Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 161145 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
545 AM CST Thu Feb 16 2017

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

&&

.AVIATION...VFR to prevail through most of the period. However, high
clouds will continue to stream over the area over the next 12
hours or so as an upper-level low- pressure system approaches
from the west. Light northerly surface winds will veer to
southeasterly by mid-day as surface high pressure migrates off to
the east. Clouds will thicken and lower this evening as moisture
continues moving into the region in association with the low, but
CIGs should remain generally VFR. By very late in the period,
rain showers with intermittent MVFR ceilings will be possible. A
lightning strike or two could also occur, mainly for BRO and
possibly HRL, but expected coverage too low to mention in TAF at
this time.

&&

.PREV DISCUSSION... /issued 416 AM CST Thu Feb 16 2017/

SHORT TERM:(Now through Friday):A mid /upper- level low,
currently seen on water-vapor imagery moving ashore near the
border of Sonora/Sinaloa in Mexico, will eventually turn
northeastward, arriving in the Big Bend region this evening and
lifting into the Red River valley by late Friday afternoon. At the
surface, high pressure currently centered over central TX will
move eastward today allowing an onshore flow to return. Low- to
mid-level moisture will increase across northeast Mexico and into
the CWA tonight, and this will allow for a slight chance of
showers, beginning as early as this evening, across the area. Rain
chances become scattered, generally east of Jim Hogg/Starr
counties, after midnight.

Have re-introduced thunder as well for these areas through mid-day
Friday, for several reasons:  Both NAM and GFS indicating possible
lead short-waves ejecting ahead of the main low/trough.  H5 temps
fall to near -18C for at least a few hours around sunrise,
steepening the lapse rates and allowing for MUCAPE in the
neighborhood of 1000 J/kg to develop.  Also, a vort max is progged
to swing through just south of the CWA early Friday morning, as it
rounds the base of the trough.  Working against the thunder threat
is a layer of drier air/capping inversion around H7, as well as a
lack of surface forcing for ascent.  So will just word isolated for
thunder over the eastern areas...with best shot roughly from 09-15Z
Friday.  Mid/upper-level trough axis swings through the CWA as the
morning progresses, moving offshore by the early afternoon hours,
effectively ending precip chances.

Inherited temperature grids for highs today and lows tonight were in
great shape, so only cosmetic changes were made. Still expect
afternoon highs near to just slightly above normal today, with temps
tonight generally in the 50s, except some low 60s near the coast.
Raised temps a few degrees for Friday afternoon, since it appears
that there will be enough clearing in the afternoon for temps to
rebound into the lower 80s, even after a possible round of showers
and a few thunderstorms in the morning.

LONG TERM (Friday Night through Wednesday):Looks like the
beginning of the long term will emulate the saying "the calm
before the storm" as the highly amplified split flow continues to
progress while showing good run to run consistency. A lot of
uncertainty Sunday through Tuesday with the approach and passage
of a large scale upper trough/low then back to a dry and
comfortable end of the long term.

Mid level ridge strengthens over the region Saturday as the
rather deep trough pounds the Pacific coast. High and dry and very
warm temperatures with 850mb temp nearing 20C which should result
in surface temperatures climbing well above 90 degrees along and
west of I-69C.

Sunday through Tuesday is to say the least uncertain and will
exhibit a period of possible adverse weather or maybe not much at
all for our region. The large upper rapidly deepening upper low over
the Desert SW and NW Mexico will be making its approach Sunday
and Monday. Model guidance (MAV/ECMWf) continues to show a
variety of moderate to high pops Sunday into Monday with timing of
when and where the best chances develop remain the biggest
question making this a difficult forecast. Moisture content/depth
looks to be the biggest factor starting our Sunday strong
southerly is expected to develop with low level moisture on the
increase. South winds may be to strong 25-35 mph and moisture may
not be sufficiently deep to get convection going during the day.
Temperatures will be above normal once again thanks to the
possible southerly wind advisory winds, especially in the Mid and
Lower RGV. As we move into Sunday night and early Monday the upper
low will be getting better organized and moisture content will be
at its peak across our region and a line of thunderstorms may
form to our NW and bring about a quick burst of convection, some
strong but not betting on severe, to the CWA. By Monday afternoon
into Tuesday both GFS/ECMWF as well WPC shows the upper low
becoming well stacked and the surface low becomes occluded over
South Texas and maybe right over our CWA. This could result in a
rapid decrease in pops Monday afternoon with possible little to no
rain Tuesday as winds turn and northwest and moisture decreases.
Temperatures will also be a concern with not much in the way of
cold air advection just some cooling due to the cold low passing
overhead. Temperatures may be to warm even though they will be 5
to 10 degrees lower then Saturday and Sunday. More fine tuning to
this period is likely as the entire system makes its entrance and
for us to get a few more model runs under our belt.

If all goes according to plan by Wednesday we will back to dry
and warm and fair conditions as high pressure will building in
from the west.

MARINE:(Now through Friday):  As of 2:50am CST, Buoy 020 reporting
NNE winds at 08G16KT with combined seas of about 3 ft.  Surface high
pressure across the lower Texas coast at this time will move
eastward today allowing winds to veer to the east and then southeast
by later this afternoon.  Wind speeds become more moderate on
Friday, especially for the outer Gulf waters, but no headlines are
expected as seas stay below 3 ft.  Periods of showers and
thunderstorms are anticipated tonight through much of Friday, with
brief heavy downpours and gusty winds possible.

Friday night through Monday...High pressure will dominate the
beginning of the long term, Friday night and Saturday, with a
light south wind and slight sea. The fair conditions are not
expected to last long with a large and strengthening storm system
approaching Sunday and taking its time moving east not clearing
the area until Wednesday morning. South wind will be on the
increase Sunday and may become stronger then what the current
forecast indicates. Small craft advisories maybe needed for the
Laguna Madre and Bay waters Sunday with wind gaining strength and
sea building across the offshore waters through at least Monday
morning. The storm system will bring a chance of showers and
thunderstorms with best chance Sunday night and Monday. Much
lighter offshore winds may develop Tuesday but seas may remain
somewhat agitated as the storm system moves very slowly
northeast.

&&

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

$$

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