Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 191744 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1144 AM CST Sun Feb 19 2017

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

.AVIATION...Satellite images and surface observations indicate
mostly high clouds across the CWA late this morning with patches
of low to mid clouds scattered across the area. Ceilings were near
2100ft at KBRO to near 2700ft at KBKS. Expect VFR conditions in
general to prevail across the Rio Grande valley and northern
ranchlands this afternoon into early evening as the atmosphere
remains capped ahead of the 500mb trough across the southwest
United States/northwest Mexico moving eastward across west Texas
and northern Mexico tonight. The cap is expected to erode across
the northern and western portions of the CWA this evening and a
line of showers and thunderstorms...possibly evolving into a
squall line...will develop this evening and move eastward
overnight. MVFR to IFR conditions will develop along and ahead of
this convective boundary tonight as low clouds/patchy fog
develops initially. MVFR to IFR conditions will continue with
showers and thunderstorms reducing ceilings and visibilities late
tonight into early Mon morning.
&&

.PREV DISCUSSION... /issued 602 AM CST Sun Feb 19 2017/
DISCUSSION...Updated for latest /12Z/ aviation discussion below.

AVIATION...A fairly busy forecast situation for the next 24
hours. Fortunately, the timing of the worst impacts for
Harlingen/Valley and McAllen/Miller look to be overnight and prior
to the first outbound and inbound flights of the new workweek.
Brownsville could be a different story, as the line/wavy band of
thunderstorms may slow toward the coast just before daybreak and
could cause delays.

For today, no real problems as any MVFR ceilings early should
scatter out by noon or so based on model soundings lifting the
inversion and mixing out moisture for all but the high levels,
where a decent cirrus canopy should dominate. Winds are already a
bit stronger than earlier forecast to begin the day and observed
speeds of 31 knots at 1K feet indicative of the gustiness, which
has kept all but high MVFR haze/fog away. The winds should dip a
bit at McAllen but likely stay up along the US 77/IH 69E corridor
in the "Valley Wind Machine" zone, and inherited forecast of 20+
knots looked reasonable.

The evening starts quiet enough with southeast winds aiding the
return of low MVFR ceilings. Trouble begins between 03Z and 06Z in
McAllen and quickly spreads to Harlingen/Brownsville toward or
just after midnight. Based on likely to categorical forecast
precipitation went with prevailing thunder with TEMPO groups for
gusty winds and lower visibility and ceilings in a three-hour
window. Capped wind gusts at 30 to 35 knots for now though could
easily be more given the convective setup of high CAPE, dry
adiabatic sounding from 850 mb to 500 mb, and a decent wind field
to boot. The quick movement of the line will work over the
atmosphere from west to east; model soundings for McAllen show low
clouds long gone before daybreak with perhaps some light rains out
of mid decks, and have added a final group to cover. Not
necessarily so for Harlingen and definitely not for Brownsville,
where prevailing thunder forecast (MVFR) was held through 12Z
Monday.
52/BSG

PREV DISCUSSION... /issued 403 AM CST Sun Feb 19 2017/
.Some Potential for Damaging Winds Hail and Brief Heavy Rainfall
Overnight...

SHORT TERM (Now through Monday): The picture for the next 24
hours is becoming more clear as atmospheric profiles and timing of
local features have come into focus. As mentioned in previous
discussions, the prime concern is for a rapidly developing line
(or line segments with bow-shaped structures) along the lee of the
Sierra Madre around sunset, peaking in intensity as it surges
across South Texas by mid to late evening through a few hours
after midnight. Following the line, unsettled weather continues a
bit longer, especially in the Lower Valley and out into the Gulf
where deep moisture remains through mid morning Monday. The
NAM/GFS were used for the meso-alpha scale details, as the ECMWF`s
coarse physical and temporal resolution (available for us) fails
to capture the location and timing of the rapidly developing
activity.

The good news? Residents will have plenty of time today to
prepare for tonight`s hazards.

For today, still plenty of dry layers between the low level soupy
air (dewpoints in the upper 60s to lower 70s) and developing
cirrus canopy to hold precipitation off. In fact, expect to see
some afternoon breaks in the lower clouds in all areas. West of
US 281, winds go slack enough and with atmosphere unstable enough
have left slight chances but only for the northern tier.
Otherwise, only lowered afternoon highs a degree or two based on
the more viscous air mass than on Saturday along with the overall
cloud cover.

By Sunset, perturbation embedded within the middle of the
southwest flow ahead of 500 mb trough moving into west Texas sets
off what appears to be a quick developing quasi-linear convective
system (QLCS) structure. The QLCS races into the Rio Grande
Plains (Zapata/Jim Hogg/Starr) county by or before 9 PM, then
continues to intensify while moving through the mid Valley and
King Ranch by midnight. Of course, the question still to be
answered is whether there will be enough lift to erode what
appears to be a 1 degree C/2 degree F cap especially for the
populated RGV. Signs are leaning more in this direction, before
the line could become somewhat disorganized and/or weaker as it
runs into the strongest cap toward the Lower Texas coast.

The biggest threat of impact from the QLCS would be damaging wind,
and with wind fields fairly impressive above the surface and the
initial dry layer above the inversion supporting wind drag, one
cannot rule out the potential for 70 mph gusts in the strongest
segments. The same drier layer and adiabatic lapse rates between
~850 and 500 mb would also support large hail; should any
cells/segments become discrete the hail size could easily exceed
1" in diameter. And downpours, though relatively brief, could drop
a quick 2 inches of rain and create an urban flood threat in poor
drainage areas. Frequent lightning is guaranteed for some if not
many along the line, as well.

The line will shift to the Lower Valley well after midnight and
momentum will probably carry it off the coast. That said, both the
00Z and now arriving 06Z GFS show a possible few hour slowdown
from Brownsville to SPI and on into the Gulf by daybreak Monday;
any back- building or "training" could become an issue for morning
commuters/school bus riders in these areas and localized flooding
could become a concern in these areas. Farther west, drier air
surges into the mid-upper Valley/Rio Grande Plains by early
morning and may well end the rain completely. For now, kept slight
chances after daybreak along/west of US 281 and ended everything
after noon with some afternoon sunshine making for a decent end to
the day. Have kept thunder in the wording for elevated convection
well behind the initial QLCS, but all should be over by sunset
Monday.

As for temperatures, ECMWF is basically out to lunch with it`s
upper 80s to lower 90s forecast as northwest flow should pick up
in the afternoon and more comfortable (but not cool by any
February definition) air arrives. With 850 temps falling to
between 11 and 13C, mixing will still warm things up but only into
the upper 70s to lower 80s based on dry adiabatic arguments.

LONG TERM (Monday night through Saturday): A mid level low will
be overhead Monday night, though the associated PVA will be just
to the east. Weakening convection will linger over the area Monday
night into Tuesday as the low shifts farther into the Gulf, but
it will be anti-climactic. Wrap around moisture will keep clouds
overhead on Tuesday before drier air from upstream finally takes
control Tuesday night. Mid level ridging will then move in aloft
while the old mid level low shifts away to the southeast. A zonal
mid level pattern will emerge over the area by mid week, with
drier air supporting seasonal weather, though a little above
normal in temperature. Benign weather will follow until a dry
front arrives from upstream late Friday, which itself will only
produce a wind shift to north- west. Used a model blend for most
elements. Highs will be mainly in the 80s while lows will be in
the 50s and 60s through the period. It should be a relatively
quiet week, weather wise.

MARINE:
Now through Monday: Marine layer will be overcome a bit by the
increasing pressure gradient ahead of both upper and surface
systems, so expect caution conditions with both southerly wind
(15 to 20 knots) and seas (nearing 6 feet) by later today and
into tonight. Best time for boating/fishing will be before noon
today.

Tonight into Monday is plain unsettled...and dangerous as well as
whatever mess comes off of the land may be even more organized
and equally hazardous across all of the Gulf waters Monday
morning, with Laguna Madre improving after noon with just a fresh
northwest wind and gradual drying. For the Gulf, background winds
will be all over the place but convection could easily produce
gale-force gusts and rough waters/locally high seas especially
through early afternoon. Best to postpone Monday trips, and
larger craft will need to be aware of everything from the winds
to blinding visibility in heavy rain as well as frequent
lightning.

Monday night through Thursday night: Winds will shift to moderate
northwest to north by Monday afternoon with the arrival of a cold
front, and will remain north through Wednesday as high pressure
spreads over the area. Winds will begin to swing or veer Wednesday
night, becoming south on Thursday with low to moderate seas.
&&

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

$$

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