Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 181145 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
645 AM CDT Sun Mar 18 2018

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


.AVIATION..."Underrunning" southerly flow overnight has been key
to keeping ceilings at MVFR for most of the night, and even light
mist has been virtually non-existent as we approach sunrise. That
said, if there`s a slight dip in winds around daybreak, some light
mist could form and ceilings could briefly dip to high IFR levels
for a couple hours post-sunrise. Thereafter, no change to the
prior forecast, which breaks clouds gradually by late morning with
hazy sunshine (no visibility restrictions however) by afternoon.
Winds a bit tricky as peak values did rise to near 20 knots on
Saturday, but gradient has further slackened and with flow above
950 mb dropping below 10 knots before shifting northwest higher
up, expect no more than 15-17 knot gusts with sustained winds
fresh but relatively light.

No changes for tonight, either - except to push back the arrival
of ceilings closer to midnight as has been the case the past few
nights. 06Z model guidance still suggesting LIFR as the dominant
cloud base for most areas, and kept this for all but McAllen (MVFR
there). With lighter winds toward daybreak ahead of the Monday
wind shift, a better opportunity for some mist but nowhere near as
low as guidance suggest for reasons given in prior aviation
discussion at 06Z. Held at 4-5SM for now...and that`s no


.PREV DISCUSSION... /issued 350 AM CDT Sun Mar 18 2018/

SHORT TERM (Today through Monday):
The spring we`ve expected, at least for the short term and through
most of the work week, is right in gear: Warmer to much warmer
than average, little to no rainfall, and drying fronts and/or
patterns that combine with the increasing sun angle to gradually
worsen drought conditions while winds and drying fuels increase
the threat for erratic wildfire behavior and growth. For the short
term, we`ll be dealing with all of these issues.

For today (Sunday)...little change to the prior forecast thinking. A
weak shear axis passes through the South Texas Brush Country and
Coastal Bend this afternoon/evening and brushes by the northern
ranchlands late this afternoon. Position of 250 mb jet and low level
potential temperature ridge are ideal for convection...and model
soundings (particularly the GFS) for Hebbronville/Falfurrias are
quite impressive.  Only problem?  Timing, and impressive drying
above the thinning to erasing clouds down low; the atmospheric
energy is indeed moisture starved soon after the potential builds.
There remains a window between 1 and 4 PM (Hebbronville to
Falfurrias) and a little after 4 PM (King Ranch area) for isolated
thunderstorms.  Should a storm or two "go", large hail/gusty winds
are supported by the atmosphere and will add to the hazardous
weather outlook.

Otherwise, the Valley remains dry with an afternoon heat "spike" as
winds turn northwest mainly west of US 281. We reached 97 in Zapata
and 94 in McAllen, with an unofficial mesowest site touching 100
near the river at Falcon Lake on Saturday, and see no reason for
this not to occur again today so will raise temperatures
accordingly.  Dewpoints have been woefully overforecast by models in
recent days out there and will lower by 5 to 10 degrees in the
pocket of northwest winds to account for reality this afternoon.
Morning clouds in most areas will burn off by or shortly after noon
so the sense of the sky will be sunny to mostly sunny.

The low clouds return tonight across the RGV and probably for a time
across the Rio Grande Plains, though the wind shift to the northwest
will arrive right around or just after sunrise out there. One last
muggy night with temperatures not falling below 70 for most
populated areas, as southeast winds kick up a hair as they have done
in recent nights.  For this reason, fog will be limited but did add
patchy wording for 3 mile type action through the Monday morning
commute in these areas.

For Monday, the wind shift quickly surges from the Rio Grande Plains
through the Lower Valley by mid morning; the slightly faster GFS/NAM
are preferred based on the fairly speedy flow aloft that brings
the "dip" which pushes the boundary through. There is just a hint
of low level moisture and lift during the late morning through
noon or so that could kick off light showers behind the wind
shift, but these too favor areas north of the RGV and won`t last
more than an hour in any one location. Soon after, clearing skies
and a cleaner look to the sky overspreads the entire region. A
fresh northwest breeze won`t do much to dent high temperatures,
though slightly lower 850 mb temperatures will bring afternoon
temperatures down about 8 to 10 degrees from today across the
Starr-Jim Hogg-Zapata region, 5 degrees cooler in Kenedy County,
with little change farther east and southeast.

LONG TERM (Monday Night through Saturday):
The long term forecast is dominated by strong H5 ridge across the
central US, keeping the main storm system track well to the north.
This means local weather will continue to be influenced more by
surface features through the upcoming week. On Tuesday, the main
surge of high pressure arrives behind Monday`s front, bringing
breezy conditions along the coast. This will also provide nearly
clear skies with highs in the lower 80s across the region. Onshore
flow returns on Wednesday as the sfc high moves across the
Mississippi Valley. This will help bring Gulf moisture back into
the region, pushing temperatures back into the upper 80s. Gusty
southeast winds continue Thursday through Saturday, with highs
reaching the upper 80s and lower 90s each afternoon.

Now through Monday: Winds and seas at 3 AM were right where we
left them on Saturday morning: around 3 feet with 11 knot
sustained and a few higher gusts from the south/southeast. The
usual diurnal dip will keep winds around 7 to 10 knots and seas 2
to 3 feet in the Gulf, with some gustiness over Laguna Madre
especially north of the bay. Similar winds overnight with a
similar gradient well ahead of the wind shift, which arrives
Monday late morning to early afternoon. Flat flow aloft translates
to a weak "bubble" high of less than 1010 mb and with little cold
air advection expect 10 knot or less wind and slight to moderate
seas at most. A pretty fine situation for boating and fishing both

Tuesday through Thursday: Gusty north winds arrive across the Gulf
waters Tuesday as a secondary surge of high pressure sweeps
southward. Winds will likely reach right around 20 knots,
necessitating short advisories for the day Tuesday. Lighter
easterly flow develops Wednesday with the ridge in the vicinity,
allowing seas to relax down to around 2 feet. Southeast return
flow sets up Thursday as the high moves east and low pressure
deepens across Kansas. Winds will reach the 15 to 20 knot range,
with wave heights increasing to 4 to 5 feet.

FIRE WEATHER...As discussed here and in outlooks the past couple of
days, Monday`s situation is locking in for at least part of the
forecast area. Current forecast indicates afternoon humidity falling
to 10 to 15 percent from western Hidalgo and much of Brooks County
through the Rio Grande Plains, with the longest duration and lowest
humidity likely in Starr, Jim Hogg, and Zapata County.  ECMWF
dewpoints continue to point to upper teens to mid 20s across these
areas, with upper 20s to mid 30s during the rapid dryout farther
east but not including areas along/east of US 77 in the Lower Valley
where some onshore component will neutralize dewpoint falls. In
fact, ECMWF still carries upper 60s in Brownsville, around 60 in
Harlingen, but around 50 in Harlingen and falling to the lower 20s
farther west and northwest. In these areas, increased downward
mixing of 20 to 25 knot winds off the surface is expected, and have
bumped 10m wind forecasts to 15-17 knots for the time of the rapidly
falling humidity.  This translates to a "15/15" rule event - but
with the weekend`s drying (12 percent RH in Hebbronville to Zapata)
we likely have dry to very dry fine fuels setting up to further
enhance the threat.  With all this in mind, will be issuing a Fire
Weather Watch for Hidalgo/Brooks westward.  Thanks for NWS Corpus
Christi for collaborating this morning.


TX...Fire Weather Watch from Monday afternoon through Monday evening
     for TXZ248>250-252-253.



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