Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 151752 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1152 AM CST Fri Dec 15 2017

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

&&

.AVIATION...Recent observational trends indicate ceilings remain
at VFR categories with an overcast deck between 6000 and 7000
feet across much of the area. Radar wind profiler shows a
northerly wind in the low levels shifting to the south at around
7000 feet, so conditions may remain at VFR a bit longer with
exception to lowered visibilities and ceilings in and around
precipitation. At this point, decided to hold off on prevailing
MVFR or lower conditions until tonight and Saturday morning when
better moistening is expected to occur closer to the surface.
NNW/NW winds will continue into Saturday with a few gusts to 20
knots possible in the afternoon.

&&

.PREV DISCUSSION... /issued 943 AM CST Fri Dec 15 2017/

DISCUSSION...Observations along the lower Texas coast and over
the Gulf waters indicate winds are ranging from 20 to 30 knots
with very few Gale-force gusts. With peak winds expected now
through this morning, decided to replace the Gale warning
everywhere with a Small Craft Advisory. Winds should diminish
along the Laguna Madre early this afternoon with hazardous seas
continuing over the Gulf waters through this evening.

In addition, the Wind Advisory has been cancelled as winds remain
below criteria. North winds may be breezy at times this afternoon
and improve this evening.

PREV DISCUSSION... /issued 548 AM CST Fri Dec 15 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Cold front has pushed through the Rio Grande Valley
with gusty north winds now occurring at all three terminals.
Ceilings have not lowered as quickly as expected, remaining
predominantly at VFR altitudes of 080-100 thus far. Suspect that
some "top-down" moistening has yet to occur. Radar VWP from BRO
shows that N-NW flow extends up to about 050, with the upgliding
SW flow above that, which is in line with the CIG`s seen so far.

Still expect the CIG`s to lower to MVFR levels later this morning,
though confidence is only medium now. Periods of light
rain/drizzle will also occur, but the axis of more persistent
rainfall is shaping up to be north of the RGV airports per radar
trends. Drizzle likely to persist overnight near BRO as a coastal
trough develops, with ceilings possibly lifting to VFR for HRL
and MFE.

PREV DISCUSSION... /issued 427 AM CST Fri Dec 15 2017/

SHORT TERM (Now through Saturday): Cold front`s impacts have been
a little underwhelming so far, with ceilings generally up around 6-8
kft. and only very light rain observed.  However, main high-pressure
surge has not yet arrived and model guidance has not changed
significantly, so will keep the main scenario in place with a few
tweaks.

There is some signal in the short-range guidance for lesser QPF with
the isentropic lifting over the frontal surface today, particularly
in the Lower Valley.  NAM/HRRR suggesting most persistent lift will
be in a swath from SW to NE across the central CWA, so will shade
highest PoP`s there for this morning.  Went ahead and added Kenedy
County to the Wind Advisory which took effect at 3am for consistency
with other coastal zones.  Still expecting temps to be stuck in the
upper 40s to low 50s areawide today.

Models have been consistent for a few days now in indicating a
relative lull in precip chances tonight.  Jet-streak now over NE TX
will have pulled away, lessening lift, and the surface boundary will
be well south of the area.  Model time-height sections also suggest
some low-level subsidence.  Would not be surprised to see some
drizzle over Cameron/Willacy counties overnight, though, as coastal
trough tries to get its act together.

Another cool, gray day in store for Saturday, though with lighter
winds vs. today.  Modest ~1015mb low develops just offshore. More
significantly, Baja upper low will have finally opened up and be
moving into West Texas by Saturday afternoon.  NW portions of the
CWA will be in pretty good position for dynamic enhancement in the
right-entrance region of a 120-knot H25 jet streak.  This should
lead to widespread showers and even a thunderstorm or two for a few
hours Saturday afternoon as some elevated instability is acted upon
by the dynamic lift.  Continued NW surface flow will keep high temps
in the 50s...perhaps 60F right along the coast, despite H85 winds
turning strong southerly in response to troughing in the lee of the
Rockies.

LONG TERM (Saturday Night through Thursday): Rapidly diminishing
opportunities for rainfall will occur during the first 12 hours of
the forecast period as a 500 mb trough quickly shifts northeast
from the Texas Big Bend to the Mississippi Valley area. Another
chance for precipitation, albeit isolated in coverage, is expected
Monday through Monday night as another mid-level disturbance works
quickly east from the Desert Southwest to the eastern half of
Texas, allowing a cold front to slip through Deep South Texas and
the Rio Grande Valley. The remainder of the forecast period will
be dry with a zonal 500 mb flow in place over the BRO CWFA.

Temperature-wise shakes out to be not too bad. After a cloud-and-
rain induced chilly Saturday night, more near-normal values are
expected through the rest of the forecast with an onshore flow
becoming reestablished and persisting with the exception of the
late Monday passage of the weak cold front.

Beyond the end of the official forecast period, long range model
guidance suggests another frontal passage and overrunning
precipitation event for the weekend before Christmas. Stay tuned.

MARINE (Now through Saturday): Dangerous marine conditions are
currently quickly developing in the wake of a cold front.  North
winds of 20-30 knots, gusting to 35-40 knots, are expected for all
of the coastal waters this morning into the early afternoon.  Seas
build to 7-10 ft.  A Gale Warning is in effect until 4pm, when the
pressure gradient will have relaxed somewhat. Small Craft Advisories
have been posted for the Gulf waters thereafter, mainly for seas
expected to remain agitated through around midnight.  A developing
coastal trough/low will keep winds offshore elevated on Saturday,
most likely at Exercise Caution levels.  While seas will be
choppy/confused, changing fetch as the low moves northward should
limit total seas to moderate levels of 4-5 ft.

Saturday Night through Thursday: The most adverse marine
conditions during the period are expected to occur Tuesday and
Tuesday night, as the passage of a cold front produces Small Craft
Exercise Caution wind and sea conditions along the Lower Texas
Coast. For the remainder of the forecast period, however, light
to moderate winds and low to moderate seas are anticipated with
alternating low and high pressure over the western Gulf of Mexico.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory for winds until 4 PM CST this afternoon for
     GMZ130-132-135.

     Small Craft Advisory until midnight CST tonight for GMZ150-155-
     170-175.

&&

$$

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