Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 290019 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
719 PM CDT Tue Mar 28 2017

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

&&

.AVIATION...Strong south winds will be the main aviation impact as
surface winds gust up to 25 knots through the evening hours. Winds
could decouple enough later tonight for low level wind shear to
develop as winds near 2000 feet approach 50 knots. LLWS weakens
by sunrise Wednesday as the low level jet lifts northeast and a
cold front approaches the RGV. The cold front is expected to move
through KMFE around noon but there is uncertainty if the front
makes it to the coast. Winds will be diminishing in advance of the
front and look to be light NW in wake of the front. Patchy MVFR
cigs are possible but models suggest stratus will be short lived
through midnight with VFR being the predominate condition
overnight. Much drier air will be working its way through the
region as a large upper level storm system passes well to the
northeast providing VFR conditions for much of Wednesday.

&&

.PREV DISCUSSION... /issued 331 PM CDT Tue Mar 28 2017/

SHORT TERM (Tonight through Wednesday Night):  Closed 500-mb low
pressure and a surface low-pressure system still expected to move in
tandem out of the Desert Southwest and across northern Texas during
the forecast period. An initial weak cold front extending southward
from the surface low-pressure system will move from west-to-east
into Deep South Texas and the Rio Grande Valley during the pre-dawn
hours of Wednesday before becoming stalling out near the Lower Texas
Coast Wednesday afternoon. 12Z runs of NAM & GFS in good
agreement that the only real precip threat with this initial front
is across the northern tier of counties between about 09-15Z
where isolated showers and thunderstorms are possible. Drier air
sweeps through in the wake of this boundary, making the temp
forecast for Wednesday a bit difficult. Duration of
westerly/downslope wind component at low levels limited in
duration. Blended latest MET/MAV guidance which seemed fairly
realistic. This had the effect of backing off high temps a couple
of degrees in the RGV, though it will still be seasonally hot,
with low-mid 90s pretty much everywhere.

Onshore flow returns fairly quickly Wednesday evening, pumping up
dewpoints back into the 60-70F range for the SE two-thirds or so of
the CWA.  With the main upper-level dynamics swinging through north-
central TX and the approach of secondary cold front, another short
window for precip/thunder exists between midnight and around sunrise
Thursday. GFS much more robust with the precip chances than NAM
at this point, so some uncertainty remains. SPC has placed the
area in a Marginal Risk for severe weather. This is a
possibility, especially if a momentum-driven squall line reaches
the northern counties. Prog soundings indicate favorable shear and
instability profiles, but overall moisture depth is shallow and a
significant cap remains. Given all this, went with 20-30% PoP`s
for 06-12Z Thursday with isolated thunder mention but downplaying
the severe threat for now.

LONG TERM (Thursday through Tuesday): The forecast period
begins with a weak cold front moving across the lower Texas coast
Thursday morning. Some lingering showers and thunderstorms will be
possible Thursday morning before subsidence increases across Deep
South Texas as a 500mb trough axis moves east over the state
Thursday into Thursday night. Weak zonal flow aloft will continue
to provide subsidence across the region ahead of the next mid-
level storm system approaching from the west. Moisture will begin
to increase across portions of the area Saturday as the next 500mb
low/trough moves over New Mexico Saturday and Saturday night. The
best opportunity for showers and thunderstorms will be Saturday
night into Sunday with rain chances ending by Sunday evening. The
mid-level low/trough axis moves over the Lone Star state Sunday
night into Monday.

MARINE:

Now through Wednesday Night: Currently missing data from both
Buoy 42020 and TABS-K, but with tight pressure gradient in place,
winds will remain hazardous across all of the Lower Texas Coastal
Waters (including the Laguna Madre) through the night. Small Craft
Advisory for Winds is in effect for the Laguna through 8am
Wednesday. Strong winds over the Gulf waters through tomorrow
morning will build seas to 7-9 feet, then slowly subside into the
evening with a SCA in effect through 7pm Wednesday. Winds decrease
to more light-moderate speeds Wednesday night as the pressure
gradient weakens in advance of a frontal boundary. SCEC for seas
likely to persist at least for the outer Gulf waters, however,
with a threat of thunderstorms as well.

Thursday through Saturday Night...A weak cold front moves
offshore the lower Texas coast Thursday morning. Winds will shift
to the northwest and increase in the wake of the front and small
craft advisories will likely be needed for the Gulf waters
Thursday. Moderate to strong northwest winds Thursday will
diminish Thursday night and turn to the northeast. Southeast and
south winds will develop Friday as surface high pressure across
the western Gulf of Mexico moves eastward. The pressure gradient
is expected to increase along the lower Texas coast Saturday with
low pressure strengthening across west Texas. Moderate to strong
southeast will prevail across the coastal waters Saturday and
small craft advisories may be needed for the Laguna Madre and
offshore waters.

&&

.PRELIMINARY POINT TEMPS/POPS...
BAYVIEW/PORT ISABEL  75  88  67  85 /   0  10  20  20
BROWNSVILLE          74  92  66  88 /   0  10  20  20
HARLINGEN            74  93  65  88 /  10  10  20  10
MCALLEN              73  97  66  89 /  10  10  30  10
RIO GRANDE CITY      71  96  63  90 /   0   0  30  10
SOUTH PADRE ISLAND   74  82  67  80 /  10  10  20  30

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory for winds until 8 AM CDT Wednesday for
     GMZ130-132-135.

     Small Craft Advisory until 7 PM CDT Wednesday for GMZ150-155-170-
     175.

&&

$$

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

59...short term/aviation
69...long term



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