Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 180911
AFDBRO

Area Forecast Discussion
National Weather Service Brownsville TX
411 AM CDT Wed Apr 18 2018

.SHORT TERM (Today through Thursday): Southerly winds have remained
elevated overnight, in the 10-15mph range, courtesy of an enhanced
pressure gradient caused by the surface cyclone moving through the
state of Kansas.  Plains high pressure in the wake of this low will
drive a modest surface cold front southward through Texas today.  As
the front approaches, the pressure gradient will gradually decrease,
with wind speeds backing slightly and falling off a bit later in the
afternoon following breezy conditions earlier in the day. With light
zonal flow aloft, temperature guidance has been remarkably
consistent, so no changes were made to today`s forecast high
temperatures.  These should be just a tad warmer than yesterday`s,
amounting to a couple degrees above normal, with Lower/Mid-Valley
temps topping out generally in the upper 80s to low 90s F.

Despite lighter winds overnight, dewpoints in the 60s area-wide will
keep Thursday morning`s lows in the upper 60s to low 70s for a
somewhat muggy feel.  Column moisture will be increasing (to ~1.5"
PW) over the northwestern counties ahead of the aforementioned front
and in advance of a weak wave in the jet-stream.  Front should be
knocking on the door of the northern CWA by sunrise Thursday. Models
show some light QPF developing in the 09-12Z window, so will keep
isolated showers (no thunder) in the forecast overnight, mainly for
Zapata/Jim Hogg/Starr counties.  Better QPF amounts are focused
inland in Mexico where the upslope component will contribute.

The relatively weak front should gradually move southward through
the CWA, reaching the Lower Valley by late afternoon.  Moisture
gradually spreads eastward but diffuses somewhat.  Models pretty
emphatic in killing off the precip chances (only isolated to begin
with) for Thursday afternoon with ridging at mid-levels moving in.
H85 temps are progged to fall 4-6C from today`s values, so have
followed along with latest guidance in taking a couple of degrees
off of Thursday`s max temps, yielding generally low-mid 80s.

.LONG TERM (Thursday Night through Tuesday): 500 mb riding will
prevail across the South Central Plains States late Thurs. A large
500 mb closed low will dig eastwards across the southwestern
States and the South Central Plains Fri/Sat and Sun. As this 500
mb closed low moves east across the Mid Miss River Valley on Sun,
a weak cold front will move through the RGV on Sun. CAA after this
fropa will remain pretty limited on Sun Night/Mon with overall
temps moderating pretty steadily into Tues. In the wake of this
first 500 mb closed low another closed 500 mb low near the Baja
Cali Peninsula will open up as a short wave trough axis and will
eventually pass over South TX late Mon and Tues. The approach and
passage of both of these 500 mb troughs/closed lows will maintain
pretty decent upper level divergence over the RGV this weekend
into early next week. However the deep layer moisture will likely
remain pretty limited through Day 7. However enough moisture will
be present to maintain the risk of isold/sct conv.

The GFS and ECMWF are in decent agreement in temps and pops up
through Sun and then diverge somewhat for the period of Mon and
Tues where the GFS is a bit stronger in the CAA coming in behind
the fropa with the ECMWF showing a warmer and wetter solution
starting Mon.

Will lean closer to the warmer ECMWF solution for temps and will
go with a blend of the GFS and ECMWF for pops through Day 7.

&&

.MARINE (Now through Thursday)...Latest observation from Buoy
42020 as of 3:40 am CDT indicates SSE winds of 19G23KT and seas of 6-
7 ft.  Have cancelled the Small Craft Advisory for Winds on the
Laguna early, as winds have fallen to the 15-20KT range, and worded
SCEC instead.  Have extended the SCA on the 20-60nm Gulf waters
until 10am, but will plan to let it expire for the 0-20nm waters as
scheduled at 7am.  Pressure gradient falls off fairly quickly
between 12-15Z this morning, but expecting it will take a few hours
for winds to respond and for seas to settle below advisory level.
SCEC-level seas likely to persist into the early afternoon farther
offshore.  More benign boating weather sets up for tonight through
Thursday, despite a weak cold-frontal passage Thursday morning.
Moderate onshore winds and seas of 3-4 ft. will be the rule.

Thursday Night through Sunday Night: With the approach of the
large 500 mb closed low from the west, a strong surface low will
develop across much of the middle portion of the country. This
will strengthen the PGF along the lower TX coastline possibly
requiring some SCA`s for either the Bay or Gulf waters through
Sat. The PGF will then start to weaken somewhat as a surface cold
front reaches and moves through the region late Sat into Sun. This
will likely lower the potential for additional SCA`s towards the
end of the CWF.

&&

.PRELIMINARY POINT TEMPS/POPS...
BAYVIEW/PORT ISABEL  85  70  79  68 /   0  10  20  10
BROWNSVILLE          87  70  82  68 /   0  10  20  10
HARLINGEN            88  68  82  65 /   0  10  20  10
MCALLEN              91  70  84  67 /   0  10  20  20
RIO GRANDE CITY      95  69  84  66 /   0  20  20  20
SOUTH PADRE ISLAND   79  71  76  70 /   0  10  20  10

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory until 10 AM CDT this morning for GMZ170-175.

     Small Craft Advisory until 7 AM CDT this morning for GMZ150-155.

&&

$$

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

Short Term/Aviation...53
Long Term...60
Upper Air/Graphicast...62



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