Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 212353 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
653 PM CDT Sat Oct 21 2017

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


.AVIATION...The fog theme remains valid for Sunday morning. Winds
will decrease to very light after midnight, remaining so through
sunrise, with dew points in the 70s. Not expecting a lot of cloud
cover to inhibit radiational effects either. The model guidance
hints at light fog across the area, perhaps more so for the brush
country and ranchlands, but there is an outside chance of locally
moderate to dense fog forming at the TAF sites for a few hours
from roughly around 5 am to 9 am. The fog will hug the ground and
may not be deep, but visibility on the ground may temporarily dip
to a mile or less, and even briefly to a half or a quarter of a
mile. That said, there is too much uncertainty with timing and
duration to place more than tempo IFR groups for visibility in
the TAFs for now, and not looking at dense fog advisory conditions
in the areas surrounding the airports at the moment. Winds should
shift to north by mid morning with the front pushing through in
the afternoon. The best chance for scattered convection will be
late morning to late afternoon as the front moves through, with a
rumble of thunder and brief gusty winds a possibility.


.PREV DISCUSSION... /issued 330 PM CDT Sat Oct 21 2017/

SHORT TERM (Tonight through Sunday Night): Main issue this evening
and overnight is the chances for fog formation across the area.
Last couple nights have seen moisture trapped right along the
surface, with driest air not until above 5000 feet. This has
produced minimal fog, generally as ground fog within the lowest 20
feet of the ground. Overnight forecast sounding shows similar
setup around sunrise, along with slightly increased moisture
pooling ahead of the approaching cold front, so have continued
with the fog forecast through around sunrise Sunday morning.

Cold front gradually progressing through the Panhandle currently,
while the H5 trough beginning to accelerate sewd off the Rockies.
This will help push the front through Texas, arriving sometime
Sunday afternoon. Models have been paring back slowly on rain
chances with the frontal passage, mainly due to the lack of upper
dynamics, which stay well to the north Sunday afternoon. But ample
moisture, a decent forcing boundary in the cold front, and daytime
heating into the 90s should still be able to spark scattered showers
and thunderstorms during passage Sunday afternoon. With quick
progression of the front, and northwest flow aloft setting up
quickly behind the trough, drier air will move in quickly Sunday
evening. This will allow temps to fall into the mid 50s across the
ranchlands and los 60s in the lower valley, depending on how long
the cloud cover takes to move out.

LONG TERM (Monday through Saturday): 500 mb ridging will prevail
over the western states with broad and deep troffing continuing
over central and eastern States. The 500 mb short wave that ushers
through the cold front on Sun will kick out to the northeast
eventually resulting in a closed low forming up over Great Lakes
region. This pattern will allow another reinforcing shot of
cooler/drier Canadian air to push into the RGV on Tues. The ridge
axis over the western States then starts to break down on Wed and
Wed Night as another strong trough axis from Canada pushes south
over the Rockies. As this trough axis strengthens over the the
central States late this week, it will push another cold front
through the region late Fri. Some decent moisture return does
occur ahead of this front and will maintain some mention low end
pops on Fri Night.

The GFS and ECMWF 500 mb fields show decent agreement in the
placement and strength of the over pattern over the lower 48
states through Day 7. Run to run consistency with the GFS MEX MOS
is pretty stable through next Sat with the ECMWF showing more
instability mainly for temps next Sat.

Overall forecast confidence in the longer range period is above
average at this time.

MARINE: (Now through Sunday night): Southerly flow will remain light
overnight, as the next front approaches from the north. Scattered
showers and a few thunderstorms are possible overnight, mainly
further away from shore. The light winds continue tomorrow morning,
slowly shifting northwesterly. The main rush of north winds arrives
later in the afternoon, with winds increasing to 20 knots around
sunset. Advisories will be needed for this event, which should
continue into Monday morning. Gusty north winds will increase to 7
to 8 feet offshore Sunday night, with rough waters on the Laguna

Monday through Thursday Night...The reinforcing shots of Canadian
air will likely maintain moderate to strong bay and Gulf winds and
seas through midweek. The best potential for SCA conditions will
be across the lower TX Gulf waters where a fairly strong PGF will
likely push the conditions there near or above SCA levels through
Wed. The PGF will then relax somewhat as the marine winds shift
with the returning S-SE flow on Thurs and Thurs Night.




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