Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 170401 AAB

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1001 PM CST Fri Feb 16 2018

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


.AVIATION...Satellite imagery and web cameras along the lower
Texas coast indicate that dense sea fog is developing across the
near shore Gulf waters. Have issued a Dense Fog Advisory for the
Laguna and Gulf waters from 0 to 20 nautical miles out until 9 AM
Saturday. Also, have updated the grids to remove pops, adjust
cloud cover, and made minor adjustments to the temp/dewpoint grids
to reflect current trends. No other changes needed.


.PREV DISCUSSION... /issued 533 PM CST Fri Feb 16 2018/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Mostly clear skies and moderate southeast winds
prevail across deep south Texas early this evening. Winds will
continue to weaken through the evening and overnight. Fog/low
stratus is expected to develop overnight which should take all
three terminals down to IFR conditions, if not LIFR at least
briefly. Some improvement can be expected by late Saturday morning
as southeast to south winds increase. Full VFR conditions should
develop by early Saturday afternoon as moderate and gusty winds
allows drier air aloft to mix down to the surface.

PREV DISCUSSION... /issued 336 PM CST Fri Feb 16 2018/

SHORT TERM (Tonight through Saturday Night): After morning
stratus/fog across the region, much of the area has cleared this
afternoon. Winds have also increased across the region as the
boundary layer is well mixed allowing for stronger winds right off
the deck to channel toward the surface. Temperatures this
afternoon were very similar to yesterday with generally 80F plus
being reported as of the 2pm hour.

A cold frontal boundary is currently located along a line from
central TX to ArkLaTex to the AlaTennGa region. The front is
losing motivation to push further into Deep South Texas as the
front becomes parallel to southwesterly flow in the northwest
quadrant of (a quite impressive) 591dam upper level ridge. Water
vapor imagery really tells the story as dry air aloft continues
across much of the Gulf Coast...southward to the Caribbean.

With continued increase in low level moisture with
southerly/southeasterly flow under a stout subsidence inversion
tonight, expect more low stratus and fog development. In fact,
pockets of dense fog are likely, especially along and east of
I69C. Of course as the front nears, winds in the northern
Ranchlands will lighten a bit. This along with moisture pooling
just ahead of the front will help initiate fog development as
well, though a little less certain.

As the front remains stalled, can`t rule out a scattered shower or
two, especially across the northern Ranchlands tonight and into
Saturday. Expect more cloud cover to reside across the region
again as the front remains fairly close. Given the increased
clouds expect temperatures to be a degree or two cooler than
today, however, with the warm air advection in place the short
term looks to remain roughly 7-10 degree above average.

LONG TERM (Sunday through Friday): A 500 mb troffing will dig
southwards over the Rockies Sun and Mon which will in turn build
strong ridging over the southeastern States and the Gulf of Mex
with the RGV sandwiched in between the two. As the trough axis
digs down south, a pretty complex surface low pressure system
will develop over the central Plains states which will interact
with the surface ridging over the East Coast to strengthen the
PGF over the south central Plains into early next week resulting
in stronger S-SE low level flow. This will maintain pretty strong
WAA over the RGV throughout the first half of next week keeping
the temps well above climo for late Feb. The western 500 mb trough
axis will then start to shift east and shear out becoming more
elongated from SW to NE over the lower 48 states with the ridge
axis remaining in place over the southeast. This troffing pushes a
cold front down into central and southern TX late Wed with the old
frontal boundary becoming stationary over the region or drifting
back northwards allowing for WAA to return to the region.

The GFS and ECMWF are in good agreement in the run up to the
potential fropa around midweek. So will go close to a consensus of
the two models for temps and pops through around Wed. The two
models then start diverging in the solution for Thurs and Fri with
the ECMWF backing off quite a bit on a frontal passage while the
GFS still maintains a decent passage with CAA continuing into the
Thurs and Fri. The ECMWF also shows a much drier longer range
forecast versus the GFS since the ECMWF is backing off on the
fropa. Will opt for a blend of the two model sets also from Wed
onwards through the end of the week, but do not have great
confidence due to the continued divergence in solutions from both

The longer range guidance also shows some decent moisture
advection and increasing CAPE values ahead of this midweek frontal
approach. So will maintain some mention of thunderstorms in the
forecast from Tues Night onwards.

MARINE (Tonight through Saturday Night): Seas will remain rather
benign through much of the short term period as light to moderate
southeasterly winds are expected. A cold front will move southward
toward the region, however, the front will eventually slow and
stall just north of our forecast area.

The other real story will be marine fog. Indeed, marine fog
occurred overnight and into the morning hours this morning across
the Laguna Madre and our southern 0-20nm zones. This afternoon,
much of this fog has mixed out with the exception of the nearshore
waters 0-20nm just north of Port Mansfield. Left the Dense Fog
Advisory for this area through 00z, however, as additional fog
development is expected this evening and especially overnight,
this advisory could very well be extended in time and area.

Sunday through Wednesday Night: The PGF will start to strengthen
on Sun and Mon as the strong surface low pressure system develops
over the central Plains states. This will likely result in
a stronger S-SE low level marine flow. These stronger southerly
winds will tend to be surpressed somewhat as both the current Bay
and Gulf water temps remain pretty cool in the upper 50s. This
will tend to limit the downward mixing of the stronger southerly
flow. However, marginal SCEC/SCA conditions could be possible in
the longer range period especially for the Gulf Waters.


GM...Dense Fog Advisory until 9 AM CST Saturday for GMZ130-132-135-



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