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Area Forecast Discussion (AFD)

Issued by Brownsville/Rio Grande Valley, TX (BRO)

                            
656 
FXUS64 KBRO 230408 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1008 PM CST Thu Feb 22 2018

.DISCUSSION...Satellite observations and local camera networks indicate
that areas of fog continue to persist over the Laguna Madre and 
Gulf waters out to 20 nautical miles. Light northeast winds have 
pushed some of the fog into the coastal land areas. Have updated
the weather grids to extend the coverage of fog across all of 
deep south Texas overnight. No major changes needed. 

&&

.PREV DISCUSSION... /issued 629 PM CST Thu Feb 22 2018/ 

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...A mix of IFR/MVFR ceilings prevail across deep south Texas
early this evening. A warm front is expected to move north across the
region later tonight, with the light onshore flow maintaining the
very moist low-level conditions. LIFR ceilings should return
later this evening and overnight for all Rio Grande Valley terminals.
Patchy fog reducing visibilities to IFR/LIFR will be possible late
tonight, mainly between 08 to 13z. Conditions will gradually improve
by mid to late morning Friday with MVFR or low-end VFR conditions Friday
afternoon as southeast winds strengthen.

PREV DISCUSSION... /issued 337 PM CST Thu Feb 22 2018/ 

SHORT TERM (Tonight through Friday Night): Cold front which pushed all 
the way through the CWA overnight has now pulled up stationary about 
40 miles offshore, oriented roughly SSW-NNE.  Model consensus now 
gradually veers the N-NW surface flow to the west of the front back 
around to onshore as the night goes on and weak coastal troughing 
ejects toward the NE. NAM (which continues as the primary basis for 
the first 24 hours of the forecast) is somewhat slower than the GFS 
to do so, but the end result will be that temperatures and dewpoints 
will creep up overnight, except over the northern/western Ranchlands 
where temps will hold more steady. Will once again have to watch 
convection moving off the higher terrain in Mexico in the SW flow 
aloft and moving across the Rio Grande, with potential impacts for 
roughly the area east of Hwy. 281/I-69C.  12Z GFS was pretty bullish 
on convection in this area, with a 0.5" precip bullseye over Zapata 
county driven by a weak wave aloft.  HRRR has been trending weaker 
with the convection in this area this evening, and forecast 
soundings do not suggest a very favorable environment.  But after 
the past two nights, will respect the recent trend and maintain 
scattered showers and isolated thunder for mainly the western three 
counties.

Also will see areas of fog along the coast, with patchy fog forming 
further inland with the more soupy airmass returning with the light 
onshore flow.  Could become dense in spots but won't try to pin down 
locations just yet.

Temperatures warm up to back up above normal Friday afternoon as SE 
flow becomes breezy with a tightened pressure gradient in place. 
Most areas will see 80s again during the afternoon, depending on 
inland penetration of the warm front.  Column RH wanes so basically 
nil rain chances during the day.  Models indicating another 
disturbance may initiate convection over the Sierra Madre again 
Friday night.  Most of this activity is progged to pass north, but 
could clip Zapata county again so will introduce isolated showers in 
this timeframe.

LONG TERM (Saturday through Thursday): A series of deep 500 mb 
troughs will dig across the southwestern and south central plains
states this weekend. This will push through another pretty mild
fropa on Sun. Surface convergence from the fropa in addition to
better moisture advection occurring ahead of the front will
increase the conv potential for this upcoming weekend. Longer
range MUCAPE values ahead of the front remain pretty unstable for
both Sat and Sun so will maintain the mention of thunder for both
days. The overall moisture levels after the fropa never decrease
that much due to pretty limited dry air advection occurring early
next week. 

Another deep 500 mb trough will dig down south along the West
Coast on Mon and Tues which will rebuild the mid level ridging
over the Gulf of Mex. This will push the old cold front back
northwards as a warm front during the first half of next week with
the better moisture values shifting steadily north away from the
RGV decreasing the conv potential later in the forecast period. 

This West Coast 500 mb closed low will then open up and will shift
east into the south central plains states late Wed/early Thurs
which will push another fairly mild cold front through the RGV.
Moisture advection ahead of this next fropa will be more limited
than the fropa this weekend and will maintain only some slgt chc
pops towards the end of next week. 

Overall model agreement between the GFS and the ECMWF is pretty
reasonable through Day 7 in the overall timing of the 500 mb
trough progressions. The biggest disagreement shows up with the
timing and the position of the last trough axis that tracks over
the south central states. The GFS shows the trough tracking much
farther north versus the ECMWF. So overall confidence in the
longer range forecast wording is above average through Wed and
then drops to average for Thurs. 

MARINE: (Now through Friday Night): Marine Weather Statement for
visibilities of 1-3 nm in fog remains in effect for the Laguna 
Madre and nearshore Gulf waters through midnight tonight. If 
conditions worsen this evening, it may be upgraded to a Dense Fog 
Advisory. Otherwise, winds will veer onshore tonight as the old 
front moves back NW as a warm front. Moderate SE winds kick in 
behind the warm front for Friday, possibly increasing to SCEC 
levels Friday night as the pressure gradient continues to tighten.
Seas build to near 6 ft. as well in the 20-60nm Gulf waters, with
4-5 ft. predominating until then.

Saturday through Tuesday Night: The PGF ahead of the next fropa
this weekend will increase enough on Sat to possibly push the Bay
and Gulf conditions up close to SCA levels due to the stronger
S-SE low level flow. Once the cold front moves through the region
on Sun the PGF will then weaken somewhat with the post frontal
winds allowing for fairly benign marine conditions to prevail into
Mon and Tues. 

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...None.
&&

$$

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