Area Forecast Discussion
Issued by NWS Brownsville, TX

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344
FXUS64 KBRO 041833 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
1233 PM CST Sun Dec 4 2016

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

&&

.AVIATION...The latest NAM forecast soundings maintains a pretty
saturated lower level inversion throughout the afternoon and
evening into the overnight hours. As the surface low and
associated cold fronts move further northeast away from the RGV
the lower level flow becomes more westerly which will start to
advect in drier air from the west which will improve
ceilings/vsbys up to MVFR/VFR levels. Until then expect IFR/LIFR
conditions to prevail for most of the upcoming TAF period with
better aviation conditions returning late in the period.

&&

.PREV DISCUSSION... /issued 602 AM CST Sun Dec 4 2016/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Generally poor flying conditions expected over the
next 18-24 hours as periods of rain occur. Moist air aloft riding
up over a shallow layer of colder air at the surface will
maintain low ceilings which will be the limiting factor for flight
rules. IFR CIG`s to prevail for the next couple of hours, with
LIFR possible. From mid-morning on, satellite trends suggest that
BRO and HRL may be able to break out into MVFR, but confidence on
this is not high. Guidance is pretty insistent that MFE will
remain socked in with IFR ceilings all day. Any thunderstorms
later on today will be isolated in coverage with best chances
near the coast. Taking into account AWC and SPC convective
forecasts, will go with TEMPO groups for -TSRA at BRO & HRL for a
few hours this afternoon before instability diminishes.

Overnight, NW winds will strengthen to breezy as a surface low
just offshore moves northward. This may stir things up enough for
CIG`s to rise to low-end MVFR levels...but again, confidence is
low so will forecast OVC010 for all three terminals.

PREV DISCUSSION... /issued 418 AM CST Sun Dec 4 2016/

SHORT TERM (Today through Monday):
Forecast has become fairly straightforward in the short-term.
Periods of light-to-moderate rain to continue (despite most land
areas being in a "lull" at the moment) through tonight as very
moist air ahead of an upper low currently centered over the
Sinaloa coast is isentropically lifted over the shallow cold
airmass that was brought in by yesterday`s/last night`s front.
Coastal areas may also receive a boost in rainfall from a surface
low will be developing offshore of the Lower Texas Coast later
today, then migrating northward tonight. Isolated thunder is still
in the works for today, though model CAPE fields suggest that it
will be limited to the coastal counties and offshore areas by this
afternoon.

Aforementioned upper low moves to just south of the Big Bend region
by late tonight, with some drier air beginning to wrap into the CWA
after midnight.  Widespread rain...moderate at times...still
possible through the evening, before PoP and QPF values trend down
somewhat for inland areas.  The coastal surface low keeps the
possibility of more widespread rain along the coast going through
the night.

Moving into Monday, the upper low finally traverses Texas to our
north and may become an open wave before being absorbed into the
mean flow.  At any rate, NAM/GFS in good agreement that a more
pronounced dry slot will be moving over the area during the morning,
which should shut off any rain chances for land areas by mid-day.

As far as temperatures go, with overcast skies and occasional rain,
Sunday`s high temps will only recover a few degrees from morning
lows, which means upper 50`s for the NW counties to near 70 at BRO.
Chilly and breezy tonight for a raw feel to the air with temps
generally in the 50s.  Some recovery on Monday as sun breaks out in
the afternoon, but still a few degrees below early December averages.

LONG TERM (Monday Night through Saturday):
More tranquil conditions return quickly Tuesday and Wednesday as
onshore flow returns to the region. Conditions will remain warm,
with highs in the lower to mid 80s areawide. The net major cold
front barrels through the region Wednesday night, as the polar
cold air speeds across the US. EC still lagging behind the rest
of the forecast models in the arrival time, so will continue with
passage between midnight and dawn Thursday. Temperatures will
plummet behind the front, going from the mid 60s ahead of the
front to the upper 40s by sunrise. temperatures will hold steady
during the day Thursday, hovering right around 50 degrees as
strong cold air advection continues. Breezy north winds will
continue Thursday night and into Friday morning, so lows at
sunrise Friday will fall to near 40 degrees, with some outlying
areas of the northern ranchlands possibly reaching the mid 30s.
surface ridge doesn`t move to the east of the area until pre-dawn
Saturday, which finally brings onshore flow back to the region and
brings moderating temperatures. This will be brief as models
noting another cold front sometime on Sunday, bringing more colder
air.

MARINE:
Now through Monday: Latest observation, as of 250 am CST, at Buoy
020 indicates ENE winds at 19G23 knots. Seas have come down just
slightly to 6 ft. during the past hour as it appears that an
easterly swell is finally diminishing. Winds may also be in a
relative lull the next few hours, but effect should be relatively
short-lived, however, as NE winds begin strengthening again by
mid- day as a surface low develops near the southern edge of the
Gulf waters. Seas to remain agitated...in the 6-7 ft. range,
before building back to 7+ ft. this afternoon and tonight, so have
extended the SCA for the Gulf Waters until midnight, with SCEC
currently effect for the Laguna Madre. An extension of the SCA is
probable as winds shift to the NW and remain strong as the surface
low migrates up the coast. NW winds and seas finally subside
somewhat by later on Mon., but SCEC/low-end SCA conditions may
persist.

Mariners should also be aware of thunderstorms over the waters
through tonight...with cloud-to-water lightning, gusty winds, and
locally further elevated seas the main threats.

Monday night through Thursday: Light marine winds will be expected
Monday night into Tuesday as the effects of the departing surface
low come to an end. This will allow seas to finally relax down to
2 to 3 feet through Wednesday. Main marine threat will be the next
cold front racing through the region before dawn Thursday. Models
still showing winds around 25 knots through the day Thursday,
which pushes seas up above 10 feet the remainder of Thursday.

&&

.PRELIMINARY POINT TEMPS/POPS...
BAYVIEW/PORT ISABEL  66  57  70  56 /  80  80  30   0
BROWNSVILLE          66  56  71  55 /  70  80  20   0
HARLINGEN            61  56  71  53 /  70  80  20   0
MCALLEN              57  55  71  53 /  70  80  10   0
RIO GRANDE CITY      57  53  71  49 /  80  80  10   0
SOUTH PADRE ISLAND   69  59  69  62 /  80  80  30   0

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory until midnight CST tonight for GMZ150-155-
     170-175.

&&

$$

This product is also available on the web at:
HTTP://WEATHER.GOV/RGV

Short Term...60
Long Term...52
Graphicast/Upper Air...62



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