and
over the land cut regions of the King Ranch by late morning, easing
inland a bit and generally clearing out the South Padre area by
early afternoon with diminishing chances toward the upper Valley/Rio
Grande Plains.  From Tuesday-Thursday, there`s even a possibility
that the "Canicular" upper ridge (parked over southeast New Mexico)
spreads across south Texas, with more subsidence and perhaps
reducing coverage to 10 percent in more areas.  As for temperatures,
with a hair more afternoon clouds and the atmosphere a bit less
"hot", expect readings a degree or so lower than this weekend but
still a little above seasonal averages.

By Friday and Saturday, model consensus and persistence indicates
enough moisture will arrive to both increase cloud cover and
precipitation coverage, beginning in the Lower Valley/Gulf toward
daybreak and spreading quickly west through the day. Even higher
mean RH/precipitable water arrives Saturday. Exactly how the rain
falls (when and where) is still a difficult call, but past events
suggest heaviest bands from pre-dawn through mid day/early afternoon
toward the coast, with amounts lessening beyond the mid Valley if
debris mid/high clouds can help stabilize things.  And there is the
actual possibility that the upper low never makes it this far and
precipitation is limited further, not to mention Harvey or his
offspring for which it is far too early to even speculate on final
outcomes. For now, will push up chances into the 40 percent range
late Friday into Saturday and nudge temperatures down a degree or
two but now as low as the GFS deterministic, just yet.

As for Eclipse Day (Monday):  Though we`re only partial (a crescent
sun), the big question remains where better sea breeze activity
fires to cover the sun - and times out with the peak around 115 PM.
Updated sky cover forecast this morning to suggest best opportunity
for fewer fair weather clouds would be at the beach (as any sea
breeze action will have jumped farther inland, and mid/high clouds
would move west, not east) and the upper Valley/ranchlands. As for
temperatures...with 50% partiality there will be some solar dimming
but expect nothing more than a steady value between noon and 2 PM
before still being able to reach current forecast highs at 3 or 4
PM. It will not be noticeable for most.

MARINE: Now through Sunday Night:  Rather isolated shower
activity noted on current radar in the southern 20-60nm zone. Buoy
020 reporting ESE winds of 6-8 knots as of 1:50pm CDT. Apart from
isolated nighttime/early morning showers favoring the outer Gulf
waters (possibly including thunder by late Sunday night),
favorable marine conditions are expected. A weak pressure gradient
over the Lower Texas Coastal Waters will drive mainly light SE
winds, keeping seas at 2-3 ft.

Monday through Thursday Night...Other than isolated showers and
perhaps a thunderstorm, favoring the 20 to 60 nm leg in most
cases, light winds (briefly moderate each afternoon on Laguna
Madre) and slight seas will be the rule through Thursday as
surface ridge continues to extend into the waters from the east.
Thursday night may see the first notable increase in coverage and
intensity (rainwise) assuming the door opens to deep southeast
flow. With high uncertainty have kept chances more conservative at
30-40 percent, but be sure to check back frequently if you have
early Friday fishing/boating excursions planned.




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