Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 242330 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
630 PM CDT Sun Sep 24 2017

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


.AVIATION...Isolated showers over the central potions of the
region are expected to dissipate within an hour of sunset. VFR
conditions to prevail overnight with exception to brief MVFR
around sunrise from any streamer bands that form over the Gulf
waters and move northwest along the prevailing southeast flow.
Models suggest increasing clouds cover with isolated to possible
scattered convection Monday but with VFR conditions the
predominate flight conditions. Light southeast flow overnight
becomes moderate Monday afternoon with some gusts approaching
20 knots.


.PREV DISCUSSION... /issued 246 PM CDT Sun Sep 24 2017/

SHORT TERM (Now through Monday night): The region will remain
trapped in an area of weakness aloft, as a mid/upper level trough
deepens across the western half of the US and a weak area of low
pressure over the northern Gulf of Mexico and the Gulf coast states
slowly drifts westward. The combination of the weakness aloft, along
with daytime heating, and the sea breeze has allowed for isolated to
locally scattered showers and thunderstorms to develop thus far this
afternoon across primarily the coastal counties. This activity
should spread further westward, diminishing into the evening hours
as daytime heating subsides. The diurnal streamer activity over the
Gulf waters will pick back up by early morning, with a shower or two
possibly making it inland. As the western trough builds eastward
Monday and Monday night, deep layer moisture will be on the
increase, with the dry pocket between 500 and 700 mb effectively
eliminated. Expect better rain chances tomorrow (30 to 40%),
especially across the western three counties, where the best
moisture will reside. Made only minor adjustments to the inherited
temperatures to adjust for increased cloudiness and pops. Highs
Monday may be a bit too high across Zapata and Starr counties, but
will be highly dependent upon exactly when and where convective
activity fires up.

LONG TERM (Tuesday through Sunday): A mid level trough will hover
over the southwest United States with a cut off low forming at its
base. Meanwhile, a low over the north central Gulf will pack it up
and shift toward Florida as a weak ridge forms over coastal Texas,
the northwest Gulf, and the lower Mississippi Valley. A persistent
weak background of surface high pressure will continue over the
north Gulf. Plains high pressure will receive reinforcement from a
Canadian air mass mid to late week, pushing a weak front into the
area. The models bring slight backing wind shifts into the picture
from Thursday morning on, and high pressure will slowly filter into
the area as the upstream high spreads south and east, but a sharp
frontal passage will be difficult to pin down. Relatively high PWat,
above two inches Tuesday through Wednesday night, will support more
widespread rain chances, but forecast QPF values are look less than
extraordinary, so did not go for heavy rain wording at this stage.
Rain chances look better over the far west, with perhaps some
destabilizing influence over north central Mexico from the upstream
cut off. Retained the inherited forecast with minor changes, and
used a model blend as the background.

Now through Monday night: Surface high pressure remains in place
over the southeastern United States through the period. This will
maintain favorable marine conditions as light to moderate
southeast winds and low seas prevail. Rain chances continue
through the period with isolated to scattered showers and
thunderstorms possible.

Tuesday through Friday night: The pressure field will remain rather
broad and flat through the long term, supporting continued light to
moderate winds and low to moderate seas. However, winds will back
from southeast to east Wednesday and Thursday as a new air mass
works into the area, and subsequent reinforcement will further back
winds to northeast Thursday night to Friday night. Ample moisture
pooling ahead of the associated weak front identified to arrive
Thursday to Thursday night will keep at least a slight chance of
showers and thunderstorms in the forecast for the marine areas.


BAYVIEW/PORT ISABEL  79  90  80  90 /  20  30  20  30
BROWNSVILLE          78  92  79  91 /  20  30  20  30
HARLINGEN            77  94  78  92 /  20  30  20  30
MCALLEN              78  96  79  93 /  10  30  20  40
RIO GRANDE CITY      77  97  78  94 /  10  40  30  50
SOUTH PADRE ISLAND   80  88  80  88 /  20  30  20  20




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