Area Forecast Discussion
Issued by NWS Brownsville, TX

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000
FXUS64 KBRO 201143 AAA
AFDBRO

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BROWNSVILLE TX
643 AM CDT SAT SEP 20 2014

.DISCUSSION...UPDATED FOR LATEST AVIATION DISCUSSION BELOW.

&&

.AVIATION...THE SLOW MOVING CONV REMAINS THE MAIN AVIATION ISSUE
FOR THE DAY. SO FAR THE CONV HAS GENERALLY MISSED THE LOWER RGV
AIRPORTS OVER THE LAST SEVERAL HOURS AS THE CONV AREAS TO THE WEST
AND NORTH HAVE COLLAPSED. BUT NEWER FORMATION HAS BEEN GRADUALLY
CREEPING TOWARDS THE TAF SITES. THE SLOW MOVEMENT OF THESE CELLS
COULD RESULT IN SIGNIFICANT REDUCTIONS TO CEILINGS AND VSBYS LATER
THIS MORNING INTO THE AFTERNOON HOURS. INITIALLY VFR CONDITIONS
WILL LIKELY PREVAIL OVER THE NEXT SEVERAL WITH THE POTENTIAL FOR
PERIODS OF MVFR OR IFR CONDITIONS INCREASING LATER THIS MORNING AND
THIS AFTERNOON AS THE DAYTIME HEATING INTERACTS WITH THE TROPICAL
AIRMASS IN PLACE OVER THE AREA. EXPECT A RETURN TO MORE VFR
CONDITIONS LATER THIS EVENING AND TONIGHT WITH THE LOSS OF THE
AFTERNOON HEATING DIMINISHING THE CONV COVERAGE A BIT.

&&

.PREV DISCUSSION... /ISSUED 408 AM CDT SAT SEP 20 2014/

..SLOW MOVING STORMS POSE FLASH FLOOD THREAT TODAY...

SHORT TERM /NOW THROUGH SUNDAY/...THE MAIN ISSUE OF THE DAY
CONCERNS THE CONV POTENTIAL OVER THE AREA. OVERNIGHT TWO MAIN AREAS
OF CONV THREATENED THE LOWER RGV. THE FIRST WAS A LARGE CONV AREA
THAT FIRED OVER THE HIGHER TERRAIN OF NORTHEAST MEXICO AND DRIFTED
SLOWLY WEST TOWARDS THE RGV. THIS AREA FINALLY COLLAPSED AS IT MOVED
EAST OF THE HIGHER TERRAIN. THE OTHER AREA WAS TO THE NORTH NEAR AN
AREA OF SURFACE CONVERGENCE OVER THE NORTHERN PORTIONS OF THE BRO
CWA EXTENDING OUT INTO THE GULF OF MEX. THIS LINE OF CONV HAS
WEAKENED SOMEWHAT BUT HAS LEFT BEHIND SOME LINGERING PATCHES OF
CONV. ALL OF THIS CONV HAS HAD ONE THING IN COMMON. THAT IS VERY
SLOW CELLULAR MOVEMENT WHICH HAS HELPED ENHANCE SOME HIGHER RAINFALL
TOTALS OVER THE NORTHERN RANCHLANDS. THE 00Z BRO SOUNDING SHOWS A
PWAT OF 2.36 INCHES WITH A CAPE OF 4685 J/KG WITH FAIRLY WEAK WINDS
AT MOST LEVELS.

THIS DEEPER TROPICAL MOISTURE WILL REMAIN ENTRENCHED OVER THE AREA
TODAY BUT WILL THEN START TO SHIFT SLOWLY SOUTH AND WEST ON SUNDAY
MOVING AROUND THE WEST SIDE OF THE 700 MB RIDGE AXIS IN PLACE OVER
THE SOUTH CENTRAL PLAINS STATES. THE HIGHER PWAT VALUES WILL BE SLOW
TO CLEAR OUT OF DEEP SOUTH TX...SO WILL MAINTAIN A MENTION OF POPS
THROUGH ALL THREE PERIODS. NAM AND GFS GUIDANCE ARE IN PRETTY GOOD
AGREEMENT WITH THE HANDLING OF THIS SCENARIO WITH THE ONLY BIG
DIFFERENCE BEING THAT THE GFS IS A LITTLE QUICKER IN DRYING OUT THE
ATMS OVER THE AREA SUNDAY NIGHT.

WILL BE POSTING AN FFA FOR STARR...HIDALGO AND INLAND CAMERON COUNTY
FOR A SERIES OF REASONS. FIRST...RADAR TRENDS THROUGHOUT THE NIGHT
INDICATE THAT ANY CONV THAT FORMS TODAY WILL HAVE VERY SLOW MOVEMENT
DUE TO WEAK STEETING FLOW ALOFT. SECOND...MODEL FORECAST SOUNDINGS
INDICATE THAT PWAT VALUES WILL REMAIN WELL ABOVE THE 2 INCH MARK
THROUGHOUT TODAY AND POSSIBLY SUNDAY ALSO. THIRD...THE COUNTIES
ALONG THE RIO GRANDE ALL HAVE FAIRLY LOW FFG DUE TO RECENT RAINS.
ANY SLOW MOVING CONV IMPACTING THESE COUNTIES COULD PRODUCE SOME
FLOODING PROBLEMS.

FOR TEMPS WILL LEAN TOWARDS THE LOWER SIDE OF THE MAV/MET HIGH TEMP
GUIDANCE CLOSER TO THE COOLER ECMWF NUMBERS AS THE HIGHER POPS AND
CLD COVER WILL TEND TO LIMIT THE DAYTIME HEATING SOMEWHAT.

LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
MODEST MID LEVEL CONVERGENCE FROM A CENTRAL PLAINS JET MAX INCREASES
500MB HEIGHTS AND EXTENDS THE CENTER OF WEST TEXAS RIDGING OVER THE
AREA. THIS SHIFTS 850MB WINDS MORE TO THE EAST NORTHEAST AND ADVECTS
IN DRIER 850/700MB AIR. KEPT THE FORECAST DRY GIVEN THIS DRIER AIR
AMID MODEST EASTERLY SURFACE FLOW AND RELATIVELY WARM TEMPERATURES.
A LITTLE BIT OF MID LEVEL MOISTURE WILL KEEP SOME CLOUDS IN THE AREA
BUT EXPECTING PARTLY SUNNY AND WARM CONDITIONS OVERALL.

TUESDAY UPPER LEVEL CONVERGENCE INCREASES MODESTLY AND A WEAK 850MB
EASTERLY WAVE REMAINS OFFSHORE IN THE BAY OF CAMPECHE AND SOUTH
CENTRAL GULF OF MEXICO. KEPT THE FORECAST DRY THIS DAY AS WELL GIVEN
THE LACK OF DEEP COLUMN MOISTURE AND A CENTRAL FOCUSING MECHANISM.
PARTLY CLOUDY AND WARM AGAIN ON WHAT WILL BE THE FIRST DAY OF
CALENDAR FALL USHERED IN ACROSS DEEP SOUTH TEXAS BY LOW 90S INSTEAD
OF MID 90S OF COURSE.

WEDNESDAY 850-700MB RELATIVE HUMIDITY INCREASES AS THE BAY OF
CAMPECHE/SOUTH CENTRAL GULF WAVE APPROACHES AND APPEARS TO SLOWLY
WEAKEN. A SHORT WAVE TROUGH DIGGING INTO THE CENTRAL PLAINS BRINGS
MODEST UPPER LEVEL DIVERGENCE TO THE AREA AND CUTS INTO THE
PERSISTENT MID LEVEL RIDGING OVER THE AREA. WITH 850-700MB RH AROUND
80% AND WEAK MID LEVEL DIVERGENCE CONTINUED THE 20 TO 30 PCT SHOWER
AND THUNDERSTORM CHANCES ACROSS THE AREA.

WEDNESDAY NIGHT INTO THURSDAY THE PLAINS TROUGH GROWS SLIGHTLY
STRONGER AND GENERATES BETTER MID LEVEL DCVA/LIFT OVER THE REGION.
WITH GOOD LOW LEVEL MOISTURE PERSISTING BEHIND THE WEAKENING 850MB
WAVE AND GOOD GFS/ECMWF AGREEMENT WENT WITH GENERALLY 30 TO 40 PCT
SHOWER AND THUNDERSTORM CHANCES...THE HIGHEST OF THE WEEK IN THE
AFTERNOON HOURS AND KEPT A 20 TO 30 PERCENT CHANCE ACROSS NEARLY THE
ENTIRE AREA IN THE OVERNIGHT WEDNESDAY NIGHT INTO THURSDAY EVENING.

FRIDAY THE POSITION OF THE UPPER LOW OVER CENTRAL AND SOUTHEAST
TEXAS FAVORS MODERATE SOUTHWESTERLY 700-800MB FLOW WHICH WILL TEND
TO WARM AND DRY THAT LAYER...LIKELY MORE THAN GUIDANCE INDICATES
CURRENTLY. KEPT POPS DOWN AT 20 PCT IN THE VALLEY...30 PCT OVER THE
RANCHLANDS WHICH IS WELL BELOW GUIDANCE CONSENSUS CONSIDERING THE
EXPECTATION OF MID LEVEL SOUTHWESTERLIES. IF THE UPPER LOW MOVES A
LITTLE FURTHER SOUTHWEST THAN CURRENTLY MODELED HOWEVER THAT COULD
INCREASE RAINFALL CHANCES BY KEEPING THE 700 MB FLOW OUT OF THE
SOUTHEAST RATHER THAN THE SOUTHWEST SO WILL CONTINUE TO MONITOR.

SATURDAY FEATURES A STRONGER EASTERLY WAVE ON THE HORIZON EMERGING
FROM THE YUCATAN PENINSULA...A LITTLE STRONGER THAN WHAT WE HAVE
SEEN THE LAST WEEK OR SO...BUT OBVIOUSLY WELL INTO THE EXTENDED
PERIOD OF THE MODEL RUN. GOOD CONSENSUS THAT THAT WAVE WILL NOT
ARRIVE THOUGH SO JUST DEVELOPED A SLIGHT AFTERNOON CHANCE OF SHOWERS
AND STORMS AMID LIGHT TO MODERATE ONSHORE FLOW. THE TYPE OF WAVE
PRESENTED IN GLOBAL GUIDANCE WOULD BE ABLE TO GENERATE MORE
ORGANIZED AND WIDESPREAD UNSETTLED WEATHER BY LATE NEXT
WEEKEND...BUT TOO EARLY TO HAVE ANY CONFIDENCE. /68-JGG/

MARINE...BROAD SURFACE RIDGING OVER THE SOUTHEASTERN STATES AND
THE GULF OF MEXICO WILL MAINTAIN FAIRLY LOW WINDS AND SEAS ACROSS
THE LOWER TX BAY AND GULF WATERS THROUGH THE REST OF THE WEEKEND.
WEAK SURFACE TROFFING MAY FORM ALONG THE LOWER TX COASTLINE LATER
TODAY INTO SUNDAY WHICH WILL ALLOW THE SURFACE FLOW TO SHIFT AROUND
TO A MOVE NE DIRECTION ON SUNDAY. NO SCA CONDITIONS EXPECTED THROUGH
SUNDAY.

SUNDAY NIGHT THROUGH WEDNESDAY...
MODERATE EASTERLY WINDS ARE FORECAST SUNDAY NIGHT THROUGH WEDNESDAY.
WIND SPEEDS MAY EXCEED EXERCISE CAUTION LEVELS ON MONDAY NIGHT. SEAS
WILL BECOME MODERATE WITH LONGER PERIOD SWELL DEVELOPING FROM A LONG
FETCH OF EASTERLY WINDS ACROSS THE WESTERN AND CENTRAL GULF OF
MEXICO...PERSISTING THROUGH WEDNESDAY. SOME SHOWER AND THUNDERSTORM
ACTIVITY IS EXPECTED ON WEDNESDAY BUT ORGANIZED AND WIDESPREAD
ACTIVITY IS NOT CURRENTLY ANTICIPATED.

&&

.PRELIMINARY POINT TEMPS/POPS...
BAYVIEW/PORT ISABEL  89  78  89  79 /  50  40  50  20
BROWNSVILLE          90  76  90  78 /  50  40  50  20
HARLINGEN            91  75  90  77 /  50  40  50  20
MCALLEN              92  76  91  77 /  50  40  50  20
RIO GRANDE CITY      93  76  91  77 /  50  40  50  20
SOUTH PADRE ISLAND   87  79  89  81 /  50  40  50  20

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...FLASH FLOOD WATCH UNTIL 4 PM CDT THIS AFTERNOON FOR TXZ248-252-
     253-255.

GM...NONE.
&&

$$

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

SHORT TERM...60
LONG TERM...68
GRAPHICAST/UPPER AIR...58





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