Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 291716
AFDBRO

Area Forecast Discussion
National Weather Service BROWNSVILLE TX
1216 PM CDT MON AUG 29 2016

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

&&

.AVIATION...northerly flow continues through much of the lowest
layers of the atmosphere over deep south TX in response to a
tropical upper tropospheric low about 200 miles south of south
central Louisiana. Visible satellite imagery is showing
considerable cumulus field across the forecast area as daytime
heating works on low level moisture in place. Have begun to see
some isolated shra develop across the region in some of the
thicker cumulus. CU/SHRA to hang around through the
afternoon/early evening hours before beginning to dissipate with
the loss of daytime heating.

Low/mid clouds will begin to redevelop from northeast to
southwest mainly after midnight tonight as a trough associated
with the TUTT low pushes southwest along the central TX Gulf
Coast. Am expecting sct shra to develop along with the increasing
clouds, but mainly after 09Z early Tuesday morning. SHRA to
continue to become more widespread as we progress toward the end
of the TAF period. VFR conditions will generally prevail.

&&

.PREV DISCUSSION... /issued 626 AM CDT MON AUG 29 2016/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Satellite imagery is showing newly-formed low clouds
over the northern half of Deep South Texas wrapping around a mid-
level disturbance over the Upper Texas Coast. Some of these clouds
may move over the aerodromes until mid-morning, when higher-level
cloudiness will form. Meanwhile, weak surface low pressure along
the Lower Texas Coast will cause surface winds to veer throughout
the day before becoming light and variable tonight. Isolated
convection is possible but remains not included in the current
TAFs. Overall, VFR is expected through the next 24 hours.

PREV DISCUSSION... /issued 429 AM CDT MON AUG 29 2016/

SHORT TERM (Today through Tuesday): A closed 500 mb low pressure
system is evident on water vapor satellite imagery spinning over the
northwest Gulf of Mexico. Model guidance suggests that this feature
will ease ever so slightly to the south-southwest today. Isolated
(20%) to scattered (50%) showers and thunderstorms are expected for
Deep South Texas and the Rio Grande Valley, with the least chance
over the southern half of the BRO CWFA (farthest from the low) and
the best chance over the northern half and especially along the
border with neighboring CRP CWFA (closest to the low). With minimal
chances of showers and thunderstorms for 75% of the BRO CWFA, felt
confident leaving well above normal daytime high temperatures but
knocked them down a couple of degrees with increased cloud cover
expected. Tonight, the 500 mb feature moves closer to Deep South
Texas and the Rio Grande Valley, but with the loss of daytime
heating, only isolated convection, if any, will be experienced
inland with more scattered activity for the adjacent coastal waters.
Mid-level low hangs offshore on Tuesday, but a return of daytime
heating and deep tropical moisture in place should give an equal
chance of scattered showers and thunderstorms both inland and
offshore at that time. High temperatures, of course, will be knocked
to more near normal levels given the forecast rain chance and mostly
cloudy skies overhead.

LONG TERM (Tuesday Night through Sunday): Indeed, a little (or a
decent amount) of rain will fall by Wednesday if already not
underway earlier this week. The question then remains: How much
and where will additional rains fall to close out the week and
perhaps stretch into the weekend? Uncertainty increases with time
but some trends look reasonable.

In the big picture, at least one good spoke of energy rotates
around the remains of the Tropical Upper Tropospheric Trough
(TUTT) which oozes southwest along the Lower Texas coast Wednesday
and along the Tamaulipas coast Thursday before becoming absorbed
into the westward rebuilding ridge to close the work week. That
spoke comes with a good slug of deep moisture, some of which has
bee producing scattered to numerous showers/storms this past
weekend and now early this morning along the northern Coastal
Bend. The inherited forecast, which was virtually unchanged from
the one we set up this time Sunday morning, carries likely (60
percent) rain chances Tuesday night over the King Ranch and
Wednesday into Willacy County, with high chance values down to
Cameron. Did not bite on the high-bias moisture ECMWF, but bottom
line for Tuesday night and especially Wednesday is Cameron County,
currently in moderate drought, should see enough coverage to bring
0.25-0.5 inches, with 0.5 to as much as 1 inch farther north.
Coverage could fade a bit farther west, but any additional heating
along/west of US 281/IH 69C could help the afternoon cause. Didn`t
get cute on timing as situation is not classic sea breeze jump,
though the Upper Valley may hold off until noon.

Temperatures Wednesday are a difficult call; for now, lower 90s
were left intact since an all-day rain is unlikely (not favored in
low-mid level northeast to north flow unless a tropical cyclone is
in play) but 88 in some spots would not surprise.

As the remains of the TUTT ease along the Tamaulipas coast
Wednesday night and Thursday, the better rain chances will be
taken with it. Both GFS/ECMWF and even NAM keep the column fairly
moist during the evening with favorable east/northeast low level
flow to bring hit and run showers and perhaps a few storms going,
so held onto the inherited 30 percent rain chances through
midnight. By late Wednesday night and Thursday, the pesky
moisture shifts to the Lower RGV but could stretch back to the
populated mid Valley by afternoon especially with a bit more
heating aiding the cause.

The last of the deep moisture hangs tough from the Rio Grande
south into eastern Tamaulipas Thursday night into Friday. The
ECMWF is a shade farther south with its leftover energy but fairly
consistent with GFS, and too close to remove chances altogether
near the coast. Kept this going into Friday morning, then retained
the sea breeze inland by afternoon.

The weekend sees the ridge return, but mid level east southeast
flow may be enough to keep some depth of moisture around,
especially near the Rio Grande. For Saturday, added isolated
morning precipitation for the eastern half of the area and Sunday
did a more typical sea breeze jump as low level flow turns more
solidly to the southeast. As for temperatures, a gradual rise each
day with triple digits returning to the mid/upper Valley perhaps
as soon as Saturday especially in areas where showers/storms are
not close by, but more likely on Sunday as southeast flow begins
to pick up and atmosphere warms with 1000-850 and 1000-500
thickness values returning to levels typical of recent weeks.

Bottom line to all of this is welcome but minor drought easing in
the Lower Valley, which may see widespread 0.5 to 1" rains with
locally higher amounts between tonight and Saturday. Similar to
2009, 2014, and 2015, the last two or three days of August will
see a multi-day streak of measurable rain.

For beachgoers, this will also be the first multi-day streak of
rain since early June; it has been a fantastic summer on South
Padre. Rip current threat will peak Wednesday and Thursday with
perpendicular flow combined with some of the swell (see marine
section below) but should abate in time for the holiday weekend
when precipitation will be fading away, especially by late morning
each day.

MARINE: Buoy 42020 reported west-southwest winds around 6 knots
with seas slightly over 1.5 feet with a period of 6 seconds at 03
CDT/08 UTC. Brief weak low pressure along the Lower Texas Coast
and an overall weak pressure gradient over the western Gulf of
Mexico will produce generally light winds and low seas for the
adjacent coastal waters of Deep South Texas and the Rio Grande
Valley throughout the forecast period. Small Craft Exercise
Caution and Small Craft Advisory continue to not be expected. The
slow southward progression of a disturbance in the middle layers
of the atmosphere just off the Texas coastline will lead to an
increase in showers and thunderstorms, especially tonight and
Tuesday.

Tuesday Night through Friday: About the only concern is how much
swell will arrive from the broad northeast flow around the TUTT
system, perhaps enhanced a smidge by the tropical cyclone in the
east central Gulf. New wavewatch model indicates 4 foot combined
seas, most of which would be swell, nudging into the eastern
waters Wednesday into Thursday. With starting points at 1 to 2
feet today and tonight, this seems reasonable. By late Thursday,
assuming the forecast track and acceleration into the northeast
Gulf is correct, seas/swell will quickly collapse.

Otherwise, light winds except occasionally moderate east
northeast winds Tuesday Night into early Thursday, with the main
issue for boating/fishing fairly good coverage of rain Tuesday
night through Wednesday night, remaining pesky south of Mansfield
right into early Friday. Not the best week for fishing, but the
weekend should improve.

&&

.BRO Watches/Warnings/Advisories...
TX...None.
GM...None.
&&

$$

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19/63


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