000
FXUS64 KBRO 181750 AAA
AFDBRO

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BROWNSVILLE TX
1250 PM CDT SAT MAY 18 2013

.DISCUSSION...UPDATED FOR 18Z AVIATION DISCUSSION BELOW.

&&

.AVIATION...GENERALLY A BROKEN RECORD FORECAST INTO SUNDAY
AFTERNOON. SLOWLY DEVELOPING/MOVING UPPER LEVEL TROUGH ACROSS THE
NORTHERN PLAINS WILL ENHANCE THE SEMI-PERMANENT PRESSURE GRADIENT
BETWEEN THE LEE OF THE SIERRA MADRE AND THE WESTERN GULF FOR ONE
MORE DAY. THAT MEANS GUSTY SOUTH/SOUTHEAST WINDS FROM MID TO LATE
MORNING THROUGH EARLY EVENING WITH SUSTAINED VALUES RAMPING INTO
THE LOW-MID 20S AND GUSTS AT OR ABOVE 30 KNOTS FOR A FEW
HOURS...FOLLOWED BY THE NOCTURNAL DROP TOWARD BUT NOT BELOW 10
KNOTS OVERNIGHT.

AS FOR CLOUDS...DRY AIR WITHIN THE INVERSION ENSURES RAPID MIX-OUT
OF THE LATE NIGHT THROUGH EARLY MORNING SOUP...WHICH WILL FLIRT
WITH THE IFR/MVFR BOUNDARY AT 1K FEET BETWEEN 06Z AND 13Z. ONCE
MIXED...SCATTERED TO OCCASIONALLY BROKEN CIRRUS PREVAILS THOUGH
HIGH LEVEL DRYING APPEARS SUNDAY AFTERNOON.

LAST BUT NOT LEAST...SMOKE FROM THE LAST OF AGRICULTURAL BURNING IN
YUCATAN STATE WILL CONTINUE ON A TRAJECTORY TOWARD THE VALLEY AND
SHOULD LEAD TO HIGH MVFR VISIBILITY RESTRICTIONS UNDER THE
INVERSION OVERNIGHT AND EARLY MORNING AS HAZE/MIST. VISIBILITY
SHOULD RISE ABOVE THRESHOLDS NEAR THE GROUND...BUT HAZE ALOFT
LOOKS TO BE IN THE CARDS FOR SUNDAY WHICH COULD RESTRICT
VISIBILITY A LITTLE IN THE MIXING LAYER...3 TO 5K FEET OR SO...ON
APPROACH.

52/BSG

&&

.PREV DISCUSSION... /ISSUED 626 AM CDT SAT MAY 18 2013/

DISCUSSION...UPDATED FOR LATEST AVIATION DISCUSSION BELOW.

AVIATION...IR SATL IMAGERY SHOWS RAGGED LOW LEVEL SC DECKS
PERSISTING MAINLY EAST OF HIGHWAY 281 AND HIGH LEVEL CIRRUS MOVING
OVER THE REGION. THE LATEST NAM FORECAST SOUNDINGS FROM THE LOWER
RGV INDICATE THAT A FAIRLY MOIST AND SHALLOW LOW LEVEL INVERSION
WILL PERSIST ON THROUGH THE EARLY MORNING HOURS LIKELY MAINTAINING
IFR CEILINGS THROUGH THE FIRST PORTION OF THE TAF PERIOD. AFTER
SUNRISE THE DAYTIME HEATING AND INCREASING LOW LEVEL MIXING WILL
ERODE AWAY THE CEILINGS ALLOWING VFR CONDITIONS TO PREVAIL. THE
NIGHTTIME LOW LEVEL INVERSION WILL THEN REFORM AGAIN LATE SAT NIGHT
POSSIBLY ALLOWING FOR SOME MORE PATCHY LOW SC FROM 06 TO 12Z SAT.

FOR AVIATION INTERESTS HEADING SOUTH INTO MEXICO MOUNT
POPOCATEPETL...LOCATED SOUTHEAST OF MEXICO CITY...HAS GOTTEN MORE
ACTIVE OVER THE PAST COUPLE OF WEEKS PERIODICALLY RELEASING ASH
CLOUDS OVER SOUTHERN MEXICO. SO FAR NONE OF THESE ASH CLOUDS HAVE
HAD ANY IMPACT TO AVIATION INTERESTS IN THE LOWER RGV. HOWEVER
PILOTS FLYING INTO MEXICO SHOULD PAY ATTENTION TO THE LATEST VOLCANIC
ASH ADVISORIES ISSUED FROM THE WASHINGTON VOLCANIC ASH ADVISORY
CENTER (VAAC). THESE ADVISORIES CAN BE FOUND AT THE FOLLOWING WEB
ADDRESS...WWW.OSPO.NOAA.GOV/PRODUCTS/ATMOSPHERE/VAAC

PREV DISCUSSION... /ISSUED 359 AM CDT SAT MAY 18 2013/

SHORT TERM /TODAY THROUGH SUNDAY/...HEAT...HUMIDITY AND HAZE WILL
BE THE ISSUE THROUGHOUT THE SHORT TERM PERIOD AS MID LEVEL RIDGING
REMAINS IN CONTROL OVER THE REGION TODAY. THIS RIDGING WILL START TO
ERODE AWAY AS A BROAD 500 MB TROUGH/CLOSED LOW PUSHES EAST ACROSS
THE WESTERN AND CENTRAL PORTION OF THE LOWER 48 STATES. ALTHOUGH THE
MID LEVEL RIDGING STARTS TO ERODE TOMORROW 1000-500 MB THICKNESSES
WILL REMAIN PRETTY ELEVATED. THE WAA WILL ALSO CONTINUE FROM THE
SOUTH ACROSS DEEP SOUTH TEXAS AS AN ELONGATED SURFACE LOW PRESSURE
SYSTEM AHEAD OF THE MID LEVEL TROFFING/CLOSED LOW MAINTAINS THE
SOUTHERLY LOW LEVEL FLOW.

THE NAM TEMP GUIDANCE APPEARS TO BE DOING THE BEST JOB REFLECTED
OVERALL TRENDS VERSUS THE ECMWF AND THE MAV WHICH APPEARS TO BE
LAGGING BEHIND ON THE MAX TEMPS. WILL BE GOING AT AND ABOVE MET
NUMBERS FOR HIGHS AND A BLEND OF THE MAV AND MET NUMBERS FOR MINS.

THE LATEST OUTLOOK FROM TCEQ INDICATES THAT SMOKE AND HAZE CONTINUES
TO ADVECT NORTH OVER DEEP SOUTH TX FROM AG FIRES BURING IN SOUTHERN
MEXICO AND CENTRAL AMERICA. SO WILL MENTION HAZE THROUGHOUT TODAY
AND SUN MAINLY ACROSS THE CENTRAL AND EASTERN PORTIONS OF THE CWA.

LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
BREEZY CONDITIONS ARE EXPECTED TO CONTINUE THROUGH SUNDAY NIGHT INTO
MONDAY WITH AN INITIAL PLAINS TROUGH MOVING NORTHEASTWARD INTO THE
GREAT LAKES AND A SECOND TROUGH DIGGING INTO THE PANHANDLES REGION.
THE WARMEST MAX TEMPS FOR THE WEEK WILL PROBABLY OCCUR MONDAY WITH A
DRY PREVIOUSLY SUBSIDED COLUMN NOT HAVING MODIFIED YET FROM THE
APPROACHING TROUGH.

THIS TROUGH WILL LEAD TO A BIT OF A COURSE CHANGE IN THE FORECAST
THROUGH THE MIDDLE PART OF THE WEEK. THE 00Z GUIDANCE SUITE HAS
CONTINUED A TREND WITH DIGGING A FAIRLY STRONG UPPER TROUGH FURTHER
SOUTH THAN IT DID EARLIER IN THE WEEK. THIS PRODUCES STRONG LAYER WARM
ADVECTION IN THE LOWEST 200MB OR SO...AT THE VERY LEAST INCREASING
LOW/MID CLOUD COVER ON TUESDAY...BUT GFS/ECMWF FORECAST SOUNDINGS
POINT TOWARDS ENOUGH MOISTURE RETURN AND LARGE SCALE ASCENT TO ALLOW
FOR ABOUT 2500 TO 3000 J/KG OF SBCAPE TUESDAY AFTERNOON OVER MAINLY
OUR NORTHWESTERN COUNTIES. IN RESPONSE TO THE TROUGH...AT LEAST
MODEST LOW LEVEL CONVERGENCE WILL BE IN PLACE WITH A WEAK SFC LOW
FORMING IN THE LEE OF THE SIERRA MADRES. CONVECTION ALSO APPEARS TO
INITIATE ALONG A FRONTAL BOUNDARY OVER THE HILL COUNTRY AND ON THE
MEXICAN MOUNTAINS AND THE FLOW PATTERN WOULD FAVOR IT MOVING INTO
DEEP SOUTH TEXAS IF IT WERE TO KEEP GOING. WITH THAT
CONSIDERED...ADDED A SCHC OF SHRA/TSRA IN THE MORE FAVORED TIME
FRAME FOR ACTIVITY...MAINLY LATE TUESDAY AFTERNOON IN THE
NORTHWEST SHIFTING EAST/SOUTHEASTWARD OVERNIGHT TUESDAY.

THE AIRMASS REMAINS UNSTABLE WEDNESDAY WITH LITTLE CAPPING IN PLACE
DUE TO SOUTHEASTERLY 850MB WINDS AND BROAD DIFFLUENT FLOW ALOFT.
SOURCES OF SURFACE CONVERGENCE ARE MORE LIMITED BUT WITH THE
REASONABLE POSSIBILITY OF PREVIOUSLY GENERATED BOUNDARIES IN THE
AREA...INCLUDING AT LEAST A SLIGHT CHANCE OF SHRA/TSRA SEEMS
PRUDENT. THE SOUTHEASTERLY LOW LEVEL FLOW CONTINUES THROUGH THURSDAY
BUT BY THEN UPPER RIDGING IS BUILDING IN AHEAD OF A PACIFIC STORM
SYSTEM AND SUSPECT SUBSIDENCE INVERSION WILL BE STRONG.

LOOKING AT THE POLAR JET STRUCTURE A FAIRLY COMPLEX EVOLUTION WILL
PLAY OUT IN TERMS OF HOW FAR SOUTH THE UPPER TROUGH WILL DIG AND
WHETHER WE WILL BE IN PLAY FOR RAIN/THUNDER. THE MOMENTUM GETS
MAINLY TO THE EAST SIDE OF THE PLAINS TROUGH AS IT MOVES INTO THE
PANHANDLES REGION MAKING ME SKEPTICAL AS TO ITS CONTINUED
SOUTHEASTWARD DIGGING...HOWEVER RIDGING BUILDING AHEAD OF A SHARP
NEGATIVELY TILTED TROUGH IN THE PACIFIC NORTHWEST WOULD PROBABLY
KEEP ENOUGH OF A KICKER JET ON THE BACKSIDE OF THE TROUGH TO KEEP IT
MOVING TO ABOUT WHERE THE GFS/ECMWF HAVE IT LOCATED. ENSEMBLE
GUIDANCE IS IN REASONABLY GOOD AGREEMENT WITH THE DETERMINISTIC RUNS
SO CONFIDENCE IS SLIGHTLY HIGHER IN A MORE PRECIP FRIENDLY PATTERN
FOR THE AREA...THUS THE CHANGE TO THE FORECAST. STILL KEPT THE
POPS LOW THOUGH DUE TO THE INHERENT COMPLEXITY OF WHAT WILL EVENTUALLY
PLAY OUT.

FRIDAY INTO SATURDAY SHARP UPPER RIDGING BUILDS INTO THE AREA.
EXPECT COLUMN THICKNESSES TO RISE WITH DRYING MIDDLE LEVELS. WE LOSE
OUR PERSISTENT SUBTROPICAL JET GENERATING LEE TROUGHING THOUGH SO
THE ACTUAL SFC FLOW MAY BE MORE EASTERLY AND ONSHORE...SO PAINTED
WARMER HIGHS OUT WEST BUT MODERATED JUST A BIT IN COASTAL LOCATIONS
IN DAYS 6 AND 7. PRECIP CHANCES LOOK VERY LOW UNDER STRONG SUBSIDENCE
INDUCED CAPPING. /68-JGG/

MARINE...TODAY THROUGH SUNDAY...THE SURFACE LOW PRESSURE SYSTEM
OVER THE CENTRAL PORTION OF THE COUNTRY WILL MAINTAIN A MODERATE TO
STRONG S-SE SURFACE FLOW OVER THE LOWER TX BAY AND GULF WATERS
THROUGH SUN. THE PGF WILL BE STRONG ENOUGH NEAR THE COASTLINE TO
PUSH CONDITIONS OVER THE LAGUNA MADRE INTO SCA TERRITORY DURING THE
LATE MORNING AND AFTERNOON HOURS. THE COOLER WATERS OVER THE GULF OF
MEXICO WILL CONTINUE TO LIMIT THE DOWNWARD MIXING OF THE LLJ OVER
THE REGION. SO WILL MENTION SCEC CONDITIONS THERE.

SUNDAY NIGHT THROUGH THURSDAY...
EXERCISE CAUTION LEVEL CONDITIONS ARE FORECAST ON THE GULF SUNDAY
NIGHT AND MAY CONTINUE ON ALL WATERS THROUGH MONDAY. WINDS ARE
FORECAST TO BECOME A LITTLE LIGHTER ON TUESDAY OUT OF THE SOUTHEAST
AND REMAIN SO THROUGH THURSDAY. SEAS WILL BE MODERATE THROUGH THE
EARLY PART OF THE WEEK...POSSIBLY FALLING TO LOW TO MODERATE BY
WEDNESDAY AND THURSDAY.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...SMALL CRAFT ADVISORY FOR WINDS UNTIL 7 PM CDT THIS EVENING FOR
     GMZ130-132-135.

&&

$$

THIS PRODUCT IS ALSO AVAILABLE ON THE WEB AT:
HTTP://WEATHER.GOV/RGV