Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 161156 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
556 AM CST Sat Dec 16 2017

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


.AVIATION...A Low pressure area develops over the SW Gulf of
Mexico this morning and move north just east of the Lower Texas
Coast tonight. Expect cigs to remain in the MVFR category with
occasional bouts of IFR during more persistent periods of drizzle
and light rain. As the Low makes its closest approach, mid
afternoon to early evening, there could be a very isolated
thunderstorm, however coverage and confidence not high enough to
add into TAFs. Ceilings may briefly rise this evening but should
remain at MVFR levels before approaching IFR after midnight. Light
northwest winds are expected to turn west this evening.


.PREV DISCUSSION... /issued 412 AM CST Sat Dec 16 2017/

..Another Cool, Cloudy, and Damp Day Across Deep South Texas...

SHORT TERM (Today through Sunday): Expecting weather conditions to
deteriorate today as a low pressure area forms along the cold front
over the SW Gulf.  Model consensus shows the low slowly deepening as
its approaches and passes over the Lower Texas coastal waters this
afternoon and evening, respectively. This low is being induced by
the Baja low being ejected NE as an upstream shortwave trough
digging south along the California coast replaces the current low
Sunday. In a nutshell, expect rain to increase in areal coverage
mainly for the western three-quarters of the CWA with the Lower
Valley see possible lesser coverage depending on how close the low
tracks near the coastal line.  This will also make temperatures a
very challenging, because if the low tracks 30-50 miles farther west
temperatures could jump into the lower 60s, albeit briefly. So with
the current thinking (surface low stays over the coastal waters)
temperatures are not expected to rise very much today and if the
rain coverage does pan out highs in the west will struggle to reach
50 with lower 50s elsewhere except immediate coast line. Confidence
is above average that the low will track east of the coast with
model consensus maintaining NW winds across the entire CWA with the
"warmer" NE-E winds staying offshore.  Back to the precipitation,
backed off on the chance of thunder and just mentioned isolated
across the coastal sections. Only the NAM is indicating any elevated
instability with the passage of a 100kt jet streak later this
afternoon, but at this time winds turn west and limited low level
drier air moves in across the far western zones. SPC has painted
limited general thunder along the coast with the proximity of the
surface low, i.e. warmer temperatures, higher dewpoints along with
the stronger jet passing just to the north.

As the low is forecast to track well to the northeast overnight and
Sunday, conditions improve significantly with rain chances ending
and temperatures warming (Sunday).  Not expecting much in way of
clearing tonight or sunshine Sunday, with a persistent southwest
flow, due to the above mention upstream shortwave trough/low,
keeping a tap of Pacific moisture aloft. With the clouds sticking
around will trend near guidance for lows sunday morning and slightly
below the guidance consensus for Sunday`s highs but all areas should
see maxes climb into the 60s.

LONG TERM (Sunday Night through Friday): The long term starts
very similar to how the short term starts in this cycle. Warm
front develops along the coast (or just inland this time). The
difference this round will be that we should (for the most part)
be on the warmer side of the front. Still, however, much of the
same can be expected with cloudy and showery conditions expected
to last through at least Tuesday. Additionally, can`t rule out a
T`storm or two during this timeframe. Again, much of the region
should be on the warmer side of the front where instability will
be present, though at this juncture think the best shot for
thunder looks to be Tuesday across our marine zones and upper
valley/ranchlands as upper level impulse moves across CenTex.

Models show a brief break by Wednesday and Thursday. Another
strong cold frontal boundary will be due into the region early
Friday. Globals are in relatively good agreement with timing and
strength of the frontal boundary at this time. We did trend temps
for the end of the period downward (away from climo) and kept POPs
a reasonable level for this early. The pattern going into next
weekend look all too familiar with more coastal
troughing/overrunning to keep our region gloomy, damp, and below
average temperature wise.


MARINE (Today through Sunday): Winds and seas have
temporarily lowered sufficiently allowing for earlier small craft
advisories to expire. Marine conditions to start off fairly benign
but are anticipated to gradually deteriorate through the afternoon
and into the evening, albeit mainly over the outer Gulf waters.
Models suggest a low pressure area to develop over the SW gulf along
yesterday`s frontal boundary and track over the coastal waters later
today. Winds will be the most challenging element and will depend on
the exact track of this low pressure area. With the consensus
indicating the low to pass closer to the coast, then east of the
coastal waters, stronger winds are likely over the outer waters
while higher seas may be directed across all gulf waters. Small
Craft Advisories may be necessary later this afternoon and this
evening but uncertainty on the track of the low and how tight the
pressure gradient may become, model guidance in just shy of SCA,
will hold off on issuing a SCA at 4 am and let the day crew evaluate
observations through the morning. The low lifts northeast over the
NW Gulf tonight into Sunday with winds and seas lowering west to

(Sunday Night through Friday): Light to moderate winds and seas
to begin the long term. As a low develops along a warm front,
expect some scattered showers to develop and perhaps a rumble of
thunder or two as instability increase, especially on Tuesday
afternoon. Marine conditions will remain somewhat quiet at least
until the next cold frontal boundary nears the region early
Friday. At that time, strong pressure gradient behind the front
will increase winds and waves with likely SCA conditions. Given
this is at the end of this forecast early, the exact specifics are
a bit murky, but confidence is above average for the end of the


BAYVIEW/PORT ISABEL  58  51  67  63 /  70  40  10  30
BROWNSVILLE          55  50  68  64 /  70  40  10  20
HARLINGEN            53  48  66  62 /  70  40  10  20
MCALLEN              51  48  65  62 /  70  30   0  20
RIO GRANDE CITY      49  45  63  57 /  70  20   0  20
SOUTH PADRE ISLAND   64  54  68  67 /  70  40  10  30




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