FXUS64 KBRO 261149 AAA
Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
649 AM CDT Mon Jun 26 2017
.DISCUSSION...Updated for latest aviation discussion below.
.AVIATION...No significant changes to the previous TAF package ore
reasoning behind it. The relatively wet forecast continues today
as Rio Grande Valley remains parked between the remaining hot
southwestern U.S. upper level ridge and a flatter ridge across the
eastern Gulf and Florida. Fairly deep moisture rolling across the
now summer-warm Gulf (sea surface temperatures in the mid 80s) in
low level easterly flow has built a decent swath of showers, most
of them along and south of the Rio Grande about 25 miles east of
the coast and slowly edging west, with others developing right on
Padre Island in Kenedy County. With morning sounding likely
unencumbered by much of an inversion, expect new convection to
fire along old boundaries including those from the Gulf, most
likely by 9 AM or so along/east of US 77/IH 69E; have kept this
thinking and given well-moistened column expect tropical-type
rains to reduce visibility briefly to IFR levels especially in
stronger cells with thunder.
For the daylight hours, previous forecast covered the timing of
convective onset well and followed this approach, introducing
thunder at Brownsville/Harlingen by 9/10 AM and McAllen by noon/1
PM. With even deeper moisture in place and fact that all terminals
received measurable rain (over 1 inch at both Harlingen and
Brownsville), went with TEMPO groups for late morning through mid
afternoon (from east to west) with IFR visibilty in thunder, then
tailed back to PROB30 for late afternoon with leftover boundaries
and worked-over areas making specifics more difficult.
Expect an evening break in all areas, but perhaps not for long
toward the coast where PROB30 was reintroduced with as time height
sections increase moisture and continued easterly flow over the
warm waters may quickly redevelop showers soon after sunset which
will move onshore sooner than early this morning. Extended the
PROB30 idea through most of the night at Harlingen and Brownsville
with reasonable chances for brief visibility restricting showers
but only to MVFR. McAllen should remain partly cloudy, with
perhaps MVFR ceilings late at night but no mentionable
restrictions for now.
Ceilings will flirt between high MVFR and low VFR with the MVFR
conditions during the heavier showers or thunderstorms.
.PREV DISCUSSION... /issued 404 AM CDT Mon Jun 26 2017/
SHORT TERM (Now through Tuesday): Weak but present 500 mb trough
strung from north to south in the western Gulf roughly along 94W
longitude will remain nearly stationary today, with deep layer
moisture continuing to peak along and east of the axis mainly to 92W
(south of the TX/LA border) today. The trough nudges west toward the
Coastal Bend and Lower Texas coastline tonight and early Tuesday,
focusing the deeper moisture closer to the coast. However, the best
moisture and forcing looks to remain offshore for the short term,
which makes for an ideal set-up for the Valley/Deep South Texas:
Enough moisture for more welcome rain, with the more sustained and
heavier rain-producing clusters remaining over the Gulf.
For the sensible weather, some diurnal considerations were added to
this forecast based on analog situations. First, late night/pre-
dawn precipitation will increase from generally low-topped
convection over the Gulf, moving west and tending to become more
isolated to scattered the farther inland one gets, with no
precipitation across the Rio Grande Plains and Upper Valley. Second,
morning heating will quickly trigger more numerous showers and some
thunder especially along/east of US 77/IH 69E, reaching to the
McAllen metro (uS 281/IH 69C) region by or shortly after noon, with
best chances for more numerous showers/storms through mid to late
afternoon, but fading back to more scattered coverage across the Rio
Grande Plains. Meanwhile, still plenty of deep moisture from the
Gulf to the Lower Valley; while marine layer arguments will reduce
coverage by mid to late afternoon, it won't end it until just before
The proximity to the 500 mb trough tonight into Tuesday will offer
the better opportunity for coverage and even a bit more rainfall
totals than today. Unlike this overnight, expect just a short break
with no rainfall over land near the coast before isolated to
scattered activity begins, before midnight. Numerous coverage could
enter the picture along/east of US 77/IH 69E before daybreak
Tuesday, but with back side of 500 mb trough never moving onshore
confidence is low for such coverage until daytime heating gets going
Tuesday and percolates these areas. The increased coverage and
boundaries will head west before noon and clusters should overspread
all but Zapata County before mid to late afternoon.
Temperatures will be tricky to say the least, driven by timing,
boundaries, residual rainfall, mid to high cloudiness, etc. On
Sunday, McAllen was the "cool" spot and solidly broke its 13 day
spell of triple-digit highs with just an 87 (despite a mid 90s
forecast). Combining the uncertainty in timing with the expectation
of plenty of convection, boundaries, and early onset, went on the
low side of the guidance suite with lower 90s just about everywhere.
As for the rainfall...previous forecasters had the idea right:
Overall, by the time all is said and done with the Sunday-Tuesday
event, all areas will end up in the 1 to 3" range on average.
"Lucky" areas (we all need rain after the 3 week "Canicula" heat
wave) will see 5+ inches. Brownsville/SPI International Airport
received 1.55" and Valley Int'l Airport (Harlingen) 1.05" on Sunday -
quite a way to break through the dry June. Another two days of this
and 5" will be in the cards, as well as a dent in the drought.
Slower moving clusters could produce upwards of 3" which would be in
the nuisance flood range for urban/poor drainage areas, so something
for commuters and drivers to be aware of each day.
LONG TERM (Tuesday Night through Sunday): Continued, but waning,
isolated to scattered shower and thunderstorm activity is expected
Tuesday night through Friday as the 500 mb weakness over the
western and northwest Gulf of Mexico persists but gradually
dissipates. Dry weather will return for Friday night through
Sunday as the mid-level weakness is replaced by 500 mb high
pressure building over the northern half of the Gulf of Mexico.
Near normal temperatures early in the forecast period courtesy of
clouds and rain will rise to above normal levels towards the end
of the period as the 500 mb high takes over.
MARINE (Now through Tuesday): If winds/seas were the only forecast,
conditions would be nearly perfect for boating/fishing trips as
light to moderate winds and slight to moderate seas will be dominant
in the "background". Unfortunately, the "foreground" includes
numerous gusty showers and isolated to scattered thunderstorms.
Areas under or near the storms will experience locally torrential
rains as well as briefly choppy waters and gusty winds over 25
knots. One thing we'll be keeping an eye on near the shore is
potential for quick developing waterspouts mainly each morning, but
perhaps into early afternoon also.
Light/variable winds on land, slow moving east to west cells from
the Gulf, and colliding boundaries over the now sufficiently warm
Gulf and Bay waters (mid 80s) are prime conditions for a few cells
to spin down a spout or two. While summer waterspouts are not as
dangerous as a spring/fall larger mesocyclone (i.e. tornado over
water event), they can still be plenty dangerous and produce gale
force gusts in and nearby.
Tuesday Night through Sunday: Small Craft Exercise Caution and/or
Small Craft Advisory winds and/or seas are expected through a vast
majority of the period as high pressure over the Gulf of Mexico
interacts with a low pressure system that will drop slowly into
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