Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 281440 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
940 AM CDT Wed Sep 28 2016

.DISCUSSION...Made a few generally cosmetic changes to the
forecast in line with current radar, sky, and temperature trends.
Despite high moisture content below 500 mb, low level northerly
flow and weak mid level easterly flow today underneath upper level
ridge will make it difficult to bring anything more than sprinkles
to the mid Valley...similar to what happened on Tuesday. For this
reason, trimmed area of mentionable rains (probability) to the
coastal counties but left sprinkles through midday out to McAllen.
Partly sunny skies farther north indicative of no mention of
rainfall so pulled from all ranchland areas west of US 77 (King
Ranch area) as well.

As for temperatures, 9 AM readings were between 69 and 73 and
adjusted near term values down to match. Bigger question will be
how does the afternoon evolve? With highest relative humidity
between 875-600 mb across most of the Valley, combined with low
level north/northwest winds, and now getting close to October,
expect thick enough cloud cover to hold and keep temperatures down
a couple degrees lower than guidance consensus from McAllen to
Brownsville. Would not surprise if low to mid 80s held once again
today, but with some lifting of the clouds especially over the
mid Valley only nudged to between 85 and 87 for now. Farther
north, with thinner clouds/more sunshine, held the upper 80s to
around 90 up there.

No changes today except to trim sky cover from north to south as
lower layer humidity sinks farther south into the Valley; in fact,
skies could well go clear across many areas but will address in
afternoon update.
52/BSG

&&

.CLIMATE...If you`re not following us on social media, note that
Tuesday September 27 was the coolest day, Valley-wide, since June
4th! This front puts to bed the persistent searing hot days that
dominated July through this past Sunday, and when sunshine returns
late this week into the weekend, temperatures will be more
pleasant and the nights will be even more so with many areas in
the 60s by daybreak each morning. While heat will return at some
point next week, the duration/persistence is no longer an issue as
wind shifts/fronts become more common as we move deeper into
October.
52/BSG

&&

.PREV DISCUSSION... /issued 638 AM CDT Wed Sep 28 2016/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...A NE-SW oriented surface coastal trough has begun to
move inland this morning producing intermittent SHRA at BRO and
HRL. This trough will continue to be the focus for any aviation
weather concerns for the next 6-9 hours. Very brief IFR CIG`s
have been reported at BRO over the past hour or two, but in
general, expect MVFR ceilings and visibility in showers.

Short-term model guidance in good agreement that aforementioned
trough will continue moving inland through around 16-18Z before
washing out/moving south of the area. This should bring periods
of SHRA to BRO and HRL with TEMPO MFVR conditions, along with a
modest wind shift from N to NNE. Leaving SHRA out of MFE for now
as confidence still low that precip associated with the trough
will progress that far inland. Also not confident that showers
will continue at BRO/HRL once the trough axis passes around 18Z,
so will drop mention after that.

After sunset, winds become very light out of the NE with VFR
prevailing through the evening hours. Slight chance of some
reduced visibilities in mist forming later at night, but
confidence not high enough to include at this time.

PREV DISCUSSION... /issued 357 AM CDT Wed Sep 28 2016/

SHORT TERM (Today through Thursday):
A weak NE-SW oriented surface trough (evident on radar and
satellite imagery) is currently positioned from the offshore
waters down through near BRO. In the near-term, adjusted PoP/Wx
through 12Z to keep aligned with these trends. Short-term model
guidance in good agreement that aforementioned trough will finally
"slosh" inland (through roughly the SE one-third of the CWA)
around mid-day today before washing out. With plentiful moisture
in place (precipitable water progged to hang in the 2.0-2.4"
range), this will be the focus for precip today. Not expecting
excessive rainfall with this activity, with generally stable
atmosphere in place beneath ridging aloft, but a quick inch of
rain in a few spots near the coast isn`t out of the question.
After coordination with WPC, the marginal risk for excessive
rainfall along the coast was dropped. Max temps will run right
around normal today.

Overnight, once the surface trough clears the area and/or
dissipates, didn`t see much to keep showers going so only
mentionable PoP`s will be in far SE CWA.  Meanwhile, aloft, a more
northerly flow will begin to establish itself, slowly at first.
Column PW dips to 1.5-1.7" north to 2.0-2.2" south.  Min temps will
be in the mid-70s for RGV and lower 70`s farther north and west.

Into Thursday, NAM/GFS both in agreement that more robust northerly
flow develops at mid-levels with drier air pushing in at the surface
(but not really a front, per se) as well with a surface high surging
down from the Plains. Atmospheric column dries significantly from
north to south on Thursday, with PW falling to 1.5" north to 2"
south by evening per the NAM.  GFS is quicker/more aggressive with
the drying. Trended PoP`s down from north-to-south through the
day...hanging onto some isolated PoP`s across the extreme southern
CWA even into the afternoon, giving a nod to the NAM and another
coastal trough coming ashore south of the area.  If GFS ends up
being closer to reality, those rain chances may need to be dropped
entirely.  Temps creep back up to the low 90`s in the afternoon.

LONG TERM (Thursday Night through Tuesday):
By Thursday night, the surge of dry air at all levels will have
commenced, which will mean the final end of rain chances for the
remainder of September and well into the first week of October. H5
ridge remains in place across Texas as the midlevel flow pattern
remains amplified and slow progressing. At the surface, high
pressure will also dominate Texas, keeping light onshore flow
continuing into early next week. Temperatures will gradually
return to mid to upper 90s through the weekend as the llvl return
flow continues. Have gone a couple degrees above guidance
consensus, as models have consistently underestimates maximum
temps during the past month.

MARINE:
Now through Thursday: Light to moderate generally NE winds will
predominate, hence seas only 2-3 ft. through the short- term with
no SCEC/SCA expected. Isolated to scattered showers and
thunderstorms will occur today which could produce brief bouts of
gusty winds and heavy rain. Showers become more isolated by early
Thursday with threat ending sometime during the day as drier air
moves in from the north.

&&

.PRELIMINARY POINT TEMPS/POPS...
BAYVIEW/PORT ISABEL  85  76  89  72 /  30  20  40  10
BROWNSVILLE          86  75  91  72 /  30  20  40  10
HARLINGEN            86  74  91  71 /  20  10  30  10
MCALLEN              87  75  94  69 /  10  10  20  10
RIO GRANDE CITY      88  73  91  67 /  10  10  20  10
SOUTH PADRE ISLAND   84  79  85  81 /  30  30  40  10

&&

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

$$

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