Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 151747 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
1147 AM CST Sun Jan 15 2017

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

&&

.AVIATION...Winds continue to zip through the region at the
surface and aloft, being drawn into the storm system passing to
the north. Winds will remain 15 to 20 knots with gusts to 25 at
the surface the rest of the afternoon, and will only draw down to
around 15 knots through the night. Daytime CU field lesser today
as moisture is focusing better off to the north and drier air
draws into the region from the southwest. Skies should remain
mostly clear overnight as minimal capping sets up locally and dry
air continues crossing the region. Breezy and gusty conditions
will return for KBRO and KHRL tomorrow morning, but KMFE will see
winds decrease and shift more southwesterly as a weak trough
moves into the western valley.

&&

.PREV DISCUSSION... /issued 555 AM CST Sun Jan 15 2017/

DISCUSSION...Updated for latest /12Z/ aviation discussion below.

AVIATION...Prolonged stream of thick high clouds (estimated at
35K feet) and winds staying up near 10 knots have killed off any
possibility of IFR ceilings or fog early this morning in the more
populated Lower/Mid Valley, though there are some pockets of IFR
to the north across the ranchlands. With daybreak just an hour
away, not too bullish on IFR with southeast flow at the surface
set to increase within a couple of hours. For now, rolled a TEMPO
MVFR period through 16Z to cover any pesky low clouds that have
been popping up from time to time.

Thereafter, forecast through 00Z largely left alone. With clouds
breaking by noon or a little thereafter, probably faster out west
where mid level dry slot arrives soonest and helps mix out any
remaining VFR cumulus, winds should crank up quickly as tight
gradient and low level jet combine to kick sustained speeds to or
above 20 knots with gusts to or above 30 knots from time to time.
Brownsville with more clouds (and lower in the VFR range) winds
may be a tad lower.

For the overnight, dry air (RH below 40 percent) reaches down
toward 900 mb, which should be good enough for at most scattered
MVFR but elected to roll clear skies for much of the night. That
will change late, perhaps as soon as midnight (06Z) but elected to
go a little later with the MVFR stratus deck. Breezy conditions
with gusts over 20 knots likely for a good part of the night and
30 to 40 knot winds between 2 and 3K feet AGL. Mechanical mixing
through the night means no low level wind shear issues.
52/BSG

PREV DISCUSSION... /issued 405 AM CST Sun Jan 15 2017/

SHORT TERM (Now through Monday): The big picture remains the same
for the southern tip of Texas through the period - we "wave" at the
more active weather that affects the northern half of Texas as
vigorous upper low now moving into Sonora Mexico early Sunday
morning loses its vigor while lifting toward El Paso this afternoon
and on into the Panhandle this evening. The passage of this system
dampens the backside of the subtropical ridge that has dominated the
Gulf and coastal Texas for the past few days, but the only effect on
the sensible weather from the system is the well-advertised return
to the bigger "Valley Wind Machine" of gusty south winds today, with
a sharper wind contrast from Valley to Rio Grande Plains/northwest
ranchlands on Monday.

As for rain-supplying moisture? Not much to be found. Yesterday`s
western Gulf shear axis did pop out 0.10 to 0.15" in the mid
Valley...a bit farther west than expected...and plenty of mid level
clouds that held afternoon temperatures down a few degrees (but
still plenty warm for mid January) than forecast. That axis has
exited into the Gulf, but lower level streamer-type showers to
spread toward the Lower Texas Gulf waters this morning and may clip
the coast so have maintained mentionable rain there.
Elsewhere...bands of mid clouds are sweeping across most of the
area.  Cannot rule out a sprinkle or light shower, with slightly
higher possibilities across the ranches so have maintained a
mentionable value (20 percent) through daybreak.

After daybreak, the mid clouds should gradually push away as dry
slot approaches ahead of the lifting short wave into west Texas.
Unlike Saturday, expect clouds to mix out a bit to more of a partly
sunny sky perhaps as early as late morning but more definitely by
noon with the chance for a complete clear-out best across the Upper
Valley/Rio Grande Plains.  With more sunshine have bumped
temperatures back to the warm values a few degrees above guidance,
with mid 80s likely in a number of spots. Records could be close in
Brownsville (83 in 2016) and McAllen/Miller (86 also in 2016). The
other story will be the wind. All models have backed off just a hair
on low level wind fields, with 28 to 32 knots between 850 and 950 mb
more common for the peak mixing hours of late morning through late
afternoon.  With viscous airmass still in place this combination
should keep winds below advisory levels (30 to 39 mph and gusty) but
not by much.  The strongest winds as usual will cover the region
between Highways 77 and 281 and spread toward Bayview in southeast
Cameron County.

The low level jet then strengthens near the coast overnight and
could reach 40+ knots in the aforementioned layer if the GFS is
correct. This will keep some mechanical mixing going to the
surface and expect at least 10 to 20 mph winds with higher gusts
through the night especially near the coast. Winds will dip late
across the Rio Grande Plains/Upper Valley but for now still
forecasting in the 5-8 mph range. This should preclude fog...but
any wind shift to light north could change that equation in a
hurry. Will defer fog potential in Zapata/Jim Hogg/western Starr
for the next shift. Moisture is lacking but low level jet should
be enough for at least thin stratus to develop after midnight. See
little reason for temperatures in the breezier areas to fall
below 70 so held minimum values there in the Lower and Mid Valley.

A plume of slightly deeper moisture passes into the Coastal Bend and
especially east and southeast Texas late tonight and early Monday
and there could be enough for some stronger storms there. The south
end of the weakened moisture plume may touch the ranchlands...and
blended forecast with that for the South Texas Brush country early
Monday for slight chances of rainshowers. By noon Monday, dry air
covers the entire upper Valley and ranchlands with light winds,
perhaps turning northerly.  With limited 850 mb temperature surge
and only a light westerly downslope flow, don`t expect a heat spike
but afternoon temperatures could reach into the mid 80s from Zapata
through western Hidalgo County. Valley Wind Machine continues for
the Lower RGV but less emphatic than today.

LONG TERM (Monday Night through Saturday): A more lengthy period
of rainfall was the most significant change to the long term portion
of the forecast for this cycle. The intrusion and stalling of a
weak cold front into Deep South Texas and the Rio Grande Valley,
followed nearly immediately by the passage of a deep 500 mb trough
through Texas, will produce scattered thunderstorms and/or showers
across the BRO CWFA from Tuesday night through Thursday night,
with pockets of isolated showers pockmarking the remainder of the
long term forecast period. With no major cold air surges likely,
daytime high and overnight low temperatures are progged to be
above normal, even with the scattered precipitation potential.

MARINE (Now through Monday): Increasingly difficult conditions will
be featured in different portions of the waters today through
Monday. First, for the Laguna Madre:  Relatively cool bay
waters...and proximity to cooler nearshore waters farther north...
should help moderate the wind machine a little this afternoon to
keep winds just below advisory (20 knot sustained and gusty) but it
will be close.  Will go with caution headline and add a "may be
needed" for advisory this afternoon just in case.  Same deal for the
warmer waters beyond 20 nautical miles, especially for tonight where
low level jet peaks and seas should be able to touch 7 feet for a
time. The 0-20 nm waters are colder and that should be enough to
hold the winds down and cut seas just enough for no headlines...but
expect at least caution tonight with arrival of low level jet as
well as the elevated seas edging westward.

Winds will gradually diminish Monday as will seas, but caution
conditions (6 foot seas and 15-20+ winds) still likely through the
morning offshore, with some gustiness on Laguna Madre as well.

Best time to fish/boat may be this morning (Sunday) nearshore and on
Laguna Madre.

Monday Night through Saturday: Increasingly favorable marine
conditions are anticipated during the forecast period as a weak
cold front becomes stationary along the Texas coastline and
lingers for a few days before dissipating. The worst wind and sea
conditions are progged to occur Monday night through Tuesday
night, when sea heights on the Gulf of Mexico waters east of Padre
Island could be high enough to require Small Craft Exercise
Caution. Otherwise, no other Caution periods or Small Craft
Advisories are expected to be needed.

&&

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

$$

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