Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 211142 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
642 AM CDT Wed Mar 21 2018

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


.AVIATION...Clear skies and light winds prevail across deep south
Texas this morning. A dry and stable environment will continue
for the next 24 hours as surface high pressure remains generally
in place. Expect VFR conditions to prevail for all three RGV
airports through late tonight. Light E-SE winds will increase
later this morning into the afternoon as surface high pressure
shifts east. Light southeast winds will prevail this evening and
overnight as high pressure moves farther east.


.PREV DISCUSSION... /issued 447 AM CDT Wed Mar 21 2018/

..Valley Wind Machine Set to Crank Up This Weekend and Beyond...

SHORT TERM (Today through Thursday): Light winds and mostly clear
skies prevail across deep south Texas this morning courtesy of
surface high pressure. Mostly sunny skies are expected today with
high temperatures in the low to mid 80s, except for the 70s near the
coast. Surface high pressure will move eastward today allowing winds
to become more east to southeast and increase as the pressure
gradient increases across the region. Temperatures tonight will
be little warmer with lows mainly in the low to mid 60s with some
increasing clouds. Slightly above normal temperatures will
continue into Thursday with high temperatures in the 80s under a
mix of sun and clouds. Rain-free conditions will prevail through
the forecast period as dry northwest flow aloft and surface high
pressure remains generally in place through Thursday.

LONG TERM (Thursday Night through Tuesday): The main story
through the period will be wind and heat. Deterministic models are
in excellent agreement with the big picture this morning as the
GFS has come around to holding onto the strong subtropical ridge
in the central Gulf by then, which holds back the progression of
the 500 mb trough that eventually is forecast to dig into the
Sonora region of Mexico beyond this forecast period. This means no
front anywhere near the southern tip of Texas...with the wind
machine expected to continue through next Tuesday as well. Prior
to early next week, the 500 mb ridge forecast is nearly identical
to that of the past few days: Building northward along the Rio
Grande Friday through Saturday then easing slowly into the western
Gulf Sunday and Monday, and toward the central Gulf Tuesday as it
feels some nudge from the developing southwest U.S. trough.

For the sensible weather, the heat will build fairly quickly as
atmosphere warms and dry air in the column mixes down each day.
Low clouds may arrive to become broken-overcast by Friday morning
and could take until noon to begin breaking up, though for now
have forecast partly cloudy vs. overcast (this may change in the
next day or two). By mid to late afternoon, mostly sunny skies
should allow the heat to spike into the mid 80s near the coast,
around 90 McAllen, and low to mid 90s Rio Grande Plains based on
forecast 850 mb temperature around 20C west and 18C east. Wind
machine will be underway but perhaps tempered by some of the early
clouds and 850-sfc winds below advisory levels. That said, 20 to
25 mph gusting 35 mph in the Brownsville-Raymondville corridor
looks likely.

Saturday and Sunday look like twin days, even as the 500 mb ridge
shifts from the longitude of Texas Saturday into the western Gulf
Sunday. Winds will stay up overnight and with some fast-scooting
clouds will keep temperatures up in the 70s in most locations
(except the ranchlands) by daybreak. The wind machine peaks these
days, and given near 40 knot winds just off the deck to start the
day and still near 30 knots in the 850-924 mb layer during peak
mixing potential for 25 to 35 mph sustained with gusts 40 mph or
higher, favoring the triangle of Weslaco-Raymondville-Brownsville
and points in between. Wind advisories are a decent bet for these
areas, though current deterministic forecast is just a hair below.
Forecast temperatures were not changed...with triple digit highs
possible along the Rio Grande near Falcon, Mid to Upper 90s from
McAllen to Zapata, and 88 to 92 elsewhere given the atmospheric
heat profile as well as mostly sunny skies after noon.

Monday is looking more like the weekend now as the 500 mb ridge is
now forecast to still be over the western Gulf, with enough deep
dry air to mix out any morning clouds and push afternoon
temperatures close to the forecast values on the weekend. Because
of some uncertainty on Day 6, only nudged values up a few degrees
but didn`t match the hotter weekend forecast just yet.

For Monday night and Tuesday, the only question left is will there
be rain? The latest runs keep Monday night largely dry, as the
main energy looks to remain well north of the Valley. Retained
slight chances along SH 285 between Hebbronville and Falfurrias.
Another sultry breezy fact could be near windy
conditions should low level jet be able to pulse to the surface
between McAllen and Harlingen/Bayview/Port Isabel. A short wave
moving through the east side of the trough may produce some
thunderstorm activity that favors central/north Texas but could
extend into south Texas; trimmed back rain chance area to the
ranchlands and Gulf waters near Baffin Bay but kept the Valley
dry. Bumped winds up Tuesday as well, and potential for another
day of 20 to 30 mph with higher gusts is now in play. Still plenty
warm for the season and upped temperatures into the upper 80s to
lower 90s for most areas and perhaps still mid 90s across the Rio
Grande Plains.

For beach goers: South to north longshore current will be rolling
along and surf waves leaning that direction will build and peak at
5 feet or higher through the weekend. The situation Saturday and
Sunday offers the possibility for rare but extremely dangerous
"reverse rips" just north of the Isla Blanca jetty, where
evacuation of water upshore is sufficient for equilibrium to
result, and currents that "backwash" toward the jetty from
north to south are the result.

MARINE: Buoy 42020 reported east-northeast
winds around 12 knots gusting to around 18 knots with seas slightly
under 2.6 feet with a period of 6 seconds at 03 CDT/08 UTC. Buoy
42045 (TABS K) reported east winds around 10 knots gusting to around
14 knots at 02 CDT/07 UTC. The pressure gradient will increase today
as low pressure develops across west Texas this afternoon. East
winds will increase today then will become southeast tonight as
surface high pressure shifts east. Moderate southeast winds will
develop across the Gulf waters tonight as the pressure gradient
strengthens across the western Gulf. Exercise Caution conditions
likely on the Gulf waters and on the Laguna Madre on Thursday.

Thursday night through Sunday Night: Overall, the combination of
wind and choppy to rough conditions on Laguna Madre and
combination of wind waves and swell on the Gulf will make for a
difficult to dangerous period for small craft boating/fishing
trips in all waters. While marine layer will hold the winds a
little bit, enough dry air mix-down particularly Saturday and
Sunday will pump speeds up to near or just above Small Craft
Advisory levels, including the nearshore waters. Diurnal
flips may be minimal...20 to 22 knots overnight but still 18-21
knots by day through the period; Laguna winds will easily pass 20
knots...perhaps 25 knots at times...Saturday and Sunday and
nocturnal low level jet could create periods of 20 to 25 knot
winds for much of each overnight Friday through Sunday. Combined
seas will build to 6 feet Thursday night into Friday and probably
average out around 7 feet for the weekend, though persistence
should produce a rare 7 to 8 foot sea and 8+ second swell in
southerly flow much of the time.

FIRE WEATHER...Today: The threat of rapid fire
spread will be a concern again today, but winds will likely not
reach Red Flag Warning criteria. However, burning is still
discouraged as the ground remains very dry. While dewpoints will
recover some today, low level easterly winds will increase this
afternoon and may become marginally elevated. Will hold off on
issuing a Fire Danger Statement at this time, and allow the next
shift to monitor the situation.

Friday through Monday: The `gut` here says that fine and medium
fuels will remain at least in the "dry" range through the period,
removing that parameter from consideration. The question then
becomes wind/relative humidity. We issue Red Flags based on 25+
mph 20 foot winds, which is in the forecast for the usual suspects
(Hidalgo/Cameron/Willacy), and humidity, which needs to be <40
percent inland and <45 percent coast, will be close (inland,
mainly) with the better sunshine and mix-down of very dry air not
too far above the surface, particularly Saturday/Sunday and
perhaps Monday, too. Current forecast is on the margin for both of
these parameters in Hidalgo/inland Cameron/inland Willacy, perhaps
extending into Brooks and Kenedy. Bottom line? It`s no time to
burn safely in the RGV and King Ranch areas.




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