Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 110529 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1129 PM CST Sun Dec 10 2017

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

.AVIATION...Little change from the prior forecast, basically
extended another six hours out. The rare parked-in-place surface
high holds through today, perhaps sliding a bit east after sunset
before making way for a reinforcing ridge come early Tuesday beyond
this forecast. Little more than patchy cirrus continuing across
south Texas from the slow moving cutoff 500 mb low that slides into
central Baja California through the day, with recently arrived GFS
suggesting the cirrus layer rising to near 40K feet Monday evening.
Winds will be light to calm, turning westerly toward daybreak (land
breeze) then shifting east at Harlingen/Brownsville Monday afternoon
- but still no higher than 5-7 knots, before fading back to
light/calm soon after sunset.

.PREV DISCUSSION... /issued 521 PM CST Sun Dec 10 2017/

DISCUSSION...Updated for latest /00z/ aviation discussion below.

AVIATION...About the easiest forecast one has to make here in the
Rio Grande Valley. The rare parked-in-place surface high remains
so right through Monday, with nothing more than cirrus continuing
to spread across south Texas from the slow moving cutoff 500 mb
low that slides into central Baja California Monday afternoon.
Winds will be light to calm overnight, edging from the west after
midnight (land breeze) then shifting east at Harlingen/Brownsville
Monday afternoon but still no higher than 5-7 knots. With superb
visibility for another day, it`s a great day to fly in the Valley.

PREV DISCUSSION... /issued 221 PM CST Sun Dec 10 2017/

.Warmer Start To The Workweek Expected...

SHORT TERM (Tonight through Monday Night): Pretty tranquil
weather to prevail across the RGV as a 500 mb Rex blocking
pattern will persists over the western States throughout tomorrow
night. Meanwhile deep 500 mb troffing will dominate the eastern
states throughout the short term. This will maintain a very dry
atms flow over the region keeping nil pops in place. High level
cld cover ejecting out from the 500 mb closed Baja Cali low to our
west will gradually thicken northern Mex and southern TX possibly
diminishing the sunshine a bit. However expect Mostly Clear skies
to prevail for the most part.

The strong sunshine should moderate the cool airmass in place over
the region steadily over the next couple of days resulting in
warming temps. However...fairly low dewpoints and surface winds
will combine with the generally clear skies to allow for good
radiational conditions tonight and tomorrow night. At this time,
not expecting any widespread freezing/frost conditions as the
anticipated dewpoints appear to remain well above the freezing
mark throughout tomorrow night.

Short Term max/min temp guidance is in decent agreement through
tomorrow night. Will go a little more on the warm side of guidance
as both the NAM and GFS appear to be too slow in the warmup through
Mon Night.

Finally will include some fog across the northern counties as MAV
and MET guidance hint at a small window of opportunity for light
fog for a few hours early Tues morning.

LONG TERM (Tuesday through Sunday):Overall, a pretty benign
pattern is setting up for a majority of the long term period.
The area will be between a strong longwave trough across the
eastern U.S. and a closed off low (part of the Rex Block) out
west. This should keep things rather zonal/if not a slight ridge
over the region through mid to late week. Models still bring a
very modest frontal boundary through on Tuesday. This should
increase the low level flow, however, no precipitation.

Another, stronger frontal boundary will slide through either late
Thursday or into the day Friday. This front may have at least a
few showers, however, kept lower POPs (especially across the
east) for now at there is still some differences in the globals
with respect to timing. Also, there is a bit of a difference in
GFS and ECMWF regarding what occurs on the north central Gulf
Coast, near the mouth of the MS River. ECMWF develops a surface
low along the front as it moves over the natural baroclinic zone
along the coastline. The GFS is less bullish. The outcome will
likely determine whether much of Friday will be cloudy (cooler) or
not as cloudy (warmer). Regardless, high pressure begins to move
east of our area by Saturday increasing southerly surface flow
once again before another weak frontal boundary due into the
region by late Sunday afternoon/evening. Frye/55

MARINE (Tonight through Monday Night): It can`t get more quiet
than this for the Lower TX marine areas as a very weak MSL field
remaining in place over the area yields up very low winds and
seas. No marine issues expected.

(Tuesday through Sunday Night): A very weak cold frontal boundary
is expected to push through early in the long term period
(Tuesday). This will cause winds to shift northerly and increase
to around 15kts or so. Seas may increase slightly, however, should
stay below SCEC/SCA thresholds. After some slight improvement
midweek, another slightly stronger cold front moves through on
Friday increasing winds to 15 to 20kts across our marine zones and
increasing wave heights to 6 feet at times. Winds and seas looks
to remain elevated through the weekend given that southerly flow
increases as a low deepens and builds southward along the eastern
spine of the Rockies. 55/Frye



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