Area Forecast Discussion
Issued by NWS Brownsville, TX

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844
FXUS64 KBRO 201403 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
903 AM CDT SAT AUG 20 2016

.DISCUSSION...After reviewing current trends, model data,
persistence, and "gut" feelings, have made several small but
notable changes to the forecast for today and tonight across the
area.

The main change was to remove and reorient areas of  mentionable
rainfall for at various times and locations across Deep South
Texas and the RGV both today and tonight. In the big picture, a
fairly large MCS continues to spin from the Big Bend toward South
Central Texas, with anticyclonic northerly flow high in the
atmosphere spinning a thick cirrus shield into South Texas
including the Rio Grande Plains of the Upper Valley.

For the rest of today, the main story for the populated RGV will
be gusty south/southeast winds and the "blast furnace" feel that
has been so common since the end of June on many days. Whatever`s
left of the morning MCS will shift east and any outflows will poop
out well north of the region. Whatever cirrus does sneak into the
Lower/Mid RGV should also fade into patches, allowing full
sunshine to work on the hot air mass (1000-500 mb thickness
nudging over 583 dm in the mid Valley) to allow temperatures to
reach triple digits west of U.S. 77.

Across the northwest corner of the area, closest to the 500 mb
shear axis oozing into south Texas, deeping moisture and closer
proximity to the old boundaries should be enough for isolated late
afternoon showers/storms in the Zapata/Starr/Jim Hogg region,
perhaps nudging into Brooks. The populated Valley will remain
rain-free. With the thicker clouds did a somewhat unusual lowering
of temperatures out in the Rio Grande Plains.

The evening/overnight forecast is a bit trickier, as mid level
flow turns a bit more from the west/northwest which could allow
any new MCS development, as well as outflows from new Sierra Madre
development depicted by the HRRR just before sunset, to provide a
better opportunity for notable rain over a larger area. Just how
far south and east that rain can get remains in question;
deterministic and ensemble guidance from both the GFS/ECMWF are
pretty bearish for the Lower and even the Mid Valley. Analog
situations suggest keeping Cameron and the Lower Texas coast dry,
which fits in with the guidance trends with a buffer of low end
rain chances for the mid Valley. Scattered to Numerous activity
should favor the Rio Grande Plains through the King Ranch,
reflected in the 12Z HRRR as well as the ECMWF and GFS.

Forecast updates are online at weather.gov/rgv (click on the map
below the graphics).
52/BSG

&&

.MARINE...Just a few tweaks to winds, to reduce daytime speeds a
hair over the Gulf (vs. the shallow Laguna) north of Port Isabel,
where upwelling and marine layer arguments will keep speeds
moderate - still in caution levels - but not as close to 20 knots
as previously forecast. Also removed mentionable precipitation
from all waters today with just a bit seeping south from the
Coastal Bend this evening/overnight. Lower Texas waters should be
rain-free overnight, and will take a closer look for the morning
fishing excursions off of Port Isabel and SPI later today to see
if and when rain needs to be mentioned.
52/BSG

&&

.PREV DISCUSSION... /issued 658 AM CDT SAT AUG 20 2016/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...IR Satl imagery shows mid and high level cld cover
from the convection over south central TX moving over the RGV
from north to south. Surface obs indicate some patchy low level SC
decks near the Weslaco area reducing ceilings down to MVFR levels.
The 00z BRO sounding shows drier air in place over the region at
most levels which will likely limit any significant ceilings from
forming over the region. So expect aviation conditions over the
next 24 hours to be mainly VFR with some potential for brief MVFR
conditions due to the lower ceilings.

PREV DISCUSSION... /issued 401 AM CDT SAT AUG 20 2016/

SHORT TERM (Now through Sunday): The 500 mb ridging that edged
into the Texas coastline over the last couple of days will start
to erode some as 500 mb troffing builds into the central U.S.
This strengthening 500 mb trough axis will in turn push cold front
into central Texas which will then likely stall as a stationary
front later this weekend. Deep layer tropical moisture will remain
concentrated along and south of the cold front/stationary front
through Saturday and Sunday which will maintain pretty significant
pops in the middle sections of the Lone Star state. The deeper
layer moisture values will drop off pretty quickly closer to the
Texas coastline where the 500 mb ridge axis has more of an
influence. The short term guidance reflects this trend pretty
dramatically maintaining likely pops for the LRD area throughout
the weekend with the lower RGV seeing mainly slight chc/low end
chance pops. So will maintain the best pops over the western
counties closer to the deeper moisture levels.

As the conv chcs increase steadily over the region the cld cover
will also be on the increase which will tend to hold down the max
temps and heat indices a bit. The short term temp/pop guidance for
the RGV is in decent agreement through Sunday. So will opt for a
general blend of the short range guidance.

LONG TERM (Sunday Night through Friday): Lingering moisture
from the subtropical jet will interact with a weak upper level
impulse and keep chances for scattered showers and a few
thunderstorms in the forecast to round out the weekend and into
early next week. Precip activity will be largely hit and miss as
overall deep layer moisture will be lacking as high pressure
builds in across the Gulf and Deep South Texas. The 500mb ridge
will set up and strengthen over the mid Gulf region through the
week...keeping subsidence and drier conditions on place. A weakly
inverted trough will bring a quick boost of moisture Thursday into
Friday...but have gone with a dry forecast as moisture levels
remain rather shallow. Temps will start out near normal...with
readings expected to increase a bit through the week due to the
decreased rainfall chances.

MARINE:

Now through Sunday: Buoy020 data from the lower TX Gulf waters
indicates that SCEC conditions are occurring right now with light
surface winds prevailing across the Laguna Madre. A pretty good
surface PGF will prevail along the lower TX coastline throughout
the weekend due to the surface low pressure system moving across
the central U.S. These moderate surface winds will maintain the
marine conditions close to SCEC criteria through Sunday. The PGF
will not be strong enough to likely produce SCA conditions for
either Saturday or Sunday.

Sunday Night through Wednesday Night: Lingering moderate ESE to SE
winds will continue over the coastal waters Sunday night as high
pressure gradually builds westward over the northeastern Gulf.
Seas will continue to be moderate before gradually relaxing later
on Tuesday and through the day Wednesday. Winds will also slacken
as the high pressure becomes centered over the Gulf...reducing the
gradient.

&&

.PRELIMINARY POINT TEMPS/POPS...
BAYVIEW/PORT ISABEL  93  81  92  81 /  10  10  40  20
BROWNSVILLE          96  79  94  80 /  10  10  40  20
HARLINGEN            98  79  96  80 /  10  10  40  20
MCALLEN             102  81  98  80 /  10  20  40  20
RIO GRANDE CITY     100  79  97  79 /  20  30  40  30
SOUTH PADRE ISLAND   89  81  87  81 /  10  10  40  20

&&

.BRO Watches/Warnings/Advisories...
TX...None.
GM...None.
&&

$$

This product is also available on the web at:
HTTP://WEATHER.GOV/RGV



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