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Area Forecast Discussion (AFD)

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

                            
041 
FXUS64 KBRO 210105 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
705 PM CST Tue Feb 20 2018

...Mesoscale Update...

.DISCUSSION...Recent radar trends have indicated some leading
stratiform from convection upstream moving into Starr and Zapata 
Counties this evening. In response to this, increased POPs this 
evening to better reflect the evolving situation. 

The thunderstorm potential looks conditional at the moment. SBCAPE
(~2000 j/kg) and low level backed winds show some potential. The
primary limiting factor is weak low level shear. HRRR has been 
bullish with convection across the west, however, as a capping
inversion increases overnight think shear will not be the only
limiting factor. If storms can maintain and move a little east
into less inhibition and away from stratiform contaminated air,
can't rule out a strong storm or two...again very conditional. 

&&

55/Frye

.PREV DISCUSSION... /issued 535 PM CST Tue Feb 20 2018/ 

..00z Aviation Update...

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...The primary story this TAF cycle will be continued
MVFR/IFR ceilings and brisk southerly winds. Overnight, cloud
bases should remain steady or lower somewhat, perhaps as low as
IFR at times for HRL. Can't rule out some very light shower
activity for HRL. As of the 00z update we are monitoring
convection across northern Mexico, however, not expecting this to
impact our TAF sites at this time. Southerly winds will pick back
up by mid to late morning on Wednesday. Any IFR ceilings should
bump to MVFR or perhaps even VFR for McAllen, especially by
afternoon. 

PREV DISCUSSION... /issued 312 PM CST Tue Feb 20 2018/ 

SHORT TERM (Now through Wednesday Night): Winds have maintained
their gusty character into early this afternoon, with few gusts
topping 35mph but remaining shy of Wind Advisory criterion.
Earlier, also had to nudge PoP's up a bit for the remainder of the
afternoon, per radar trends over the eastern counties and near-
term model guidance over the west.

In the larger sense, Texas remains caught in the SSW flow between
the upper-level trough over the western CONUS and the strong high
parked off the SE CONUS; this won't change drastically in the 
short-term. This evening, GFS and HRRR are pretty bullish with 
scattered showers and a few thunderstorms over Zapata/Jim 
Hogg/Starr counties courtesy of a "ripple" in the H5 flow. NAM is 
a bit further north with the ripple, but have leaned away from it 
for the near-term. Otherwise, another warm and humid night is in 
store with only slight chances of showers for the remainder of the
area. Temps for tonight and Wednesday were changed very little 
from the inherited forecast. Regarding Wednesday, flow aloft veers
to more WSW and becomes a bit drier, making the column moisture a
bit more diffuse. Have kept only isolated coverage PoP's through 
Wednesday afternoon. Temps were teased down just a bit, but most 
areas will see low-mid 80s with some afternoon breezes (but down a
notch from today with the PGF weakening ahead of an approaching 
front).

Speaking of the next cold front, it will be pushing southward
through Texas on Wednesday. 12Z GFS moves it into the
northern/western counties between 00-03Z on Thursday and stalls it
there. 12Z NAM is actually a hare slower but keeps slowly plowing
it forward through the night, essentially bisecting the CWA from
NE-SW by early Thursday morning. Per coordination with other Texas
offices, have leaned into NAM solution for winds/temps/dewpoints
starting Wednesday evening. Some uncertainty here, though. NAM
pushed the last front too far south...but, this time the high
pushing it is stronger (around 1040mb), though still tracks much
more east than south. So, there will be a fairly tight gradient in
min temps from NW to SE Thursday morning, with mid-upper 50s near
APY to around 70 for BRO. Scattered showers and isolated
thunderstorms are expected along and behind the front, with more
isolated activity ahead of it.

LONG TERM (Thursday through Tuesday):  Return flow should redevelop
at the coast on Thursday as a West Coast short wave trough pushes
into the Southwest, supporting pressure falls over the southern
High Plains, with rain chances decreasing as a remnant frontal
boundary retreats north. At least that was the original
thinking...per some late-breaking collaboration with other Texas
offices, will lean more toward the colder NAM solution (though
with some uncertainty) for Thursday/Thursday night, which will 
leave quite a gradient in temps across the CWA with the boundary 
stalled out. Front should finally lift back northward early in the
day on Friday.

The short wave trough will swing deep over the Southwest by 
Friday, with continuing southwest flow aloft over the CWA. Thus, 
the focus for convection will remain well to the north of the Rio 
Grande Valley Friday as the next cold front moves toward south 
Texas on Saturday. A surface cold front is forecast to push 
through Saturday night, but as mentioned the mid level trough will
be well to the north of the area at that time, and unable to 
provide significant supporting upper dynamics. Elevated convection
will develop across the CWA Saturday night with the front. 
Moderate north to northeast winds will develop on Sunday behind 
the front, with lingering showers. The GFS and the ECMWF differ 
slightly on the location/progression and strength of the front, 
with the ECMWF slower and weaker than the GFS on Sunday. 
Differences seem to decrease somewhat Sunday night, however, with 
high pressure settling over the area. High pressure will slide 
east on Monday with winds veering to east and southeast by 
Tuesday. Mostly cloudy skies will develop and stick around Friday 
and beyond, with a chance of rain on Sunday and Monday. The newer 
GFS run keeps temperatures lower by 5 to 10 degrees from Sunday 
afternoon through Tuesday, though still near to slightly above 
normal.

MARINE (Now through Wednesday Night): Mainly moderate SE winds
will persist through much of the short-term, with some backing to
more easterly possible ahead of a cold front Wednesday night over
the northern waters. However, persistent SE fetch across the 
western Gulf will keep Gulf seas elevated, especially for the 
waters 20-60nm out. 6-7 ft. seas will be common, with SCEC likely 
to continue and possibly reaching SCA criterion tomorrow night. 
Scattered showers and thunderstorms also expected for the northern
waters Wednesday night.

Thursday through Sunday night:  Moderate southeast to south winds and
moderate to high seas will prevail through the long term forecast as 
high pressure to the east dominates the Gulf. Forecast wave heights 
are coming in slightly higher than the short period wind driven 
heights one might expect for the predicted wind strength, and it is 
possible that some east to southeast longer period fetch component 
is being incorporated.  Hence, periods of small craft should 
exercise caution to low end small craft advisory conditions will 
be the rule. A front will move into the area Saturday night into 
Sunday, shifting winds to east and then northeast.

&&

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

$$

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Update...55/Frye