Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 161556
AFDBRO

Area Forecast Discussion
National Weather Service Brownsville TX
956 AM CST Tue Jan 16 2018

.DISCUSSION...The Arctic front has moved through all of Deep South
Texas and the Rio Grande Valley a few hours ahead of schedule.
Temperatures have fallen 15 to 25 degrees just a few hours in wake
of the front. Hebbronville has already reached freezing (32
degrees) and the rest of the Ranchlands should reaching freezing
at or shortly after Noon. The intense cold air advection and
latest NAM suggest most of the RGV seeing freezing temperatures
around 6 pm. With the earlier onset of freezing temperatures we
have moved up the timing of the Freezing Warnings and Wind Chill
Advisories along with Winter Weather Advisories. No freezing
rain/drizzle is being reported at this time but with temperatures
at 32 degrees and drizzle beginning to develop in Hebbronville
freezing of bridges, overpasses, some roadways could be imminent.



&&

.PREV DISCUSSION... /issued 545 AM CST Tue Jan 16 2018/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...The passage of an arctic cold front is expected
between 13-14z this morning. Strong winds and low cloud decks with
light precipitation are likely in the wake of the front. MVFR
ceilings expected to continue through the 24 hours. Ceilings may
lower to below 1K feet at times. Will add the mention of frozen
precipitation at all aerodromes for tonight, around 03z at MFE
and around 06z at BRO/HRL.

PREV DISCUSSION... /issued 439 AM CST Tue Jan 16 2018/

.Arctic air pushing towards the Rio Grande Valley...

SHORT TERM (Today through Wednesday): The arctic cold front currently
extends from near Laredo to Corpus Christi to Sabine Pass. The cold
front will continue to surge south into deep south Texas by mid-
morning. The high temperatures for today have already occurred since
midnight or will likely take place between sunrise and mid-morning
before the strong arctic cold front brings much colder air into the
region. Temperatures will fall quickly through the day into the 30s
everywhere and below freezing in the northern Ranchlands in the
afternoon. Strong north winds will develop in the wake of the front
this morning and continue through the afternoon. Wind gusts between
40 to 45 mph will be possible near the coast between 5 am through
the afternoon. Therefore, have pushed up the start time on the Wind
Advisory, starting at 5 am, for coastal Cameron, coastal Willacy and
Kenedy counties through the early this evening.

The combination of cold temperatures and strong north winds
will produce very cold and even dangerous wind chills. Wind chill
values are expected to drop into the teens and 20s overnight and
will likely remain in the 20s for the entire area on Wednesday. Low
temperatures tonight will range from the upper 20s across the
northern ranchlands, around 30 in the Rio Grande Valley, and the mid
to upper 30s near the coast. North winds will remain moderate to
breezy near the coast on Wednesday as a coastal trough develops.
High temperatures Wednesday will be in the mid to upper 30s, except
around 40 along the beaches.

Additionally, freezing temperatures will help to produce a chance of
sleet and freezing rain across the entire area. The main question on
wintry precipitation is how much moisture will be available. The NAM
continues to be the driest, with low level dry air moving into the
western portions of deep south Texas by this evening. The GFS is
keeps some moisture lingering through Wednesday afternoon,
especially near the coast, courtesy of the coastal trough. Will
leave the previous POP and QPF grids intact and will let the day
shift decide on any changes. A few hundredths of an inch of
accumulation of ice is still possible. No changes were made to
winter weather headlines at this time.

LONG TERM (Wednesday Night through Monday): The cold Arctic airmass
will persist over the RGV into Wed night/Thurs morning maintaining
near to slightly below freezing temps through early Thurs. Pretty
significant PVA occurring out ahead of a 500 mb short wave diving
southeast out of the Desert SW will also help enhance UVV over the
region maintaining the threat for pcpn. Enough near or just below
freezing temps will linger over the region late Wed/early Thurs to
justify the continued mention of a possible wintry mix. The temps on
Thurs will then warm back up above freezing for the rest of the day
Thurs allowing for any pcpn to fall as liquid.

The 500 mb short wave will close off near the central U.S./Mexico
border region Wed and Thurs and will continue pushing SE towards the
RGV. This closed 500 mb low will pass over the RGV late Fri moving
out into the west Gulf of Mex by late Saturday. This will allow mid
level ridging to build over the region resulting in warmer and drier
conditions for the rest of the weekend into Mon. Until this closed
low clears the region deeper layer moisture will linger over the
region resulting in generally cloudy conditions. The approach of
the 500 mb trough/closed low from the west and the persistent
coastal troffing near the lower TX coastline will keep pops in the
forecast through the end of the work week.

The GFS and ECMWF differ somewhat on the timing and location of
the trough axis moving out of the Desert SW with the GFS moving
the feature through a little faster and on a more northerly track
versus the ECMWF`s more southerly and slower track. Despite this
difference between the models, run to run comparisons for both
forecast models temps and pops show pretty good agreement through
Day 7. So will opt for a general 50/50 blend between the two
models for the longer range forecast wording. Overall confidence
in the longer range forecast wording is above average.

MARINE (Now through Wednesday): Marine observation platforms along
the lower Texas coast indicate generally light winds early this
morning. A powerful arctic cold front will sweep through the lower
Texas coastal waters this morning. Dangerous wind and sea conditions
will rapidly develop and prevail through the period in the wake of
this front. Made some adjustments to the speed up the timing of the
frontal passage based on the latest RAP13 and HRRR models. Now with
the strong cold front expected to move into the northern portions of
the marine area around 11z, will push up the start time of the Gale
Warning to 5 AM. Frequent gusts of 34 knots can be expected with
hazardous seas of 7 to 11 feet developing by this afternoon. The
Gale Warning is in effect through this evening, with Small Craft
Advisories likely to be posted for late tonight and Wednesday,
especially for the Gulf of Mexico waters. Coastal troughing and
moisture along and behind the front will lead to very cold rain
showers today through Wednesday.

Wednesday Night through Saturday Night: The Northerly surface
flow over the lower TX marine locations will steadily decrease
with a warmer southerly flow returning Thurs night and persisting
through Sat Night. The PGF over the Laguna Madre will likely
remain weak enough to keep conditions down below SCA levels into
the weekend, but expect Gulf seas to likely range in SCEC/SCA
criteria through Sat Night.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...Winter Weather Advisory until noon CST Wednesday for TXZ248>251.

     Freeze Warning until noon CST Wednesday for TXZ248>251.

     Wind Chill Advisory until noon CST Wednesday for TXZ248>251.

     Wind Advisory until 6 PM CST this evening for TXZ251-256-257.

     Winter Weather Advisory from 6 PM this evening to 9 AM CST
     Wednesday for TXZ252>255.

     Freeze Warning from 6 PM this evening to 9 AM CST Wednesday for
     TXZ252>255.

     Wind Chill Warning from 10 PM this evening to noon CST Wednesday
     for TXZ252>255.

     Wind Chill Advisory until 10 PM CST this evening for TXZ252>255.

     Wind Chill Advisory until noon CST Wednesday for TXZ256-257.

GM...Gale Warning until midnight CST tonight for GMZ130-132-135-150-
     155-170-175.

&&

$$

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

59...Short term



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