Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 020548 AAB

Area Forecast Discussion...UPDATED
National Weather Service BROWNSVILLE TX
1148 PM CST Thu Dec 1 2016

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


.AVIATION...Light east winds with cloud decks ranging between 3500-
6000 feet prevail across Deep South Texas late this evening. Light
east winds will shift to southeast by morning. Low level moisture
will continue to increase over the area overnight. MVFR ceilings
will develop later tonight, with IFR ceilings possible towards
sunrise along with light fog. Conditions expected to slowly
improve mid- to late morning with stronger south-southeast winds.
VFR conditions will be possible at MFE and possibly HRL by late
Friday afternoon. MVFR ceilings return to the area Friday evening
as southeast winds continue.


.PREV DISCUSSION... /issued 552 PM CST Thu Dec 1 2016/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Light east winds with cloud decks between 3500-5000
feet prevail across Deep South Texas this evening. Light east
winds tonight will shift to southeast by Friday morning. Low
level moisture will increase over the area tonight. MVFR ceilings
will develop this evening, with IFR ceilings possible after
midnight, along with light fog, persisting through daybreak before
slowly improving with stronger south southeast winds. VFR
conditions will be possible at MFE by late afternoon Friday.

PREV DISCUSSION... /issued 258 PM CST Thu Dec 1 2016/

SHORT TERM (Tonight through Friday Night): Zonal flow aloft will
become more southwesterly tonight as a short wave trough digs
south along the West Coast and cuts off over Northwest Mexico.
Plentiful East Pacific moisture will pump over the area into the
ridge building over East Tex and the Lower Miss Valley, supporting
overrunning. At the surface, high pressure will spread out over
the Southeast United States under the ridge, interacting with
pressure falls upstream over Mexico being induced by the cut off
low. The result will be a weak surface trough forming over South
Texas, in reality the return of a warm front on Friday, with
generally light to mdt local winds shifting to southeast. Rain
chances will increase as well, becoming fairly robust on Friday,
especially over the marine areas. Amounts may range from a few
hundredths to a few tenths over a 12 hour period, though pockets
of heavier accumulation will be possible. Went with thunderless
chances tonight, but with the boundary moving back north and the
upper divergence ahead of the developing cut off, added thunder
for Friday and Friday night. Temps will be well above normal in
this scenario, with 60s tonight and then even 70s Friday night,
with high temps Friday from the mid 70s to near 80.

LONG TERM (Saturday through Thursday): The longer range period
looks pretty interesting over the RGV as a large and deep 500 mb
cut off low digs southwards over northern Mexico and pushes
towards the region this weekend. This 500 mb low will clear out of
the area early next week allowing a cooler airmass to build over
the region for early next week. Following the passage of this 500
mb closed low another sharp and large 500 mb trough will dig
across the western and central states late in the forecast period.

Big problems exist in timing differences with the 500 mb troughs
and closed lows in addition to the passages of the surface fronts
especially starting on Sunday. The latest ECMWF guidance is much
faster with the eastward progression of the troughs/closed lows
versus the GFS. Accordingly the GFS does not push any major cold
front through the area until late Tues/early Wed.

The models do have good agreement in the passage and strength of
the northern Mexico closed 500 mb low which provides better
dynamics over the region and increasing moisture values through
Sunday and Monday. So will go with better pops through the weekend
and on Monday with generally lower pops expected from Tuesday

Overall confidence is not good at all today concerning the longer
term forecast trends. Run to run consistency with the GFS
guidance is pretty stable. However this is offset by the very poor
agreement between the ECMWF and the GFS on timing issues. So
temps will be a blend of the longer range model guidance through
day 7.

MARINE:  (Now through Friday night): High pressure over the Lower
Mississippi Valley will spread out and cover the Southeast United
States, providing an east fetch of moderate winds and moderate but
slowly building seas. Lower Texas Coast winds will veer to southeast
on Fri and will strengthen to fresh Fri night with a weak inverted
surface trough over the area, adding enough wind component to get
winds and seas into SCEC territory.

Saturday through Tuesday...A stronger southerly low level flow
ahead of the 500 mb closed low digging across northern Mexico will
maintain conditions near SCEC/SCA levels for Saturday and Sunday.
As the trough axis digs further northeast and east away from the
region the Surface PGF will then weaken allowing the winds and
seas to remain calmer Mon and Tuesday.




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