Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 161801 AAB

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1201 PM CST Sat Dec 16 2017

.DISCUSSION...Updated for latest /18z/ aviation discussion below.


.AVIATION...It`s all downhill ceiling/visibility-wise from here
as the low temperatures and still some dewpoint separation first
closes up (100 percent RH) and as the "cold dome" gradually erodes
and winds lighten further especially overnight through nearly all
of Sunday morning, temperatures and dewpoints will remain steady
then rise stubbornly and slowly together through midday. With the
still chilly temperatures (though not quite as cold as Friday
evening) holding through the morning and contrast from land to
offshore waters (low to mid 70s) locking in with the low December
sun angle, expect only marginal improvement before the end of the
forecast (noon Sunday).

Model guidance is trending in this direction, so confidence
increased in the pessimistic forecast. That, and observations
spreading slowly west from the coast of IFR to LIFR ceilings
first, with visibility to follow due to the lowering ceiling and
light land to sea winds, make the argument as well.

For Brownsville and Harlingen, ceilings are IFR now and will fall
to LIFR within a hour or two either side of midnight - and could
well hold there right through noon Sunday as guidance suggests -
with the light north wind locking in the "dammed" cold air.
Visibility is a bit trickier and will depend on just how low
ceilings go. Confidence is a bit higher near the coast where
temperatures/dewpoints could be just a couple ticks higher than at
McAllen/Miller...but guidance suggesting prolonged LIFR/VLIFR
ceilings farther west which would argue for visibility a bit lower
than guidance. For now, dropped visbility to IFR in all areas (1-2
miles) through 9 AM Sunday and raised to MVFR (3 miles)

Conditions beyond noon Sunday will depend on how quickly winds can
veer to the east or southeast. Given that there is little synoptic
push for this to happen, the NAM-12 light north winds may hold
right through the afternoon and keep ceilings down and visibility
fuzzy. Will defer to next shift for these decisions.




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