Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 071858 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1258 PM CST Thu Dec 7 2017

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

&&

.AVIATION...The aviation conditions over the next 12 to 24 hours
will be pretty hazardous as a strong 500 mb trough axis pushes
further southeast towards the RGV. Abundant moisture remains in
place over the region with very strong CAA persisting at the lower
levels of the atms. The morning BRO sounding shows very saturated
conditions prevailing up through the 700 mb level with a strong
inversion evident. South TX radar sites show the pcpn shield
steadily expanding across the area with surface obs already
showing IFR conditions in place at all three RGV airports. These
conditions will not improve any throughout the short term and
could potential drop down to LIFR levels for several hours later
tonight. The concern shifts towards the potential for some light
snow mixing in with the rainfall as the continued CAA drops the
temps aloft down to below freezing. So will include some tempo
groups for -SN through the overnight hours through sunrise
tomorrow. The conditions will not improve until possibly late in
the upcoming TAF period as the 500 mb trough clears through the
region allowing for drier air to filter into the region from the
north and northwest. Expect MVFR/VFR conditions to return late in
the TAF period.

&&

.PREV DISCUSSION... /issued 529 AM CST Thu Dec 7 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Very low cloud decks and widespread light
precipitation will continue through the TAF period. Surface winds
are expected to increase as high pressure building into Texas
interacts with low pressure over the west-central Gulf of Mexico.

PREV DISCUSSION... /issued 336 AM CST Thu Dec 7 2017/

SHORT TERM (Now through Friday): Let`s cut right to the chase and
discuss hazards. A Wind Chill Advisory has been posted for all of
Deep South Texas and the Rio Grande Valley from just before midnight
tonight until a couple of hours after sunrise Friday morning, as
moderate to breezy surface winds combine with overnight lows in the
30s to produce temperatures that will feel as if they are in the mid-
20s to around 30 degrees.

Secondly, surface high pressure building into the Lone Star State
will interact with surface low pressure over the west-central Gulf
of Mexico to increase surface winds from mid-morning today until
around mid-morning Friday. Winds are expected to be strong enough
that a Wind Advisory has been posted for this time frame for Kenedy
County and the coastal portions of Cameron and Willacy Counties. The
strongest winds are likely to occur for the entirety of Padre Island
from the mouth of the Rio Grande River up to Baffin Bay. Please
refer to the short term portion of the marine forecast discussion
below to see what effects these winds will also have on the Lower
Texas coastal waters.

Finally, a Freeze Watch and a Winter Weather Advisory were weighed
for portions of the BRO CWFA for tonight through early Friday
morning, but opted to punt these hazards to the incoming shift, who
will consider newer model guidance and issue them (or products
stronger than those, i.e., a Freeze Warning and Winter Storm
Warning) if they are deemed even necessary.

Overall, the slow approach of a deep 500 mb trough will continue the
overcast skies and excellent chance of precipitation, even possibly
frozen for some areas, from now through Friday morning. The passage
of this trough late Friday will finally begin to clear cloudiness
and reduce and eventually end the chance of rain.

LONG TERM (Friday Night through Wednesday): Behind the 500mb
trough axis Friday night, much drier air air will move into Deep
South Texas with northwest flow aloft bringing strong subsidence
through the weekend. At the surface, high pressure will move
overhead with lightening winds and clear skies Saturday morning.
The main concern continues to be the possibility of a freeze
across portions of the Rio Grande Valley and especially the
Ranchlands if winds can drop off enough overnight. There is some
model disagreement with the NAM bringing freezing temperatures as
far east as Hidalgo and Cameron counties and the GFS being much
warmer with above-freezing temperatures everywhere. Decided to go
with a blend of the NAM and ECMWF as the GFS seems to be a warm
outlier. Will have to keep an eye on the possibility of a Freeze
Watch/Warning.

This weekend, mid-level ridging of high pressure will try to
build in with persistent northerly flow at the surface through
early next week. Thus, below-normal temperatures are expected with
highs in the upper 60s and lows in the 40s. A deep trough will
move into the Great Lakes region by the middle of next week, which
may bring our next cold front Tuesday into Wednesday. Low
moisture will keep rain chances near zero with temperatures slowly
increasing to near normal towards the end of the period.

MARINE (Now through Friday): Buoy 42019 reported north-northeast
winds around 25 knots gusting to around 31 knots with seas of 10.5
feet with a period of 8 seconds at 02 CST/08 UTC. Adverse marine
conditions early this morning will deteriorate even further later
this morning and through midday Friday as surface high pressure
building into Texas interacts more intensely with slow-moving
surface low pressure over the west-central Gulf of Mexico. A
combination of Small Craft Advisories and a Gale Warning for the
Lower Texas coastal waters will be replaced by a Gale Warning for
all of the waters as winds increase and seas build to dangerous
levels. Mariners are urged to consult the most recent Coastal Waters
Forecast for the latest information and navigate the Lower Texas
Coast only if absolutely necessary from now through noon Friday.

Friday night through Tuesday: Surface high pressure over Central
and West Texas will continue to bring light to moderate northerly
winds across the coastal waters on Saturday. Hazardous seas may
warrant a Small Craft Advisory through the early part of the day
Saturday before seas are able to improve from north to south. Low
seas and light to moderate winds will bring much more favorable
marine conditions Sunday into early next week.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...Winter Storm Warning from midnight tonight to 9 AM CST Friday
     for TXZ248>250-252.

     Wind Chill Advisory from 11 PM this evening to 8 AM CST Friday
     for TXZ248>257.

     Winter Weather Advisory from midnight tonight to 9 AM CST Friday
     for TXZ251-253>257.

     Wind Advisory until noon CST Friday for TXZ251-256-257.

GM...Gale Warning until noon CST Friday for GMZ130-132-135-150-155-
     170-175.

&&

$$

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

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



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