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Area Forecast Discussion (AFD)

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

FXUS64 KBRO 142341 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
641 PM CDT Sun Oct 14 2018

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


.AVIATION...TAFs reflect deteriorating conditions Monday with a 
cold front arriving in the lower valley by afternoon. The main 
threat for the morning and early afternoon will be increasing 
shower and thunderstorm chances. With that will come the 
possibility of strong, gusty winds, occasional to frequent cloud 
to ground lightning, and brief, locally heavy rainfall. Except 
for the possibility of reduced ceilings and visibilities in 
moderate to heavy rainfall, Initial TAFs will be VFR, though 
brief, lower MVFR ceilings will be possible around dawn. Low VFR 
ceilings will develop Monday morning, and will then drop to MVFR 
with the arrival of the front and colder air. Arrival of the front
at MFE around 18Z and at Brownsville around 21Z is an estimate 
for now, subject to updates. Afternoon airport conditions will be
complicated by reduced, MVFR ceilings, and lower visibility in 
mist and possible heavy rain. Ceiling improvement above MVFR not
expected Monday evening.


.PREV DISCUSSION... /issued 326 PM CDT Sun Oct 14 2018/ 

SHORT TERM (Now through Monday Night):  The season's first strong 
cold front is still on pace to move through Deep South Texas on 
Monday.  Ahead of said front, expect a mild, balmy night with SE 
winds of 5-10 mph.  Precip-wise, most areas should stay dry through 
tomorrow night, though a quick-hitting coastal shower is in play 
with enhanced streamer activity expected over the Gulf.  A few 
showers could also ride out ahead of the front proper and reach 
Zapata/Jim Hogg counties by daybreak on Monday.

Expected timing of frontal passage continues to nudge forward in 
time.  Leaning strongly toward the NAM/Hi-Res ARW for this FROPA, as 
they tend to perform better in these situations. Timing:  Through (or 
at least moving through) northern/western Ranchlands by 15Z, into 
the mid-Valley by around 18Z, through BRO by 21Z, and offshore by 
00Z.  Gusty north-northwest (20-25mph, with a brief period of gusts 
up to 35 mph possible) winds will accompany the front, which will 
immediately begin ushering much colder air. Most all locations will 
experience their high temps for the day during the morning.  Knocked 
these high temps down a few degrees (into the mid-80s) for the 
populated RGV with the faster timing of the front. By sunset, 
current forecast reflects temps ranging from 50F for NW areas to 60-
ish in southern Cameron County (mid-60s SPI).  The northern/western 
Ranchlands could see an extended (24+ hours) period of wind-chill 
readings in the 40s beginning around noon on Monday.

As far as rainfall potential goes, expect a band of heavier showers 
with some thunder along and perhaps just behind the front.  A few 
storms could be a little on the strong side initially, with gusts up 
to 40mph and small hail, but severe storms aren't expected.  Moist 
airmass (PW's of 2.0-2.2 inches) will be riding up over the front, 
with isentropic lifting, so elevated convection (at least isolated) 
could keep going into Monday evening in the midst of more more 
widespread light/moderate rain.  Models, as well as WPC guidance, 
have backed off just slightly with totals through 12Z Tuesday. Still 
looks like widespread totals generally in the 1-2" range, though. 
Will have to watch for any areas that receive multiple thunderstorm 
dumps for localized flooding, but not really seeing the rain rates 
being maintained long enough for a widespread threat.

The light rain/showers continue through Monday night, as a coastal 
trough begins to develop along the front and southwest flow aloft is 
maintained.  Intensity should ratchet down a bit, though.  Low temps 
will range from the upper 40s to low 50s in general, making for a 
pretty miserable feel outside.

LONG TERM (Tuesday through Sunday): As the long term period 
begins, temperatures continue their free fall behind the cold 
front with lows Tuesday morning bottoming out into the upper 40s 
in the ranchlands and low 50s in the RGV. Temperatures are slow to
rebound, with stronger cold air advection continuing through 
Wednesday, keeping high temperatures in the 50s and low 60s. 
Persistent northerly flow through the weekend will hold high 
temperatures well below normal. 

Any elevated convection behind the front will likely reside or be
moving off the coast by Tuesday morning with a coastal trough 
forming offshore in the wake of the front. With a closed upper 
level low expected in the Desert SW and persistent ridge over the 
eastern Gulf, a steady stream of tropical moisture is expected 
across the region, enhancing rain chances through the week. Model 
guidance shows PWAT values near 2.0-2.25 Wednesday evening into 
Thursday along coastal locations as a surge of moisture and mid 
level disturbance work around the ridge to the east. 

Would expect the next best chance of convection to be with this 
feature Wednesday night and early Thursday, with locally heavy 
rainfall possible. Total rainfall Tuesday through Thursday will 
range from near 1-2 inches across the northern and western 
portions of the CWA, 2-3 inches in central portions and mid to 
upper valley, and over 3-5 inches possible along the immediate 
coast and portions of the lower valley. 

An additional round of moisture Friday night into Saturday ahead
of a boundary or weak front Saturday evening will provide another
opportunity for locally heavy rainfall near the coast, with PWAT
values near to just above 2 inches. Additional rainfall values
near 1-2 inches possible along the coast Friday into Saturday. If
the models come into agreement on a front Saturday evening, rain
chances and amounts would likely increase across the region. 

MARINE (Now through Monday Night):  SCEC in place currently for the 
Laguna Madre; otherwise, moderate SE winds and moderate seas to 
remain in place until a strong cold front arrives in the coastal 
waters around noon tomorrow.  North winds will quickly pick up to 20-
25 knots for both the Laguna and Gulf, with frequent gusts up to 
30KT. A few gale force gusts cannot be ruled out, either.  SCA has 
just been issued from noon tomorrow (Monday) through 7 AM Tuesday, 
though this is just for starters and will almost certainly be 
extended. Wave guidance has backed off bit on sea heights, but will 
quickly build to hazardous (7+ ft.) levels by later on Monday 
afternoon and be maintained through the short-term.

Tuesday through Sunday: Northerly winds increase and continue
through Thursday. Seas build steadily, peaking Wednesday or
Wednesday night, keeping high and rough seas through the week.
High end Small Craft Advisories are likely to continue into
Thursday with some gale gusts possible Tuesday through Wednesday 
night. Next weekend could get a little interesting with early 
model runs increasing swells across the Gulf. Have toned this 
feature down a bit from the model output, but could once again 
need high end SCAs. 


GM...Small Craft Advisory from noon Monday to 7 AM CDT Tuesday for 



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