Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 151708 AAB

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1208 PM CDT Thu Mar 15 2018

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


.AVIATION...Multi-layered low clouds are supporting MVFR to low
VFR ceilings now at the TAF sites. High pressure over the
northeast Gulf is interacting with low pressure over eastern
Colorado to support moderate south southeast winds locally.
Continued onshore flow the next 24 hours will likely maintain the
current conditions. North south mid level ridge axis over the
south central US will continue to shift east, with west or west
southwest mid level flow aloft supporting the elevated cap over
the area, and locking in low level moisture and a saturated layer
between 800 and 900 mb.


.PREV DISCUSSION... /issued 618 AM CDT Thu Mar 15 2018/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Main updates this cycle were to lower wind speeds
slightly today and introduce lower cloud ceilings late Thursday
night into early Friday morning. CIG`s remain VFR currently, but
have been trending down the past several hours and should lower to
MVFR within a few more hours as low-level atmospheric moisture
continues to increase. A few light showers may occur near the
terminals today, mainly between 15-21Z, as weak upper-level
disturbance passes just north of the region. However, little-to-
no impacts are expected. Low cloud deck may mix/scatter out to
VFR late afternoon for HRL/MFE. MVFR CIG`s progged to return after
sunset, with SREF probabilities and MOS guidance hinting that IFR
deck may develop late tonight (after 06Z), particularly at

PREV DISCUSSION... /issued 351 AM CDT Thu Mar 15 2018/

SHORT TERM (Today through Friday):
Going forecast in good shape. Weak ripple in zonal flow aloft,
long-advertised by the GFS model, appears to be real, as noted in
water-vapor satellite imagery and RAP analyses. This wave,
currently traversing the Big Bend area of TX, is progged to pass
just north of the CWA around mid-day. Meanwhile, low-level
moisture will continue to increase, especially nearer the coast,
as SSE return flow becomes better established. Weak lift with the
wave should act to produce isolated light showers, mainly during
the middle part of the day (9am-3pm). Tightened surface pressure
gradient between a low emerging into the lee of the central
Rockies and the northern Gulf high will also bring breezy
conditions to the coastal counties. Afternoon temp forecast just a
little tricky, as model progs dry the atmospheric column from
west-to-east, following the passage of the aforementioned ripple.
Given that temps "overperformed" just slightly yesterday, and the
expected ~4C increase of H85 temps vs. a day ago, went ahead and
added a degree to max temps for all land areas. Coastal counties
and northern ranchlands expected to see mid- upper 70s, with low
80s in the Upper Valley where clouds thin/clear earlier in the

Not much to write home about overnight.  Southerly winds remain
somewhat elevated, pumping up dewpoints into the mid-60s even well
inland.  Correspondingly, overnight lows will also be in the mid-
upper 60s, for a muggier feel than we`ve seen lately.

Zonal flow aloft and a drier column, along with continued WAA, will
allow for a fairly sharp warmup (10 degrees or so vs. today) on
Friday.  Dryline also progged to penetrate eastward to the vicinity
of Hwy. 281/I-69C during the afternoon.  Temps should rise into the
low 90s pretty easily west of the dryline, with mid-80s on the other
side and upper 70s along the coast.  As has been mentioned in
previous AFD`s, will need to keep an eye on the dryline for any
possible thunderstorm development.  While a cap around H85 should
keep a a lid on any convection, an axis of fairly impressive CAPE
(2,000-2,500 J/kg) is forecast by both the NAM and GFS to develop
just in advance of the dryline.  Both models also forecast deep-
layer (surface to 6km) shear of 40-50 knots, so if a storm or two
can get going (admittedly, this is highly conditional), there is
potential for strong to severe cells.  Last night`s 00Z hi-res
models were unimpressed, though, so will just keep silent 10% PoP`s
in the official forecast.

LONG TERM (Friday Night through Wednesday): The zonal flow will
continue into the beginning of the weekend, which will continue
the warm regime for the spring breakers. Highs will again reach
the mid 80s near the coast to the lower 90s inland both days.
Models are hinting a very weak streamer-type showers right along
the coast both Saturday and Sunday morning. With the minimal
nature of the showers and low overall area of effect for these
showers, have kept PoP forecast to silent 10s for now. Best chance
of shower of the two days would be Sunday, with some minor
enhancement possible as a vort races across south Texas. As the
next midlevel trough finally progresses east, it will drag the
tail end of a weak front through south Texas on Monday. There will
be a modest shift of winds to the northeast, and it should temper
temperatures a bit, but highs will still reach the lower 80s
Tuesday and Wednesday behind the front. Moisture remains limited
to the near sfc layer, so showers are unlikely during passage.

Now through Friday: Pressure gradient will tighten today as a
surface low emerges into the lee of the Colorado Rockies before
moving away toward the Upper Midwest on Friday. Small Craft
Exercise Caution may need to be advertised on the Laguna Madre
this afternoon, and is likely for the 20-60nm Gulf waters into
tonight. Winds and seas in this area may briefly approach Small
Craft Advisory (SCA) levels tonight, but confidence not high
enough at this juncture for any headlines. More moderate southerly
winds and seas expected by the daytime hours on Friday due to the
somewhat loosened pressure gradient.

Also of note is that Buoy 42020 has been returned to service as of
yesterday afternoon, after an extended outage related to Hurricane
Harvey last August.  Latest observation, as of 2:50am CDT, was for
SE winds at 14 knots and seas of about 2 feet.

Saturday and Monday: Modest southeast flow will continue for the
region over the weekend, generally remaining in the 10 to 15 knot
range. The long fetch along the coast will allow seas to churn up
a bit, with swells around 4 feet. A minor cold front passage is
expected Monday morning, with north winds around 10 knots in its
wake. This will keep the churn going for another day, keeping seas
3 to 4 feet.




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