Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 181756 AAB

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1156 AM CST Sat Feb 18 2017

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


.AVIATION...Sunny skies with moderate winds prevail across Deep
South Texas late this morning with subsidence aloft. Breezy
south-southeast winds and VFR conditions will continue through
this afternoon. MVFR ceilings expected to develop after sunset as
low level moisture increases. All Short-term guidance suggest
that IFR ceilings should impact local airports mid to late evening
and will continue through the overnight hours. Patchy fog is
possible but dense fog is not expected as winds should remain
elevated. Low cloud decks and patchy fog expected to mix out
around 16z Sunday as winds increase.


.PREV DISCUSSION... /issued 619 AM CST Sat Feb 18 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Dense fog has developed at the RGV terminals within
the past hour, with visibilities down to 1/2SM or less. AWW`s in
effect for BRO & HRL for the next couple of hours. With 25KT
winds just off the deck at 1000 ft. AGL observed by 12Z BRO raob
and VWP, thinking fog should mix out (at least back up to IFR)
pretty quickly after sunrise. Guidance is hanging on to IFR CIG`s
through much of the morning for BRO & HRL, though. Breezy SSE
winds kick in late morning, with VFR expected for HRL & MFE but
MVFR ceilings nearer the coast at BRO as low-level moisture
continues to increase. Short-term guidance insistent on bringing
back IFR CIG`s by mid-evening which should last through the
remainder of the TAF period.

PREV DISCUSSION... /issued 406 AM CST Sat Feb 18 2017/

SHORT TERM (Now through Sunday):  Very warm and breezy conditions
will prevail in the short-term.  For today, a shortwave mid/upper-
level ridge axis will pass overhead, resulting in very warm
temperatures...several degrees above the mid-80s realized in the
Mid/Lower RGV on Friday. Not expecting any records to fall, though
BRO will be close (87F forecast vs. 88F record).  Will hold off on
any Fire Danger Statements today.  RH values fall to 25-30% over
Starr/Zapata/Jim Hogg counties this afternoon, but 20-ft. winds
progged to stay at 10mph or less.

Overnight, flow backs to S-SW through pretty much the depth of the
troposphere as the strong upper-level trough that has been producing
copious precip for SoCal moves inland to NW Mexico and the SW CONUS.
Near-surface moisture continues to increase tonight on SE winds,
with dewpoints climbing to the mid-60s to near 70 across the area.
30-35KT winds develop at H85, continuing the warm-up at low-levels.

Another very warm and breezy day in store for Sunday, as the upper-
level trough and surface cold front continue moving closer to Deep
S. Texas.  Latest model guidance indicating that tomorrow will be
even a bit warmer than today.  Combined with the muggy air,
afternoon heat indices will touch the mid-90s during the afternoon
in the Mid/Lower Valley.  The weather could become a little more
interesting by later Sunday afternoon, though most of the action
will hold off until the long-term portion of the forecast. With
the steepening mid-level lapse rates and increased moisture, some
instability develops, with an axis of MLCAPE values of around
2000- 2500 J/kg over the CWA with 0-6km bulk shear of 35-45KT.
However, the aforementioned warming at H85 yields a strong cap
(significant CIN), which will be difficult to overcome until the
stronger forcing arrives during the evening. Will maintain the
mention of thunder Sunday afternoon but have lowered PoP`s some
from the inherited broad-brushed 30% values. Best chance through
00Z Monday would seem to be across the northern tier of counties
if a lead short-wave disturbance ejects ahead of the main trough,
as the 00Z GFS is suggesting.

LONG TERM (Sunday Night through Friday): A deep, narrow, and
slightly negatively tilted mid level short wave trough will
stretch from north to south over the high plains Sunday night.
Some of the energy will split to the north, but the base of the
axis will move overhead Sunday night into Monday with an
associated surface cold front. The supply of moist Gulf air and
steeper lapse rates suggested by mid level cooling and lower mid
level heights will support higher instability. The combination
will bring a marginal risk of severe weather.

The best rain chances will be Sunday night into Monday. Could see
some initial storms fire over eastern sections Sunday evening due
to general instability, but most of the activity will likely
occur later Sunday night as the upper trough axis moves into the
picture and ties into the surface cold front. Forecast QPF amounts
seem rather paltry, with only about a half an inch of rain
progged, with the best upper dynamics still north of the area, but
a few stronger cells could result in locally heavy rainfall. The
rain will affect the Upper Valley Sunday night, with the
concentration shifting east as the front moves through Monday
morning, reaching the coast by noon.

In the small chance that strong to severe storms do develop,
damaging wind gusts of 60 to 80 mph would be possible, especially
if a line formed up on approach to the coast. Hail up to golf ball
size could develop in larger, stronger, individual cells. Even an
isolated tornado could be possible.

The entire event should transpire relatively quickly. Mid level
ridging will start to build in Monday evening after the trough
shifts east. Dry northwest to north winds will ensue, and mostly
clear skies will be the norm overnight. Temperatures will be more
moderate Monday night through mid week, though still above normal,
as high pressure spreads over the area with dry weather. High
temperatures will then be in the lower to mid 80s with overnight
lows mainly in the 50s.

Used a model blend as there were no significant outliers early in
the long term. After Monday, ridging will build in aloft with
high pressure spreading over the area at the surface. A weak cold
front with limited upper support and bringing little more than a
wind shift and slightly cooler weather will move through on Friday
at the end of the long term.


Now through Sunday: Buoy 42020 reporting S winds of 12 to 14 KT
and combined seas of 3 ft. as of 2:50am CST. These moderate winds
and slight seas are expected to continue through the day today.
Winds increase somewhat overnight, especially for the Gulf waters,
as an approaching surface trough tightens the pressure gradient.
SCEC wording may be needed. SCEC becomes more likely for all of
the coastal waters on Sunday, as the trough/cold front move
closer. Seas build to 6 ft. for the Gulf waters 20-60nm out.

Sunday night through Wednesday night: Moderate south winds and
slightly elevated seas of 4 to 6 feet will be in play Sunday night,
with small craft should exercise caution conditions possible on the
Gulf. Winds will decrease ahead of an approaching cold front by Mon
morning however, though seas may continue to be slightly elevated.
Winds will shift to moderate northwest to north by Monday afternoon
with the arrival of the cold front, and will remain north as high
pressure spreads over the area. There is a slight potential for
rougher marine weather Monday morning before noon if any stronger
convective activity develops along the front. This would mean
strong, gusty winds, rough seas, lightning, and heavy rain. In
either case, conditions will improve Monday afternoon in the wake of
the front.




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