Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 210303

Area Forecast Discussion
National Weather Service Brownsville TX
1003 PM CDT Sun May 20 2018

.DISCUSSION...Updated near-term Pop/Wx/QPF grids to account for
radar trends. Bumped up to high-end scattered coverage through 06Z
across northern Jim Hogg and Brooks counties. Left rest of western
zones at 30% for now, as still some uncertainty as to whether
clusters of storms about 100 miles west of the Rio Grande will be
able to survive if/when they move off of the higher terrain. Text
products updated and sent.


.PREV DISCUSSION... /issued 645 PM CDT Sun May 20 2018/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Main forecast challenge is whether the line of showers
and thunderstorms, currently located about 130 miles NW of the
MFE-BRO corridor, can hold together as it approaches. Some of
this activity could merge with/be enhanced by storms moving off
the higher terrain in Mexico as well. In line with current short-
term forecast thinking, will introduce showers and possible
thunder at MFE after around midnight. Showers also a possibility
for HRL and BRO, but confidence significantly lower at this time.
Otherwise, SE winds of 10-15KT with MVFR ceilings will prevail
overnight. Conditions may improve to VFR mid-day Monday; however,
another round of isolated showers and thunderstorms is possible in
the afternoon, again favoring MFE.

PREV DISCUSSION... /issued 322 PM CDT Sun May 20 2018/

SHORT TERM (Tonight through Monday Night): Main forecast issue is
the possibility of rainfall overnight into tomorrow morning, and
how far east it will progress. Currently a broken line of
thunderstorms exists from Del Rio east to Houston, which is slowly
progressing southward. Meso models and recent trends indicate a
better chance for this line to propagate southward along the river
into Deep South Texas. Outflow will continue to help this line
southward, but associated H5 wave will be shifting NEWD through
the rest of the day and tonight, so thunderstorm activity is not
expected to quite hold it intensity as it arrives. Still, Deep
gulf moisture across the region and minimal capping, especially
the western valley, will help the remnants of these thunderstorms
into the region this evening. Have kept a good chance for
precipitation going for the overnight hours, with southerly flow
continuing to bring moisture northward. The next wave that was
expected early tomorrow morning is now expected more during the
afternoon hours, so any boundaries and low level moisture
remaining may be enough to spark more shower and thunderstorm
activity. Have returned PoP chances to the Monday forecast for
this potential.

LONG TERM (Tuesday through Sunday): The large 500 mb closed low
situated over the Desert SW states on Tues will move out to the
northeast on Wed while another large 500 mb closed low will move
into the West Coast from the eastern Pacific Thurs and Fri.
This closed low will also move up to the northeast this coming
weekend pushing into the Pacific NW state by Sunday. Meanwhile,
over the Gulf of Mex a broad weakness in the 500 mb pattern will
persist throughout much of this upcoming work week. Both the GFS
and ECMWF models maintain this broad 500 mb troffing over the Gulf
of Mex through next weekend with the ECMWF showing stronger
development over the mid level troffing versus the GFS. This
alignment of troughs/closed lows to the east and northwest will
maintain persistent 500 mb ridging over the RGV throughout the
upcoming work week. This will maintain above normal temps and
rain free conditions throughout next weekend. The broad troffing
over the Gulf of Mex will maintain the best moisture levels and
associated conv potential over the eastern half of the Gulf of
Mex with the western Gulf remaining on the dry side of the trough

The GFS and ECMWF 500 mb heights are in pretty good agreement for
the synoptic pattern through Day 7. Run to run and model to model
comparisons of the GFS/ECMWF MOS temps and pops throughout next
weekend is pretty consistent with both model sets trending towards
gradually increasing heat for the RGV this upcoming week.

Overall confidence in the upcoming forecast period is above

MARINE (Now through Monday night): With the lack of a
significant surface feature in the central Plains, gradient has
weakened enough to allow winds to relax, and will continue to
slowly decrease during the next 36 hours. Winds tonight will
remain around 15 knots offshore, with lighter winds along the
coast. Winds pick back up to 15 knots along the Laguna tomorrow,
but all areas drop to 5 to 10 after sunset as surface ridging
briefly shifting westward across the northern Gulf.

Tuesday through Friday Night: Fairly broad surface ridging
extending into the Gulf of Mex this upcoming week will maintain
winds and seas below SCA levels.




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