Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 171108 AAA
AFDBRO

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BROWNSVILLE TX
608 AM CDT FRI APR 17 2015

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

&&

.AVIATION...MVFR CONDITIONS COVER THE LOCAL TAF SITES WITH
LOWERED CEILINGS AND PATCHY LIGHT TO MODERATE FOG AS ABUNDANT LOW
TO MID LEVEL MOISTURE SUPPORTS LOW CLOUD AND PATCHY FOG FORMATION.
SHOWERS AND THUNDERSTORMS ARE FORECAST BY AFTERNOON AS MID LEVEL
ENERGY MOVES OVERHEAD...DESTABILIZING THE LOWER ATMOSPHERE AND
IMPACTING THE LOCAL TAF SITES WITH LOWER FLIGHT CATEGORIES. A
SLIGHT RISK OF SEVERE WEATHER HAS BEEN ADVERTISED BY SPC FOR THE
AREA OF THE TAF SITES TODAY AND TONIGHT...WITH A MIX OF POSSIBLE
SEVERE WEATHER HAZARDS...INCLUDING LARGE HAIL AND DAMAGING WINDS
BUT ALSO WITH FREQUENT LIGHTNING AND HEAVY DOWNPOURS. UNCERTAINTY
CONTINUES ON TIMING AND COVERAGE OF ANY CONVECTION...BUT THE
LATEST MODEL DATA INDICATES THAT MORE THAN ONE ROUND WILL BE
POSSIBLE AS STRONG WEATHER DEVELOPS OR REDEVELOPS. WITH ALL THE
POTENTIAL WEATHER IN THE AREA...FLIGHT CONDITIONS WILL LIKELY BE
BELOW VFR MUCH OF THE NEXT 24 HOURS.

&&

.PREV DISCUSSION... /ISSUED 424 AM CDT FRI APR 17 2015/

SHORT TERM /TODAY THROUGH SATURDAY/...

THE THREAT OF STRONG TO SEVERE WEATHER WILL CONTINUE THROUGH THE
SHORT TERM. A MID LEVEL CUTOFF LOW OVER THE FOUR CORNERS WILL MOVE
SLOWLY TOWARD COLORADO...WITH SOUTHWEST MID LEVEL FLOW OVER DEEP
SOUTH TEXAS AND THE RGV. FARTHER ALOFT ABOVE THE AREA...THE SUB
TROPICAL JET IS ALSO ADDING A DESTABILIZING INFLUENCE AS IMPULSES
EMBEDDED IN THE JET MOVE IN THIS DIRECTION. THE LOW LVL ENVIRONMENT
WILL REMAIN MOISTURE RICH IN THE SHORT TERM WITH STEEP LAPSE RATES
AND ELEVATED CAPES. AS DAYTIME HEATING TAKES HOLD...INSTABILITY WILL
INCREASE...WITH SHOWERS AND THUNDERSTORMS DEVELOPING BY THIS AFTN.
MID LEVEL LIFT APPEARS TO MAXIMIZE THIS AFTERNOON AND EVENING AS
ENERGY MOVES AROUND THE BASE OF THE MID LEVEL LOW. FREQUENT LTNG...
GUSTY WINDS...HEAVY RAINFALL AND SMALL HAIL HAVE ALL BEEN OBSERVED
WITH RECENT STORMS...AND SIMILAR WORDING HAS BEEN CONTINUED IN THE
FORECAST PRODUCTS. DAMAGING WINDS AND LARGER HAIL WILL BE POSSIBLE
WITH ANY STORMS THAT BECOME SEVERE. SPEAKING TO THAT...SPC HAS THE
CWA IN A SLIGHT RISK TO AN ENHANCED RISK OF SEVERE WEATHER TODAY
THROUGH SATURDAY. WILL NOT ISSUE ANY FLASH FLOOD WATCHES AT THIS
TIME AS THE FLASH FLOODING THREAT SHOULD BE LOCALIZED IN NATURE.
FLASH FLOODING MAY BECOME AN ISSUE WITH LARGER STORMS...IF MOVEMENT
SLOWS...OR IF A LONGER TERM EVENT MATERIALIZES.

OTHERWISE...MOSTLY CLOUDY SKIES WILL CONTINUE WITH LIGHT TO MODERATE
SOUTHEAST WINDS. HIGH TEMPS WILL BE ABOUT AVERAGE IN THE MID 80S...
WITH OVERNIGHT LOWS IN THE 70S...SEVERAL DEGREES ABOVE NORMAL.

LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...

THUNDERSTORM CHANCES CONTINUE THROUGH MOST OF THE LONG TERM AS DOES
A CONDITIONAL SEVERE WEATHER RISK.

SATURDAY NIGHT INTO SUNDAY THE NOSE OF A BROAD 110 TO 130 KT JET
MAXIMUM WILL BEGIN TO PUSH ACROSS THE AREA. DRIER MID LEVEL
WESTERLIES SHOULD RESULT IN LESS CLOUD COVER ACROSS MOST OF THE AREA
DURING THE DAY WHICH WOULD ALLOW AFTERNOON TEMPERATURES TO APPROACH
90 DEGREES. THERE IS HOWEVER A WEAK BOUNDARY INDICATED BY
NAM/GFS/ECMWF THAT THE GFS IN PARTICULAR INITIATES CONVECTION ON IN
THE LATE AFTERNOON. THE PRIMARY QUESTION FOR SUNDAY WILL BE WHETHER
THAT SHALLOW/WEAK BOUNDARY PROVIDES ENOUGH CONVERGENCE TO GET
PARCELS THROUGH A PRETTY STABLE NEAR SURFACE LAYER TO ABOUT 4500 FT
WHERE CONSIDERABLE INSTABILITY IS AVAILABLE. GUIDANCE CONSENSUS IS
ABOUT 3000 J/KG OF MIXED LAYER CAPE WITH 40 TO 45 KNOTS OF 0-6KM
BULK SHEAR WITH MINIMAL STORM MOTION VECTORS. THIS WOULD FAVOR SLOW
MOVING BUT INTENSE THUNDERSTORMS CAPABLE OF LARGE HAIL/DAMAGING
WINDS AND BRIEF TORNADOES. THIS RISK IS CONDITIONAL AND HIGHER EAST
OF HIGHWAY 281. IT IS A...NOT SURE IF STORMS WILL DEVELOP...BUT IF
THEY DO THEY WOULD LIKELY BE SEVERE SITUATION.

MONDAY THAT BOUNDARY PUSHES THROUGH WITH THE CORE OF THE
AFOREMENTIONED JET MAXIMUM NOW EAST OF US PUTTING US IN AN AREA OF
MODEST DIVERGENCE ALOFT. GFS/ECMWF DEPICT GOOD 850MB SOUTHEASTERLY
RETURN FLOW HELPING TO NUDGE THE BOUNDARY TO THE NORTH THROUGH THE
DAY BUT IN DOING SO CLOUD COVER APPEARS TO INCREASE LIMITING SURFACE
TEMPERATURES AND KEEPING OVERALL INSTABILITY MORE MODEST. THE
EXCEPTION TO THIS IS PERHAPS THE FAR SOUTHWESTERN REACHES OF DEEP
SOUTH TEXAS AND THE ADJACENT MEXICAN PLATEAU WHERE BETTER HEATING IS
EXPECTED. KEPT SCATTERED SHOWERS/TSTORMS WITH ABOUT A 30 PCT CHANCE
IN THE FORECAST DURING THE DAY INCREASING TO 40 PCT OVERNIGHT WITH
THE BOUNDARY SHIFTING NORTHEASTWARD. THUNDERSTORMS WILL LIKELY
INITIATE ON THE SIERRA MADRE DURING THE DAY MONDAY AND TRACK SLOWLY
EASTWARD INTO A MODERATELY BUT INCREASINGLY UNSTABLE AIRMASS.

TUESDAY UPPER LEVEL FLOW IS RAPID BUT MORE ZONAL. SURFACE FLOW IS
EXPECTED TO SHIFT EASTERLY WITH WEAK ASCENT IN THE 850MB LAYER
KEEPING AT LEAST SOME CLOUD COVER PRESENT. THE ONSHORE EASTERLY FLOW
WILL BE A LITTLE COOLER AND OVERALL INSTABILITY IS EXPECTED TO BE
THE WEAKEST ON TUESDAY ALTHOUGH MEXICAN INITIATED THUNDERSTORMS MAY
THREATEN THE AREA AND COULD STILL BE RELATIVELY STRONG GIVEN THE
STRONG DEEP LAYER SHEAR AND MODEST ML/MUCAPES.

FAIRLY CONSIDERABLE DIFFERENCES BEGIN TO EMERGE BETWEEN THE
ECMWF/GFS SOLUTIONS BY WEDNESDAY. THE ECMWF FAVORS A MORE CLOSED LOW
OVER CALIFORNIA AT THIS TIME WHILE THE GFS SHOWS A SHARPER OPEN WAVE
OVER THE COLORADO RIVER VALLEY WITH A STRONGER 120-130KT JET MAXIMUM
AHEAD OF IT. THE ECMWF WOULD PUT THE REGION IN A MORE DIVERGENT
OVERALL UPPER LEVEL FLOW BUT BOTH MODELS INITIATE AFTERNOON
CONVECTION AMID A HOT AND HUMID AND UNSTABLE AIRMASS WITH SOME HINT
OF A RESIDUAL LOW LEVEL BOUNDARY WORKING INTO THE INCREASING LEE
TROUGHING ON THE SIERRA MADRES. KEPT POPS BELOW GUIDANCE CONSENSUS
AS THIS PATTERN FREQUENTLY FAVORS PM THUNDER OVER MEXICO THAT ONLY
SLOWLY MOVES EASTWARD AFTER SUNSET. WITH THAT SAID HOWEVER THE
CONSIDERABLE BOUNDARY LAYER MOISTURE PROGGED TO BE AVAILABLE FORCED
ME TO AT LEAST KEEP 40-50 POPS OVER MOST OF THE AREA IN THE
AFTERNOON.

THE LARGE ECMWF/GFS DIFFERENCES CONTINUE THROUGH THURSDAY. THE GFS
ENSEMBLE MEAN IS CONSIDERABLY CLOSER TO THE DETERMINISTIC GFS THAN
THE ECWMF SO THE ECMWF WAS PREFERRED FOR THE FORECAST PACKAGE. THIS
CALLS FOR A GENERALLY PERSISTENT PATTERN WITH ISOLATED TO SCATTERED
SHOWERS/THUNDERSTORMS IN THE FORECAST IN THE AFTERNOON RATHER THAN
THE GFS DEPICTION OF A COLD FRONT CUTTING INTO THE WESTERN HALF OF
DEEP SOUTH TEXAS RESULTING IN MORE WIDESPREAD CONVECTION. /68-JGG/

MARINE...

SATURDAY NIGHT THROUGH TUESDAY... MODERATE SOUTHEAST FLOW IS
EXPECTED TO BECOME MORE EASTERLY THROUGH THE EARLY PART OF THE WORK
WEEK WITH LOW TO MODERATE SEAS FORECAST. THE PRIMARY HAZARD TO
MARINERS WILL BE THUNDERSTORMS EXPECTED TO DEVELOP OFF AND ON
THROUGH THE PERIOD. MARINERS ARE ADVISED TO PAY CLOSE ATTENTION TO
THE LATEST FORECASTS.

TODAY THROUGH SATURDAY...LIGHT TO MODERATE SOUTHEAST WINDS AND LOW
TO MODERATE SEAS WILL PREVAIL THROUGH SATURDAY. HOWEVER...SHOWER AND
THUNDERSTORM CHANCES WILL INCREASE AS AN UPPER DISTURBANCE MOVES
CLOSER AND HELPS TRIGGER STRONG WEATHER ALONG THE COAST WHICH WILL
THEN MOVE OFFSHORE. SOME OF THE STORMS WILL BE STRONG TO SEVERE...
WITH HIGHER WINDS AND SEAS IN AND NEAR THUNDERSTORMS.

&&

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

$$

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