Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 132255 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
555 PM CDT Wed Mar 13 2024

...New AVIATION...

.SHORT TERM...
(Tonight through Thursday night)
Issued at 300 PM CDT Wed Mar 13 2024

A deepening upper level trough over the Desert Southwest looks to
form a closed 500mb low by Thursday night as it breaks from the
Polar Jet. This feature will maintain southwesterly flow aloft
through the short term period. A mid-level shortwave traversing the
southwesterly flow aloft looks to kick of another weak surface low
on the leeward side of the Sierra Madre Mountains early Thursday
morning. This will result in tightening pressure gradients and
breezy southerly to southeasterly surface winds Thursday, especially
along the coast and Lower Rio Grande Valley. By Thursday night
pressure gradients look to relax as the shortwave moves out of the
area and the surface low dissipates. Likewise, lighter winds are
expected Thursday night.

Some marine fog or haze could be possible tonight along the coast,
however winds will likely be too strong for dense fog to develop and
move inland.

&&

.LONG TERM...
(Friday through next Wednesday)
Issued at 300 PM CDT Wed Mar 13 2024

*Key Messages
-Hot Friday becomes a seasonably cool start to next week
-Unsettled conditions should bring some rain, favoring the Brush
Country and upper Valley late this weekend but spreading across the
region early next week.

For most of the period, the atmosphere is dominated by a closed-off
500 mb low that takes up residence in the southern Nevada/southern
California/Arizona area over the weekend, then weakens slowly early
next week while cycling around these areas.  East of the trough,
broad southwest to west-southwest flow persists, with weak short
waves ejecting from the parent trough and sliding through
west/central/north Texas every couple of days. By the weekend, a
developing broader trough in the northern jet stream slides across
the upper Great Lakes, and helps develop a weak downstream (and
farther south) ridge over the southern Plains.  This pattern ushers
a sagging front through central and eventually south Texas, which
will ultimately be a focus for at least some rain across the region,
beginning over the weekend across the northern ranchlands and
reaching the Valley by late Sunday/Sunday night, with additional
perturbations between Monday and Wednesday that *might* provide
better lift for more widespread precipitation during this period.
There is plenty of divergence in the global model suites, so details
of exactly where and how much rainfall will need to be ironed out
later this weekend.

As for the forecast...Friday sees another hot afternoon across the
mid/upper Valley, and plenty humid closer to the coast.  One
potential fly in the ointment is whether an overnight weak 500 mb
disturbance - perhaps convectively driven - can produce a mini-
thunderstorm system in the lower Valley before daybreak which
quickly scoots offshore by mid morning. Deterministic GFS has been
bullish on this, while the ECMWF has a more minor moisture shield
and limited forcing, with no appreciable QPF. Blends are more
bearish and given the uncertainty have left mentionable
precipitation out of the forecast in the lower Valley early
Friday...but situation will need to be monitored.

Southeasterly gradient collapses overnight into Saturday as surface
winds back to the east...but deeper moisture is pinned to the
ranchlands/upper Valley where rain chances have been retained. One
thing to watch is a possible (small) convective burst along/just
north of the wind shift line. Models favoring this farther north
(hence the better potential for the ranchlands) but any farther
south could allow residuals to touch the Valley...though plenty over
late night stability would reduce to showers or sprinkles.

Saturday will see morning clouds give way to afternoon sunshine with
lighter easterly flow overall, and with the air mass cooling
slightly, high temperatures should stay just below 90 for the
Valley...a touch cooler across the ranchlands.

A better opportunity for a convective system...favoring the Rio
Grande Plains through the Brush Country...is late night Saturday. As
500 mb heights fall a bit and the southwestern U.S. trough nudges
into Arizona, something could fire along the lee of the Sierra Madre
west of Eagle Pass and potentially move southeastward (as such
events tend to do).  Given the initial instability and the region
getting just into the climatologically favored period for severe
weather, it would not surprise to see a modest wind/hail threat
overnight there...with the higher stability across the Valley
allowing any system to fade to showers with a few rumbles at best
into early Sunday morning.

Afternoon northeasterly flow Sunday should signify the start of the
true airmass change, holding highs down to seasonable levels (upper
70s ranchlands, lower 80s Valley) along with a 20-30 percent chance
for showers and thunderstorms.

Thereafter, unsettled but modestly cooler conditions prevail Monday
through Wednesday with a broad northeast to east flow taking over.
As mentioned above, uncertainty is the story for potentially
beneficial rains Tuesday and Wednesday...the wetter solution would
also hold temperatures some 5 to 7 degrees below average, which is
currently the forecast.

Finally...with the longer-fetch and stronger northeast flow Monday
through at least Tuesday, beach equipment managers may need to take
stock and consider moving items behind the dunes for potential run-
up especially at high tide. We`ll have a better idea as we get
closer. Otherwise...higher surf and intense rips will develop by
Monday and continue through Wednesday.

&&

.AVIATION...
(00Z TAFS)
Issued at 553 PM CDT Wed Mar 13 2024

MVFR to VFR conditions will occur at the aerodromes through the
next 24 hours. A breezy to windy onshore flow will also occur with
precipitation not expected.

&&

.MARINE...
Issued at 300 PM CDT Wed Mar 13 2024

Now through Thursday night...Stronger southerly winds this afternoon
are creating adverse conditions on the Laguna Madre and northern
portions of the nearshore Gulf waters. A Small Craft Advisory for
the northern Laguna Madre and nearshore Gulf waters north of Port
Mansfield will run from 3-9PM today. Additionally, Small Craft
Should Exercise Caution headlines are in place for the southern
Laguna Madre. Moderate winds are expected to return tonight as
pressure gradients slacken. A mid-level shortwave moving overhead
Thursday morning will kick of a weak surface low along the Sierra
Madre Mountains. This will again tighten pressure gradients Thursday
afternoon as it moves further east before dissipating Thursday
evening. This could necessitate additional Small Craft Advisories or
Small Craft Should Exercise Caution headlines for the Laguna Madre
Thursday afternoon. Moderate southerly winds are again expected to
return Thursday night. Moderate seas and moderate winds are expected
over the offshore Gulf waters through the period.

Warm humid air advecting over relatively cooler seas could support
the development of some marine fog or haze tonight. However, winds
look to remain on the stronger side, which would hinder the
development of dense fog.

Friday through Monday Night...The slowly backing from from southeast
to east and northeast Friday through Sunday should keep wind speeds
below 15 knots through the period...though any early Friday
convective disruption could push out stronger gusts (temporarily
from the east) along with choppy to rough and briefly high waves.
Combined seas will remain moderate...generally 3 to 4 feet...through
Sunday. As the gradient picks up Sunday night and especially Monday,
northeast winds will increase to near or just above 20 knots and
seas build toward 7 feet...and Small Craft Advisories will become
more likely especially Monday and Monday night.

&&

.PRELIMINARY POINT TEMPS/POPS...
BROWNSVILLE             71  84  71  86 /   0   0   0  10
HARLINGEN               69  88  71  89 /   0   0   0  10
MCALLEN                 71  91  73  91 /   0   0   0  10
RIO GRANDE CITY         69  95  71  94 /   0   0  10  10
SOUTH PADRE ISLAND      69  76  69  77 /   0   0   0   0
BAYVIEW/PORT ISABEL     69  84  69  83 /   0   0   0  10

&&

.BRO WATCHES/WARNINGS/ADVISORIES...None.

&&

$$


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