Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 210001 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
601 PM CST Fri Jan 20 2017

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

.AVIATION...The main issue in the current suite of TAFs is the
potential for dense fog and LIFR conditions overnight. Model
guidance suggests this fog will develop around midnight tonight,
but the density may very well be determined by the strength of
the surface winds. Tomorrow, the passage of a pre-frontal trough
will produce clear skies and veering winds with full VFR.
&&

.PREV DISCUSSION... /issued 254 PM CST Fri Jan 20 2017/

SHORT TERM (Tonight through Saturday Night): A weak surface
low/trough has developed today over the eastern CWA, with some
cyclonic spin evident in radar and satellite imagery. Precip is
mainly confined to Kenedy county and the northern offshore waters
at this time (just as forecast). This activity will continue
moving off toward the NE, with all precip exiting the area by late
this afternoon. Wind-shift the the NW on the backside of the
trough has already moved through the mid-Valley, so not expecting
any more showers over the population centers in Hidalgo and
Cameron counties.

For tonight, an upper-level trough axis passes with a sliver of
residual low-level moisture remaining in place, with the rest of the
tropospheric column becoming quite dry.  Light SSE surface winds and
absence of lower clouds initially, along with a near-surface
temperature inversion, strongly suggest another round of fog
tonight.  Have focused the fog wording over the eastern two-thirds
of the CWA, as forecast RH`s don`t dip below 90% over most of
Zapata, Starr, and Jim Hogg counties.  MOS guidance is on board in
developing dense fog across the mid-Lower Valley.  Confidence is
pretty high, but will let swing shift pin down the timing on a Dense
Fog Advisory.

For Saturday, a well-organized storm system will be developing
rapidly with a strengthening mid- and upper-level trough over the
Rockies combining with a deepening surface low over the Southern
Plains. This vertically stacked low-pressure area passes well to the
north of the Deep South Texas.  A more westerly component to the
surface winds is expected, allowing for surface dew points to lower
and temperatures to rise on Saturday due to the downsloping effect
off the higher terrain in Mexico. Left inherited max temperatures
alone (running 3-4 degrees above guidance).  Records will be in
jeopardy; please see Climate section below.

Relative humidities will be crashing in the afternoon.  Wind speeds
may approach marginal levels for a Fire Danger Statement for a few
hours, mainly for zones in Zapata and Jim Hogg counties.  Pretty
confident that Red Flag Warning criteria will not be met, so will
let midnight shift evaluate further as needed.

A dry cold front will sweep through the area from the NW after
midnight on Saturday night.  Winds become breezy at 15-20mph by
daybreak, but much stronger winds aloft (~50KT at 850mb) won`t yet
be mixing down to the surface.  Models showing a few more degrees of
cooling (than previously indicated) right behind the front, so
lowered Sunday morning low temps accordingly.

CLIMATE:  Near-record high temperatures are expected on Saturday.
Record highs for Brownsville, Harlingen, and McAllen are 90/87/91F,
respectively. Forecast highs are 89/91/93F.

LONG TERM (Sunday through Friday): A cold front will have already
gone through the CWA to start the long term period...with axis of
the mid level trough over the Arklamiss stretching back towards
the Deep South Texas. Models indicate strong 850mb and 500mb
winds...55kts and 75kts respectively...across the region.
Furthermore...extremely dry air aloft will be in place. Given
clear skies and surface heating...do expect that strong winds and
dry air will be mixed down through the day Sunday. Minimum RH
values will easily fall into the low to mid teens across most of
the region...with reading in the upper teens to mid 20s along the
coast. Surface winds will are expected to be in the 25 to 35 mph
range...with gusts from 40 to 50 mph. A Wind Advisory will likely
be needed within the next 24 hours. Meanwhile....as in previous
discussions...there will be a continued threat of wildfires. The
cured fuels...dry winds...and strong winds will create ideal
conditions will for any fires to spread rapidly out of control
Sunday...and burning of any kind is adamantly discouraged.

The pressure gradients will weaken later on Sunday as the deep 500mb
low moves across the SE CONUS...with mid level ridging building in
and moving across the Southern Plains for the first half of the
week. Flow aloft will be from the NW with surface flow switching
back to the SW to SE. Onshore flow remains limited and any
significant moisture return is not expected.

By mid to late week...the progressive weather continues as the
ridge moves east and yet another 500mb trough diver across North
Texas. This will help push another cold front Wednesday into
Thursday. Given the overall dearth of moisture...it is likely that
this will be a dry front as well. Winds will not be as strong as
the previous system...however given the continued lack of rain and
dry air...there may be some additional wildfire concerns.

After several days of above normal temperatures through mid
week...reading should be cooler with this second front. In
fact...we should see values right around normal to round out the
work week.

MARINE:

Today through Saturday night: Marine fog persisted into mid-day
on the Laguna Madre and near-shore Gulf waters, but webcams on SPI
show that visibility has continued to improve and is now well
above 1 nautical mile. Another rough of marine fog is likely
tonight for the same areas.

Otherwise, generally favorable marine conditions are expected for
the daytime on Saturday as light to moderate offshore flow continues
and seas run at 2-4 ft.  NW winds quickly ramp up after midnight
Saturday night as cold front sweeps through the coastal waters.  A
Gale Watch remains in effect beginning at 3am Sunday.

Sunday through Wednesday: Strong winds behind a cold front will
keep Gale conditions in the forecast through at least Sunday
evening. Sustained NW winds of 30-35kts are expected...with gust
to 45kts. Seas will build to 10 to 13 feet. Even after Gale
conditions lessen...SCAs will likely be needed through Monday
morning. Surface high pressure will settle in across the Lower
Texas Coastal waters. Winds be light and variable Monday before
the gradient tightens as the high shifts east and more moderate S
winds turn. SCAs maybe needed on Tuesday. Another cold front will
pass through the coastal waters on Wednesday...with SCAs possible
as stronger NE winds expected.
&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...Fire Weather Watch from Sunday morning through Sunday afternoon
     for TXZ248>257.

GM...Gale Watch from late Saturday night through Sunday evening for
     GMZ130-132-135-150-155-170-175.
&&

$$

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