Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 211141 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
641 AM CDT Sun May 21 2017

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


.AVIATION...At the 3 air terminals MVFR conditions are
predominate at sunrise are likely to become VFR mid to late
morning and persist through the early evening before MVFR ceiling
redevelop. A stationary front over the Northern reaches of Deep
South Texas will be the focus for todays and tonight`s convection
however showers or thunderstorms could form anywhere at any time.
The strongest and most widespread convection is expected later
tonight, once again favoring areas north and west of the TAF
sites. Prob30 groups were added to the TAFS overnight due to the
increase coverage of thunderstorms. Winds will remain light east-
southeast gusty near any convection.


.PREV DISCUSSION... /issued 439 AM CDT Sun May 21 2017/

SHORT TERM (Today through Monday):  With a surface front to linger
across the Northern and Western Ranchlands today through Monday,
and several disturbances within a vigorous mid/upper southwest
flow, the chance for showers and thunderstorms will be elevated.
Best rain chances remain along and north of the frontal boundary
with lower chances across the Rio Grande Valley, but none the less
some of the rain that does develop will be capable of producing
beneficial rainfall.

There remains a concern for strong to severe thunderstorms and heavy
rainfall mainly across the north and west tonight into Monday
morning. Confidence is still on the low side with differences among
the GFS/ECMWF on a disturbance moving across South Texas. GFS seems
to dampen out the 50H shortwave trough indicating scattered
convection with moderate rainfall potential while the ECMWF shows a
stronger shrtwv with a more organized MCS feature along and north of
the frontal boundary with very high rainfall potential. In any case,
the atmosphere to destabilize with daytime heating and with the
approach of the southern-stream jet energy convection to form over
the Sierra Madre or even along the front this evening and overnight
and possibly lingering into Monday morning. SPC has included all
areas of Deep South Texas within a Marginal risk of severe
thunderstorms but better conditions will extend along the frontal
boundary itself. Shear values and instability will be sufficient to
support strong updrafts within any thunderstorm capable of producing
large hail and strong gusty winds. Also, decent direction shear is
seen in point forecast soundings that might support an isolated meso-
cyclone. Heavy rainfall potential exist over the northern
ranchlands as some training of thunderstorms may develop in this
area producing 2 to 4 inch rainfall amounts.

LONG TERM (Monday Night through Saturday): Southern-stream jet
over Texas through early Wednesday will keep that part of the
forecast unsettled, with quieter weather through the remainder of
the week and into the first part of Memorial Day weekend.

Showers and thunderstorms may be ongoing at the start of the
long-term period, with a convective boundary providing a focus for
lift as stated in previous AFD. Activity should favor the NE part
of the CWA Monday night, but will, of course, depend upon where
any boundary sets up. Marginally severe wind and heavy rain could
accompany any storms, especially near the coast, as moisture will
remain in good supply with precipitable water values in the
1.8-2.0" range and 0-6km bulk shear running 35-45 knots with the
fairly strong flow aloft.

00Z runs of GFS/NAM/ECMWF in fairly good agreement on bringing a
cold front through the CWA sometime Tuesday afternoon, though
there are some timing differences. Expecting mainly isolated
convection to develop ahead of the front, though forecast
soundings suggest a cap will be in place much of the day. SPC has
placed the entire CWA in a Marginal Risk for severe weather on
Tuesday, with ample buoyancy and bulk shear values supportive of
large hail and damaging winds...again, if the cap can be broken.

Showers and thunderstorms are also likely to accompany the front
proper Tuesday evening/night, though GFS/ECM models holding best
QPF south of the border where upslope flow will be a greater
contributor to lift. Have focused on the 00-06Z Wed timeframe
across the SW half of the CWA. Went above guidance with 50% PoP`s
to match up with relative maximum in WPC QPF values. For what
it`s worth, quick look at the 06Z NAM shows a slower progression
of the front, with widespread moderate to heavy rain from about
03-09Z, so clearly still some uncertainty on the details.

At any rate, precip threat should end by mid-day Wednesday for all
areas, as NW flow aloft advects drier air into the atmospheric
column. Successive model runs keep limiting the amount of cooling
behind the front, with mid-upper 80s now expected for highs in
the RGV on Wednesday. Upper-level ridging takes hold for the
remainder of the period, first with a transient short-wave ridge
on Thursday, followed by the northward building of the subtropical
high out of Mexico for Thursday into Friday. Latest blended
guidance bumps up daily max/min temps by a couple of degrees over
the previous forecast, which seems reasonable. Breezy
southeasterly winds also return quickly on Thursday, and more so
on Friday and Saturday, as high pressure strengthening over the
Gulf couples with surface troughing in the lee of the Rockies.

MARINE (Now through Monday):  Conditions at Buoy 42020 at 3 AM show
southeast winds around 10 knots and seas around 4 feet. East to
southeast winds and seas expected to remain steady state through the
period with a stationary front over South Texas maintaining a weak
pressure gradient. Isolated to Scattered showers and thunderstorms
today, tonight and Monday.

Monday Night through Thursday Night: Pressure gradient remains
fairly slack across the Lower Texas Coastal Waters into Tuesday
evening with a frontal boundary north of the area. This will
yield light-to-moderate winds and moderate seas of generally 3-4
ft. A better-defined frontal boundary moves into the waters from
the north late Tuesday night/early Wednesday morning, with a
brief period of SCEC conditions possible. SE winds strengthen to
moderate to fresh by Thursday afternoon, with SCEC likely for both
the Laguna and Gulf. SCA`s may be needed offshore Thursday night
as seas build to near 7 ft. Thunderstorms may produce gusty winds
and downpours through Tuesday night as well.


BAYVIEW/PORT ISABEL  88  78  89  76 /  20  20  30  40
BROWNSVILLE          91  80  92  77 /  20  20  30  40
HARLINGEN            92  76  92  75 /  20  30  30  40
MCALLEN              95  79  95  76 /  20  30  30  20
RIO GRANDE CITY      92  76  92  75 /  30  40  30  40
SOUTH PADRE ISLAND   81  79  82  77 /  20  20  30  40




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