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152
FXUS64 KBRO 200515
AFDBRO

Area Forecast Discussion
National Weather Service Brownsville TX
1215 AM CDT Tue Jun 20 2017

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

&&

.AVIATION...VFR conditions to prevail through the TAF period. East
to southeast maximum wind gusts to near 20 knots can be expected
through the afternoon hours before beginning to subside toward
sunset. Light/variable winds during the late night/early morning
hours.

&&

.PREV DISCUSSION... /issued 632 PM CDT Mon Jun 19 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Aside from a brief period of MVFR possible during the
overnight hours tonight, VFR is likely through the next 24 hours.

PREV DISCUSSION... /issued 305 PM CDT Mon Jun 19 2017/

SHORT TERM (Tonight through Tuesday Night): Some relatively modest
changes made to the short-term forecast, but conditions for most
folks will remain hot and dry.  Mid-level high centered over AZ/NM
will be the major factor, though uncertainty still remains over the
developing tropical/subtropical disturbance currently north of the
Yucatan peninsula.  Also, a band of moisture associated with what
was a dying frontal boundary over North Texas has been somewhat
reinvigorated by convection over that area.  Flow around an upper-
level trough over the NW Gulf is expected to rotate this band of
moisture through the area tomorrow, with the 12Z GFS favoring the
morning hours and the NAM bringing it through in the afternoon. Went
ahead and introduced measurable PoP`s (broad-brushed 20% values)
for tomorrow afternoon, leaning toward the NAM timing and diurnal
heating effects.  Showers would be brief and passing and not affect
most areas.  A rumble or two of thunder could also occur, though.

Lest it be forgotten, the heat will stay on as well. SPS issued
earlier today panning out well, with heat indices running 102-108F
as dewpoints have mixed out to the upper 60s-low 70s in the mid-
Lower Valley. Lows tonight once again in the mid-70s generally.
High temps tomorrow should be in line with those observed today,
or even a touch cooler as H85 temps/1000-500mb thicknesses slip
just a bit, so not expecting Heat Advisory criterion to be met.

A bit more uncertainty creeps into the forecast by Tuesday night,
but models appear to be moving toward a consensus of the tropical
disturbance (or likely a tropical cyclone by then) lying offshore of
the Louisiana or far SE Texas coasts by early Wednesday morning.
This should leave the CWA mainly in subsidence, so have left the
land areas dry through 12Z/Wed.

Coastal run-up/overwash could become an issue by later Tuesday as
swells build and we move closer to New Moon phase...but again, there
is uncertainty regarding this and most effects would hold off until
the long-term portion of the forecast.

LONG TERM (Wednesday through Monday):Latest 12Z the model and
NHC consensus continuing to indicate the tropical low (Invest
93L), possible cyclone, to pass northeast of the our CWA limiting
any impacts to the mainly the gulf coastal waters and the
immediate coast Wednesday and Thursday and possibly lingering into
Friday. The disturbance remains disorganized with the strongest
wind field and convection remaining well east of a broad area of
circulation centered just north of the Yucatan Peninsula. As long
as moderate shear over the Southern and Central Gulf of Mexico is
maintained the less likelihood of significant development is
anticipated. With the current scenario and increased confidence of
the system moving into the NW Gulf, northeast of our region, the
afternoon package will adjust the areal coverage of pops farther
east while reducing the percentages downward. Main impact for
inland areas through Friday will continue to be the extreme
temperatures which are likely to get hotter if the tropical system
remains along the consensus track. Large cyclonic circulation and
subsidence favoring the western side of the tropical low coupled
with the anticyclonic flow around the "Canicula" ridge over the SW
Deserts should provide a dry northwest to north flow allowing for
compressional heating. Latest guidance has bumped up afternoon
high a few degrees

As mentioned the main impacts will be for interest along the surf
zone and over the Marine Gulf waters. Rip current risk will likely
already be in the high category and breakers are expected to be
increasing Wednesday and continue to be hazardous Thursday.
Seas/Swells will be building over the coastal waters and some wind
gust may approach gale force/tropical storm force if the system
strengthens.

Once this system moves inland models suggest an inverted trough
moving west across the Southwestern Gulf into NE Mexico/S Texas late
in the weekend and early next week. Both the GFS/ECMWF and Canadian
are quite bullish on increasing moisture and the chance of rain. At
this point and time will maintain 20 percent Sat-Monday but later
shifts will need to increase pops if this trend persist. An increase
in cloud cover and easterly flow off the Gulf will also allow for
temperatures to moderate down towards seasonal normals,
hopefully, down as much as 4 to 8 degrees.

Fire Weather...There is some concern on how dry it may become
Wednesday and Thursday on the western side of any tropical system
that forms over the Gulf of Mexico. Fuels are already dry with the
extreme heat and lack of rain and forecast is trending much drier
and no significant rainfall the middle of week. Surface humidity
bottom out 20-40 percent, or slightly lower, Wednesday and
Thursday afternoon. Winds are not expected to be strong but gusty
conditions in the afternoon may result in some elevated fire
threat. Also, heat will be extreme and is forecast to be hotter
then recent days. Temperatures to approach or exceed 100 degrees
across the Lower Valley and range 104-109 in the Upper Valley and
Western Ranch lands.

MARINE:(Now through Tuesday Night):  Modest onshore winds and
slight seas in store through most of tonight before winds shift more
N-NW in response to falling pressures over the central Gulf in
association with the aforementioned tropical disturbance.  N winds
may reach 15-20 knots by tomorrow afternoon over the Gulf waters,
but of much bigger impact will be the expected swell generated by
the tropical disturbance.  Swell waves will, of course, depend on
how long the circulation lingers in any one place relative to the
Lower Texas coastline, but wave guidance seems reasonable in
building waves to 7-8 ft. by tomorrow evening and on into Wednesday,
so Small Craft Advisories are probable for that timeframe.  Mariners
should stay alert to the latest forecasts and advisories as the
greatest impacts from the tropical disturbance are expected to be in
the coastal waters.

Wednesday through Saturday...Forecast is highly dependent
on future track and strengthening of the tropical disturbance in the
Southcentral Gulf. Model consensus shows a developing tropical
cyclone moving northwest towards the SE Texas coast but confidence
is below average with the future strengthening of this system. in
any case, mariners can expect an increase in swells and swell period
Wednesday and Thursday. Winds are more uncertain with low confidence
on how strong the system may get and on its closest approach to the
Lower Texas coastal waters, if tropical development does actually
occur. Mariners should prepare for the most reasonable outcome with
the potential for gale force gusts in the outer waters, 20-60nm,
Wednesday night into early Thursday if the model consensus remains
on its current projections. With the system expected to move well
north of the region Friday southerly flow is expected to increase
with a moderate to marginally strong southeast flow and seas
resuming for next weekend.

&&

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

$$

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HTTP://WEATHER.GOV/RGV

19/69