Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 160441 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1041 PM CST Mon Jan 15 2018

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

.AVIATION...The mention of frozen precipitation has been kept out
of the upcoming issuance of TAFs, as it appears that it will not
affect the aerodromes until after 06Z Wednesday. Tranquil VFR
conditions will continue through the overnight hours until the
passage of the arctic cold front mid-morning Tuesday. Strong winds
and low cloud decks with light precipitation are likely in the
wake of the front. Cloud decks could easily be lower than the 1K
feet level indicated.
&&

.PREV DISCUSSION... /issued 541 PM CST Mon Jan 15 2018/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Tranquil and VFR weather conditions are expected
tonight in advance of the mid-morning Tuesday passage of a
powerful arctic cold front. The front will produce degraded
aviation conditions with strong winds, precipitation, and low
ceilings. The next issuance of TAFs tonight may need to include
the mention of frozen precipitation for Tuesday evening and
especially Tuesday night.

PREV DISCUSSION... /issued 416 PM CST Mon Jan 15 2018/

SHORT TERM (Now through Tuesday night):A deep 500mb trough will
dig into the eastern U.S. with a dam low over the Great Lakes
region Tuesday into Wednesday. At the surface, a low is tracking
across the southern Great Lakes with a strong cold front moving
across much of the interior portions of the country. Strong
southerly flow continues across South Texas ahead of it with
temperatures climbing into the mid 70s with winds gusting to 25
mph this afternoon. Tonight, high temperatures for the day will
likely take place in the mid-morning hours as a strong arctic cold
front brings much colder air through mid week. Temperatures will
fall through the day into the 30s everywhere and below freezing in
the northern Ranchlands in the afternoon. A Freeze Warning goes
into effect for Zapata, Jim Hogg, Brooks and Kenedy counties at 3
PM CST Tuesday with below-freezing temperatures possibly through
noon on Wednesday. Some model guidance is showing the potential
for temperatures to drop into the mid 20s in the northern areas,
so have nudged lows down a couple degrees from the previous
forecast. Later in the day, a Freeze Warning goes into effect at 9
PM CST for the Rio Grande Valley through 12 PM CST Wednesday with
temperatures dropping into the low 30s and upper 20s.

In addition to the cold ambient temperatures, life-threatening wind
chill values are expected Tuesday into Wednesday. A Wind Chill
Advisory goes into effect for all of Deep South Texas early
afternoon on Tuesday with a Wind Chill Warning for the Rio Grande
Valley (Cameron, Willacy, Hidalgo, Starr) starting at 10 PM CST.
Wind chill values are expected to drop into the teens overnight and
will likely remain in the 20s throughout the entire day Wednesday.

A tricky part of the forecast will be the potential for light wintry
precipitation. Model soundings show surface temperatures dropping to
below freezing Tuesday afternoon, but a couple of warm noses from
850 to 700mb will likely keep any precipitation mostly freezing rain
and sleet. With the temperatures in place for some light glazing,
the question is how much moisture will be available. The NAM remains
the driest, bringing very high dew point depressions Wednesday
morning and afternoon, which would limit any accumulations. The GFS
and ECMWF are a bit wetter with a coastal trough keeping moisture
over Deep South Texas longer through Wednesday afternoon. Even light
drizzle could cause issues on roadways and other elevated surfaces,
but a few hundredths of an inch of accumulation of ice will be
possible. A Winter Weather Advisory in effect for all inland areas
Tuesday afternoon and evening through the overnight.

Behind the front, models bring a 1036 to 1040mb high into South
Texas, also producing strong winds across mainly the eastern half of
the area. Winds are expected to gust to 40 to 45 mph Tuesday morning
through the afternoon, so a Wind Advisory has been issued for
coastal Cameron, coastal Willacy and Kenedy counties Tuesday morning
through the early evening. North winds will remain moderate to
strong through the day on Wednesday as coastal troughing develops
and remains in place.

LONG TERM (Wednesday through Monday):Very cold air will be well
entrenched across the region with 1040mb surface ridge settling
briefly over South Texas. This ridge is ushering in very low dew
points out west with still a lingering coastal trough just off the
Lower Texas coast. This creates the challenge of where to cut off
the precipitation line Wednesday. With a wide range of pops among
the model guidance will continue to mention 20 percent pops,
across the NW half of the CWA but with no accumulation. The
precipitation will likely be in the form of very light drizzle and
likely in the form of freezing drizzle with temperatures below
freezing at least Wed morning. Chance pops remain across the
Eastern half of the region where the coastal trough will have more
influence and deeper moisture resides. Once caveat is if the
surface ridge has more influence over the coastal trough all rain
may temporarily end until later Wednesday night through Friday
morning when an upper trough invigorates the coastal trough.
Temperatures range of only 3-5 degrees Wednesday to bring daytime
highs just over freezing for a few hours. The surface ridge begins
to head east as a potent midlevel shortwave trough approaches
west Texas. Models diverge at this point with the GFS showing an
open wave and EC indicated a closed low which will be slower
moving across South Texas. In either case, rain chances increase
Wed night and Thursday and may linger into Friday morning with the
strengthening of the coastal trough. Challenge here is how much
influence will this trough have on temperatures. first
temperatures may dip back below freezing Wednesday night/Thu am
and this brings freezing precipitation back into play. No
advisories at this time but will be looking at this more carefully
the next few days. Slight Moderation of temps is expected
Thursday but the increase in rain may play a role in keeping high
a few degrees lower.

The coastal trough moves east and the large polar high moves east
Friday with east to southeast winds return in vengeance later Friday
and Saturday. Much warmer temperatures by Saturday will make this
cold snap a memory.  Another trough brings another front to the
region Sunday with only a modest cool down Monday.

MARINE:(Now through Tuesday night): Strong southerly flow will
continue this afternoon as the pressure gradient tightens ahead of a
strong cold front. Winds tonight will lighten before the front moves
through Tuesday morning. The Gale Watch has been upgraded to a
warning for all marine areas off the lower Texas coast beginning at
9 AM CST. Frequent gusts of 34 knots can be expected with hazardous
seas of 7 to 11 feet developing by late Tuesday afternoon. Very poor
boating conditions will continue through Wednesday morning. Coastal
troughing and moisture along and behind the front will lead to very
cold rain showers Tuesday and Wednesday.

Wednesday through Friday...Hazardous marine conditions
continue for Wednesday with a slow recover Thursday and Friday. A
coastal trough or low pressure area to form near and possible over
the coastal waters Thursday and moves east Friday. This may play a
role in the overall winds and seas, but in any case marine
conditions to remain agitated and rather rough through the end of
the week with issuance of periodic Small Craft Advisories.
&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...Winter Weather Advisory from 3 PM Tuesday to noon CST Wednesday
     for TXZ248>251.

     Freeze Warning from 3 PM Tuesday to noon CST Wednesday for
     TXZ248>251.

     Wind Chill Advisory from noon Tuesday to noon CST Wednesday for
     TXZ248>251.

     Wind Advisory from 9 AM to 6 PM CST Tuesday for TXZ251-256-257.

     Winter Weather Advisory from 9 PM Tuesday to 9 AM CST Wednesday
     for TXZ252>255.

     Freeze Warning from 9 PM Tuesday to 9 AM CST Wednesday for
     TXZ252>255.

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

     Wind Chill Advisory from 3 PM to 10 PM CST Tuesday for
     TXZ252>255.

     Wind Chill Advisory from 3 PM Tuesday to noon CST Wednesday for
     TXZ256-257.

GM...Gale Warning from 9 AM Tuesday to midnight CST Tuesday night for
     GMZ130-132-135-150-155-170-175.
&&

$$

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