Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 211143 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
643 AM CDT Thu Sep 21 2017

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

&&

.AVIATION...A FEW to SCT low clouds will continue to stream across
the region this morning, with an occasional BKN cig possible. Once
the atmosphere mixes out and winds increase, VFR conditions
should prevail through the afternoon. Meanwhile, convection is
expected to be a little more widespread today, however have left
VCSH given the uncertainty of if or when any TAF sites will be
impacted.

&&

.PREV DISCUSSION... /issued 410 AM CDT Thu Sep 21 2017/

SHORT TERM (Today through Friday): Isolated showers are once again
ongoing over the offshore Gulf waters this morning. A moist
atmosphere is evident based on a PWAT of 1.94" of off the 00Z
sounding. Meanwhile, the preliminary and non-operational GOES-16
Total Precipitable Water product shows a swath of 1.9-2.1" values
across portions of Deep South Texas and the Lower RGV. The overall
weather pattern will begin to change today as the ridge of high
pressure aloft shifts southward and flattens out over the next 36
hours as a large H5 trough digs across the Western CONUS.
Additionally, a weak shortwave will meander near the ARKLATEX
keeping some instability over Northern Texas and weak lift aloft.
This general pattern could lead to an increase in convection today
with more scattered to possibly widely scattered showers and maybe
an isolated thunderstorms. Models do differ with moisture and POPs
with the GFS on the drier side and the ECMWF indicating more robust
chances. Have split the differences and have keep 20 to 30% for land
areas with somewhat higher chances over the waters. Some chances for
convection will continue through Friday as well. Meanwhile, have
made little changes to temps, with the warm, above normal values
continuing.

LONG TERM (Friday Night through Wednesday): A major 500 mb trough
is expected to close off over the western states while an
elongated ridge axis will build over the Miss and Ohio River
Valleys. This ridging will shift east as the western closed low
builds east throughout the longer range forecast period. The 500
mb trough axis will then start tapping deeper layer tropical
moisture from the south advecting it northwards into the Midwest
and the southern Plains States starting this weekend. As the
western 500 mb closed low pushes east it will pull a cold front
down into northern and central TX by next Wednesday. The approach
of the western closed upper level low and the associated gradual
increase of deeper layer moisture will likely result in some
steadily increasing pops throughout Day 7. As the deeper layer
moisture increases and the 500 mb ridging weakens over the
region...thickening cld cover and decreasing 1000-500 mb thickness
values will allow for overall temps to lower a little over the
Day 3 through 7 range.

So for average temps...will lean towards above climo and guidance
earlier in the period and will then trend closer to climo and
model blends later in the longer range period. For pops will go
closer to a model blend for the longer range.

Model to model consistency between the ECMWF and the GFS for the
500 mb fields show pretty decent agreement in the development and
eastward movement of the West Coast closed low. Both models are
also in reasonable agreement in the levels of moisture return
later in the forecast period. Run to run consistency with both the
MEX and Euro guidance is pretty stable. So overall forecast
confidence is above average at this time.

MARINE: (Now through Friday): No significant changes to the marine
forecast over the next 36 hours with broad surface high pressure
remaining in place northeastern Gulf of Mexico. A generally weak
surface pressure gradient will maintain light to moderate southeast
winds and low to moderate seas through the period. Increased showers
or storm will be possible today and Friday.

Friday Night through Monday Night: Persistent surface ridging
over the eastern States will maintain light to moderate SE winds
and pretty benign seas through Sunday. No SCA conditions expected.

&&

.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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