Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 180034 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
634 PM CST Wed Jan 17 2018

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


.AVIATION...Satellite imagery continues to show overcast skies
across South Texas with moderate winds out of the north. A few
patches of sleet or freezing drizzle will be possible tonight, but
confidence is low on any aviation impacts in the Rio Grande
Valley. A coastal disturbance will strengthen during the day
Thursday with ceilings dropping to MVFR and likely IFR in the


.PREV DISCUSSION... /issued 346 PM CST Wed Jan 17 2018/

..Third frigid outbreak of the Winter of 2017/18 continues into
Thursday morning...

SHORT TERM (Tonight through Thursday Night): A challenging forecast for
tonight into Thursday with a coastal trough strengthening with the
approach of a mid level low pressure trough.

Cold air in place and not going anywhere fast with 1038 sfc high
settling near the Arklatex. Mid level trough over New Mexico moves
SSE tonight transitioning into a mid level low and slowly moving
across Texas Thursday/Friday. Winds at 925-850mb turn SE-S over the
next 12-36 hours increasing moisture up to 700mb. Forecast point
soundings from MFE to BRO become saturated overnight with only the
surface and boundary layer showing some dew point spread. Light
drizzle is expected to develop later tonight with more widespread
rain spreading east to west Thursday and Thursday night. Big
question for tonight and Thursday morning is if temperatures fall
below freezing in areas where the drizzle develops.  A minimal
freeze is possible (due to potential wet bulbing affect) for a few
hours (3am-9am Thu best case) over NW Cameron, portions of Willacy
and Hidalgo Counties during the time drizzle may form which could
result in patches of freezing drizzle. Will keep the area out of
Winter Weather advisory at this time due to low probability of
significant impacts but an SPS will be issued to highlight the
probable impacts. Colder temperatures north and west will warrant a
freeze warning overnight and the cold and some wind hanging on will
warrant a wind chill advisory over the Lower RGV to the coast.

As mentioned rain chances increase Thursday starting off as drizzle
with probably widespread rain in the afternoon and tomorrow night.
All model pop guidance suggest best pops with the best convergence
along the coast with lower chances out west. Rain amounts could
possible approach a quarter inch or more in the Lower Valley with
WPC indicating qpf values nearing one inch Thursday night into
Friday. Thrusday`s highs will be highly dependent on
rainfall/drizzle and lowering clouds. As in past events,
temperatures will have a difficult time rising in a low overcast
with drizzle and northwest winds. Expect temperatures to only have a
limited diurnal range especially in the Mid and Lower Valley where
conditions are likely to be worse. Max Temps have been lowered below
guidance with low to mid 40s.

LONG TERM (Friday through Wednesday): Cold airmasses in mid
January are notoriously stubborn when there is either minimal
forcing or a quick return of southerly (or drying westerly) flow and
such will be the case at the start of the long term period. The good
news for those looking forward to spring-like weather is both types
of flow...low level southerly and mid to upper level westerly
flow...will arrive en masse during the weekend. Another front will
follow to begin next week but not to fear; this one comes dry with a
return back to seasonable temperatures (generally lows in the 40s to
lower 50s and highs in the upper 60s and 70s) Monday through
Tuesday. Some uncertainty remains next Wednesday with differences in
handling of energy moving toward west Texas...and given uncertainty
chose to keep the Day 7 forecast as is for now.

On Friday, short wave that produces light precipitation and perhaps
a bit more moderate precipitation Thursday night swings through east
Texas and Louisiana quickly early Friday.  This will quickly end any
remaining showers/rain across the mid/Upper Valley and ranchlands
pre-dawn but could see pesky precipitation hang on toward noon near
the coast, and longer over the Gulf. With light northwest flow
favored to hang tough behind the departing coastal trough -
especially given stubborn clouds as well as the chilly nearshore
waters - have leaned into the chillier NAM-12 solution with highs
likely to struggle into the low or mid 50s area-wide and perhaps not
reaching 50 between San Ygnacio and Hebbronville.

Drier air spreads across the region Friday night. At the same time,
winds lighten to near calm while warm advection begins in earnest
above the surface. Low ceilings and perhaps dense fog are in play,
but have not included due to uncertainty on how the cloud cover
plays out (as well as whether temperatures sneak up or slowly fall)
but will add to the hazardous weather outlook to cover the threat
from late night into mid morning Saturday.

Transition days can be tricky to forecast and Saturday is just such
a day. Despite increasing southerly flow two areas for cooler
temperatures will need to be watched:  Coastal/beaches, which is
most likely to achieve, due to the low 50s nearshore water
temperatures that, despite sunshine, may hold highs in the low 60s
on the Island and perhaps to Port Isabel. The second area is the
Upper Valley to the Rio Grande Plains. We typically look to the NAM-
12 for hints at cool air "damming" and that situation is not showing
its hand at this point.  Given a cooler start, do have highs lower
than across the RGV - but pushed into the low to mid 70s with
expectation of more sunshine and southeast flow to pick up just
enough by mid afternoon.

Springlike weather arrives quickly Saturday night, with temperatures
staying mild in the 60s in most areas overnight as notable but not
too strong south/southeast flow picks up ahead of approaching full-
atmospheric trough. Sunday looks warm and windy to end any leftover
cabin fever and get outside to enjoy a mix of sunshine and clouds
with temperatures soaring into the lower 80s in most areas. Current
land wind forecast in the "wind machine" area is capped around 20
knots (20 to 25 mph) but there are indications that 25 to 30+ mph
winds from late morning through late afternoon could
occur...something we`ll update over the next few days.

The next front arrives Sunday overnight; GFS is a hair faster this
run than ECMWF but decided to leave midnight timing for now until we
get a bit closer.  With the midnight timing, Monday lows will likely
still be relatively mild but closer to seasonable levels across the
Rio Grande Plains ranchlands. Monday looks to be a near perfect day
with plenty of sunshine, low humidity, but still pleasant
temperatures with much lighter winds than on Sunday.  Clouds could
increase Tuesday and Wednesday with veering low level flow to the
east then southeast; temperatures look to settle into seasonal
values (low 70s by day, 40s to low 50s by morning) though Wednesday
could prove slightly lower if clouds and rain develop...not forecast

MARINE: mall Craft Advisories for the Gulf
waters will be extending through Midnight with residual sea at
around 7 feet. Coastal trough develops near the coastal waters
Thursday strengthening and drifting closer to the coast Thursday
night and Friday. This may allow for stronger easterly winds in the
far offshore waters but should drive a moderate sea into the near
shore waters. Winds may stay below advisory levels however sea may
exceed SCA levels late Thursday into Thursday night. Drizzle and
light rain redevelop tonight and increase in coverage and intensity

Friday through Monday - With the chilly near shore waters (50s)
contrasting the still warm offshore waters (lower 70s), expect
contrasts in wind/seas to develop beginning Friday night and
continuing through Sunday. If past is precedent, southerly speeds
could be some 10 to 15 knots less over the 0-20 nm leg compared
with the offshore leg, and nudged differences in this direction
for most of the period from Friday night to Saturday night. NWPS
sea height forecasts felt this a bit with moderate to perhaps even
slight seas nearshore, before enough stronger southerly flow and
propagation of seas from off to nearshore should push levels close
to advisory levels (7 feet) by Sunday; winds may also be
sufficient for advisory over Laguna Madre too.


TX...Freeze Warning from 10 PM this evening to 9 AM CST Thursday for

     Wind Chill Advisory until 10 AM CST Thursday for TXZ254>257.

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



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