Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 251941

Area Forecast Discussion
National Weather Service Brownsville TX
241 PM CDT Mon Sep 25 2017

.SHORT TERM (Now through Tuesday night): South Texas remains
between the H5 ridge just offshore and the large-scale along the
Rockies. Morning sounding showed PW values of 2.02 inches, which
is similar to GOES16 sounder readings this afternoon. As the ridge
shifts east slightly late tonight, the leading edge of the trough
will begin to push more moisture into the region aloft. Along
with this will be shortwave impulses rotating through the region,
adding instability to the atmosphere. Tonight, rain chances will
remain low, as surface forcing will be minimal with no boundaries
to work with. Tomorrow will see a better shot for showers and
thunderstorms as the next shortwave arrives and influences the
seabreeze. Best chances for rainfall will be further inland closer
to the trough, once the seabreeze gets moving. Increased cloud
cover closer to the surface and increased cirrus aloft will hold
temps down a couple degrees from today, but will still reach the
low to mid 90s ahead of the advancing seabreeze.

.LONG TERM (Wednesday through Monday):

A cut off low should set up over the southwest United States by
Wednesday while ridging centered over the northwest Gulf spreads
out. A persistent weak background of surface high pressure will
continue over the north Gulf, providing a moderate east flow for the
CWA. Plains high pressure will push a weak front into north Texas on
Thursday. The front will arrive Thursday night with a local wind
shift to northeast. The front will push offshore on Friday. High
pressure will move out of Canada over the Great Lakes on Friday, a
portion of which will ultimately spread south over the lower
Mississippi Valley and into east Texas and the northwest Gulf,
reinforcing the ongoing high pressure pattern and prolonging
northeast winds. The proximity of the cut off low could provide a
higher strong weather threat to the upper valley in the early stages
of the long term, but the models seem less eager to do so today
compared to yesterday. Nonetheless, slightly higher pops were
retained for portions of the upper valley. Rain chances over the
remainder of the CWA will be better predicated on the ample low
level Gulf moisture forecast, generally at or above two inches near
the coast, except for a brief decrement on Sunday. Should see a mix
of clouds and sun throughout, maybe leaning toward mostly cloudy,
with near normal temperatures.  Retained the inherited forecast with
minor changes, and used a model blend as the background. The GFS was
slightly wetter in a couple of the periods, but comparable to the
ECMWF in most other respects.


Now through Tuesday night: Minimal chance in the forecast for the
next 36 hours across the northwest Gulf as high pressure remains
well to the northeast, causing modest onshore east to southeast
flow. Wave action will remain consistent through tomorrow in the 2
to 3 foot range. An increase in fetch length later tomorrow will
bring some higher longer period swells closer to shore Tuesday
night. Streamer shower activity will continue tonight and again
tomorrow night, mainly just before dawn.

Wednesday through Saturday night: The pressure field will remain
rather broad and flat through the long term, supporting continued
light to moderate onshore 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 from the north. Subsequent high
pressure reinforcement will further back winds to northeast Thursday
night. Ample moisture pooling ahead of the associated weak front
identified to arrive Thursday night will keep at least a slight
chance of showers and thunderstorms in the forecast for the marine


BAYVIEW/PORT ISABEL  80  91  80  89 /  20  30  30  30
BROWNSVILLE          79  91  78  91 /  20  30  30  30
HARLINGEN            78  94  78  91 /  20  30  30  30
MCALLEN              80  95  79  93 /  20  30  30  40
RIO GRANDE CITY      79  95  78  93 /  20  40  40  30
SOUTH PADRE ISLAND   82  87  81  87 /  20  30  30  30




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