Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 161120
AFDBRO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BROWNSVILLE TX
620 AM CDT THU APR 16 2015

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

&&

.AVIATION...MID TO HIGH CLOUDS ARE STILL STREAMING ACROSS THE CWA
IN THE SOUTHWEST FLOW ALOFT...WITH MULTI-LAYERED LOW CLOUDS AT THE
TAF SITES...WITH BRO CURRENTLY MVFR. GUIDANCE SUGGESTS A RANGE OF
CIGS FROM IFR TO VFR WITH CEILINGS FILLING AND FADING IN THE MVFR
BRACKET. LIGHT TO MODERATE SOUTHEAST WINDS WILL KEEP THE MOISTURE
ADVECTING INTO THE AREA TODAY...SUPPORTING THE LOWER CLOUD LAYERS.
THE LOSS OF THE CAP TODAY WITH DAYTIME HEATING WILL PROMOTE SHOWER
AND THUNDERSTORM ACTIVITY IN THE AREA THIS AFTN AND EVENING...
ENHANCED BY UPPER INSTABILITY. A BIT OF FOG AND EVEN SOME IFR
CIGS MAY DEVELOP NEAR THE COAST TONIGHT AS MOISTURE LEVELS REMAIN
ELEVATED AND FOG IS SUPPORTED BY WETTED SURFACES. LIGHT TO MDT
SOUTHEAST WINDS WILL CONTINUE BUT SHOULD BE LOW ENOUGH TO ALLOW
COASTAL FOG TO DEVELOP TONIGHT.

&&

.PREV DISCUSSION... /ISSUED 406 AM CDT THU APR 16 2015/

LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
ORGANIZED SEVERE WEATHER/FLASH FLOODING RISKS PERSIST.

NAM/GFS/ECMWF PROG A CLOSED UPPER LOW OVER COLORADO EMBEDDED IN A
BROAD TROUGH FRIDAY NIGHT. A JET ROUNDING THE BASE OF THIS TROUGH
APPEARS TO HAVE TWO SEPARATE RELATIVE MAXIMA...THE FIRST WHICH WILL
BE PUSHING THROUGH FRIDAY NIGHT. GOOD UPPER DIFFLUENCE AND MODEST
SPEED DIVERGENCE YIELD POSITVE LARGE SCALE ASCENT AND STRONG
INSTABILITY WITH WARM AND HUMID SURFACE AIR IN PLACE. GUIDANCE
SUGGESTS AN MCS FORMS DURING THE DAY OVER SOUTH CENTRAL TEXAS AND
PUSHES INTO THE REGION FRIDAY NIGHT. GFS/NAM FORECAST SOUNDINGS SHOW
ABOUT 3000 J/KG OF MLCAPE WITH VERY STEEP MID LEVEL LAPSE RATES IN
PLACE...ON THE ORDER OF 8 C/KM AMID FAVORABLE DEEP LAYER AND
SOMEWHAT FAVORABLE LOW LAYER SHEAR AS WELL. VERY LARGE HAIL WILL
PROBABLY BE THE LEADING THREAT...ESPECIALLY ACROSS THE NORTHERN HALF
OF THE CWA...WITH A SECONDARY THREAT OF DAMAGING WINDS AND POSSIBLY
A TORNADO...ESPECIALLY IF CONVECTION GETS GOING OVER DEEP SOUTH
TEXAS BEFORE SUNSET.

SATURDAY THE REGION LOOKS TO BE UNDER THE GUN AGAIN WITH THE SECOND
RELATIVE MAXIMUM IN THE AFFOREMENTIONED UPPER LEVEL JET MOVING INTO
THE AREA DURING THE LATE AFTERNOON AND EVENING. THE LEFT FRONT
QUADRANT OF THIS STRONG JET CORE IS PROGGED TO BE ALTERING THE
HEIGHT FIELDS OVER THE AREA PINCHING A SHORTWAVE TROUGH TOGETHER AS
IT MOVES EASTWARD. THIS WOULD AGAIN RESULT IN VERY STRONG LARGE
SCALE ASCENT. AT LEAST INITIALLY THE AREA WILL BE IN A FAVORABLE
ENVIRONMENT FOR SEVERE WEATHER WITH WESTERLY WINDS ALOFT DRYING THE
MID LEVELS AND FURTHER STEEPENING LAPSE RATES. AT THE SAME TIME
HOWEVER SOUTWESTERLY WINDS AT 850MB BEGIN TO RESPOND TO THE LARGE
SCALE ASCENT BY INCREASING LEADING TO DRYING AND WARMING IN THAT
LAYER AND AN INCREASED CAP. THE POTENTIAL FOR SEVERE WEATHER AND
THUNDERSTORMS DEFINITELY APPEARS TO EXIST ON SUNDAY BUT THE THREAT
MAY BE MADE MORE MARGINAL BY PREVIOUS DAYS CONVECTIVE ACTIVITY IF A
LARGE MCS SWEEPS ACROSS THE ENTIRE AREA WORKING OVER THE AIRMASS OR
POTENTIALLY LINGERS CLOUD COVER IN THE MORNING. TRYING TO ESTIMATE
WHETHER THE LARGE SCALE ASCENT PRESENT WILL BE SUFFICIENT TO LIFT
OUT BUILDING CAPPING AT 850MB IS ALSO CHALLENGING AND LIMITS
CONFIDENCE IN SEVERE POTENTIAL. AGREE WITH SPCS DAY 3 OUTLOOK OF
MARGINAL RISK FOR NOW.

SUNDAY GFS/ECMWF DO INITIATE ANOTHER ROUND OF THUNDERSTORMS UNDER
THE CORE OF ABOUT A 110 TO 130KT JET MAXIMUM WITH NO SIGNIFICANT
SYNOPTIC SCALE FORCING ALOFT. HOWEVER AS 850MB WINDS RELAX AS LARGE
SCALE FORCING WEAKENS SOUTHEASTERLIES PULL MOISTURE BACK INTO THE
REGION UP AGAINST DRIER AIR TO THE WEST THAT HAD BEEN WARMED/DRIED
BY DOWNSLOPING SOUTWESTERLIES. THIS CREATES A SORT OF DE FACTO DRY
LINE THAT APPEARS TO SERVE AS THE FOCUS POINT FOR THUNDERSTORMS.
BELIEVE THIS RISK IS EVEN MORE CONDITIONAL AND KEPT OPS IN THE 30 TO
UP TO 40 PCT RANGE ALTHOUGH INSTABILITY AND SHEAR WOULD FAVOR
STRONG/SEVERE AGAIN ON SUNDAY IF EVERYTHING COMES TOGETHER.

MONDAY THE RIGHT REAR QUADRANT OF THE SLOWLY EASTWARD MOVING JET
MAXIMUM SETS UP OVER THE AREA. GUIDANCE INITIATES CONVECTION ON THE
SIERRA MADRE AND ADVECTS IT IN THROUGH THE LATE AFTERNOON AND
EVENING AND KEEPS ENOUGH LARGE SCALE ASCENT PRESENT TO KEEP AT LEAST
A CHANCE OF THUNDER IN THE FORECAST OVERNIGHT. SHEAR/THERMODYNAMIC
PROFILES WOULD AGAIN BE SUPPORTIVE OF SEVERE WEATHER IF
THUNDERSTORMS GET GOING...PROBABLY A BIT MORESO THAN SUNDAY.

TUESDAY CONVECTION APPEARS TO GENERATE FROM RESIDUAL BOUNDARIES AND
DAYTIME HEATING AND KEPT APPRECIABLE POPS IN THE FORECAST WITH RICH
LOW LAYER MOISTURE AND NO CAPPING SOUTHWESTERLY FLOW TO LIMIT
DEVELOPMENT. WEDNESDAY INTO THURSDAY THERE IS SOME DISPARITY BETWEEN
THE GFS/ECMWF ON THE TRACK OF A FAST MOVING SHORTWAVE TROUGH WITH
THE GFS BRINGING IT FURTHER SOUTH AND POSSIBLY HAVING A LITTLE
LARGER IMPACT ON OUR OVERALL PATTERN EITHER WAY...KEPT AT LEAST SOME
CHANCE OF SHOWER AND THUNDERSTORM CHANCES IN THE FORECAST.

IN ADDITION TO THE ORGANIZED SEVERE WEATHER THREAT THERE IS A THREAT
OF FLASH FLOODING DUE TO CONTINUED ROUNDS OF THUNDERSTORMS. FOR NOW
THOUGH ABOUT ALL I CAN COMFORTABLY SAY IS THAT FLASH FLOODING IS AT
LEAST POSSIBLE...BUT THE INDIVIDUAL CONVECTIVE THREATS ARE TOO
DISORGANIZED AND RELATIVELY FAST MOVING TO HONE IN ON ANY ONE AREA
ESPECIALLY IN A MORE THAN LOCALIZED SENSE. FLASH FLOOD GUIDANCE
REMAINS LOW WITH SOIL CONDITIONS SATURATED SO IT COULD EVOLVE
QUICKLY INTO A MAJOR ISSUE. /68-JGG/

MARINE...
TODAY THROUGH FRIDAY...LIGHT TO MODERATE SOUTHEAST WINDS AND LOW TO
MODERATE SEAS WILL PREVAIL THROUGH THE SHORT TERM AS BROAD HIGH
PRESSURE CONTROLS THE NORTHWEST GULF. SHOWERS AND THUNDERSTORMS WILL
BE PRONE TO DEVELOPMENT WITH DAMAGING WINDS...FREQUENT LIGHTNING...
HEAVY RAINFALL AND SMALL HAIL BEING THE POSSIBLE THREATS TO
MARINERS.

FRIDAY NIGHT THROUGH TUESDAY...
MARINE CONDITIONS WILL REMAIN STABLE THROUGH THE PERIOD...WITH
SEAS OF 3 TO 4 FEET AND SOUTHEAST FLOW 10 TO 15 KNOTS EACH DAY.
THUNDERSTORM ACTIVITY WILL BE THE MAIN MARINE WEATHER
CONCERN...ESPECIALLY ON FRIDAY NIGHT AND POSSIBLY AGAIN EACH
AFTERNOON AND EVENING THROUGH TUESDAY. ANY THUNDERSTORM ACTIVITY
WILL LIKELY INCLUDE GUSTY WINDS...FREQUENT LIGHTNING AND THE
POSSIBILITY OF HAIL.

&&

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

$$

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