Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 011555 AAB
AFDBRO

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BROWNSVILLE TX
955 AM CST SUN MAR 1 2015

.DISCUSSION...FEBRUARY CLOSED LIKE A COLD LION...AND MARCH IS
CERTAINLY IS COMING IN LIKE ONE RELATIVE TO WHAT OUR TEMPERATURE
AND SENSIBLE WEATHER SHOULD BE. COASTAL TROUGH CONTINUES TO
ENHANCE LIGHT NORTHWEST FLOW UNDERNEATH THE COLD DOME THAT SETTLED
ACROSS THE ENTIRE VALLEY LATE FRIDAY NIGHT. WITH NOSE OF SURFACE
HIGH LOCKED DOWN ACROSS THE RIO GRANDE PLAINS SEE ONLY MARGINAL
MODERATION FOR THE REST OF THE DAY. NAM-12 AND OTHER MESOSCALE
MODELS CONTINUE TO DESTROY THE GLOBAL MODEL SUITE THIS MORNING IN
HANDLING THE RAW CHILLINESS...SO NO REASON TO DEVIATE AND HAVE
REPOPULATED TEMPERATURE FORECAST WITH THE COLDER VALUES...NUDGED
A BIT LOWER BASED ON EXPECTATION FOR LIMITED CLOUD DECK LIFTING
ACROSS THE MID/UPPER VALLEY.

NEW FORECAST HAS AFTERNOON HIGHS MAINLY IN THE 50S...BUT DID POKE
A COUPLE OF 60+ AREAS ALONG THE RIO GRANDE IN STARR COUNTY OWING
TO THE NAM`S TIGHT WARMING GRADIENT...WHICH LOOKS TOO OPTIMISTIC
BUT PERHAPS REALISTIC IF SURFACE WINDS CAN BACK JUST A LITTLE
TOWARD THE NORTH/NORTHEAST.

I`VE ALSO ADJUSTED THE OVERNIGHT VALUES DOWN SEVERAL DEGREES. IN
THE BIG PICTURE...CALIFORNIA/NEVADA TROUGH HAS SLOWED DOWN WHICH
HAS PUMPED UP THE SUBTROPICAL 500 MB RIDGE INTO THE EASTERN
ROCKIES. AT THE SAME TIME...UPPER LEVEL CONVERGENCE SLIDES THROUGH
THE DAKOTAS THIS EVENING AND INTO THE UPPER MISSISIPPI VALLEY BY
MONDAY MORNING. WHAT DOES THIS HAVE TO DO WITH THE RGV? PLENTY.
THAT UPPER CONVERGENT/CONFLUENT AREA BRINGS A REINFORCING SHOT OF
HIGH PRESSURE INTO THE CENTRAL PLAINS THIS EVENING...WHOSE `NOSE`
WILL EXTEND DOWN THROUGH WEST-CENTRAL TEXAS AND ALONG THE LEE OF
THE SIERRA MADRE. THIS ENSURES A MINOR RE-COOLING THIS EVENING AND
OVERNIGHT AND LIKELY WILL SET THE STAGE FOR A CHILLY START TO THE
WORK WEEK. EXPECT TO SEE DRAMATIC CHANGES TO MONDAY`S HIGH
TEMPERATURE FORECAST...WHICH CURRENTLY ADVERTISES SPRING /UPPER
70S TO AROUND 80/ BUT WILL MORE LIKELY BE MIRED IN THE RAW CHILL
FOR YET ANOTHER DAY.

THAT LION IS PROVING VERY STUBBORN!
52/BSG

&&

.MARINE...SMALL CRAFT ADVISORY HAS EXPIRED AS SEAS DIPPED BELOW 6
FEET WELL OFFSHORE WHERE SWELL HAS BEEN MOST INFLUENTIAL. THAT
SWELL BEGAN TO DROP AS WELL...NOW DOWN TO AN 8 SECOND PERIOD.
EXPECT BENIGN CONDITIONS...OUTSIDE OF THE POTENTIAL FOR LOCAL
DENSE FOG...INTO MONDAY EVEN THOUGH THE NORTH WINDS WILL GET A
BOOST BUT SHOULD REMAIN BELOW ANY ADVISORY OR CAUTION LEVELS WITH
SEAS CONTINUING TO DIP.
52/BSG

&&

.PREV DISCUSSION... /ISSUED 540 AM CST SUN MAR 1 2015/

DISCUSSION...UPDATED FOR LATEST AVIATION DISCUSSION BELOW.

AVIATION...IFR/LIFR CONDITIONS WL PERSIST THROUGH MOST OF THE
NEXT 24 HOURS ACROSS DEEP SOUTH TEXAS. WARM MOIST AIR WL CONTINUE
TO OVERRUN SHALLOW COOL AIR. THE RESULT WL BE AREAS OF LIGHT RAIN
AND DRIZZLE...AND ALSO A THICK STRATUS DECK WITH AREAS OF FOG. A
SLIGHT IMPROVEMENT IN CIGS/VSBYS IS EXPECTED THIS AFTERNOON BEFORE
CONDITIONS DETERIORATE AGAIN BY THIS EVENING.

PREV DISCUSSION... /ISSUED 412 AM CST SUN MAR 1 2015/

SHORT TERM /NOW THROUGH MONDAY/...A MID LEVEL TROUGH IS CURRENTLY
OVER THE MID CA COAST WHILE A WEST TO EAST FLOW IS IN PLACE IN THE
MID LEVELS ACROSS MUCH OF THE SOUTHERN PLAINS. SURFACE COASTAL
TROUGH IS ALMOST STATIONARY OVER THE GULF WATERS WHILE THE CANADIAN
HIGH WRAPS AROUND IT WITH A NORTHWEST TO NORTHERLY FLOW IN PLACE
ACROSS THE CWA. A VERY LIGHT RAIN OR DRIZZLE IS DEVELOPING OVER THE
GULF WATERS AND MOVE INTO THE LOWER AND MIDDLE VALLEY...BUT IT
REMAINS LIGHT. WITH THE COLD AIR MASS IN PLACE ACROSS THE AREA...
LOW LEVEL CLOUD COVER WILL REMAIN THICK IN THE BOUNDARY LAYER. IN
THE SHORT TERM...THE MID LEVEL TROUGH OVER CA COAST WILL SINK
FARTHER SOUTH...RESULTING IN AN AMPLIFYING RIDGE OVER THE SOUTHERN
HIGH PLAINS AND A REBUILDING SURFACE HIGH OVER THE CENTRAL
PLAINS.

TODAY...CONTINUED CLOUDY AND COOL WITH LIGHT WINDS...AND WITH
TEMPERATURES IN THE UPPER 50S TO LOWER 60S. TONIGHT...THE UPPER
LEVEL FLOW WILL BECOME MORE SOUTHWEST WITH THE COASTAL TROUGH
SHIFTING EAST. LOW TEMPS WILL BE IN THE 50S. RECONSTITUTED HIGH
PRESSURE OVER THE PLAINS WILL SEND A WEAK REINFORCING AIR MASS
SOUTH...MAINTAINING NORTH WINDS A BIT LONGER. TEMPS WILL WARM INTO
THE LOWER 70S ON MONDAY. DEEPENING LOW PRESSURE OVER THE HIGH PLAINS
WILL HELP REESTABLISH A SOUTHEAST FLOW MONDAY AND MONDAY NIGHT.

LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
STILL FORECASTING A STEADY WARM UP UNTIL A MID WEEK COLD FRONT
PLUNGES US INTO CLOUDY AND CHILLY WEATHER ONCE AGAIN.

A LARGE UPPER LEVEL TROUGH WILL BE ADVANCING SOUTHEAST TOWARDS THE
NORTHWEST MEXICAN COAST MONDAY NIGHT INTO TUESDAY. THIS WILL
INTERACT WITH LOW LEVEL TROUGHING OVER THE SOUTH CENTRAL GULF OF
MEXICO TO PRODUCE A BREEZY/WARM/WINDY DAY TUESDAY. WIND GUSTS OF 30
TO 35 MPH REMAIN IN THE FORECAST THIS MORNING...ESPECIALLY EAST OF
HIGHWAY 281. SOUTHWESTERLY 700/850MB WINDS WILL DRY AND WARM OFF THE
MOUNTAINS...AND MIX INTO THE BOUNDARY LAYER AND AMID PARTLY CLOUDY
SKIES HELPING TEMPERATURES INTO THE MID TO UPPER 80S EXCEPT FOR THE
IMMEDIATE COAST.

SOUTHWESTERLY FLOW AHEAD OF THE TROUGH WILL POSITION ITSELF TO
CREATE MORE UPPER DIVERGENCE OVER SOUTH TEXAS THROUGH TUESDAY
EVENING AND BROAD SURFACE TROUGHING WILL FORM IN THE LEE OF THE
SIERRA MADRE MOUNTAINS...SHIFTING OUR WINDS MORE TO THE EAST. A FEW
SPRINKLES MAY DEVELOP OVERNIGHT UNDER A VERY STRONG TERRAIN INDUCED
RESIDUAL INVERSION BUT OPTED TO KEEP THE FORECAST DRY FOR MEASURABLE
PRECIP.

UPPER RIDGING AMPLIFIES SHARPLY OVER THE PACIFIC NORTHWEST WEDNESDAY
MORNING AS A SHORTWAVE TROUGH DIVES INTO THE CENTRAL PLAINS. THE
RESULTING COMBINATION OF UPPER CONVERGENCE FORCES A 1035 MB HIGH
OVER MUCH OF THE CENTRAL US WHICH USES COLD AIR DAMMING PROCESSES TO
SEND AN AIRMASS...SOURCED NEAR THE POLES...SOUTHWARD ACROSS TEXAS.
THE RESULTING ARCTIC AIRMASS ARRIVES DURING THE DAY WEDNESDAY. THERE
REMAINS AT LEAST SOME TIMING DISCREPANCIES IN ACTUAL FRONTAL
TIMING...BUT FOR THE FORECAST CONTINUED TO BE A LITTLE FASTER THAN
THE GLOBAL MODEL CONSENSUS. THIS IS DUE TO THE TENDENCY FOR SUCH
SYSTEMS TO MOVE FASTER THAN MODELED AMID THE LOWER VERTICAL
RESOLUTION OF GLOBAL MODELS AND THE PREEXISTING TROUGHING OVER THE
AREA...SO AT LEAST IN THE LOWEST 3000 TO 5000 FT IT WILL FACE LITTLE
RESISTANCE.

CONFIDENCE CONTINUES TO INCREASE THAT THIS FRONT WILL PRODUCE A
STARTLING DROP IN TEMPERATURES AS IT ARRIVES. THE FORECAST CONTINUES
TO CALL FOR HIGHS APPROACHING THE MID 80S WEDNESDAY AFTERNOON
BEFORE PLUNGING INTO THE MID TO LOWER 40S BY WEDNESDAY NIGHT. WIND
CHILLS ARE IN THE 30S AREA WIDE...AND AS THIS SYSTEM COMES WITHIN
VIEW OF THE NAM AND ITS BETTER HANDLING OF ARCTIC AIR MASSES... EXPECT
WE MAY NEED TO GO LOWER WITH THE TEMPERATURES. MOIST SOUTHWESTERLY
FLOW ALOFT AND COASTAL TROUGHING NEAR THE SURFACE AMID CONTINUED
COLD ADVECTION WILL KEEP HIGHS FROM REBOUNDING ABOVE 50 DEGREES ON
THURSDAY. GFS/ECMWF ALSO CONTINUE TO POSITION SOUTH TEXAS IN A
DIVERGENT PATTERN ALOFT ON THE RIGHT REAR QUADRANT OF A STRONG
SPEED MAX TO THE EAST OF THE SLOWLY ADVANCING PARENT TROUGH. AT
THE SAME TIME A STRONG OVERRUNNING PATTERN IS APPARENT IN THE
800/700MB LAYER SO RAISED RAIN CHANCES AND KEPT A PERSISTENT
MENTION OF DRIZZLE.

CONTINUED TO CARRY DEWPOINTS LOWER THAN GUIDANCE CONSENSUS GIVEN THE
ARCTIC AIRMASS...THIS PUTS DEWPOINTS IN THE MID TO UPPER 20S WITH
PRECIP FALLING THROUGH THE COLUMN. MODEL SOUNDINGS SHOW A RELATIVELY
DRY LAYER IN THE LOWEST 50 TO 100 MB OF ATMOSPHERE THAT MIGHT BE
PRIME FOR WET BULB COOLING IF ENOUGH PRECIPITATION DEVELOPS. WET
BULB ZERO TEMPERATURES...EVEN CALCULATED TO OUR LOWER FORECAST
VALUES ARE STILL ABOVE FREEZING BUT ITS CLOSE ENOUGH TO KEEP AT
LEAST SOME EYE ON FOR FREEZING RAIN IF THE AIRMASS IS ESPECIALLY
COLDER THAN ADVERTISED.

THURSDAY NIGHT INTO FRIDAY THE UPPER JET MAX MOVES OUT OF THE AREA
BUT RELATIVELY ROBUST SOUTHWESTERLY FLOW WITH A FEW RIPPLES SHOULD
BE SUFFICIENT TO KEEP COASTAL TROUGHING IN THE AREA. KEPT RELATIVELY
STRONG NORTHWESTERLY WINDS IN PLACE AND KEPT RAIN/DRIZZLE CHANCES
HUGGED TO THE COASTLINE BEFORE INCREASING AGAIN FRIDAY NIGHT AS
850MB WINDS STRENGTHEN AS THE MAIN TROUGH/SEMI CLOSED LOW BEGINS
ADVANCING OVER NORTHERN MEXICO. SFC CONVERGENCE/COASTAL TROUGHING
INCREASES AND GFS/ECMWF SHOW GOOD 300K ISENTROPIC LIFT...SUFFICIENT
AT LEAST FOR SOME MENTION OF RAIN FRIDAY NIGHT INTO SATURDAY.
CONTINUED A SLOW WARMING TREND FRIDAY INTO SATURDAY WITH SFC FLOW
SHIFTING EASTWARD AND BEGINNING TO COME OFF THE WATER RATHER THAN
THE CENTRAL PLAINS...BUT WE WILL LIKELY KEEP THE LOW CLOUDS AND
DRIZZLE UNTIL THE TROUGH AXIS CLEARS SATURDAY EVENING. THIS SENDS
ANOTHER SURGE OF HIGH PRESSURE/FRONT THROUGH. ALTHOUGH IN THIS CASE
THE COLD ADVECTION APPEARS WEAKER AND THE FRONT DRIER. TEMPERATURES
ARE PERHAPS ABLE TO BOUNCE BACK INTO THE 60S BY SUNDAY. /68-JGG/

MARINE...
NOW THROUGH MONDAY...ELEVATED SEAS ARE NOW COMPOSED ALMOST
ENTIRELY OF EASTERLY SWELL. THE GOOD NEWS IS THAT WAVE HEIGHTS ARE
SLOWLY DECREASING AND WILL END THE GULF ADVISORY TO EXPIRE THIS
MORNING. A COASTAL TROUGH IS STILL POSITIONED OVER THE FAR OUTER
WATERS...BUT IT SHOULD FILL LATER TODAY AND TONIGHT AS WINDS OVER
LAND AND THE NEAR SHORE VEER DUE TO SUPPORTING HIGH PRESSURE TO
THE NORTH SHIFTING EAST. COMPLETE DISSIPATION WILL BE SLOW TO
OCCUR HOWEVER...AND A WEAK SIGNATURE MAY LAST THROUGH THE ENTIRE
SHORT TERM. WAVE HEIGHTS WILL REMAIN MODERATE TOWARD THE END OF
THE SHORT TERM...WITH LIGHT NORTH EAST WINDS NEAR SHORE AND
SOUTHEAST WINDS TOWARD THE OUTER EDGE OF THE LOWER TEXAS COAST
MARINE DELINEATION AT 60 NM.

MONDAY NIGHT THROUGH FRIDAY... SOUTHEAST WINDS ARE EXPECTED TO
BECOME STRONGER MONDAY NIGHT INTO TUESDAY. WIND SPEEDS AND SEAS MAY
REACH SMALL CRAFT ADVISORY CRITERIA ON THE GULF OF MEXICO MONDAY
NIGHT AND TUESDAY AND THE LAGUNA MADRE TUESDAY. WINDS ARE FORECAST
TO SHIFT TO THE EAST SOUTHEAST AND BECOME MORE MODERATE TUESDAY
NIGHT INTO WEDNESDAY. A STRONG COLD FRONT ARRIVES DURING THE
DAY/EVENING WEDNESDAY BRINGING STRONG NORTHWEST WINDS AND ROUGH SEAS
THAT ARE EXPECTED TO LARGELY PERSIST THROUGH FRIDAY. SMALL CRAFT
ADVISORY CONDITIONS ARE LIKELY FOR PORTIONS OF WEDNESDAY LASTING
THROUGH FRIDAY WITH GALE CONDITIONS POSSIBLE.

&&

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

$$

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