Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 111242 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
642 AM CST Sun Dec 11 2016

.DISCUSSION...Updated for latest /12Z/ aviation discussion below.

&&

.AVIATION...A bit busier of a forecast dealing with IFR conditions
on both ends of the time period. First, for this morning (through
15Z), nearly calm winds combined with steady/slowly rising
temperature *and* dewpoint from the chilly past couple of days as
well as light southerly flow sinking the inversion toward ground has
brought IFR to LIFR conditions with ceiling/visibility (McAllen)
and ceiling elsewhere to start this morning. With winds slow to
pick up expect IFR conditions to continue through 15Z for the
Lower Valley terminals and perhaps through 17Z at McAllen where
surface mixing will take longest.

After the fog dissipates and ceilings lift, the second concern is
wind. Model guidance and low level wind fields showing a good
surge of southerlies for Brownsville and Harlingen, and by noon
expect 20+ knot sustained with gusts just under 30 knots. McAllen,
with less gradient, will see increased winds but several knots
lower. Ceilings will scour out in all areas by mid afternoon.

Then comes tonight. Atmospheric conditions setting up for a good
radiation dense fog event across Deep South Texas with very dry
air above the warm/moist near surface immediately overtop what had
been chilled ground for the past few days. The best opportunity
for 1/4 mile visibility and 200 (or less) foot ceilings should
remain just west and north of all terminals...but it will be close
with the discriminating factor the strength of the surface winds.
For now, situation tends to favor IFR/LIFR ceiling and visibility
with the lower values in Harlingen and McAllen where conditions
will be a bit more favorable. Conditions could come as early as 9
or 10 PM in some spots but rounded up to midnight to keep forecast
short. Later shifts will be able to fine tune the forecast but
potential for delays is high for general aviation flights.
52/BSG

&&

.PREV DISCUSSION... /issued 402 AM CST Sun Dec 11 2016/

SHORT TERM (Now through Monday): It`s out of winter and into
"spring" beginning later this morning in the Valley and by this
afternoon across the ranchlands/Rio Grande Plains as surface high
continues to weaken though will remain lodged from the Upper
Valley through the ranchlands, which will combine with the return
or the warmer and more humid air to set up a dense fog event later
tonight through mid morning Monday.

Aloft, zonal flow dominates, and with this comes deepening dry
air which will gradually return sunshine later today especially
along/west of Highway 77, with the sunshine returning by late
Monday morning after the fog burns off. One thing to keep an eye
on is the lack of wind north and west of the RGV each day, which
could play a few small tricks with afternoon temperatures.

For the sensible weather: Today, expect the low clouds and
patchy/areas of fog, most pronounced north and west of the Valley,
to erode by late morning and clear out completely by afternoon.
The surge of warmer air will kick up southerly winds for much of
the population, with speeds potentially gusting to 30 mph along
the US 77/IH 69E corridor from noon through sunset. Quite a
different feeling day today compared with Saturday, with afternoon
temperatures in most locations 20-25 degrees higher...upper 70s
to lower 80s...so put away the jackets and return to spring
clothing to the rotation if venturing out.

If headed to the beach, could be one more day of elevated water
levels to reckon with. That said, it appears that a slow veer to
the southeast in wave direction as well as falling sea/wave levels
offshore during the afternoon should bring water levels back
yards below the dune line at high tide. For now will let the
coastal flood statement expire but given the returning sunshine
and warmth a few more folks may be on the beach this afternoon, so
will be keeping a close eye.

Tonight has dense fog written all over it, particularly in
locations that don`t see the wind machine which will include all
of the ranchlands and extend through the rural farm communities of
Hidalgo County as well. Guidance highly suggestive of the fog,
but confidence high based on weak pressure gradient that arrives
in these areas ahead of subtle wind shift zone that eases into
south Texas. For the Lower/Mid Valley, just enough southerly flow
should keep widespread dense fog from forming, though expect areas
of fog in general. With the calm winds and temperatures falling
with the dewpoints, the fog should form around or even just before
midnight in the rural areas and the Valley a bit later. Dense Fog
advisories are likely for at least the rural areas and perhaps
more areas, as workweek school and general traffic will be back in
play.

Given the length of night and low level moisture locked in, it
will take several hours for the fog to dissipate with the
ranchlands Rio Grande Plains possibly taking until just before
noon for all of it to dissipate. Thereafter, another mainly sunny
afternoon with more uniform temperatures - though still a shade
cooler (mid to upper 70s) across the ranches with lower to even
mid 80s across the Valley, where winds will be down a bit compared
with Sunday afternoon.

LONG TERM (Monday night through Saturday): The mid level pattern
will remain relatively zonal through much of the long term. Both the
GFS and the ECMWF agree in principle with that scenario. Only late
in the game, roughly Thurs and beyond, will a sharp short wave
trough dive south through the Pacific Northwest and rotate or lift
across the Great Basin. That will support backing in the mid level
pattern to southwest over the fcst area, with a late next weekend
surface front in the offing, but that is just past the end of this
cycle.

While the trough is descending into the CONUS, cold air from Canada
will move down into the Northern Plains on Tuesday. The outbreak
will initially move south along the western side of an upper polar
low over Central Canada, but will be on its own after that, moving
south into Central texas by Wednesday in both long range models, the
GFS and the ECMWF. There will be a brief, mild cool down on Thursday
with the modified air mass and front pushing into the area, with a
wind shift to northeast. Rain chances will remain low through the
forecast with only a slight chance to chance on Thursday. With high
pressure shifting quickly east by Fri, southeast winds will develop
once again, and the previous boundary will move north again as a
warm front. Rain chances on Friday, while possible, were kept on the
low end.

Tuesday and Wednesday will be warm with a few clouds. Temps will be
mainly in the 60s at night and will reach into the lower 80s during
the day for some areas. Slight rain chances will develop Thursday
with increasing cloud cover as a cold front pushes toward the area.
Temps will recover Friday and Saturday as both models advertise
southeast winds, and will push warm Gulf air back over the area. A
storm system will move out of the Rockies and across the plains
Friday into Saturday, with a cold air outbreak racing south with an
expected arrival early Sunday.

MARINE:
Now through Monday...Seas remain elevated across the outer waters
and moderate southerly flow touching advisory level gusts (20+
knots) as of 3 AM. Remaining swell (8 second period) and only a
slow turn of wave direction to the east southeast should keep near
or just above 7 foot seas through the morning over the outer
waters. For this reason will extend the advisory (seas) until 3 PM
this afternoon based on the latest gridded forecast.

Conditions will only gradually subside overnight but should
finally drop to 3 to 5 feet by Monday. Aside from the morning fog,
conditions in Laguna Madre should be fine for fishing on Monday.
Today may see a brief window before the winds kick up, and small
craft exercise caution will be added to this area for the
afternoon.

Monday night through Thursday night...Light to moderate southeast
to south winds and low to moderate seas will prevail through mid
week. Broad high pressure will cover the north and east Gulf
during this time, supporting the onshore flow. On Thursday, fresh
high pressure will move south over the North Gulf, resulting in
light to moderate east winds across the Lower Texas Coast, which
will become moderate southeast by Thursday night.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory until 3 PM CST this afternoon for GMZ170-
     175.

&&

$$

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