Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 211153 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
653 AM CDT Mon May 21 2018

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


.AVIATION...Scattered showers and thunderstorms continue to move
eastward across the lower Rio Grande Valley behind a weakening
MCS early this morning. Ceilings have been challenging due to
varying rainfall intensities, but the overall trend is gradually
downward. VCTS will be possible from BRO to HRL the next couple of
hours with VFR to minor MVFR ceilings. Conditions will improve
later this morning and afternoon, but additional convective
development will be possible. VRB winds in thunderstorm activity
this morning should shift back to the ESE at around 10 to 15 knots
this afternoon. Models indicate ceilings may drop back to MVFR
late tonight with skies clearing early Tuesday morning.


.PREV DISCUSSION... /issued 509 AM CDT Mon May 21 2018/

SHORT TERM (Now through Tuesday): An MCS moved through South
Texas this evening and is producing intense convection across
Tamaulipas with torrential rain and frequent lightning just south
of the U.S. border. A few pockets of heavy rain on the north end
of the line has made its way into portions of the RGV, but the
overall trend is gradual weakening. After this convection winds
down later this morning, another chance of showers and storms will
be possible through the afternoon, as deep moisture will still be
in place across Deep South Texas. There is some uncertainty with
the amount of coverage, but model soundings show MLCAPE of around
2000 J/kg and minimal capping, so any leftover outflow boundaries
from tonight should be enough to spark additional development. Any
storms that do develop may produce locally heavy rainfall with
precipitable water values ranging from 1.75 to 2.00".

A mid-level low over Southern California will lift northeastward
Tuesday night into Wednesday, leaving a ridge in place over South
Texas. Rain chances will decrease on Tuesday with above-normal
temperatures continuing. Highs will range from the lower 90s in the
lower RGV to upper 90s in the upper RGV and ranchlands. Daytime
temperatures combining with dew points in the lower 70s will result
in Heat Indices ranging from 100 to 105 Fahrenheit.

LONG TERM (Tuesday Night through Sunday): A 500mb trough/low
across the western United States and a 500mb ridge across
northeast Mexico will provide a split flow across the state
Tuesday with southwest flow aloft across west and northwest Texas
and northwest flow aloft across south Texas. Low to mid level
moisture is progged across northeast Mexico northward into west
Texas and the Texas panhandle Tues night into Wednesday so will
mention some isolated convection across extreme western portions
of the CWA Tues night and across the coastal waters Wednesday. The
split flow aloft is expected to prevail across the state Wednesday
into Thursday before the upper ridge amplifies across northern
Mexico and west Texas Friday. Low to mid level moisture will
diminish across the western half of the state Thursday and
continue to diminish Friday especially as subsidence increases
across south TX with either a 500mb trough or 500mb low across the
eastern Gulf of Mexico Thursday retrograding westward across the
central Gulf Friday. The global models differ on the development
and timing of the upper level trough/low Friday into Saturday with
the ECMWF bringing a closed 500mb low slowly northwestward across
the northwest Gulf of Mexico through the rest of the forecast
period and the GFS progging a broad 500mb trough across the
central Gulf of Mexico Friday through Sunday. Regardless of the
model differences...subsidence will prevail across southwest TX
with not much in the way of rain chances through the rest of the
forecast period. Expect above normal temperatures Tues night
through Sunday especially for next weekend.

MARINE (Now through Tuesday): Moderate southeast winds will
continue on Monday with seas gradually subsiding to 2 to 4 feet by
Tuesday night as the pressure gradient relaxes. Showers and
thunderstorms will be possible today with abundant moisture across
the area. A few storms may produce strong wind gusts and cloud to
water lightning.

Tuesday night through Friday...Light to moderate east to
southeast winds will prevail across the coastal waters Tues night
into Wednesday with surface high pressure across the western Gulf
of Mexico and low pressure on the lee side of the Rockies. The
pressure gradient will be weak across the lower Texas coast
Thursday and Friday. Light east to southeast winds should prevail
across the western Gulf of Mexico Thursday and Friday.




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