Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 230233
AFDBRO

Area Forecast Discussion
National Weather Service Brownsville TX
933 PM CDT Thu Jun 22 2017

.DISCUSSION...Today was one of the hottest days since records have
been kept across the Rio Grande Valley. Mid June 1998 seems to
have some comparative value - it wasn`t that everyone reached
triple digits, but that *all* inland locations (more than 25 miles
from the coast) appeared to have reached or surpassed 105. The
McAllen 111 was just one of several "second tier" century mark
levels; Edinburg/South Texas International also hit 111, as did
our own tested equipment at Falcon Dam (110). Even normally lower
temperature spots achieved, such as Weslaco/Mid Valley which
reached an almost unheard of 106.

The question, then, is what for Friday? With top soils "flash"
dried given the current heat wave and the peaking provided by
back-side subsidence behind TS Cindy, the atmospheric parameters
look nearly identical to today - and nearly all of those
parameters will go straight into heating with nothing more than a
few fair weather cumulus (which will lift toward high mixing
heights again). Even though an "old school" fixture, the forecast
1000-500 mb thicknesses are only a hair below the observed values
today (near 586 dm in McAllen and 584 dm to the coast) and 850 mb
temperatures are forecast to surge to between 24 and 28C -
which dry adiabatically argues for 39 to 43C (from east to west)
or 103 to 109 once again. Current forecast is just a hair below
this...but when combining a 24 hour period of SSE flow expect
surface dewpoints to come close to guidance suggestions - upper
60s to lower 70s (mid 70s near the coast) except around 60 across
the Rio Grande Plains.

Gridded forecast show 2 to 5 hours of 111+ heat index for the RGV
counties (Starr, Hidalgo, Cameron, Willacy) and saw no reason to
wait so have triggered the advisory to catch the evening news
cycle and have people ready before a Friday when more people tend
to be out and about. The advisory may need to be expanded into
Brooks and Kenedy County which are on the fence, and will defer to
next shift to coordinate the connection to the coastal bend.

Remaining text products will be out shortly.
52/BSG

&&

.PREV DISCUSSION... /issued 702 PM CDT Thu Jun 22 2017/

DISCUSSION...Updated for latest /00Z/ aviation discussion below.

AVIATION...Mainly a wind forecast once again for the next 24
hours. Though Cindy and remnants are trekking north through the
Arklatex overnight, subsidence behind the remaining swirl will
continue for one more day, leaving mainly clear skies overnight
with perhaps a few passing cumulus fractus late as surface to 850
mb flow is just a hair east of south. Not expecting any surprise
MVFR ceilings at this point. For Friday, any cumulus decks will
quickly lift into VFR and by afternoon nothing more than FEW is
expected, raising to 7 or 8 thousand feet at McAllen.

The bigger story is wind. Overnight low level winds hold between
20 and 30 knots and some of this will mix right through midnight
and perhaps beyond, so kept gusts over 20 knots through late night
before sustained winds dip well after midnight. Tight gradient
supported by one more day of west Texas heat-enhanced low will
bring another round of near or just above 20 knot sustained with
gusts in the 25+ knot range for much of the late morning and
afternoon.
52/BSG

MARINE...Winds on Laguna Madre still cranking at 6 PM with gusts
at the land cut (Rincon del San Jose) still over 30 knots and even
in the more marine-layer influenced Bay area were gusting to 27
knots at the USCG station on the bayside of South Padre. Current
conditions and forecast as well as aforementioned low level jet
support advisory conditions (20 knot sustained) for several more
hours and extended advisory through 11 PM for now. May need to
extend further if low level jet keeps cranking but for now expect
a slight downward trend to caution.

Remainder of waters on track as winds have bumped over 20 knots at
Buoy 42020 and seas recovered over 6 feet but now transitioning to
a wind chop event with background swells turning the direction to
the southeast as the night progresses.
52/BSG

PREV DISCUSSION... /issued 258 PM CDT Thu Jun 22 2017/

Short Term (Tonight through Friday Night): Main story and
challenge is the extreme record heat today and anticipated high
heat indices expected Friday. Monthly and all-time record Maximums
being threatened this afternoon. Downsloping south to southwest
winds and the influence of the western ridge are combining to
produce the dangerous heat. A strong pressure gradient has
developed today and is expected to persist tonight and Friday as
departing tropical storm Cindy, high pressure over the Gulf and a
thermal low over west Texas all combine producing the gusty winds.
The persistent stronger winds will keep temperatures elevated
tonight with minimums anticipated to remain in the 80s. Winds are
expected to turn south to southeast Friday which will in turn
increase the dew points but not lower temperatures substantially.
Heat indices are indicated to range between 110-115 degrees across
several locations across the RGV and Western Ranch lands and
possibly stretch northward which will likely warrant a heat
advisory for Friday afternoon. Another very warm and sultry night
is in store for Friday night with the moisture deepening and
pooling in advance of front nearing the northern Gulf coast.
Streamer showers could develop after midnight over the gulf and
could make their way towards the coast by sunrise.

Long term (Saturday through Thursday): Look for a broad 500 hPa
trough across the upper Midwest with anchoring ridges centered over
northwest Mexico and Florida. A shallow weakness will be over the
west Gulf stretching from the Bay of Campeche to Louisiana. A
northward extension of the ridge over northwest Mexico into the
Pacific Northwest will translate slowly east while deeper troughing
develops over the the eastern Great Lakes, which will also shift
slowly east. The mid level over deep South Texas and the RGV will
scarcely be altered. This means that a summer pattern of weak to
moderate east to southeast winds will prevail. Ample moisture, with
pwat values forecast to be around two inches near the coast and near
an inch and a half inland, will ensure a mix of clouds and sun,
warm, slightly above average temperatures and a regular supply of
showers and mostly garden variety thunderstorms. Some of this will
be diurnally driven sea breeze convection, but moisture moving up
the northeast coast of Mexico from what is the cauldron of the
southwest Gulf will provide late night and early morning streamer
activity. The weakness aloft will certainly not act very strongly to
suppress this activity over the next week. High temperatures may
tend to be less extreme than they have been recently due to
increased cloud cover and the presence of shower activity, but heat
index values may still be near the century mark in the afternoons.

Fire weather...Modest relative humidity recovery tonight with
southerly flow remaining gusty. Pressure gradient remains strong
for Friday with humidity levels inching upward as wind direction
turns more southeast. Min RH drops between 25-45 percent in the
afternoon with 20ft winds ranging between 15-20 mph, with higher
gusts, enough to maintain an elevated threat for fire weather
conditions.

MARINE:
Tonight through Friday Night: Pressure gradient will continue to
strengthen tonight as high pressure builds across the Gulf of
Mexico and combines with Low pressure over West Texas and the
remnants of Tropical Depression Cindy. Small Craft advisory
conditions to continue or develop tonight mainly over the Gulf
waters with exercise caution conditions over the Laguna tonight
before increasing to SCA Friday mid-morning. Conditions gradually
improve Friday night as a weak front approaches the northern gulf
coast weakening the pressure gradient.

Saturday through Tuesday: Light to moderate east to southeast winds
and low to moderate seas as the Gulf returns to a seasonal
pattern.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...Heat Advisory from 1 PM to 5 PM CDT Friday for TXZ252>257.

GM...Small Craft Advisory for winds until 11 PM CDT this evening for
     GMZ130-132-135.

     Small Craft Advisory until 7 PM CDT Friday for GMZ150-155-170-
     175.

&&

$$

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