Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 141722 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1222 PM CDT Thu Sep 14 2017

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

.AVIATION...VFR will continue to be the rule through the next 24
hours. Winds will diminish from moderate to light levels before
increasing again tomorrow.
&&

.PREV DISCUSSION... /issued 644 AM CDT Thu Sep 14 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...VFR prevails for another day as high pressure builds
in from the west. Low level moisture continues to increase but
remains shallow. surface inversion strengthens tonight with a
possibility of some patchy low level clouds developing along with
patchy mvfr ground fog. The lower clouds and and fog are only
expected to last an hour or two either side of sunrise Friday.
Light southeast winds early increase this afternoon with a
occasional gusts to 20 knots.

PREV DISCUSSION... /issued 429 AM CDT Thu Sep 14 2017/

SHORT TERM (Now through Friday): Mid-level ridge will continue
to work slowly eastward as the trough over the Midwest pulls
northeast and the shortwave trough over the W Gulf fills in. This
will continue to provide rain-free and hot conditions across the
CWA today through Friday. Models keep the atmosphere rather dry
despite the establishment of the southeast surface wind. The 500mb
ridge and the persistent deep northerly flow aloft will continue
to provide subsidence allowing for a subtle mixing out of the
shallow surface moisture each afternoon while dew points recover
overnight. As a result, warmer overnight lows this morning and
Friday morning to range from 5 to 10 degrees warmer than the last
several nights. As for high temperatures, Wednesday highs were
slightly warmer then expected (2-3 degrees), and with the
atmospheric conditions about the same today and Friday, maximums
should be very similar ranging from the mid-upper 90s eastern
counties to lower-mid 100s (101-106) mid and upper Valley as well
as the northern and western Ranchlands more typical to mid-Summer
like conditions.

LONG TERM (Friday Night through Wednesday): No major changes made
to the long-term portion of the forecast. In general, upper-level
ridging will hold over Deep South Texas keeping late
(astronomical) summer heat in place. Model consensus is for an
influx of deeper moisture into the area Sunday into Monday, with
the best chances for rain occurring on those two days. However,
models disagree somewhat on the quality of this moisture return,
so some corresponding uncertainty on shower/thunderstorm chances.

The quiet weather conditions in the short-term will continue for Friday
night. High pressure aloft with very dry mid-levels will still be
in place. A large-scale trough is progged to be over much of the
western CONUS, with a tropical system (likely to become TD-16E or
Tropical Storm Norma) well south of the tip of Baja California. On
Saturday, the GFS develops a modest southerly flow at mid-levels,
between the tropical disturbance and the upper ridge, which is
displaced eastward by the larger-scale trough. Column moisture
quickly deepens Saturday night, though it does not appear to be
directly associated with the tropical system. Precipitable water
values reach the 2.0-2.2" range, which would be near the 90th
percentile of climo if it verifies. MEX PoP`s are 30-40% for
Sunday, favoring the eastern half of the CWA. A secondary low
forms offshore of SoCal on Monday, which causes the tropical
feature to quickly eject NE toward the vicinity of ELP.

Meanwhile, the ECMWF holds the ridge a bit stronger over the area,
which deflects more of the deep moisture to the south and west. RH
only deepens to about H7, so lower PoP`s are suggested. The low
offshore of CA doesn`t show up in the ECM solution; rather, the
tropical feature moves more slowly around the eastern periphery of
the mid-level high until it is absorbed by a rejuvenated western
CONUS trough. Given the uncertainty, was hesitant to lean too far
into the GFS solution, so basically just nudged up PoP`s by 5%
during the Sunday-Monday period, keeping them in the slight chance
to lower-end chance category. Both GFS and ECM show the upper
high building back westward over the area, to a greater or lesser
extent, by later on Monday. Still should be enough moisture around
for a few sea-breeze/diurnal showers and storms, though.

Kept slight chance PoP`s in east of Hwy. 281/I-69C for Tuesday,
despite semi-flat ridging aloft taking firmer hold and the
moisture eroding significantly. Beyond that, held PoP`s to silent
levels with broad mid-level high in control.

Bigger story may end up being the late-summer heat, leading up to
the autumnal equinox on 22 Sep. Aforementioned high aloft will
keep temps running 3-5 degrees or so above climatological norms,
while persistent SE surface flow maintains dewpoints in the 70s F
except far northwest. Kept MFE highs at 99F for Mon-Wed, so the
century mark will remain well within reach. Heat indices crank
into the 103-108F range each afternoon from Sunday through
Wednesday.

MARINE (Now through Friday):  Light southeast winds and low seas
are expected for the remainder of the work week with the pressure
gradient across the Western Gulf remaining rather weak. 1016mb high
over Florida to combine with lower pressure across Western Texas
keeping winds and sea at favorable levels.

Friday Night through Tuesday: Persistent surface ridging over the
SE CONUS, coupled with lower pressure over the Rockies, will
drive mainly moderate SE winds across the Lower Texas Coastal
Waters. Winds could reach Small Craft Exercise Caution levels
offshore Monday night and over the Laguna Madre on Tuesday. Seas
remain at 2-3 ft. until this time, then building to around 4 ft.
&&

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

$$

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