Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 171736 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1236 PM CDT Sat Jun 17 2017

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

&&

.AVIATION...Mainly VFR the next 24 hours, with possible brief
early Sunday morning MVFR. May also see debris high clouds
drifting south over the area. A mid level high height center over
North Central and Northwest Mexico, ridging east over Texas,
coupled with Gulf surface high pressure, will dominate aviation
conditions. Southeast winds will decrease to moderate tonight but
will again become breezy to windy and gusty on Sunday, supported
by gradient tightening upstream lower pressure. Forecast soundings
show a lowering inversion with a decreasing dew point depression
from midnight to mid Sunday morning. Though the inversion base
will drop below 3 kft, the question will be whether there will be
enough moisture/low clouds to support ceilings. Believe any low
clouds thick enough to produce ceilings will be fleeting, and thus
hard to pinpoint. The best time frame, though, will be early through
mid Sunday morning before stronger winds develop and mix out the
inversion/boundary layer. Will keep prevailing VFR in the TAFs for
now.

&&

.PREV DISCUSSION... /issued 629 AM CDT Sat Jun 17 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Mostly clear skies with southeast winds around 10
knots prevail across Deep South Texas this morning. VFR conditions
expected to continue for the next 24 hours as high pressure
remains in place. Breezy conditions will develop later this
morning with gusts around 25 knots in the afternoon. Southeast
winds will gradually decrease to around 15 knots by late this
evening.

PREV DISCUSSION... /issued 447 AM CDT Sat Jun 17 2017/

SHORT TERM (Today through Sunday): A 500 mb ridge over the southwest
United States and northern Mexico will provide subsidence across
Deep South Texas through the period. Rain-free conditions and above
normal temperatures will prevail across the region through the short
term. High temperatures today and Sunday are expected to range from
the low to mid 90s near the coast to the triple digits across the
western areas, mainly along and west of US 281/I-69C corridor. Heat
index values this afternoon and Sunday afternoon will be in the 105
to 110 degree range at most inland locations. Surface low pressure
across west Texas interacting with high pressure across the Gulf of
Mexico will support moderate to breezy southeast winds today and
Sunday. Low temperatures tonight will generally be in the upper 70s
to around 80 under partly cloudy skies. Relative humidity values
across the west expected to range from 25 to 35 percent this
afternoon with winds at 20 feet 10 to 15 mph and drying fuels. Thus,
monitoring fire weather parameters should be sufficient for now,
with no supplemental fire weather products needed.

LONG TERM (Sunday Night through Friday): The forecast for the
start of the next work week is fairly simple with continued July-
like heat with a slight lessening of surface winds as surface
pressure fields are a bit changed with weak surface high nosing
down into Oklahoma/North Texas but also spreading higher pressures
to west Texas, and likely elongated Gulf low backing winds from
east to northeast in much of the western Gulf. Temperatures
perhaps a degree lower than Sunday but plenty of triple digit
readings along/west of US 281/IH 69C especially near the river,
and mid to upper 90s elsewhere. Could see more mixed layers of
clouds but another day of no rainfall.

For Tuesday through the remainder of the week, forecast is a bit
more difficult - but looking at the big picture clears things up a
bit. Once again this morning, only the ECMWF is rotates the large
blob of tropical moisture to the west and northwest - easing into
the Valley Tuesday night and covering all of Deep South Texas
Wednesday, peaking on Thursday and Friday. All other models, led
by the GFS/GFS Ensemble but followed closely by the Canadian GEM,
NAVGEM, and UKMET favor a much more sheared system which is akin
to a hybrid TUTT/500 mb trough in the central Gulf which
initially takes the blob of deep moisture now in the north central
Caribbean and "zips" it along the east side of the shear axis - as
they have all week - to cover the eastern third of the Gulf and
include the Florida Suncoast through parts of the Panhandle
through Tuesday and Wednesday. After the primary moisture surge
departs with the core of the TUTT energy, additional moisture
hangs back over the Bay of Campeche by Wednesday night/Thursday
and could make a slow west/northwest migration as a second TUTT
(though weak), eventually reaching the Valley and (cross fingers)
providing some chances of rain by next weekend.

We`ve been scratching our heads about the persistence of the
ECMWF - even though its surface feature solution has bounced
around the past few days. What the deterministic model appears to
be doing is rapidly overcoming the 500 mb shear axis with a
westward building Bermuda-type 500 mb ridge by Monday and continue
through mid week. The back-building ridge then steers the deeper
moisture and the associated large gyre (atmospheric circulation
covering hundreds of miles) *westward* beneath it, ultimately
bringing inches of rainfall to the Texas coast.

Meteorologically and climatologically this doesn`t compute too
well. First, it is late June (not late August) and 500 mb shear
axes, especially when associated with the tail of a fairly robust
deeper trough moving into the eastern U.S., are more difficult to
dislodge. Second, analog situations and climatology favor any
initial surface low acting like a "zipper" along the shear axis,
which focuses the lift on the favorable positive side of the
vorticity center(s) and thus the eastern third of the Gulf. Third,
cooler than average sea surface temperatures in the north/northeast
Gulf waters could also be a factor in helping hold the shear axis
in place with only a gradual warming and transitioning of the
entire column as the week wears on.

With all this in mind, have elected to ditch any "blends" and lean
into the model consensus (effectively removing the ECMWF from the
mix) which keeps fair and hot weather as the general theme from
Tuesday through Thursday. Rain chances were removed Tuesday -
though will still need to watch a *southward* moving plume from
the very weak front that reaches northeast Texas Monday night -
for a stray shower or storm across the ranchlands. For consistency
with prior forecasts and the eventual westward edging of the
southern TUTT system that oozes into the Bay of Campeche/Southwest
Gulf Wednesday and Thursday, retained low rain chances across
eastern portions with a slow movement toward the mid/upper Valley.
Note that if the deterministic runs of the aforementioned models
pans out, the dry air circulating around the rejuvenated
"Canicula" heat ridge would mean no rain chances at all...and in
line with the very dry GFS which keeps 12-hourly chances at or
below 10 percent until the very end of the period.

As for temperatures...followed previous forecast(s) closely and
actually bumped up afternoon highs west of US 281 due to better
opportunity for no rainfall Wednesday-Thursday next week.

Finally, and most importantly: Despite the bearish (on rainfall
and stronger winds/coastal impacts) forecast, anytime low pressure
organizes in the central Gulf it will radiate out swells in all
directions, and the threat for true rip currents will be critical
as early as Tuesday afternoon and likely peaking on Wednesday,
with residuals through the week. Still uncertainty on tidal run-up
but potential increases Wednesday from the mid-Gulf low and
perhaps Thursday from a secondary Campeche low when combined with
rising high tide levels. Something to keep an eye on with the
beach season in full swing and the aforementioned July-like heat
continuing unabated.

MARINE (Now through Sunday): Buoy 42020 reported south winds around
19 knots with gusts near 21 knots and seas of 3.9 feet with a period
of 5 seconds at 03 CDT/08 UTC. Moderate southeast winds and moderate
seas will prevail through the period. Low pressure across west Texas
interacting with high pressure across the Gulf will support SCEC
conditions on the Laguna Madre today and Sunday and on the Gulf
waters today and tonight.

Sunday night through Wednesday:
Using the forecast reasoning
above, quieting conditions will arrive Sunday night into Monday as
the gradients all slacken and any seas/swell won`t have arrived
yet. This may be the squeeze day for fishing/boating with light
winds and slight seas.

Tuesday through Wednesday will continue to see light to briefly
moderate backing winds (east to northeast) across Laguna Madre
with perhaps some increase to near caution levels in the offshore
legs south of Mansfield Tuesday night and Wednesday. Seas will be
highly dependent on how much swell develops and propagates west
which may be a function of whether a tropical cyclone comes out
of the initial gyre in the central Gulf Tuesday into Wednesday.
For now, capped seas at 4 to 6 feet over the southeast waters and
2 to 4 feet elsewhere, though swell period may reach 8 to 9
seconds which would be a better situation for folks as wind waves
will be limited, if not near zero, for part of the time.

Being closer to the source of moisture may increase rain chances
especially Tuesday night and Wednesday - but if the
GFS/Canadian/NavGem models are correct, the Gulf may stay dry
through mid week.

&&

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

$$

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