Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 160439 AAB

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1039 PM CST Fri Dec 15 2017

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


.AVIATION...Gap in overall precip shield across the region has
built in this evening, as noted by decreasing radar echoes and
higher cigs for KMFE. Radar does show that precip is building
back in from the south and southwest, with ---dz currently at
KBRO. Have kept -dz for KBRO and KHRL through the night, with dz
and lower cigs returning later this evening for KMFE. Lower cigs
and dz are forecast to continue across the region tomorrow. Light
north winds will continue through the next 24 hours at 5 to 10kts.
Transition layer from northerly winds to southerly winds has
started to lower, with north winds confined to the lowest 2 to
3000 feet.


.PREV DISCUSSION... /issued 519 PM CST Fri Dec 15 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Widespread -dz continues to sweep northeastward across
the region this evening, as good overrunning continues behind the
cold front. Winds remain from the north at the surface, but are
beginning to decrease, while winds just off the surface remain
northerly at 20 to 25kts. Winds abruptly shift to southerly at
7000 feet, and jump above 40kts above that level. Expect this
pattern to continue through the night and into tomorrow as the
surface high slowly moves into our area. Models continue to show
the ra/dz effects to wane after sunrise tomorrow, keeping
visibility reasonable. Cigs will remain around 3000 through the
day, but shouldn`t fluctuate much below this level.

PREV DISCUSSION... /issued 252 PM CST Fri Dec 15 2017/

SHORT TERM (Now through Saturday night): In the wake of the cold
front last night, temperatures have cooled since midnight into
the low 50s and upper 40s and remained steady the last couple of
hours. In addition, widespread drizzle and light showers continue
to move northeastward across much of Deep South Texas with very
little in the way of amounts so far. A better push of surface
high pressure into South Texas this afternoon has increased winds
along the coast with a few gusts possibly up to 30 mph. Cool and
messy conditions will continue through the short term period with
moderate northwest winds and likely chances of rain through
Saturday night.

The main synoptic player is a 500mb low near Baja California,
which will finally break away and eject northeastward into North
Texas Saturday afternoon. With very moist air through the column,
Saturday will likely be another wet day. The best chance of rain
will be in the afternoon with a couple embedded shortwaves and an
upper level jet streak of 90 to 100 kts moving overhead. At the
surface, models are in good agreement developing a coastal low,
which will also enhance precipitation and keep winds breezy along
the coast. A few thunderstorms are possible, but instability is
only modest at this point.

Temperatures will be well below normal with temperatures steady or
slowly falling this evening into the upper 40s for most areas.
Overcast skies and precipitation will keep highs down on Saturday
into the low to mid 50s (normal highs are in the low 70s).

LONG TERM (Sunday through Friday): Ridging off the West Coast
extending northeast into the Pacific Northwest is alternatively
blocking meridional flow and then allowing it. In other words,
Gulf of Alaska lows are tending to energize and track south over
the ridge and then down the West Coast. Sunday morning presents
one of the lows, essentially cut off, over the extreme Southwest
U.S., supporting southwest flow aloft over Texas and a continued
feed of upper moisture.

The low will eject over West Texas on Tuesday, as the next mid
level trough begins to make its way south along the West Coast,
eventually stagnating over Northwest Mexico. In the mean time,
Canadian air pushes south and moves through the CWA as a cold
front Friday night. This part is educed from the current model
presentation. The middle of the week is more murky. The GFS is
quicker to move the first mid level low over the southwest across
the southern Plains, thus allowing weak high pressure to filter
south over the CWA. The ECMWF is a little more sluggish with the
low, thus preventing/delaying a wind shift ala the GFS. The ECMWF
keeps more of the energy to the north as well. In both cases the
week will end up mostly cloudy and warmer than average by about
ten degrees.

Now through Saturday night: Adverse marine conditions will
continue in the wake of a strong cold front with moderate to
strong NNW winds this afternoon. Small Craft Advisories are in
effect for the Laguna Madre through at least 4 PM CST and midnight
CST for the Gulf waters. Coastal low pressure developing on
Saturday may enhance the gradient with borderline Small Craft
conditions again. Seas will begin to improve Saturday night and
subside to 3 to 5 feet.

Sunday through Wednesday night: Light north winds will veer to
south by Monday with high pressure spreading over the northwest
Gulf and shifting east. The GFS is trying to switch winds to north
on Tuesday behind a more quickly ejecting shortwave trough, while
the ECMWF hangs onto south winds longer. By Wednesday or Thursday
the models line up again with south winds. Regardless of the wind
direction, wind strength will be light to moderate.


GM...Small Craft Advisory until midnight CST tonight for GMZ150-155-



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