Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 151807 AAC
AFDBRO

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BROWNSVILLE TX
107 PM CDT WED APR 15 2015

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

.AVIATION...SATELLITE IMAGES AND SURFACE OBSERVATIONS INDICATE LOW
TO MID LEVEL CLOUDS ACROSS THE CWA EARLY THIS AFTERNOON. CEILINGS
WERE NEAR 1200FT AT KAPY TO NEAR 440FT AT KBRO AND KBKS. EXPECT
VFR CONDITIONS TO PREVAIL THE REST OF THE AFTERNOON WITH ISOLATED
SHOWERS POSSIBLE ACROSS PORTIONS OF THE RIO GRANDE VALLEY AND
NORTHERN RANCHLANDS. MVFR CONDITIONS WILL DEVELOP TONIGHT AS
ABUNDANT LOW TO MID LEVEL MOISTURE ACROSS THE AREA ALLOWS LOW
CLOUDS AND PATCHY FOG TO DEVELOP LATE TONIGHT INTO EARLY THURS
MORNING.
&&

.PREV DISCUSSION... /ISSUED 629 AM CDT WED APR 15 2015/
DISCUSSION...UPDATED FOR LATEST AVIATION DISCUSSION BELOW.

AVIATION...VFR CONDITIONS WITH A TOUCH OF MORNING FOG WILL
BECOME MVFR TODAY AS CEILINGS LOWER. CURRENT SATELLITE IMAGERY
SHOWS CLEARING TO THE NORTH OF THE CWA WITH MAINLY MID AND HIGH
CLOUDS STILL TRACKING FROM WEST TO EAST OVER MUCH OF THE AREA. A
FEW RADAR RETURNS PERSIST OVER THE UPPER VALLEY AND TO THE SOUTH
ALONG THE NOW STATIONARY FRONTAL BOUNDARY JUST SOUTH OF THE AREA.
VERY LIGHT WINDS WILL PREVAIL THIS MORNING AND TODAY WITH LITTLE
PRESSURE GRADIENT. LIGHT FOG WILL REDUCE VSBY A BIT THIS MORNING.
HIGH PRESSURE WILL BUILD OVER THE NORTHWEST GULF TODAY...SETTING
UP A LIGHT TO MODERATE EAST TO SOUTHEAST WIND BY TONIGHT. IN THE
MEAN TIME...STILL NEARLY SATURATED LOWER LVLS WILL SUPPORT LOW
CLOUD REFORMATION THROUGH TDY WITH A FEW COASTAL SHOWERS POSSIBLY
DEVELOPING THIS MORNING AND AFTERNOON. WITH DECKS AT OR BELOW 3
KFT...MVFR CONDITIONS WILL DOMINATE.

PREV DISCUSSION... /ISSUED 401 AM CDT WED APR 15 2015/

SHORT TERM /NOW THROUGH THURSDAY/...A FRONT PUSHED THROUGH THE CWA
YESTERDAY AND CURRENT SATELLITE IMAGERY SHOWS SOME CLEARING OVER THE
RANCHLANDS AND BRUSH COUNTRY...BUT INDICATIONS ARE THAT UNSETTLED
WEATHER IS NOT OVER IN THE SHORT TERM. CONDITIONS MAY TAKE A PAUSE
TODAY AT LEAST...WITH PARTLY TO MOSTLY CLOUDY SKIES AND HIGH TEMPS
IN THE LOWER 80S FOR MOST AREAS. RESIDUAL LLVL MOISTURE AND ENERGY
IN THE STILL SOUTHWEST FLOW ALOFT WILL PROBABLY BE ENOUGH TO TRIGGER
A FEW SHOWERS AND THUNDERSTORMS BY THIS AFTERNOON...HOWEVER.

AFTER A BRIEF PAUSE IN THE ACTION TDY...CONDITIONS WILL DESTABILIZE
AGAIN MOVING FORWARD INTO TONIGHT AND THURSDAY. AN UPPER LOW WILL
SINK INTO THE FOUR CORNERS REGION...WITH SOUTHWEST FLOW REMAINING
ABOVE THE LOCAL AREA...AND JET SUPPORT IN THE UPPER ATMOSPHERE ABOVE
THAT. LOW LEVEL MOISTURE WILL REMAIN SUBSTANTIAL AND AVAILABLE FOR
CONVECTION. AN INCREASE IN DAYTIME SURFACE TEMPS INTO THE MID 80S
THURSDAY WILL ALSO SUPPORT DESTABILIZATION AND OVERTURNING. DAMAGING
WINDS AND HAIL WITH LOCALLY HEAVY RAINFALL WILL BE POSSIBLE THURSDAY
AS SPC PLACES A GENERAL/SLIGHT/ENHANCED CONVECTIVE OUTLK FROM SOUTH
TO NORTH OVER THE CWA. WITH THE CONTINUED CONVECTIVE ACTIVITY WHICH
WILL EXTEND INTO THE LONG TERM...A FLASH FLOOD WATCH MAY BE NEEDED.
WILL HOLD OFF ON THAT DISCUSSION FOR NOW AS DAY SHIFT WILL BE IN A
BETTER POSITION TO ASSESS THE NECESSITY...INCLUDING THE TIMING AND
EXTENT.

LONG TERM /THURSDAY NIGHT THROUGH THROUGH TUESDAY/... ESPECIALLY
ACTIVE PATTERN PERSISTS WITH DAILY RISKS FOR SEVERE THUNDERSTORMS
AND POSSIBLY FLASH FLOODING EACH DAY THROUGH MONDAY...UNLESS THE
ECWMF IS RIGHT...AND I THINK IT MIGHT BE...IN WHICH CASE THE CHANCE
PERSISTS THROUGH THE WEEK.

GFS/NAM/ECMWF ARE IN PRETTY GOOD AGREEMENT WITH THE SYNOPTIC PATTERN
THU NIGHT/FRIDAY WITH A LARGE RUMBLING CLOSED UPPER LOW OVER THE
FOUR CORNERS REGION AND A LEADING JET MAXIMUM ROUNDING ITS BASE
FROM LOUISIANA TO THE SOUTHERN TIP OF THE BAJA CALIFORNIA. THIS
KEEPS THE TRAIN OF STRONG LOW AND MID LEVEL MOISTURE POURING INTO
THE REGION. DOWNSLOPING 800-700MB SOUTHWESTERLIES DO BUILD A
CAPPING INVERSION WHICH WILL MUDDY THE WATERS A BIT. HOWEVER
STRONG LARGE SCALE ASCENT ASSOCIATED WITH GOOD SPEED DIVERGENCE
AND DIRECTIONAL DIFFLUENCE COUPLED WITH WEAK WARM AIR ADVECTION IN
THE LOWER LEVELS SHOULD LIFT THIS CAP AND STEEPEN OUR LAPSE RATES.
RICH LOW LEVEL MOISTURE WITH DEWPOINTS IN THE LOW TO MID 70S YIELD
ABOUT 3000 J/KG OF MLCAPE ON FORECAST SOUNDING WITH A LITTLE BIT
LESS SATURATED PROFILE LOOKING A LITTLE MORE FAVORABLE FOR
DAMAGING WIND GUSTS AND LARGE HAIL THAN OUR LAST FEW SETUPS. FOR
THE FORECAST PAINTED BETTER POPS A LITTLE FURTHER NORTH...WHICH
SEEMS TO LINE UP WELL WITH WHERE THE GFS/ECMWF CONVECTIVE
PARAMETERIZATION IS SETTING OFF...PRESUMABLY FROM THE EFFECTS OF
DRY/WARM AIR IN THE MID LEVELS POSSIBLY KEEPING THE CAP IN PLACE.
SPC HAS THE ENTIRE AREA IN A SLIGHT RISK/15% COMBINED THREAT FOR
DAY 3 AND I HAVE ZERO ARGUMENT WITH THAT. THE ACTUAL CHANCE OF
STORMS IS A LITTLE HIGHER AROUND HEBBRONVILLE AND FALFURRIAS BUT
IF STORMS DEVELOP THEY COULD BE SEVERE ANYWHERE WITH LARGE
INSTABILITY AND FAVORABLE SHEAR PRESENT. THE RISK FOR FLASH
FLOODING WILL ALSO BE PRESENT ALTHOUGH RAINFALL RATES MAY NOT BE
QUITE AS EFFICIENT WITH PAST STORMS WITH A SLIGHTLY LESS SATURATED
COLUMN PRESENT AND THE STORMS WILL LIKELY BE A LITTLE FASTER
MOVING. PREVIOUS DAYS CONVECTION BOTH TODAY AND TOMORROW MAY ALTER
AVAILABLE INSTABILITY SO KEEP TUNED FOR UPDATES.

SATURDAY APPEARS TO FEATURE ANOTHER RISK FOR STRONG TO SEVERE STORMS
WITH THE LEFT FRONT REGION OF A 130 KT WEST NORTHWESTERLY JET MAX
MOVING OVER THE AREA COUPLED WITH THE BASE OF AN INCREASINGLY
NEGATIVELY TITLED SHORTWAVE TROUGH AS THE PREVIOUSLY MENTIONED
FOUR CORNERS LOW BEGINS TO OPEN UP. THIS WEST NORTHWESTERLY FLOW
IS OUR MOST CLIMATOLOGICAL FAVORABLE FOR SEVERE WEATHER BUT AT
LEAST IN GFS PROJECTIONS AN 850MB TROUGH SETS UP JUST BEFORE PEAK
HEATING THAT BRINGS IN NORTHWESTERLY 850MB WINDS AND MUCH DRIER
AIR THROUGH THAT LAYER WHICH MIGHT MAKE CONVECTIVE INITIATION
TOUGHER. THE ECMWF DOES GO AHEAD AND GENERATE PLENTIFUL CONVECTION
AND THE GFS ALSO STILL GENERATES SOME. THE POSSIBILITY OF
WIDESPREAD CONVECTION IN PREVIOUS DAYS OBVIOUSLY WILL WEIGH INTO
FINAL SEVERE WEATHER POTENTIAL AS WELL. THE GIST OF MODEL
CONSENSUS HOWEVER IS THAT THERE WILL STILL BE FAVORABLE LARGE
SCALE ASCENT AND SURFACE MOISTURE AND HEATING COUPLED WITH
FAVORABLE SHEAR PROFILES AND WE WILL NEED TO WATCH CLOSELY FOR
SEVERE WEATHER POTENTIAL.

SUNDAY THE ZONAL JET SITS OVERHEAD WITH 250MB SPEEDS AROUND 125KTS.
THERE IS SOME UNCERTAINTY TO HOW MUCH MOISTURE WILL BE AVAILABLE IN
THE 850MB LAYER WHICH MAY PLAY A ROLE IN STORM ORGANIZATION BUT
GFS/ECMWF BOTH INITIATE THUNDERSTORMS OVER DEEP SOUTH TEXAS ALONG A
SURFACE FRONT THAT WAS GENERATED BY THE PUSH OF THIS NORTHWESTERLY
UPPER LEVEL JET. THIS IS ANOTHER PRETTY CLIMATOLOGICAL FAVORABLE
SEVERE WEATHER PATTERN SO...LIKE BROKEN RECORD WE WILL NEED TO
WATCH FOR THE POTENTIAL ON THIS DAY.

MONDAY THE GFS SENDS THAT FRONT THE REST OF THE WAY THROUGH THE AREA
DRYING AND COOLING THE SURFACE ENOUGH THAT NO STORMS APPEAR TO
DEVELOP UNDER THE STRONG ZONAL JET STREAK BUT THE ECMWF ACTUAL HAS
ANOTHER SHORTWAVE TROUGH PUSHING THROUGH AND GENERATE WIDESPREAD
SHOWERS AND THUNDERSTORMS. PREFER THE ECMWF GIVEN THE VERY
PROGRESSIVE UPPER LEVEL PATTERN IN PLACE LAST WEEK AND KEPT SHOWER
AND STORM CHANCES IN THE FORECAST BOTH TUESDAY AND WEDNESDAY. AT
THIS POINT THE BEST STRATEGY MAY BE TO TAKE THE THUNDERSTORM
CHANCES OUT WHEN WE QUIT GETTING THUNDERSTORMS EVERY DAY.

MAX TEMPS RUN INTO THE MID TO UPPER 80S BEFORE CONVECTION STOPS THE
UPWARD PROGRESSION EACH DAY. /68-JGG/

MARINE...
TODAY THROUGH THURSDAY...A FEW COASTAL SHOWERS WILL BE POSSIBLE
TODAY WITH AN OLD FRONTAL BOUNDARY JUST OFFSHORE. LIGHT NORTHEAST
WINDS WILL STRENGTHEN TO MODERATE SOUTHEAST TONIGHT AS HIGH PRESSURE
REESTABLISHES ACROSS THE NORTHWEST GULF. SEAS WILL REMAIN LOW TO
MODERATE. UNSETTLED WEATHER WILL CONTINUE...WITH SHOWERS AND
THUNDERSTORMS POSSIBLE.

THURSDAY NIGHT THROUGH MONDAY... MODERATE SOUTHEASTERLY WINDS ARE
FORECAST THROUGH FRIDAY...AND THEN A WEAK FRONT APPROACHES THE
AREA BECOMING STATIONARY SOMEWHERE IN THE AREA THROUGH THE
WEEKEND. LIGHT TO MODERATE WINDS ARE EXPECTED WHETHER OR NOT THE
FRONT PASSES BUT THE EXACT WIND DIRECTION WILL BE DIFFICULT TO PIN
DOWN THROUGH THE WEEKEND. THERE ARE HEALTHY CHANCES FOR SHOWERS
AND THUNDERSTORMS...INCLUDING STRONG THUNDERSTORMS IN THE FORECAST
THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK.
&&

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

$$

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

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