Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 121749 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1149 AM CST Tue Dec 12 2017

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

&&

.AVIATION...Weak cold front drifts through the region this
afternoon, with continuing northerly flow expected. North winds
will briefly have some gusts in the lower teens, but for the most
part will remain around 10kts. Skies remain mostly clear, with
only passing CI overhead. tranquil conditions continue overnight,
with light and variable winds and mostly clear skies. Winds will
remain light and variable through the day Wednesday.

&&

.PREV DISCUSSION... /issued 527 AM CST Tue Dec 12 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...A cold front will pass through Deep South Texas and
the Rio Grande Valley today, producing increasing and veering
winds initially before they diminish and back later on. Cloud
cover will be confined to high cirrus out of Mexico. VFR is
expected through the next 24 hours.

PREV DISCUSSION... /issued 421 AM CST Tue Dec 12 2017/

SHORT TERM (Now through Wednesday):  The latest surface analysis
indicates a cold front over central Texas from Carthage to San
Angelo to Kermit. This front is in association with deep 500 mb
troughiness moving through the eastern half of the United States. A
gradual north to south progression of this front through the BRO
CWFA is expected today, but the atmosphere is likely to remain too
dry to support any precipitation with the frontal passage. Surface
high pressure building in the wake of the front will work in tandem
with weak 500 mb ridging to maintain the dry weather tonight and
Wednesday. Near normal daytime highs but below normal overnight
lows are also anticipated.

LONG TERM (Wednesday Night through Monday): There is a fair
amount of uncertainty in the details of the long-term portion of
the forecast. However, this uncertainty doesn`t translate into
any really impactful weather, as airmasses of Pacific origin will
dominate much of the CONUS, per WPC discussions. Tried not to
deviate too much from the going forecast.

Significant differences in model guidance emerge as early as
Thursday, when a cold front is still progged to move southward
through the CWA during the latter part of the day and into the
evening. 00Z GFS indicates an "oozer"-type boundary, with modest
moisture pooling ahead of it, with PW in the vicinity of BRO
rising to ~1.25" (vs. climo values right around 1"). As far as QPF
signal, GFS picking up on only light QPF offshore and along the
coast, whereas the ECMWF shows a few tenths of an inch falling in
a band ahead of the front, and the NAM is totally dry. Leaning
away from the ECM on the FROPA (and for much of the forecast), as
it solution doesn`t seem very realistic. Where adjustments were
made, favored National Blend overall. (Superblend guidance also
seems to be weighting the ECM relatively low locally; another
strike against it).

Probably the biggest differences between the GFS and ECM occur in
the Friday-Saturday period. GFS shows a slight cooldown (though
with temps still near normal) with rapid column drying on Friday.
Latest ECM guidance holds temps in the 50s F all day! Again, not
buying this with Pacific airmasses dominating. Blended guidance
still keeps temps in the 60s area-wide for the coolest day of the
forecast period. On Saturday, ECM solution has old Baja low being
picked up by a larger-scale mid-latitude trough; GFS has the Baja
low briefly phasing with the larger trough but then hanging back
weak troughiness in the same region...with repercussions for the
rest of the forecast period. At the surface, breezy SSE winds
develop as the boundary lifts back northward.

Into Sunday, both models show a shortwave trough aloft swinging
through New Mexico, which eventually drives another front into
Deep South Texas late Sunday night/early Monday morning. However,
this time around the GFS is wetter, with the aforementioned Baja
trough providing more moist SW aloft. Due to the uncertainty, have
capped precip wording at slight chance, favoring the coast and
offshore during this period. Unsurprisingly, the model differences
continue Tuesday, so have continued with near-normal temps and
slight chance PoP`s at best.

MARINE (Now through Wednesday):  Buoy 42019 reported west winds
around 16 knots gusting to around 18 knots with seas slightly under
1.5 feet with a period of 3 seconds at 02 CST/08 UTC. A cold front
currently draped across central Texas will pass through the Lower
Texas coastal waters from north to south today. Moderate winds and
seas will develop today and tonight due to the passage of the front,
with lighter winds and lower seas anticipated tomorrow as the center
of surface high pressure settles over the region in the wake of the
front. Even with the frontal passage, Small Craft Exercise Caution
and Small Craft Advisory are not likely to be needed.

Wednesday Night through Sunday: Significant uncertainty exists in
the wind (and hence, seas) forecast through much of the period. A
cold front will move southward through the coastal waters later on
Thursday with increasing northerly flow in its wake, possibly
requiring Small Craft Advisories (SCA). Winds quickly veer back
to SE and pick up on Saturday, again building seas to near SCA
levels by Saturday night and continuing into Sunday.

&&

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

$$

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