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Area Forecast Discussion (AFD)

Issued by Brownsville/Rio Grande Valley, TX (BRO)

FXUS64 KBRO 140523 AAB

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1223 AM CDT Sun Oct 14 2018

...06z Aviation Update...

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


.AVIATION...VFR conditions will prevail for all TAF sites of Deep
South Texas through this cycle. Only concern looks to be gusty
southeasterly winds, especially from late morning through around
sunset on Sunday. Winds may gust up to 20-25kts at times. We will
need to watch toward the end of the cycle for perhaps an isolated
shower or two, especially closer to the coast, however, given 
uncertainty and low probabilities at this time left any mention of
convection out of the forecast.


PREV DISCUSSION... /issued 255 PM CDT Sat Oct 13 2018/ 

SHORT TERM (Now through Sunday Night):  Relatively quiet weather in 
store for the short-term period, at least relative to the strong 
cold frontal passage on tap for Monday (see Long Term section, 
below). In the meantime, hot, humid, breezy, and mainly rain-free 
(at least until late Sunday night across the northern-tier counties) 
conditions will continue.

For tonight, winds should stay up around 10mph, courtesy of an 
enhanced pressure gradient between high-pressure ridging over the SE 
CONUS and a low in the Panhandle region.  Continued transport of 
tropical air into the region near the surface will keep dewpoints 
elevated as well, generally in the mid-upper 70s.  So, it will be a 
sultry night with overnight lows only falling to around 80 for 
Brownsville and McAllen (which would be record high mins for 14 
Oct).  Moisture remains bottled up at H85 and below, however, so 
little-to-nothing in the way of showers expected.

More of the same in store for Sunday.  Temps spike up another degree 
or two (vs. today) ahead of the cold front.  Going for 94F at BRO, 
which would tie a record for the date, if it verifies.  No relief to 
speak of from the humidity, either.  Heat indices progged to reach 
the 104-108F range for the mid/Lower RGV during afternoon.  Column 
RH remains pretty diffuse, and with little forcing, will hold PoP's 
at silent levels.  NAM has been showing a shortwave racing across 
the CWA around mid-day, sparking a few showers and storms, but it 
seems to stand alone in this regard, and so was discounted here.

The long-advertised cold front should still be north of the CWA at 
12Z Monday.  Moisture will begin to pool ahead of it, though, with 
fairly strong S flow at H85 and backing flow at mid-levels.  Will 
keep widely scattered PoP's across the northern tier after midnight, 
with better coverage offshore, as this looks to be at least a good 
setup for enhanced streamer showers.

LONG TERM (Monday through Saturday):  Long term period begins with
an approaching cold front pushing through South Texas on Monday. 
Models are consistent with a strong front, but differ slightly on 
timing. The latest NAM solution sticks to a mid-afternoon arrival 
into the populated areas of Deep South Texas with the ECMWF and 
GFS models trending later into Monday evening with each new run. 
The biggest impact from a later timing will be Monday's afternoon 
temperatures. The northwestern ranchlands may slide into the upper
60s by the afternoon while the RGV tops off into the mid and 
possibly upper 80s before taking the plunge Monday evening into 
Monday night. 

Regardless of timing, this front will usher in well below normal
temperatures and keep things mostly cooled off into the weekend, 
with a slight rebound back into the mid and upper 70s on Friday 
and Saturday before a weaker cold front swings through. 

With a closed upper level low expected in the Desert SW and
persistent ridge over the eastern Gulf, a steady stream of 
moisture is expected across the region, enhancing rain chances 
through the week. Model guidance show PWAT values near 2-2.25 
inches on Tuesday, 2.25-2.50 Wednesday evening into Thursday, and 
again Saturday across mainly coastal portions of Deep South Texas 
and the lower RGV. With an overrunning tropical moisture setup, 
cooler surface temperatures, and thick cloud cover, periods of 
more steady moderate to heavy rainfall at times can be expected 
Tuesday and Wednesday night into Thursday. 

The best chance of convection would occur along and behind the
cold front on Monday into Tuesday, remaining mostly offshore late
Tuesday and Wednesday, before possibly working into the coastal 
and central counties again Thursday into the weekend as a coastal 
trough of low pressure begins to form. Any thunderstorm could 
produce heavier rainfall, potentially elevating any flooding 
concerns. Total rainfall amounts could exceed 2-3 inches in many 
eastern and central locations from Monday evening into Thursday, 
with an additional 2 to possibly 3 inches possible by Saturday 
near the coast and lower RGV. 

MARINE (Now through Sunday Night):  Buoy 42020 reporting winds from 
the SSE at 18G23KT with seas near 7 ft. as of 1:40 PM CDT.  Winds 
from platforms around the Laguna Madre have been hovering right 
around 20G25KT early this afternoon.  Earlier, had hoisted a Small 
Craft Advisory (SCA) for all marine zones through 7 PM CDT. SCA may 
need to be extended into the overnight hours for portions of the 
Gulf waters, but looks pretty marginal for meeting criteria, so will 
let evening shift make the call on any extension.  Pressure gradient 
relaxes a bit for vs. today for Sunday daytime, though periods of 
SCEC possible for the Gulf waters in the morning and the Laguna for 
midday/afternoon.  SE winds increase again on the Gulf Sunday night, 
ahead of a cold front moving through central TX, with SCEC-level 
winds likely to be realized again.

Monday through Saturday:  Cold front moves through late Monday 
afternoon or Monday evening through Monday night with moderate 
southeast winds and seas persisting until then. The north surge
moves across the coastal waters Monday night with north to 
northeast winds increasing and continuing through Wednesday. Seas 
to build steadily peaking sometime Tuesday into Wednesday, keeping
high and rough seas through the week. High end Small Craft 
Advisories are likely from Monday night through Wednesday with a 
low end chance of Gale Warnings being issued with some periods of 
frequent gale gusts possible Tuesday into Wednesday.




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