Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 131129 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
529 AM CST Wed Dec 13 2017

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

.AVIATION...Generally backing winds will occur during the TAF
period as surface high pressure moves off towards the east. As the
mid-level flow overhead becomes more prominent out of the west-
southwest, high cloudiness will increase.

.PREV DISCUSSION... /issued 351 AM CST Wed Dec 13 2017/

SHORT TERM (Now through Thursday):  Weak 500 mb ridging over the BRO
CWFA will quickly fade during the period and will be replaced by a
west-southwest to east-northeast mid-level flow overhead as filling
500 mb low pressure drops into the Baja Peninsula. This pattern will
result in rapidly increasing cloudiness, with no rain to be found
until Thursday afternoon as a cold front eases into Deep South Texas
and the Rio Grande Valley. Despite the increased cloud cover,
temperatures will moderate to near to above normal levels as an
onshore flow redevelops and intensifies ahead of the cold front.

LONG TERM (Thursday Night through Tuesday): 00Z runs of GFS and
ECMWF models continue their trend of converging solutions, as
first noted in yesterday afternoon`s AFD. In fact, they now agree
rather well, at least through the weekend.

The most active weather is expected to occur during the first 36
hours or so of the long-term period. Long-lived cutoff low,
apparent in WV imagery spinning over the northern Sea of Cortez
currently, will be "re-energized" as another shortwave drops into
it on Thursday night. Mid-level subtropical moisture tap ahead of
this feature will be maintained. Meanwhile, at the surface, a cold
front will progressing southward through Deep South Texas Thursday
evening, setting the stage for isentropic lifting. The region will
also be in the right-entrance region of a jet streak, providing
additional lift. So Friday has the makings of a rather chilly,
rainy day. (No, it won`t be a repeat of our snow event from the
previous Friday.) Dropped temps and upped PoP`s a bit more, in
line with latest guidance. With GFS/ECM in much better agreement,
didn`t see much reason to fight Superblend. One area where there
is still some disagreement is where the axis of heaviest precip
will fall. Overall event QPF totals are generally in the 0.5-1.5"
range, but GFS (and NAM) favor the southern CWA, while the ECM
puts more rain across the northern tier counties. Temps will
slowly fall through the day on Friday, due to cloud cover, rain,
and cold-air advection, with afternoon readings in the upper 40s
to near 50F.

Precip chances/QPF taper off Friday night into Saturday as the
front sags well south of the Rio Grande. The old upper low
finally ejects quickly to the NE, reaching central TX Saturday
night. Raised PoP`s over the NW counties Saturday evening as some
of the upper energy may clip this area. Looked at forecast
soundings for any thunder potential, but atmosphere appears to
remain too stable. Saturday`s temps will still struggle to reach
60F, or about 10 degrees below normal for mid-December.

A drier SW flow aloft kicks in Saturday night, pretty much shutting
off mentionable precip chances. Temps on Sunday afternoon should
warm back near normal before another, weaker cold front arrives
Sunday night. Not expecting much impact from this one, apart from
perhaps some light coastal showers and temps once again being
knocked down to a few degrees below normal. For Tuesday and
Wednesday, fairly weak high pressure at the surface and quasi-
zonal flow aloft should allow temps to remain moderate (at or just
above normal) with nil chance for rain...though there are some
model differences in the pattern aloft to be ironed out yet.

MARINE (Now through Thursday):  Buoy 42019 reported north winds
around 10 knots gusting to around 14 knots with seas of 3 feet with
a period of 5 seconds at 02 CST/08 UTC. The center of surface high
pressure will move east from Texas and across the Gulf of Mexico,
with a cold front easing into the Lower Texas coastal waters at the
end of the forecast period. Light to moderate winds and low to
moderate seas will prevail, with Small Craft Exercise Caution and
Small Craft Advisory not likely to be needed.

Thursday Night through Monday: A robust cold front will move
southward through the Lower Texas coastal waters on Thursday
night, with strong NNE winds in its wake. Small Craft Advisories
(SCA) are likely to be in effect into Friday night as seas build
to 8-9 ft. in the offshore Gulf waters. Rainy conditions are also
expected during this period. Winds and seas improve, becoming
more moderate for much of the weekend. A second, weaker cold front
is progged to move through the waters Sunday night, with Small
Craft Exercise Caution conditions likely with SCA possible for



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