Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 221144 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
644 AM CDT Fri Sep 22 2017

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

&&

.AVIATION...Ongoing morning convection will keep the chances for
SHRA or VCSH over the next few hours with occasional low clouds
bringing MVFR conditions. Otherwise, clouds bases will lift by
late morning as the CU field sets up with the possibility of
convective activity through the day. Winds will also be gusty once
again as the flow take on a more easterly component.

&&

.PREV DISCUSSION... /issued 426 AM CDT Fri Sep 22 2017/

SHORT TERM (Today through Saturday): Showers and some embedded
thunderstorms continue across portions of the coastal waters,
northern coastal counties, and northwestern Ranchlands early this
morning. Meanwhile, isolated showers are noted across areas in the
Lower to Middle RGV. Enough low level moisture combined with weak
forcing aloft due to the presence of a shortwave, has been enough to
keep these shower/storms ongoing. However, do expect most activity
to remain offshore or closer to the coastline with more instability
over water at night.

For the last day of the work week, deep south Texas remains under a
broad area of weak ascent with a large H5 trough digging across the
northwestern CONUS. Meanwhile, weak ridge has shifted south over
Mexico with a weak area of low pressure aloft over the northeastern
Gulf. Because of this, it will be possible for a few weak impulses
to move through the region around the periphery of the large western
low, and cause convection to initiate during the heat of the day,
aided by any lingering boundaries or seabreeze generation. However,
water vapor imagery and model guidance does suggest that a pocket of
mid level drier air will move in aloft through the day, which would
act to cap the atmosphere. Because of this uncertainty, have keep
slight to low end chance POPs in the forecast this morning, with
general 20% in place this afternoon. Can`t rule out an isolated
stronger storm, but not expecting a repeat of yesterday. Similar
conditions will be in place for Saturday with continued chances for
convection.

As far as temps, values have continued to run above normal and will
continue the above guidance trend.

LONG TERM (Saturday Night through Thursday): A major 500 mb
closed low will continue digging southwards over the western
States while ridging will shift to the northeast. This closed low
eventually opens up and lifts out the northeast while another 500
mb closed low reforms over the Desert SW Wed and Thurs. As this
secondary 500 mb closed low reforms ridging will then shift west
over the RGV late in the forecast period. The persistent upper
level closed lows over the western states will start advecting
deeper layer moisture from southern Mexico up north into the
middle and upper Midwest into next week. The ECMWF and GFS
guidance tends to keep the bulk of this moisture shunted west of
the RGV for much of the longer range forecast period. However
enough of this moisture may advect over the RGV to maintain some
conv threat over the region next week.

At the surface...the steady progression of the western states
closed upper level lows will push a cold front down into central
and southern Texas late in the longer range forecast period. The
approach of this cold front will help enhance the conv potential
and the cld cover late next week which in turn could help lower
temps a bit.

The ECMWF and GFS numerical guidance both indicate pretty good
agreement for temps and pops through Days 3 through 7. Also run to
run consistency with both models is also pretty stable through Day
7. So will go with a model blend for temps/pops throughout the
long term period.

Overall confidence in the longer range forecast wording is above
average at this time.

MARINE: (Now through Saturday): A shortwave aloft, combined with
plenty of low level moisture will keep scattered showers and a few
isolated thunderstorms over the Lower Texas Coastal Water this
morning. Meanwhile, weak surface high pressure over the Gulf will
maintain light to moderate southeast winds and low to moderate seas
through the period.

Saturday Night through Tuesday Night...Surface ridging over the
eastern States will maintain a light to moderate SE low level flow
across the lower TX marine locations. This will produce generally
low to moderate seas across the region through Tues. No SCA
conditions expected.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...None.
&&

$$

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