Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 141206 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Brownsville TX
606 AM CST Sun Jan 14 2018

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


.AVIATION...VFR TAFs expected. High pressure will remain in
control today, though a slow eastward shift will allow a light to
moderate southeasterly flow to develop. A thin wedge of moister
air between 900 and 850 mb trapped under an isothermal layer will
provide the ingredients for increasing cloud cover near the coast
today. The VFR ceilings may thicken tonight with a deeper moisture
layer, with southeast winds increasing to moderate on Monday.


.PREV DISCUSSION... /issued 430 AM CST Sun Jan 14 2018/

SHORT TERM (Now through Monday): Mid level ridging will remain
in place over the West today, while surface high pressure
continues to settle across the local area. Winds will become more
southeasterly today as the main part of the high pressure cell
centered over the midwest shifts east with the southern section
still extending back over south Texas. After a cool early morning
start in the 30s and 40s, high temperatures will warm well into
the 60s to near 70 degs today as low level moisture moves back
over the area and dew points rise. Overall cloud cover will
increase today as well, especially near the coast.

The slow but steady warming trend will continue tonight into Monday
as southeast winds become better established. Low temperatures
tonight will rest a couple degrees on either side of 50, while high
temperatures on Monday should make it into the lower 70s. All this
as surface high pressure remains in control and ridging out West
drifts slowly east and narrows in size. A deep mid level low will
drop out of Canada to the Great Lakes region by Monday, with a cold,
high pressure air mass dropping south from the Canadian plains and
continuing south over the American plains on Monday. This cold front
will be the challenge for the long term forecast, see below, as it
reaches deep south Texas and the RGV Tuesday morning.

LONG TERM (Monday Night through Saturday): On Monday night,
atmospheric moisture will be pooling ahead of the aforementioned strong
cold front. This will yield a slight chance of showers, mainly
along the coast and across the northern zones. Temps overnight
will generally be in the 50s, which is a bit above normal. All of
that will change as the cold front progresses southward during
the daytime on Tuesday. Some slight differences in timing amongst
NAM/GFS/ECMWF, but temps in the northwestern ranchlands are
expected to peak around 50 degrees or so early in the day, then
falling into upper the 30s by sunset. The Lower RGV may reach the
lower 60s prior to FROPA. A chance of showers will accompany the
front, with the GFS more bullish on precip chances compared to the
other guidance. Post- frontal, isentropic lifting will keep light
rain and drizzle chances going through Tuesday night.

Latest 00Z run of GFS has trended colder with the post-frontal
airmass, coming into better agreement with the ECM and NAM. Temps
will fall into the 30s most all areas by late Tuesday evening. At
this point, looks "touch-and-go" for a freeze warning as temps
hover in the low 30s for several hours. Aloft, forecast soundings
now suggest that a wintry mix of precip (light rain, sleet,
and/or freezing drizzle) will be possible across the
northern/western ranchlands; this is also supported by statistical
guidance. Still some uncertainty as to how widespread precip will
be up there by the time the coldest air arrives. NAM and ECM
generally drier with PoP`s as dewpoints crash into the teens.
Conditional probabilities of sleet/freezing drizzle quite high,
though, per National Blend statistical output. After coordination
with CRP, have inserted this wintry mix into the forecast from
about midnight through mid-morning Wednesday. Note that precip in
the more populated RGV is expected to remain all liquid at this
time, though future trends will need to be monitored. A wind chill
advisory may also be needed CWA-wide for a similar timeframe as
values fall into the 20s.

Precip chances wane somewhat into Wednesday as the isentropic
lifting loses steam and drier air becomes entrenched over the
western areas. Coastal troughing keeps a chance of rain or perhaps
drizzle nearer the shore, however. Will be another miserable,
gray day with temps only in the upper 30s to 40s. Rain chances
spread back inland on Thursday as an upper-level trough approaches
from West Texas and the surface coastal trough moves northward.
Should see some recovery in temps as CAA winds down, but the
amount of recovery is uncertain. Rain chances should pretty much
end later in the day (except offshore) when the upper trough axis
swings through.

Short-wave ridging moves in for Friday for probably the most
pleasant day of the week. Increasing sunshine should allow temps
to recover into the 60s, despite weak northerly flow. Saturday
looks warmer yet, though with breezy to windy conditions
developing as a surface cyclone in the lee of the Rockies
tightens the pressure gradient. Extended guidance indicates next
(probably dry) front passes sometime in the Saturday night-next
Sunday night window.

MARINE (Now through Monday):  High pressure over the northwest Gulf
will result in light to moderate east southeast to southeast winds,
along with low to mdt seas. Winds will increase slightly on Monday
as Gulf high pressure interacts with lower pressure inland.

Monday Night through Friday: Favorable marine conditions expected
Monday night with moderate SE winds and seas of 3-4 ft. before a
strong cold front arrives mid-day Tuesday and ushers in an
extended period of adverse winds and seas. Small Craft Advisories
are a virtual certainty by mid-day Tuesday. A window for gale
force gusts is possible for a few hours behind the front, but
still some uncertainty on this. Seas quickly build to 8-9 ft. on
the Gulf by Tuesday evening. A coastal trough develops Wednesday
and moves up the coast on Thursday, relaxing the gradient locally
somewhat but keeping seas agitated. SCA`s may need to continue all
the way through the daytime hours on Friday.




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