Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 202359 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
559 PM CST Sat Jan 20 2018

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


.AVIATION...The two primary issues over the next 24 hours are wind
and ceilings. As mentioned in discussion below, low level jet
kicks up a few notches overnight, with forecast speeds possibly
reaching 40 knots in the 2 to 3 thousand foot level and continuing
above 30 knots well after sunrise Sunday when mechanical mixing
will be able to push some of these winds down - in gusts, at a
minimum. For now, did not include a WS/18040 parameter due to
some uncertainty for overnight mechanical mixing...but should
surface winds end up close to or just below 10 knots toward
daybreak will consider adding to the 06Z package later this

By mid morning Sunday - assuming mechanical mixing has not begun
in earnest sooner - it will get underway quickly. Sustained winds
near 20 knots are likely as soon as 15Z and well over 20 knots by
noon, perhaps as high as 25 knots at Harlingen/Valley which
typically blows strongest in "Valley Wind Machine" events. Added
30+ knot gusts to both Brownsville/SPI and Harlingen/Valley; while
winds may diminish slightly by sunset, no need to add a one-hour
group at this point.

As for ceiling/visibility...model soundings are pretty consistent
on a period of mid-MVFR ceilings developing later tonight and
persisting through noon Sunday inland and probably longer at
Brownsville, lifting just above the VFR line from Harlingen to
McAllen. While there will likely be periods of Scattered
conditions, left Broken as prevailing for now.


.PREV DISCUSSION... /issued 242 PM CST Sat Jan 20 2018/

SHORT TERM (Now through Sunday night): Low-level jet continues
to mix to the surface today, with breezes jumping along the coast.
These breezes are expected to remain elevated after sunset,
running about 10 knots. This will help keep the lower atmosphere
mixed, inhibiting most fog formation. Some light fog may occur
late tonight, especially in the far western valley where winds
would be lighter, but not expecting dense fog anywhere. LLVL jet
strengthens tomorrow as the next front approaches. This will mix
to the sfc late morning, with winds of 20 to 25 knots possible in
the lower valley and around 20 knots in the upper valley during
the afternoon. A Wind Advisory is being considered for the lower
valley is wind forecast increase any further. The front then
arrives after midnight tomorrow night. The front is a weaker
front, with minimal change in temperature, so northerly winds in
its wake will be light, around 10 knots. There may be enough
forcing along the front and upper level support to spark a few
showers, mainly along the coast with the passage, but they will be
few and far between, more likely focused offshore.

LONG TERM (Monday through Saturday): A weak cold front will
arrive Monday before dawn. Northwest winds will strengthen to
moderate and, though a few coastal showers will linger through
the morning, the weather should clear up by Monday afternoon, with
high temperatures reaching the lower 70s.

Significant weather systems for the RGV and deep south Texas will
be sparse during the long term, leaving temperatures to stabilize
near normal for much of the period. After the front on Monday, a
cut off low will hang back over northwest Mexico Monday night
through Thursday before ejecting northeast. The high pressure
behind the Monday front will shift east, but some reinforcement
from high pressure over the intermountain West will be possible on
Wednesday. The result will be a persistent north to northeast
wind. As well, deeper moisture, partially as a result of
developing surface low pressure over the southwest Gulf will surge
north to increase rain chances Tuesday night through Thursday
night. Once the upstream cut off low ejects across Texas on
Friday, a larger scale trough will swing south from the pacific
Northwest into the central Plains. The base of the trough axis
will lift across northeast Texas , with a trailing surface front
arriving locally Saturday night. The EMWF and GFS models are in
reasonable agreement with this timing.

Now through Sunday night: Cooler waters are helping keep the
southerly winds from jumping, as opposed to the warmer adjacent
land areas. Southerly winds are around 10 knots for the Laguna and
nearshore waters, and will remain steady overnight. Tomorrow,
stronger winds are forecast along the coast, which may be able to
reach near 20 knots even with stable near surface layer. Small
craft will need to exercise caution tomorrow, and advisories may
be needed for the Laguna and waters close to shore if winds can
stably reach 20 knots. The next cold front is forecast to reach
the northwest Gulf after midnight Sunday night. The northerly push
is not expected to be a strong one, with winds only reaching 10
to 15 knots from the north in its wake. The front will bring a
chance for showers during passage, but they will likely be few and
far between.

Monday through Thursday night: Northwest winds will be in place
Monday, strengthening during the day in the wake of a cold front.
Small craft advisory conditions will be possible on the Gulf
waters with winds near 20 knots and seas pushing toward seven ft.
Moderate winds will slowly veer to northeast and east Monday night
through Tuesday night with improving conditions as high pressure
spreads over the area. Reinforcing high pressure will help tighten
the gradient Wednesday through Thursday, with exercise caution to
low end small craft advisory conditions on the Gulf during this
period, as well as increased marine convection as deeper moisture
moves up from the south. A wind shift to southeast will be
possible Thursday night into Friday in response to pressure falls
in the high Plains.




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