Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Brownsville Texas
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 rain showers 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 rain showers to bro and hrl with tempo mfvr conditions, along with a
modest wind shift from north to north-northeast. Leaving rain showers 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.
Previous 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 southeast 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 southeast County Warning Area. Meanwhile, aloft, a more
northerly flow will begin to establish itself, slowly at first.
Column precipitable water 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 precipitable water 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
County Warning Area 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. 500 mb
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.
now through thursday: light to moderate generally NE winds will
predominate, hence seas only 2-3 ft. Through the short- term with
no scec/Small Craft Advisory 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.
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