Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 070448 AAB

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1048 PM CST Wed Dec 6 2017

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


.AVIATION...Not much change in overall aviation forecast through
the next 24 hours. Cigs remain 500 to 700 feet through the night
and into tomorrow, with continuous light rain and drizzle. North
winds continue at the surface 15g25kts with abrupt wind shift
around 5000 feet to southeasterly winds at 25kts. Rain activity
expected to become heavier later tomorrow as the midlevel trough
approaches. The trough will bring along colder air just off the
surface, so frozen precipitation and icing will be a concern at
all locations, especially after sunset Thursday.


.PREV DISCUSSION... /issued 552 PM CST Wed Dec 6 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Cigs remain dense between 500 and 1000 areawide,
expect for a brief thinning out at KMFE in the past hour. Radar
notes -dz starting to fill in areawide, and models continue to
increase coverage and intensity of dz through the night. As dz
increases, expect cigs to fluctuate downward, settling right
around500 feet later tonight into tomorrow morning. Gusty north
winds at the surface of 15G25kts swap abruptly at 5000
feet,becoming southerly at and above that level. Expect this
transition level to slowly rise overnight as surface cold pool
continues to push southward underneath the warmer airmass aloft.

PREV DISCUSSION... /issued 246 PM CST Wed Dec 6 2017/

SHORT TERM (Tonight through Thursday Night): The cold air outbreak
will continue through the short term. Low overcast cloudiness
with periods of light precipitation will be the norm. Temperatures
will be interesting, with the NAM guidance for BRO heading south,
i.e. colder, compared to the GFS guidance. The numbers for the
upper valley appear more in line with each other. Still, the NAM
for HBV brings in a chance of snow for Thursday night which we are
not seeing in the GFS numbers. The ECMWF now also suggests a line
for brief light snow Friday morning a little farther south than
just the ranchlands and brush country. This possibility was
incorporated somewhat into the forecast, though only along the
northwest, and will bring the line just a skosh farther south.
Additional updates may be needed, however. The mid level trough
upstream over the desert Southwest will translate slowly east, but
it won`t clear the area during the short term, so the dreary
weather should stay locked in through the period. Thursday night
will see low temperatures mainly in the 30s, with wind chill
values ranging from the upper 20s across the ranchlands and brush
country to the lower 30s for the lower valley.

LONG TERM (Friday through Wednesday): The major 500 mb trough axis
that will dig southeast across the southern U.S. states will
clear through the RGV on Friday allowing for drier air to push
into the region bringing an end to the pcpn. Afterwards 500 mb
ridging will prevail over the western states while deep troffing
will prevail over the eastern half of the county throughout the
weekend into early next week. Another series of fast moving 500 mb
short waves will then dive out of Canada which will push through
another reinforcing shot of cooler and drier air over the region.

As the 500 mb ridge axis prevails to our northwest, the overall deep
layer flow pattern will remain pretty dry which will maintain near
zero pops and will also in turn allow for temps to steadily warm
throughout the period due to the increased daytime heating.

The ECMWF and GFS models remain in good agreement in the handling of
the 500 mb pattern throughout Day 7 and accordingly show close
agreement with temps/pops throughout the extended range period. Also
run to run consistency with both models is also pretty stable. So
overall confidence in the longer range forecast wording is above
average. Will go close to a 50/50 blend of the ECMWF and GFS for
both temps and pops.

The combination of brisk surface winds and low temps for this
upcoming weekend could push the wind chill values down close to
advisory levels for portions of the RGV for Fri and Sat mornings.

MARINE (Now through Thursday night): Strong high pressure will
continue to spread across the area in the short term, supporting a
tight marine gradient. A weak surface low will form offshore at the
center of the residual frontal system, helping to maintain a better
gradient. Winds will conservatively gust to gale force over portions
of the Gulf in the near term, though there could be periods of more
marginal conditions. Thus, posted a gale warning offshore for now.
Winds may be weaker closer to shore, but may eventually gust to gale
force. Winds across the Laguna Madre appear more likely to remain
below gale force.

Friday through Monday Night...The strong PGF later this week will
remain pretty strong on Friday as strong surface ridging spreads
over the Gulf of Mex. This will maintain the potential for SCA
conditions to persist at least across the lower TX Gulf waters
into Fri. The PGF will then weaken gradually after Fri through
next Mon which will allow for the winds and seas to steadily
improve with conditions generally staying below SCA levels.


GM...Small Craft Advisory for winds until 6 AM CST Friday for GMZ130-

     Gale Warning until 6 AM CST Friday for GMZ170-175.

     Gale Watch from 6 AM CST Thursday through late Thursday night
     for GMZ150-155.

     Small Craft Advisory until 6 AM CST Thursday for GMZ150-155.



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