Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 152348 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
548 PM CST Thu Feb 15 2018

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

&&

.AVIATION...Skies have cleared with the loss of daytime heating.
Evening balloon sounding showed peak winds near 30 knots from the
south at 2000 feet. Winds will decouple soon, with surface winds
dropping to around 5 knots. Still expecting some sea fog to build
back into BRO and HRL tonight, but the lack of sea fog along the
coast currently will delay arrival of the fog until later tonight.
Southerly flow will be lighter tomorrow as a weak front stalls
just to the north. Southeast winds will only rise to around 10
knots during the day.

&&

.PREV DISCUSSION... /issued 320 PM CST Thu Feb 15 2018/

SHORT TERM (Tonight through Friday Night): The main forecast
challenge remains the extent and thickness of fog over the next
two nights. Otherwise, short-term forecast remains on-track with
regard to temps/PoP`s, with minimal changes made. Subsidence
beneath a mid- level high centered right over the Gulf of Mexico
continues the rain- free conditions, until perhaps later on Friday
night when a frontal boundary stalls somewhere near the northern
tier of the CWA. Low- level S/SE flow will keep both min and max
temps around 10 degrees above climo again on Friday. High
temperatures expected to be just a couple of degrees off the
readings being observed this afternoon, owing to a dip in H85
temps and a somewhat weakened pressure gradient in advance of the
front lessening warm advection (as well as keeping the breezes
down a bit from today). Extended the slight chance rain showers
southward a little after 06Z on Saturday, with the 12Z NAM
bringing the cold front right into the LRD vicinity before it
pulls up stationary.

Fog forecast tricky again tonight (and Friday night).  With plenty
of near-surface moisture beneath a very dry layer just above, but
winds remaining at 7-10mph, looking at more of a low-stratus
condition for the mid-Lower Valley.  Some mist will also be about,
but expecting visibilities to generally run 2-4SM, so will word
patchy fog over roughly the eastern two-thirds of land areas later
tonight. Mist/fog should again mix out mid-morning on Friday, then
reappear Friday evening. Thinking it has a chance to become more
dense across the northern ranchlands, as wind speeds deaden ahead of
the front, with more patchy coverage again the Lower Valley and near
the coast by Saturday morning.

LONG TERM (Saturday through Thursday): A broad 500 mb trough axis
will move SE across the central Plains States on Sat which will
push a mild cold front into southern TX this weekend. The front does
not appear to have enough forward momentum to clear through the RGV
leaving the area under an E to SE low level flow with the best
moisture fields located more centered over central and southeastern
TX. However the front will be close enough this weekend to keep
portions of Deep South TX under the threat of some isold/sct conv.
After this 500 mb trough clears the region later this weekend
another stronger trough will then dig south across the West Coast
and the Rockies through early next week. As this trough digs further
south over the desert SW early next week, 500 mb ridging will in
turn build over the southeastern States, the Gulf Coast and the
western Atlantic. The trough digging over the western States will
also spin up an elongated surface frontal system over the central
Plains which will start boosting up the southern low level flow over
the region increasing the inland winds. This 500 mb trough axis then
elongates out from southwest to northeast which tends to erode the
ridge axis over the east a bit. This will be enough to push a cold
front through the RGV next Wed. In the wake of the fropa, some
coastal surface troffing may linger long enough and interact with
residual deeper layer moisture to maintain the conv chcs through
Thurs. Model guidance maintains some pretty decent CAPE values ahead
of this Wed fropa. So will maintain the mention of thunder for both
next Wed and Thur.

The persistent low level WAA throughout the longer range period will
maintain the overall temps well above climo through next Tues. The
CAA after the fropa on Wed will hold done temps close to or a little
below climo for Wed and Thurs.

The GFS and ECMWF models show good agreement in the overall long
term pattern through Wed then begin to diverge somewhat for Thurs.
The ECMWF is coming in much colder after the front on Thurs versus
the GFS with pretty good agreement showing up for pops through Day
7. Will go with a general blend of the two model sets through the
longer term period through Wed. And will then lean towards the
warmer GFS temps for Thurs as the ECMWF has flip-flopped a bit for
high temps over the last couple of model runs.

Overall confidence for the longer range forecast wording is above
average despite the differences showing up late next week.

MARINE (Now through Friday night): Wind observations around the
Laguna Madre generally holding in the higher-end Small Craft
Exercise Caution range (17-20 knots, with some higher gusts), so
held off on any short-fuse Small Craft Advisories.  Appears that the
marine layer is helping to hold winds down just a bit, with 20G30KT
reported recently just inland at KPIL.  Light-moderate SSE winds
will be the rule over the waters from this evening onward, with the
pressure gradient gradually decreasing ahead of cold front (though
the front is not expected to actually move through the waters). Seas
to mostly run 3-4 ft., with perhaps some 5-footers over the outer
waters this evening.

Issued a Marine Weather Statement (MWS) for fog on the 0-20nm Gulf
waters just after 1pm, valid until 9pm.  Satellite imagery indicates
a persistent fog bank out extending out about 15nm.  Visibility is
difficult to ascertain, but from Gulf-facing webcams on SPI, does
not appear to be particularly dense (at least very near the shore),
so am estimating 1-3nm in the MWS.  Laguna Madre appears quite clear
at this time, so left it out.  There is a good possibility of
another Dense Fog Advisory being issued for the nearshore Gulf
waters and Laguna this evening, running into Friday morning, per
persistence and trends from model visibility progs.  Moist SSE flow
over the cooler waters in this area certainly supports this.  Areas
of fog may persist on the Gulf waters through the day on Friday, and
again progress into the Laguna/coastal areas later on Friday night
as winds slacken and possibly back a bit more onshore ahead of the
approaching front.

Saturday through Tuesday Night: The PGF will be weak for Sat as
the surface ridge moves further eastward. The PGF will then start
to strengthen on Sun and Mon as a strong surface low pressure
system develops over the central Plains states resulting in
stronger S-SE low level marine flow. These stronger southerly
winds will likely be held down as both the current Bay and Gulf
water temps remain pretty cold in the mid 50s. This will tend to
limit the downward mixing of the stronger southerly flow. So
conditions may remain marginal for SCA conditions in the longer
range period.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...None.
&&

$$

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