Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 042344 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
544 PM CST Sun Dec 4 2016

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

&&

.AVIATION...IFR/LIFR conditions expected to prevail across Deep
South Texas this evening into the overnight hours. As the
surface low and associated cold front move further northeast away
from the area the lower level flow becomes more westerly which
will start to advect in drier air from the west. This will result
in improving ceilings/vsbys up to MVFR/VFR levels by mid to late
morning with full VFR conditions early Monday afternoon.

&&

.PREV DISCUSSION... /issued 337 PM CST Sun Dec 4 2016/

..Roller-Coaster Weather To Dominate First Week of December...

SHORT TERM (Tonight through Monday Night): The 500 mb closed low that
is moving towards south Texas today will move over the region later
tonight into Monday. This will provide the region with plenty of
vorticity advection aloft to maintain good lift. This better PVA
occurring over the region will interact with plenty of deep layer
moisture as indicated on the 12z NAM forecast soundings. So
expect the overall coverage of light rain/rain showers to steadily
increase through the night. All three short term model sets point
towards nearly the 80 to 100 % range through the next 12 to 18
hours and will stick close to this model trend. As the 500 mb
trough axis and associated surface frontal system work their way
northeast along the TX coastline drier air will start filter into
the region from the west to southwest across most layers of the
atms resulting in the pops dropping down quickly and the cld cover
steadily eroding away. The increasing sunshine will allow the
cooler airmass over the region to start moderating somewhat and
expect high temps tomorrow to move up close to the 70 mark across
the lower RGV with mid to upper 60s expected across the northern
counties. The clearing skies...lighter surface winds and fairly
low dewpoints on Monday night will allow the min temps to range in
the 40s west to the 50s east.

The better dynamics moving over the RGV with the closed 500 mb low
will maintain the outside threat of some isold t-storms. So far the
lightning activity has been confined to the more central and eastern
portions of TX today. However as the closed low gets a little bit
closer...isold t-storms cannot be ruled out totally. So will
maintain this mention in the late afternoon and early evening hours.

Short term model guidance remains in pretty good agreement and will
stick pretty close to a model consensus throughout tomorrow night.
Overall confidence in the short term forecast wording is above
average.

Will post a coastal hazard message as tidal obs along the lower
Texas coastline indicate that current tides are running about 1.5
1.7 feet above predicted levels. Web cams verify this showing the
waves breaking pretty high up on the SPI beaches. Our next high tide
near Port Isabel is around 830 pm tonight. So will issue the new CFW
to caution any folks venturing out on the beaches this evening.

LONG TERM (Tuesday through Sunday): Overall, just a few tweaks
to the bulk of the workweek forecast and into the weekend. The big
story will be a true early season arctic-sourced cold front by
Wednesday night/early Thursday, and just how cold the air and
"feels like" temperatures get. Either way, all models are in solid
agreement that the coldest morning of the young winter will
arrive on Friday.

Tuesday is a Chamber of Commerce early December advertisement for
Winter Texans and anyone who yearns for sunny mild weather with
light winds after it being either too warm/humid (Saturday) or a
bit too nippy and gray (today). Preferred the warmer ECMWF with
full sunshine and 850 temperatures rising to ~15C which should
bring lower 80s back to the Valley and upper 70s to the northern
ranchlands after a cool start. As the surface high slides into
the Gulf, light southeast flow returns as zonal 500 mb flow
briefly turns west- southwest. Plenty of deep dry air to mix out
any broken cloud decks early in the day, and continued low level
warming with the afternoon sunshine should bring temperatures into
the mid 80s for the Valley and near 80 farther north.

The front sweeps across the entire region Wednesday night as big
bowl-shaped 500 mb trough heads into the upper Mississippi Valley,
with 1044+ mb surface high surging down the Front Range of the
Rockies. Slight timing differences between the ECMWF/GFS with the
ECMWF a hair slower; given the arctic nature of the front
continued to lean toward the faster GFS with this update.

Blustery northerly flow dominates Thursday, with all indicators
pointing to falling temperatures for at least the first half of
the day and likely falling or maintaining steady values for the
Valley during the afternoon, which nearly 100 percent relative
humidity through 700 mb in a classic "steel gray" modified arctic
sky will block out the sunshine, with some ceiling raising up to
the north and west as drier air mixes in. Exactly what those
temperatures will be remains unclear. Continuing to favor the cold
GFS given the source region of the air mass along with the
overcast conditions...believe the ECMWF guidance is leaning too
heavily on 850-925 mb temperatures/thicknesses with its early day
low to mid 60s values. So, expect non-diurnal temperatures
beginning in the 50s south and 40s north to dip into the mid to
upper 40s in the Valley during the afternoon but perhaps rise
toward 50 with raised ceilings across the Ranchlands. The 20+ mph
winds combined with the cold temperatures will make it feel closer
to 40-44 during the afternoon...time for the winter clothes for
sure for the local residents.

As for precipitation...removed all mention of thunder as any
precipitation follows the front and atmosphere is very stable with
any rain a light, occasional type enough to wet the ground but not
much more. Best chances, still below 50 percent, are for the RGV
late Wednesday night through Thursday evening.

The coldest night arrives overnight Thursday and into early
Friday. The biggest fly in the ointment for how cold it can get is
both continued winds, particularly in the Valley, and still the
potential for a 3-5K foot ceiling there as well. Thinner clouds up
north may be enough for some sheltered locations to edge near
freezing. That said, elected to just nudge temperatures down a
degree or two across all areas, with advection 30s everywhere but
still just above 32 for the ranchlands. For the Lower Valley,
10 to perhaps 15 mph winds will bring wind chills into the lower
30s in the Valley and upper 20s ranches, just above advisory
criteria. Something to keep an eye on as the actual modification
of the air mass shows its hand farther north Tuesday or Wednesday.

Friday looks to remain a bit chilly, and sunshine may find it hard
to make an appearance in the Valley where low level saturation
remains tough to break. This could keep periods of light
rain/drizzle going which will add to the winter holiday "feel" for
a second day. For now, have low to mid 50s but with light winds
that could potentially continue from the north/northwest due to
cool land/warmer Gulf arguments, would not surprise to see
readings in the RGV struggle to break 50.

That changes for the weekend, as the moisture layer finally erodes
in return flow with sunshine mixing out the clouds and allowing
temperatures to return toward seasonable levels (70 to lower 70s)
Saturday and back into the 70s for Sunday.

MARINE: (Now through Monday night)...The close proximity of the
coastal surface low to the lower TX coastline this evening and
tonight will maintain a strong PGF across the Gulf Waters
throughout tonight into Monday morning. The surface low will then
start to shift out to the northeast later in the day on Monday and
Monday night. This will allow the PGF to relax finally which will
cause the Gulf winds and seas to calm down nicely. However will
extend the SCA through midday Monday as some large swells may
linger into the morning hours. Also depending on the timing of the
northeast movement of the surface low the west to northwest flow
on the backside of the surface low may also persist near SCA
levels.

Tuesday through Saturday: Get your boating and fishing fix in
Tuesday or Wednesday, as conditions beyond will be unfit for all
small craft operators and very tough for larger ships. Rapidly
subsiding seas/winds Monday night along with plenty of Sunshine
Tuesday make this *the* day to get out. Wednesday will follow
nicely as well at least for the morning with light winds and
slight seas.

The arctic-sourced front Wednesday night surging across a western
Gulf with water temperatures still some 2 to 4 degrees above
average (lower to mid 70s) for early December guarantees frequent
Gale force Gusts, likely to get near 40 knots at times, from late
Wednesday night through the day on Thursday. Seas will respond in
rapid fashion, reaching 10 feet by daybreak and perhaps to 14 feet
at some point Thursday before slowly subsiding Thursday night but
still plenty hazardous and well above 7 feet.

Seas should slip below 7 feet during the day Friday or Friday
evening, but likely continue to be elevated with plenty of
easterly swell developing as the still-strong surface high moves
to the Eastern Seaboard. Good conditions should return to Laguna
Madre in time for next weekend.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory until noon CST Monday for GMZ150-155-170-
     175.

&&

$$

This product is also available on the web at:
HTTP://WEATHER.GOV/RGV

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