Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 260553

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1153 PM CST Sat Feb 25 2017

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


.AVIATION...Winds will continue to veer from currently easterly
direction to SE the rest of the night as surface high pressure
centered over the Arkansas currently migrates eastward. This will
increase low-level moisture, with MVFR ceilings still expected to
develop around 09Z at all terminals.  These ceilings should lift
back to VFR by mid-morning. Also cannot rule out a brief shower or
sprinkle between about 12-18Z on Sunday as an old frontal boundary
moves back northward through the RGV as a warm front. Will not
mention in TAF`s at this time, though, as confidence still not very
high in this spotty activity.  Drier air aloft works back in for
Sunday evening for clearing skies.


.PREV DISCUSSION... /issued 544 PM CST Sat Feb 25 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Winds will veer from NE to SE overnight as surface
high pressure over the ArkLaTex region migrates eastward. This
will increase low-level moisture, with MVFR ceilings developing
after midnight at all terminals. NAM guidance suggesting LIFR
CIG`s, but will discount this for now (with SREF IFR
probabilities only running 10-30%) and lean toward the GFS MOS.
Also cannot rule out a brief shower or sprinkle between about
12-21Z on Sunday as an old frontal boundary moves back northward
as a warm front. Will not mention in TAF`s at this time, though,
as coverage expected to be isolated and operational impacts

PREV DISCUSSION... /issued 336 PM CST Sat Feb 25 2017/

SHORT TERM (Tonight through Sunday Night): Northeast winds will veer to
southeast and decrease to light tonight as high pressure centered
over the Southern High Plains shifts east. This will bring coastal
moisture back into the region, with overnight low temperatures in
the 50s and 60s. Patchy fog may develop across the ranchlands and
brush county. The reversal in wind flow will essentially bring a
residual surface boundary back over the area as a warm front, and
isolated showers will be possible from early Sunday morning through
the day, particularly for eastern sections. High temperatures will
make it into the 80s Sunday, and mostly cloudy morning skies will
clear out to partly cloudy as southeast winds increase to moderate.
Patchy fog will be a little more widespread Sunday night as rich low
level moisture remains in play, but light southeast winds may be
enough to stymie a more significant fog event. Overnight low
temperatures will be mainly in the 60s as a warming trend continues
into the early long term.

LONG TERM (Monday through Saturday): For the mid range (Monday
through Wednesday) the current forecast was little changed as
February goes out with "hot" style and quite appropriately given
the record temperatures we`ve for today`s one-day break
in the unusual heat this month. March will come in like a rather
hot "lamb" but changes are afoot soon after as split atmospheric
flow creates a more early spring-like setup in the steering
pattern...and yes an increasing opportunity for rain along with
the lowering temperatures by the end of the work week.

For Monday and Tuesday, the heat builds as forecast with
subtropical 500 mb ridge building north and west from the Gulf to
clip the Texas/Mexico border Tuesday into Tuesday night. Unlike
the dry heat that kept skies cloudless and humidity very low all
the way to the coast from Tuesday through this morning, courtesy
of broad west- northwest flow above the flat 500 mb "dry" ridge
that formed behind the early to mid week low latitude 500 mb
trough that gave Florida some rain, the Monday- Tuesday setup is
more traditional with a "Valley Wind Machine" setting up and
peaking on Tuesday with winds perhaps gusting over 30 mph, but
more notably warm/muggy evenings and overnight with wake-up
temperatures remaining above 70 for the RGV and perhaps some of
the ranchlands each morning through Wednesday. Could still see a
few spots poke 100 on Tuesday afternoon along the Rio Grande from
McAllen west, but for now have capped just below and in line with
the hotter ECMWF Guidance; even out west, where humidity will drop
the airmass will be more viscous which makes 100+ a little more of
a challenge.  Time will tell.

For Wednesday, other than slight timing difference among the
models, the sensible weather idea is similar: Tail of any decent
moisture favors the coastal counties out into the Gulf, and
despite shifting winds to the north out west plenty of dry to
superadiabatic mixing should push temperatures along the Rio
Grande from McAllen to Falcon into the lower 90s as dewpoints do
drop off nicely, and upper 80s farther north. This will likely be
our next day for erratic fire behavior and potential spread; stay
tuned for updates as situation evolves.

As mentioned above, recent GFS runs came around to the frontal
push shown by the ECMWF the past few days into Thursday, a
response to the carving out of a eastern US "bowl" shaped trough
(and also a return to some late winter temperatures there). For
this reason, removed rain chances Wednesday night through midday
Thursday before reintroducing a sliver along the Lower Valley
coast into the southern Gulf waters in response to deeper moisture

Thereafter, GFS and ECWMF are in decent agreement with a fairly
quick ramp-up of deep layer moisture Thursday night and especially
Friday, as southwest flow aloft ahead of trough easing east out of
Baja fills the column. GFS in particular becomes quite wet, and
would not surprise if late Friday into Saturday feature periods of
moderate rain which would be quite welcome for many after the
desert-dry of recent days. Too soon to jump on anything more than
50% chances and have capped lower/mid Valley at this value with
climatologically lower values farther west. QPF for now ranges
near 0.50" for the event, though 1" not out of the question as per
WPC latest Day 6-7 guidance. The clouds and rain overrunning a
relatively "cool" air mass will likely keep daytime temperatures
below average Friday, and perhaps Saturday depending on whether
winds can veer toward the southeast. Not sure this will happen and
for this reason kept thunder out of the Friday/Saturday period for

One final potential hazard to consider toward next weekend is
tidal overwash/run up toward the dunes at SPI. As broad and strong
surface ridge establishes over the Ohio and Mississippi Valleys,
long and strong east/northeast fetch covers the entire Gulf and
could well flood the beach at high tide at some point late next
week/weekend. Details will be fleshed out as the week progresses.

MARINE: Northeast winds remain fresh this afternoon, and wave
heights offshore are staying stubbornly just above 7 ft., so will
extend the advisory offshore until 6 pm. Winds will slowly
decrease through tonight, however, and moderate southeast winds
will return on Sunday as surface high pressure centered over the
Southern High Plains shifts east. Wave heights will remain
slightly elevated on the Gulf Sunday, and small craft should
exercise caution conditions may persist. Isolated coastal showers
will be possible on Sunday.

Monday through Thursday: Generally light to moderate winds and
seas 5 feet or less will prevail Monday as pressure gradients are
weak while 500 mb ridge back builds. The nocturnal low level jet
by early Tuesday should be enough for 15 to 20 knots and gusty
along with some 6 foot seas especially beyond 20 nm out, as well
as potential for wind/chop issues on Laguna Madre by late morning
and afternoon. After a brief lull late Tuesday night into midday
Wednesday, north/northeast flow arrives by afternoon.

Then conditions go downhill, particularly over the Gulf, as
gradient tightens quickly behind the front. Both models have upped
their game and 25 to perhaps 30 knot sustained winds are not out
of the question overnight Wednesday into Thursday, courtesy of
the 1030+ high diving into east Texas through the day associated
with the developing 500 mb "bowl" trough moving through the east
coast. This is more like it for a winter-type system and with
models coming into agreement expect to see 8 to 10 foot full blown
seas by Thursday along with a few Gale force gusts from time to
time in the Gulf.




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