Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 030544 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
1144 PM CST Fri Dec 2 2016

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

&&

.AVIATION...Ceilings have been slow to crash this evening despite
model soundings insistence for mvfr conditions to have already set
up as of 06Z.  That said, still enough confidence to keep mention of
low MVFR conditions toward daybreak at all sites with light
southeast winds keeping any notable fog/mist away.

No change to the Saturday forecast, as latest NAM and GFS continue
trend of keeping precipitation at bay for the daylight hours with
the recently arrived 00Z rain chances below 5 percent in Brownsville
and Harlingen between 18Z and 00Z.  Still looks on track for a few
peeks of sunshine through mainly broken VFR skies, and wind speed
guidance generally in line with prior thinking; 18 to 22 knots
sustained with a few gusts near 30 knots along the US 77 corridor
with a bit lower values around McAllen.

Expect MVFR conditions to return soon after sunset Saturday with
winds diminishing quickly ahead of the front that begins to return
to the ranchlands during the evening. As for mentionable
precipitation, only McAllen has a shot toward midnight; Harlingen
and Brownsville should hold off until after.  For now just called it
showers and will let later shifts evaluate for thunder.

&&

.PREV DISCUSSION... /issued 936 PM CST Fri Dec 2 2016/

DISCUSSION...shortly after 6 pm made several minor but important
changes to the short term forecast to best handle rainfall trends
into saturday evening. Consensus and trends of GFS/ECMWF/NAM all
bring warm front gradually north of Deep South Texas overnight and
especially on Saturday before northward progress hangs up in
advance of the eastward movement of the 500 mb cutoff low into
Sonora and Sinaloa. For the rest of tonight, the position of the
front along with decent 250 mb jet divergence suggest the better
development/coverage sets up from the SR 285/SR 16 corridor
northward into the Brush Country and Coastal Bend. That implies a
sharp cutoff to notable precipitation, and have sharply shaded
rain chances from near 60 percent far north to 10-20 percent in
the Lower Valley.

For Saturday, the prior forecast was in decent shape but rain
chances were once again too high for the populated RGV where a
period of breezy to even windy southerly flow behind the warm
front will also come with a pocket of relatively dry air. All
guidance indicates rain chances lower than 10 percent in this
window particularly during the afternoon and early evening and
have cut rain chances to silent 10 percent or lower. This ensures
the plethora of afternoon and early evening activities will be
dry. The same can`t quite be said for the ranchlands which will
remain close enough to the front and associated energy to keep a
modest mention of precipitation, though big rains and thunder are
less likely when compared with points north into the Coastal Bend.

Saturday evening should remain dry in the Lower/Mid Valley but the
approach of a moisture plume combined with decent upper level
divergence in the right rear quad of an approaching 110+ knot jet
will increase rain chances from northwest to southeast as the
night progresses. Have maintained wording as "numerous showers and
isolated thunderstorms" as questions remain on whether the main
rains will fall in elevated convection behind the front or along
the front. SPC has region in marginal threat given the decent
upper level dynamics, but too soon to get specific on exactly how
things will evolve. Highest chances and QPF will continue across
the ranchlands. Current temperature forecast for the start of a
cooldown across the ranchlands before daybreak Sunday is intact,
though all models are showing a seasonally-typical shallow
sharpness to the front. Would not be surprised to see Zapata and
Hebbronville in the mid 50s or lower by daybreak Sunday with
steady temperatures thereafter, while the Valley remains in the
balmy 70s.
52/BSG

AVIATION...**Delayed from 00Z TAF package** Low level soupy air
mass and model sounding forecasts all line up to bring in MVFR
ceilings by mid evening, expected to continue through the night.
Just enough surface wind (~10 knots) supported by gradually
increasing southerly flow off the deck should keep ceilings from
dropping overnight. For Saturday, overcast higher MVFR ceilings
will lift to VFR by late morning/early afternoon but only fade to
broken, with any further scattering for McAllen. The main concern
will be wind. Guidance supports sustained winds near or just above
20 knots along Highway 77 and perhaps a shade lower in McAllen;
gusts may depend on how much skies can break out but forecast wind
speeds just off the surface near 35 knots may be able to overcome
the clouds especially with area deep into the warm sector.
52/BSG

MARINE...Enhanced easterly flow just north of the warm front has
been effective in bringing fairly high seas onto the central and
south Texas coast, with Buoy 19 (south of Freeport) reaching near
11 feet earlier this evening. Some of these carried over to Buoy
20 east of Mansfield which almost touched 7 feet around sunset.
Since then, lighter east/southeast flow has arrived and seas have
dipped closer to 6 feet in the waters east of Mansfield and closer
to 5 feet at TABS Buoy (42045) east of South Padre.

With overnight winds expected to hold in the 10-15 knot range and
seas ticking down a bit further, have removed the advisory for the
remainder of the overnight.

This will change again on Saturday as the surface low intensifies
and gradient tightens; both wind and seas should edge over the 20
knot/7 foot line at some point and advisories will likely need to
be hoisted again. For now, will headline the coastal waters to
cover.
52/BSG

PREV DISCUSSION... /issued 337 PM CST Fri Dec 2 2016/

SHORT TERM (Tonight through Saturday Night): High dew point, mild
low level air will continue to pump north across the CWA tonight.
Rain chances will be on the increase, and ripples in the
overrunning southwest flow aloft, along with cooler temps aloft,
may be enough to help trigger some stronger isolated thunderstorms
across the Ranchlands and Brush Country. Higher rain chances will
remain focused primarily across the northern counties Saturday
and Saturday night as well, with somewhat better mid level forcing
developing as a cut off low sinks south into Northwest Mexico and
then moves east toward the CWA. Divergence ahead of the low will
fan out over Central Texas and South Texas, keeping rain chances
elevated during the short term, and a marginal risk of severe
weather has been posted by SPC for the entire CWA on Saturday.
Though forecast QPF amounts across the area are modest, in the
hundredths or tenths of an inch, locally heavy downpours, again
only if stronger storms develop, will be possible. Winds will
remain light to mdt out of the southeast, becoming breezy near the
coast Saturday afternoon. Temps will be well above normal, in the
60s and 70s, with low 80s across the area on Saturday. A cold
front is due on Sunday, during the first period of the long term.

LONG TERM (Sunday through Friday): A strong 500 mb closed low
will swing across northern Mexico and southern Texas Sunday and
Monday providing good divergence aloft and uvv. Good deep layer
moisture will also pool ahead of this closed low which will
interact with the good dynamics provided by the closed low to
boost up the pcpn chcs early in the longer range period. There are
some concerns that the atms could destabilize enough ahead of the
closed low to help fire off some isold stg/marginally svr t-storms
on Sunday. However will not mention them in the ongoing forecast
due to uncertainties as to when and where any stg/svr development
could occur Sun into Sun night. As the 500 mb closed low exits
the region Mon and Mon Night a little cooler airmass will build
into the region lowering temps down close to climo for the area.
The RGV will then come under the influence of a more zonal 500 mb
pattern through Tues and Wed which will allow the temps to warm
somewhat as a southeast low level flow returns. Then another large
and broad 500 mb trough axis digging across the central portion
of the country will drive a strong cold front into the South
Central Plains states on Wed and Thurs setting the stage for a
more significant cool down towards the end of next week.

The ECMWF and GFS models are in a little better agreement today
versus yesterday but fairly large differences still persist mainly
for temps on Sunday and for the timing of the strong fropa later
next week. The ECMWF shows a much cooler solution for the high
temps on Sunday with this model favoring a quicker exit of the 500
mb low and faster CAA building in over the region. Meanwhile the
GFS lifts out the closed low more slowly which holds off the CAA
until much later in the day. The longer range models then show
better agreement for the middle portion of next week for both
temps and pops with the ECMWF showing a stronger fropa and
accordingly a stronger Canadian airmass towards the end of next.

So will opt for a general blend of the two longer range model sets
through day 7. Overall confidence is pretty decent for pops
especially for the weekend. However confidence is much lower for
high temps on Sun and for the post frontal passage temps later
next week.

MARINE: (Now through Saturday night): Although local winds may veer
away from east during the short term, moderate or better east winds
will still persist over much of the Northeast Gulf basin, driving an
east swell toward the Lower Texas Coast. As a result, SCEC to low
end SCA conditions will be possible for the entire short term
period.

Sunday through Wednesday...The PGF will be strong enough from
Sunday through Monday which will maintain SCA conditions mainly
for the offshore waters. The PGF will then weaken on Tuesday and
Wednesday as the 500 mb closed low and associated coastal surface
troffing start to move away from the region. So expect calmer
marine conditions to prevail then. However the next strong cold
front which will move through the area later next week will likely
set the stage for another round of strong SCA conditions starting
Wed night and continuing into Friday.

&&

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

$$

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