Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 170515 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1115 PM CST Sat Dec 16 2017

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

&&

.AVIATION...The 00Z BRO sounding shows a pretty deep and saturated
inversion below the 850 mb level over the RGV. This will tend to
hold the low ceilings in place over the area throughout the
overnight hours. As the 500 mb short wave to our west lifts out to
the north and northeast overnight and tomorrow, drier air will
start filtering in from the west eventually allowing some MVFR
conditions to potentially return Sun afternoon. However the
saturated low level inversion will likely remain pretty stubborn
over the next 24 hours despite the drier air advection. So expect
another round of IFR/LIFR ceilings later in the upcoming TAF
period as the moist low level inversion rebuilds after sunset
tomorrow evening. The light winds and narrow surface temps and
dewpoints will also maintain some fog potential late tonight into
Sunday night. Model guidance is also hinting at the potential for
some locally dense fog during the same period especially in the
MFE and HRL areas.

&&

.PREV DISCUSSION... /issued 736 PM CST Sat Dec 16 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...The 00Z BRO sounding shows a pretty deep and saturated
inversion below the 850 mb level in place over the RGV. This will
tend to hold the low ceilings in place over the area throughout
the overnight hours. As the 500 mb short wave to our west lifts
out to the north and northeast overnight and tomorrow, drier air
will start filtering in from the west eventually allowing some
MVFR conditions to return late in the current TAF set. The light
winds and narrow surface temps and dewpoints will maintain some
fog potential late tonight into Sun morning. Expect IFR/LIFR
conditions to prevail for all three RGV airports for the night.

PREV DISCUSSION... /issued 327 PM CST Sat Dec 16 2017/

.Chilly Damp Weather Slow to Depart This Weekend...

SHORT TERM (Tonight through Sunday Night): Dreary and damp
conditions continue this afternoon, with 500 mb low ejecting quickly
through west Texas this afternoon and into Oklahoma overnight while
dampening out. Southwesterly flow continues aloft, however, as
another southwest U.S./Northwest Mexico trough crosses along the
international border between Arizona/New Mexico by Sunday night as
the subtropical ridge keeping South Florida nice and mild today
spreads into the western Gulf.

Deep South Texas, being caught in the "squeeze" between these two
systems aloft - and sitting in a classic "cold air damming"
situation from the current chilly air mass which is keeping low to
very low clouds locked in place east of the Sierra Madre, will see
very slow moderation on Sunday based on analog cases at this time of
year when the sun is near its nadir for our part of the world.
Bottom line?  Even after the quick and final shot of rain ends
quickly this afternoon, low clouds and fog/mist will be difficult if
not impossible to scour out Sunday. With another stripe of mid level
moisture arriving Sunday, that will also prevent sunshine from
breaking through - ensuring a still cool-ish day.  The NAM-12, which
has been one of the best models in these situations, was followed
closely through the short term.

For the sensible weather: Light to moderate rain will continue to
prevail across the ranchlands/upper Valley through 4 or 5 PM before
quickly exiting stage northeast and eventually providing more
helpful rain to parts of the upper Texas coast tonight into early
Sunday - areas that have seen dryness return.  The 2.5 day rain
event for places like Jim Hogg and Brooks County looks to add up to
2+ more inches in some areas, eradicating what`s left of the
drought/dry conditions there and a nice surprise for a December that
at one point looked to end at or below average. No longer the case,
when including some more rain by late next week/weekend.

Farther east, more occasional rain will end as forcing exits
quickly, leaving low clouds, fog, and light west/northwest winds for
the overnight.  Current temperatures in the mid 40s to lower 50s may
nudge down a degree or so overnight but that`s about it. Guidance
shows already low cloud bases (500 to 900 feet) to fall further and
perhaps dip below 200 feet, which would end up a de facto fog event
just about anywhere. Even though chilly temperatures are not
necessarily conducive, the crashing ceilings should aid in
perceptible fog of 1-3 mile visibility so added to the entire land
mass.

Sunday will begin with dreary overcast and fog with some drizzle
just about everywhere, and it won`t necessarily be gone by noon as
would be typical in late winter and spring.  Therefore, have
maintained mention through noon with a very slow rise in
temperatures. With light north winds more likely to prevail the
entire day as the low clouds only lift a few hundred to a thousand
feet, it would not surprise if 60 is not achieved in many areas. The
NAM covers this well, keeping the ranchlands locked into the 50s
with readings creeping over 60 in the Lower Valley. As mentioned
above, elected to lean strongly this way and disregard the warmer
blends as even the GFS has been trending toward no discernable
wind shift to the east or southeast Sunday afternoon. This is a
far cry to earlier optimistic Sunday forecasts, but follows the
trend and the climatology of the pattern.

For Sunday night, the chilly air continues to gradually erode.  This
is when it gets interesting, with low temperatures at or just below
Sunday`s highs, as dewpoints remain or rise to meet them.  Upper 50s
is often a "sweet spot" for winter fog as warm advection above the
cold dome develops, and guidance signals are very much in line with
a potential dense fog episode late Sunday night through the Monday
morning commute.  Did not add "dense" to the forecast just yet but
will highlight in the hazardous weather outlook. As for
precipitation?  The GFS shows a stripe of deeper moisture but
forcing is weak; still, elected to keep a mention of slight chances
across the eastern half of the area, mainly east of US 281/IH 69C,
Sunday night.

LONG TERM (Monday through Saturday): The synoptic pattern at
the beginning of the work week will include another Baja California
mid-level trough/low pressure system and a strong high over the
eastern Gulf of Mexico. In between, southwest flow aloft and weak
surface troughing will continue across Deep South Texas and along
the coast. High moisture in the column will keep a good amount of
cloud cover across us through the first half of the week. Models are
somewhat in disagreement with how long the overcast, cool conditions
will persist into Monday. The NAM wants to keep a more notable
coastal low with northerly flow and much cooler temperatures,
whereas the ECMWF and GFS bring temperatures back into the upper 70s
and low 80s. Decided to stick somewhere on the higher side of a
blend of the three with temperatures in the mid 70s and lower
(isolated) rain chances. Also added fog through early Monday morning
with high surface and soil moisture and light winds.

On Tuesday, confidence is higher that temperatures will warm up with
stronger S/SSW flow returning. A cold front is expected to move
through late in the day after peak heating with northwest flow very
briefly returning. The surface high will move quickly to the east,
bringing southeast flow and a better moisture return by the end of
the week. Confidence is modest right now with our next cold front by
the end of the week, with the latest 12Z model guidance. The ECMWF
brings a very weak trough through Central Texas with very low rain
chances and the GFS continues to dig another deep low over the
Desert Southwest with much cooler and wetter conditions for the
weekend. Until models come into better agreement, decided to lean on
the wetter, cooler GFS side due to pattern persistence.

MARINE: Now through Sunday night: Surface low/trough splitting
the nearshore and offshore waters (beyond 20 nm) with caution
conditions (15 to 20 knots and gusty from the southeast) hanging
on a little while longer before back side northwest winds overtake
the situation. Current forecast covers this well, and will
maintain the current trends through this evening. Seas have likely
risen toward 6 feet, as buoy 19 (in better gradient, but
developing a northeast swell) well east of Matagorda carried 6.2
foot seas and, as prior discussion mentioned, these should briefly
carry into the 20 to 60 nm leg through early to mid evening.

Thereafter, northwest winds fade to north/variable overnight into
Sunday, then turn northeast or east beyond 20 nm out Sunday
afternoon while remaining light north close to shore in the cooler
waters and especially on Laguna Madre. Light/variable winds will
continue Sunday night near shore and Laguna Madre, with a slight
uptick in southerly flow over the warmer waters farther east where
air mass recovery gets going in earnest.

Monday through Friday night: Light to moderate winds will
continue over the coastal waters with scattered showers and
possibly a few thunderstorms through Tuesday. A weak front will
move through late Tuesday with brief northwest winds returning and
higher seas to 3 to 5 feet. The next cold front on Friday could
be strong with Small Craft Advisory conditions possible.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...None.
&&

$$

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

Short Term/Aviation....60
Long Term/Upper Air...64


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