Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 171718

Area Forecast Discussion
National Weather Service Brownsville TX
1218 PM CDT Sat Mar 17 2018

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


.AVIATION...VFR conditions in place over the western half of the
forecast area with MVFR CIGS in place over the eastern half of
the forecast area amidst strong southerly flow at the low levels.
CIGS will slowly rise through the afternoon with daytime heating.
The heating will also help to decrease overall CIG coverage, but
generally areas of CIGs will continue through the afternoon. The
decrease in clouds over the east will be short-lived as CIGS will
begin become more widespread after sunset and lower into the
IFR/LIFR category overnight. CIGS will also build westward
overnight into far western portions of the forecast area. Will see
restrictions to VSBYS develop down into the MVFR/IFR categories
mainly over the eastern half of the forecast area after 09Z
tonight. VSBYS/CIGS will improve somewhat in the last few hours
of the TAF period as daytime heating returns Sunday morning.


.PREV DISCUSSION... /issued 639 AM CDT Sat Mar 17 2018/

DISCUSSION...Updated for latest /12Z/ aviation discussion below.

AVIATION...Low level humid southerly flow will continue to
creating sub- inversion IFR/low MVFR ceilings through mid morning
hours before warming sunshine and veering winds above the surface
to the west or southwest gradually lift and scour out the
ceilings by late morning to early afternoon. One difference from
Friday afternoon should be winds, which look to be a hair lighter
this afternoon than on Friday with pressure gradient lacking an
enhancing stronger southern Plains surface low and instead the
more traditional minor Valley Wind Machine situation with fresh
sustained winds and gusts holding below 20 knots. As for
misty/fuzzy visibility...we haven`t really seen restrictions with
winds remaining near 10 knots all night, though did at a TEMPO
group for MVFR mist (BR) at Harlingen early this morning.
Brownsville may be close with ceiling having fallen to LIFR but
left out for now as ceiling may bounce up to IFR which would keep
visibility from dropping below 6SM.

This evening looks to be a carbon copy of Friday evening with
mainly clear skies dotted with patchy low MVFR cumulus fractus.
GFS soundings and model signals now suggest IFR ceilings arrive
around midnight or so and continue through daybreak; added TEMPO
for LIFR based on what we`ve seen early this morning but with
winds near 10 knots only dipped visibility to MVFR for now.

PREV DISCUSSION... /issued 352 AM CDT Sat Mar 17 2018/

SHORT TERM (Today through Sunday):
There`s plenty of green in our Valley as St. Patrick`s Day 2018
brings another muggy morning with plenty of low clouds, followed
by increasing afternoon sunshine through somewhat hazy skies as
overall airmass is little changed from this time Friday. Zonally
flat 500 mb ridge continues over and south of Deep S. Texas/RGV
today which maintains weak pressure patterns with a broad but weak
surface ridge centered over the eastern Gulf, with the typical
lee side of the Sierra Madre trough. Friday`s winds kicked up a
bit more than expected in the Lower Valley with a few peak gusts
over 30 mph, but one difference with today was the passing
Southern Plains surface low that probably helped out the gradient
a bit. Such a low does not exist today, so expect more of a fresh
breeze that will go slack west of US 281 this afternoon. With the
warming airmass and abundant afternoon sunshine on Friday,
temperatures soared into the upper 80s to lower 90s region wide
and expect the same today. Out west, dew points overachieved their
"crash" falling into the mid to upper 40s from Zapata through Jim
Hogg County. While air mass has modified a bit this morning,
models are not handling the drying downslope flow so dropped
dewpoints (and hence afternoon humidity) some 5 degrees from prior
forecast and in line with latest suite of deterministic guidance
(low to mid 50s). The combination of hot temperatures and low
humidity will continue to build drought stress, as well the coming
bone-dry situation early next week (more on this in the long term
discussion) across the Upper Valley/Rio Grande Plains.

For tonight, another repeat performance as mostly clear evening
quickly turns to low clouds - possibly a bit sooner than early this
morning but enough to keep enough breeze to minimize fog potential
to patchy at best.  Trimmed back fog area to the more typical rural
locations where winds will be lighter, and raised temperatures a bit
based on current low to mid 70s (330 AM) under the low decks; only
the northern tier will drop below 70.

On Sunday, the gist of the forecast will remain the same for the
populated RGV - with another cloudy muggy start giving way to
increasing sunshine by late morning and especially during the
afternoon.  A slug of very dry air (<25% humidity above 850 mb)
arrives across the region during the afternoon, and northwest to
west flow also increases especially above 850 mb.  This will dry out
the atmosphere and could aid a heat "spike" west of US 281 along the
Rio Grande and through the Valley ranch areas. For this reason,
maintained the slightly higher afternoon temperatures especially out
west.  Along the northern tier, will be watching some impressive jet
dynamics that match up with a 850 mb potential temperature ridge.
While deep layer moisture is generally lacking, there`s just enough
between 1 and 4 PM to possibly kick off or keep any discrete
thunderstorm cells through the South Texas Brush Country as
soundings are uncapped and quite unstable. Better chances are
between Laredo and Corpus (still not all that great).

For the beaches:  Friday was a preview of the near-perfect "bookend"
Spring Break 2018 weekend and today and Sunday won`t disappoint. The
marine layer will favor the usual afternoon full clearing skies,
and lesser gradient and lower seas reduce the longshore current
threat to low today. Temperatures should edge up to 80 as well,
and UV Index will be the main issue both days, reaching very high

LONG TERM (Sunday Night through Friday):
Very quiet pattern setting up for the region through the next week
as the upper level flow continues to deflect weather systems well
to the north. The wave passing through Oklahoma will drag a weak
front southward into the region Monday morning. This will shift
winds to the north and draw in a  dry airmass. Couple that with
some downsloping, temperatures will rise into the low to mid 90s
Monday afternoon. North winds will be strongest in the western
valley, reaching near 15 mph. This coupled with the very dry
conditions will bring the threat for critical fire weather
conditions. The surface high continues to build southward Monday
night into Tuesday, with slightly cooler temperatures and clear
skies. The surface high is offshore by the end of the day
Wednesday, allowing southeast flow to return, drawing Gulf
moisture back into the lower levels of the atmosphere. The next
induced surface low in the Panhandle Thursday will tighten the
gradient considerably, bringing breezy conditions for the latter
half of the week.

Now through Sunday: Seas and winds have come down over the past
24 hours, with both buoys in the 20-60 nm leg below 4 foot seas
and 10 to 15 knots overnight. Expect these winds to dip further
with the usual diurnal flip-flop, and seas should drop to around 3
feet. Wind waves much less an issue today as swell seems a bit
more dominant, so boating/fishing trips are ideal for this time of
year. Benign conditions will prevail through Sunday as wind field
remain slack resulting in slight to moderate seas.

Monday through Wednesday: The next front crosses the northwest
Gulf Monday morning. This initial passage will be fairly weak,
with north winds remaining around 10 knots. Sea state will remain
relaxed during passage, with seas 2 feet or less. The surface high
finally builds into the region Tuesday morning, when the main
northerly wind surge arrives. Winds will reach 20 knots from dawn
through the later afternoon hours before decreasing Tuesday night.
This gusty but brief wind surge will bring swells up to 4 to 6
feet for the Gulf. Winds quickly turn easterly Wednesday as the
surface high approaches the northern Gulf. Winds will remain 10 to
15 knots, allowing seas to relax down to around 3 feet.




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