Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 180755 AAC
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
155 AM CST Sun Feb 18 2018

.DISCUSSION...Updated for latest marine discussion below.

&&

.MARINE...Surface observations and vessel reports indicate marine
fog continues to deteriorate across the nearshore Gulf waters and
Laguna Madre. Have added areas of dense fog for these zones and
posted a Dense Fog Advisory through 10 AM CST.

&&

.PREV DISCUSSION... /issued 1102 PM CST Sat Feb 17 2018/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Light southeast winds with a few passing low to
mid level clouds prevail across deep south Texas late this evening.
Low stratus and patchy fog will develop overnight. IFR conditions
are expected at all Valley airports late tonight due to patchy fog
and low stratus. Conditions will improve by mid morning on Sunday as
southeast to south winds increase. Gusts of 25 to 30 knots will be
possible in the late morning and afternoon. Winds will gradually
lower to around 15 knots after sunset Sunday.

PREV DISCUSSION... /issued 545 PM CST Sat Feb 17 2018/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Moderate southeast winds and mostly clear skies
prevail across deep south Texas early this evening. Winds will
gradually diminish to between 5 to 10 knots later this evening and
overnight. Low stratus and patchy fog will develop later tonight.
IFR conditions are expected at all Valley airports overnight due
to patchy fog and low stratus. Conditions will improve by mid
morning on Sunday as southeast to south winds increase. Gusts of
25 to 30 knots will be possible in the late morning and afternoon.

PREV DISCUSSION... /issued 321 PM CST Sat Feb 17 2018/

SHORT TERM (Tonight through Sunday Night): After another mostly
cloudy start to the day, much of the cloud cover has scattered
leaving only low hanging low topped CU across Deep South Texas.
Similar to yesterday, strong southerly winds have been realized with
some gusts as high as 30mph or so in the heat of the afternoon.
Winds will become less gusty after sunset, however, surface pressure
gradient increasing between low pressure over N Mexico and high
pressure over the Florida Peninsula will keep winds around 10-15mph
tonight.

A surface cold front will stall just to our north this evening.
This may provide some increased cloudiness for the northern
Ranchlands and perhaps a stray shower, however, confidence in any
measurable precipitation is very low and gets lower as you head
southward through the forecast area.

With low level troughing from the weak low across northern Mexico
and the upstream baroclinic zone between warmer land and cooler
water, this should be enough once again for overnight/morning
stratus development. Stat guidance is pinging fog once again for a
coastal areas, however, like this morning think the models are
overdoing this just a bit, especially if winds remain 1) somewhat
elevated and 2) parallel to the coast.

Sunday looks mostly dry. NAM is pinging some very light QPF,
however, it is an outlier with POPs. Kept a silent 10 going just in
case. Outside of that the primary story will be windy conditions.
The forecast area will approach wind advisory conditions during the
afternoon when the wind potential is maximized. Beyond pressure
gradient, strong afternoon heating will help mix the 35-40kt LLJ
right off the deck down toward the surface. Went ahead and bumped
winds and gusts overnight as confidence grows. This of course is a
good bit above guidance and respective blends, which have have
consistently been undercutting winds.

Sunday night the frontal boundary begins to lift northward slowly
away from the region. Winds will remain brisk given the continued
strong pressure gradient at the surface, but significantly less
gusty overnight as mixing becomes limited. Temperatures through the
short term will remain well above seasonal averages for February.

LONG TERM (Monday through Saturday): 500 mb ridging will prevail
over the Gulf Coast region and the southeastern States Mon and Tues
which will maintain temps well above climo. Meanwhile to the
northwest a deep 500 mb trough axis will dig down south over the
Rockies towards the South Central Plains and Desert SW. As this 500
mb trough deepens it will in turn spin up pretty substantial
surface low pressure system over the central Midwest which will
interact with the broad surface ridge over the eastern states and
the western Atlantic to crank up the southerly low level flow over
the RGV on Mon and Tues. Pretty decent moisture advection occurs
ahead of this developing surface system in the central states, but
most of the deeper layer moisture remains pooled to the north of
the RGV. So anticipate only some slgt chc pops mainly across the
northern tier of counties for either day.

The forecast then becomes trickier on Wed. Around midweek the 500
mb trough over the Rockies becomes more elongated from SW to NE
as a series of short waves digs through the long wave trough axis.
This tends to squash down the 500 mb ridge over the southeast and
shift it a little more to the east. This sets up the RGV for a
possible fropa. The problem is that the GFS and ECMWF diverge
greatly on whether or not this will occur. The ECMWF shows a much
weaker fropa and keeps the area well in the 80s to near 90s for
highs for Wed and 70s for highs Thur. Meanwhile the GFS has been
persistent in showing a stronger fropa for the region with temps
dropping into the 50s and 60s for highs on Thurs.

Both models then return back to better agreement for temps for Fri
and Sat as the low level WAA kicks in pretty quickly as the post
frontal surface ridge shifts quickly east.

The GFS and ECMWF are in better agreement for pops with the
approaching midweek cold front with both models cranking up some
pretty decent numbers for the Wed and Thurs period. Both models
increase the deeper layer moisture advection ahead of the fropa Wed
and hold these moisture levels in place into Thurs which may
interact with some coastal surface troffing to maintain the risk of
conv over the RGV after the front.

Longer range CAPE values remain pretty elevated for both Wed and
Thur of next week. So will maintain the mention of some thunder from
Wed on through Fri.

Will go with a general blend of the GFS and ECMWF for most of the
longer range temps and pops due to the pretty good agreement between
the two models. Overall confidence in the longer range forecast
pretty is relatively high. However the exception is the temps for
Thurs. Confidence here is pretty low due to the wide range of model
solutions. Will be leaning towards the warmer temps shown in the
ECMWF as believe the GFS may be overdoing the CAA after the fropa
Wed.

MARINE (Tonight through Sunday Night): Patchy fog will continue
right along the immediate coastline this evening and then become
more dense overnight across the nearshore Gulf waters and the Laguna
Madre. Otherwise, generally benign seas expected through the
evening. Increasing pressure gradient will increase low level flow
Sunday, however, given the cooler water (marine layer), think that
much of the stronger winds will have a tough time making it to the
surface. Still, enough wind (generally around 15kts or so) will be
present, which will increase wind waves just a bit with wave heights
approaching 3-5 feet.

Monday through Thursday Night: The PGF may strengthen the
southerly low level flow enough on Mon and Tues to possibly push
the marine locations close to SCA levels ahead of the approaching
cold front. The PGF will then weaken somewhat on Wed as the west
to east oriented cold front pushes into south central TX possibly
lowering down the Bay and Gulf swell activity. After the fropa,
the gradient flow will not be that strong with a general moderate
northerly surface flow allowing the swells to gradually diminish
somewhat heading into Thurs and Thurs Night.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Dense Fog Advisory until 10 AM CST this morning for GMZ130-132-
     135-150-155.

&&

$$

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

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