Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 181130
AFDBRO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BROWNSVILLE TX
630 AM CDT SAT APR 18 2015

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

&&

.AVIATION...CHALLENGING AND LOW CONFIDENCE CONVECTIVE FORECAST
TODAY. BALLOON ON THE WAY UP SHOWS ABUNDANT INSTABILITY AVAILABLE
ABOVE A STRONG CAP. IF WE ARE ABLE TO INITIATE CONVECTION IT COULD
BE QUITE STRONG WITH HAIL AND WIND A THREAT BUT CAP MAY BE JUST
STRONG ENOUGH TO HOLD BACK. THUNDER COULD DEVELOP BY THE LATE
MORNING HOWEVER AND IT MAY IMPACT THE AIRFIELDS THROUGH THE
AFTERNOON. IFR CIGS WILL SLOWLY IMPROVE THROUGH THE AFTERNOON
BEFORE DETERIORATING AGAIN AFTER SUNSET. /68-JGG/

&&

.PREV DISCUSSION... /ISSUED 356 AM CDT SAT APR 18 2015/

SHORT TERM /NOW THROUGH SUNDAY/...

RIGHT NOW...REWORKED THE NEAR TERM FORECAST BASED ON OBSERVED TRENDS
FOLLOWING THIS PAST EVENINGS CONVECTION. COMBINING THE TREND IN THE
PAST NIGHTS RAOB WITH LAPS/RAP SOUNDINGS AND THE RMS ERRORS ON THE
BROWNSVILLE VWP BELIEVE AIR JUST ABOVE THE SURFACE HAS WARMED AND
DRIED CONSIDERABLY. HOWEVER ALL NEAR TERM/CONVECTION ALLOWING MODELS
KEEP A WESTERLY WIND UP THROUGH 15 TO 18Z TODAY WHICH FURTHER DRIES
AND WARMS THAT LAYER. BROWNSVILLE VWP SHOWS WINDS ARE ALREADY
SHIFTING BACK TO THE SOUTHEAST AHEAD OF THE ARRIVAL OF THE MAIN
UPPER LEVEL TROUGH AXIS. THIS SHOULD STOP ANY CAPPING BUILDUP AND
SLIGHTLY MOISTEN THIS LAYER. GOOD SPEED DIVERGENCE CAN BE PICKED OUT
OF GOES DERIVED SATELLITE WINDS JUST AHEAD OF AN AREA OF COOLING
CLOUD TOPS OVER NORTHEASTERN MEXICO. AMSU/SSMI PRECIPITABLE WATER
ESTIMATES SHOW AN AREA OF PRESUMABLY BETTER MID LEVEL MOISTURE
ASSOCIATED WITH THIS ADVANCING FEATURE. VERY STEEP MID LEVEL LAPSE
RATES WERE OBSERVED LAST EVENING AND THAT LIKELY HASNT CHANGED.
SURFACE MOISTURE HAS POOLED THE THICKEST OVER THE SOUTHEASTERN
CORNER OF THE CWA WHERE DEWPOINTS APPROACH 75...WHILE MID TO UPPER
60S ARE OBSERVED OVER THE FAR NORTHERN CWA.

TODAY...NAM/GFS/ECMWF ARE IN GOOD AGREEMENT THAT THE BEST LARGE
SCALE ASCENT/Q-VECTOR CONVERGENCE WILL ARRIVE EARLY IN THE DAY AHEAD
OF PEAK HEATING. HOWEVER THE LARGE SCALE ASCENT SHOULD BE STRONG
ENOUGH TO STEEPEN LAPSE RATES ALTHOUGH SURFACE HEATING LOOKS AS IF
IT WILL BE LIMITED BY CLOUD COVER THROUGH THE EARLY PART OF THE DAY.
DEEP LAYER SHEAR SHOULD BE FAVORABLE THROUGH MOST OF THE DAY.
STARTED INCREASING THUNDERSTORM CHANCES BY 9 TO 10 AM AND ADDED
SEVERE WORDING TO ABOUT THE NORTHERN HALF OF THE CWA WHERE THE BEST
MIX OF SHEAR/INSTABILITY IS FORECAST TO BE PRESENT. STRONG
CONVECTION MAY GET GOING ELSEWHERE IN THE CWA BUT THE RISK APPEARS
MORE CONDITIONAL OVER THE RGV METRO AREAS.

TONIGHT...THE LEFT EXIT REGION OF A JET MAXIMUM MOVING NORTHEAST
OVER MEXICO ARRIVES TONIGHT BUT THE ASCENT IT PROVIDES DOESNT LOOK
LIKE IT WILL BE ABLE TO OVERCOME CONVERGENCE BEHIND THE DEPARTURE OF
THE LEADING SHORTWAVE TROUGH AND THE WESTERLY 850MB WINDS SWEEPING
IN DRIER AIR THROUGH THE AREA. DRIED THE FORECAST OUT A BIT TONIGHT.

SUNDAY...AS HAS BEEN THE THEME FOR THE PAST FEW DAYS NAM/GFS/ECMWF
DEPICT A FRONT/DRYLINE ACROSS THE CWA WITH MOISTURE POOLED MAINLY
EAST OF HIGHWAY 281. WITH DRIER MID LEVEL AIR FORECAST TO BE PRESENT
UNDER THE CORE OF ABOUT A 110 TO 120 KT WESTERLY JET MAXIMUM CLOUD
COVER SHOULD BE LESS AND DAYTIME TEMPS SHOULD CLIMB INTO THE MID 90S
FAR WEST AND UPPER 80S CLOSE TO THE COAST. THIS WILL BUILD UP A LOT
OF SURFACE BASED INSTABILITY AND IF THUNDERSTORMS CAN FIRE ALONG THE
BOUNDARY THEY COULD BE QUITE INTENSE WITH FAVORABLE DEEP LAYER SHEAR
PROFILES ALSO PRESENT. CONSIDERABLE UNCERTAINTY REMAINS HOWEVER ON
WHETHER OR NOT CONVECTION WILL BE ABLE TO INITIATE WITH A RELATIVELY
UNREFINED BOUNDARY AND NO LARGE SCALE FEATURES TO AID IN INITIATION.
/68-JGG/

LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...THE 500 MB CLOSED LOW
THAT WAS LOCATED OVER THE DESERT SW STATES THIS WEEKEND WILL SHIFT
EAST ON SUN AND SUN NIGHT. THIS WILL DROP A COLD FRONT THROUGH
DEEP SOUTH TX ON MON RESULTING IN A BRIEF SHIFT OF THE SURFACE
WINDS TO THE NORTHEAST. AS THE 500 MB TROUGH AXIS EXITS THE LOWER
MISS RIVER VALLEY MON NIGHT AND TUES THE OLD COLD FRONT WILL
BECOME STATIONARY AND WILL MEANDER AROUND SOUTH TX THROUGH WED.
ANOTHER PACIFIC CLOSED LOW WILL FORM UP OFF OF THE CALI COAST LINE
EARLY NEXT WEEK. THIS 500 MB CLOSED LOW WILL THEN MOVE INTO THE
DESERT SW THURS NIGHT INTO FRI. VARIOUS POCKETS OF 500 MB PVA WILL
EJECT OUT FROM THE WEST COAST 500 MB TROFFING/CLOSED LOW MOVING
OVER SOUTH TX. THESE AREAS OF VORTICITY ADVECTION WILL BE MOVING
OVER SOME AREAS OF DEEP MOISTURE RETURN THROUGH AROUND WED WHICH
WILL MAINTAIN SOME POTENTIAL FOR SCT CONV ESPECIALLY THROUGH WED
NIGHT. FROM THURS THROUGH FRI 500 MB RIDGING WILL GRADUALLY BUILD
OVER DEEP SOUTH TX REDUCING THE DEEPER LAYER MOISTURE AND TRENDING
DOWN THE CONV POTENTIAL SOMEWHAT.

FOR POPS WILL LEAN TOWARDS THE WETTER ECMWF. TEMPS WILL BE A
GENERAL 50/50 BLEND OF THE ECMWF AND THE GFS MEX MOS THROUGH DAY
7. MODEL TO MODEL COMPARISON BETWEEN THE ECMWF AND THE GFS 500 MB
FIELDS LOOK PRETTY DECENT THROUGH NEXT FRI IN THE OVERALL
PLACEMENT AND MAGNITUDE OF THE 500 MB SYNOPTIC FEATURES. SO
OVERALL CONFIDENCE IN THE LONGER RANGE OUTLOOK IS ABOVE AVERAGE
THIS MORNING.

MARINE...NOW THROUGH SUNDAY... MODERATE SOUTHEASTERLY WINDS ARE
FORECAST THROUGH SUNDAY WITH LOW TO MODERATE SEAS EXPECTED TO
CONTINUE. THUNDERSTORMS WILL BE POSSIBLE AGAIN DURING THE DAY AND
EARLY EVENING TONIGHT...AND POSSIBLY AGAIN ON SUNDAY.

SUNDAY NIGHT THROUGH WEDNESDAY...WITH THE OLD COLD FRONT
MEANDERING NEAR THE LOWER TX COASTLINE EARLY NEXT WEEK...EXPECT
A MODERATE GENERAL ONSHORE SURFACE FLOW TO PREVAIL THROUGH WED.
EXPECT THE LONGER TERM WIND AND WAVE ACTION ALONG THE LOWER TX
COASTLINE TO REMAIN BELOW SCA CRITERIA.

&&

.PRELIMINARY POINT TEMPS/POPS...
BAYVIEW/PORT ISABEL  83  71  86  72 /  40  20  20  30
BROWNSVILLE          84  70  87  72 /  40  20  40  30
HARLINGEN            85  70  90  71 /  40  20  40  40
MCALLEN              87  69  92  72 /  20  20  30  30
RIO GRANDE CITY      90  68  94  71 /  30  20  20  30
SOUTH PADRE ISLAND   80  72  79  72 /  40  20  20  30

&&

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

$$

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