Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 301755 AAB
AFDBRO

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BROWNSVILLE TX
1255 PM CDT SUN AUG 30 2015

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

&&

.AVIATION...THE SEA BREEZE HAS PUSHED WEST OF HRL RIGHT NOW...
BUT A FEW SHOWERS AND AN ISOLATED WEAK THUNDERSTORM OR TWO MAY
LINGER NEAR THE BRO AREA. RADAR SHOWS MODERATE...SHORT LIVED
DOWNPOURS WITH THE OTHERWISE RATHER SPARSE ACTIVITY...THOUGH
INTERSECTING OUTFLOW BOUNDARIES NEED TO BE WATCHED FOR QUICK
BUILDUPS. THAT SAID...TAFS SHOULD REMAIN VFR THE NEXT 24 HOURS.
INSTABILITY ALOFT AND INCREASING MOISTURE MOVING UP FROM THE
SOUTH WILL EVENTUALLY SUPPORT MORE MARINE AND COASTAL SHOWER
ACTIVITY...WITH RAIN CHANCES NUDGING UP A BIT ON MONDAY.

&&

.PREV DISCUSSION... /ISSUED 637 AM CDT SUN AUG 30 2015/

DISCUSSION...UPDATED FOR LATEST AVIATION DISCUSSION BELOW.

AVIATION...LOW LEVEL MOISTURE ON THE INCREASE WITH SHOWERS
SHOWING UP SOUTHEAST OF KBRO OR JUST OFFSHORE MOVING NORTH.
CLOUDS TO INCREASE SHORTLY BEFORE SUNRISE WITH UNCERTAINTY IF ANY
SHOWERS MOVE INLAND. THERE IS A LOW PROBABILITY OF SCT-BKN VFR
CONDITIONS ALONG WITH A FEW SHOWERS LATE MORNING AND EARLY
AFTERNOON AS THE ATMOSPHERE BECOMES A LITTLE MORE UNSTABLE.
SHOWER COVERAGE IS EXPECTED TO BE ISOLATED WITH ONLY A MENTION OF
VICINITY IN KBRO AND KHRL WITH KMFE POSSIBLY BEING ADDED TO THE
MIX IF SHOWERS MAINTAIN THEIR INTEGRITY. CLOUDS TO INITIALLY
DECREASE IN COVERAGE LATE IN THE AFTERNOON AND EARLY EVENING WITH
A POSSIBILITY OF MVFR CIGS DEVELOPING AFTER MIDNIGHT AS GULF
MOISTURE CONTINUES TO INCREASE. SOUTHEAST WINDS TO REMAIN LIGHT
WITH A FEW GUSTS IN THE AFTERNOON APPROACHING 20 KNOTS.

PREV DISCUSSION... /ISSUED 354 AM CDT SUN AUG 30 2015/

SHORT TERM  /TODAY THROUGH MONDAY/...A NEWLY FORMED MID LEVEL LOW
PRESSURE AREA IS SEEN ON WATER VAPOR SATELLITE LOOPS OVER CENTRAL
TEXAS THIS MORNING. THIS LOW WAS NOT PREVIOUSLY INDICATED BY THE
29/00Z FORECAST MODEL PACKAGE. PREVIOUS FORECASTS HAVE BEEN
CONCENTRATING ON THE MID LEVEL LOW/TROUGH ACROSS THE NORTH CENTRAL
GULF AND THE DEEP SOUTH STATES. AS THE NEW TEXAS LOW BECOMES BETTER
DEFINED AND MOVES INTO SOUTH TEXAS OVER THE NEXT 24-48 HOURS THE
EASTERN LOW LIFTS NORTHEAST OUT OF THE PICTURE AND NO LONGER IS A
PLAYER. ALL THREE MODELS NAM/GFS AND ECMWF NOW SHOW THE BASE OF THIS
LOW TRACKING AS FAR SOUTH AS THE NEIGHBORING MEXICAN STATES OF
COAHUILA AND NUEVO LEON WITH THE TROUGH AXIS EXTENDING THROUGH THE
CWA AND INTO NORTHEAST TEXAS.  FORECAST SOUNDINGS AND TIME HEIGHTS
INDICATE A GRADUAL INCREASE IN LOW LEVEL MOISTURE TODAY AND TONIGHT
WITH A MORE SIGNIFICANT SURGE MONDAY INTO TUESDAY.

FOR TODAY THERE REMAINS A NEARLY 10KFT DEEP LAYER OF DRY AIR BETWEEN
800-500MB WHICH WILL BE SLOW TO ERODE AS LOW LEVEL MOISTURE
TRANSPORT WILL BE LIMITED ABOVE 850MB /5KFT/LEVEL. A FEW LOW TOP
SHOWERS MAY FORM ALONG A WEAK SEA BREEZE BUT DO NOT THINK THERE
WOULD BE SUFFICIENT LIFT OUTSIDE OF DIFFERENTIAL HEATING AND THE CAPE
DOES NOT LOOK ALL THAT IMPRESSIVE TO SUSTAIN UPDRAFTS FOR THUNDER.
TEMPERATURES START OFF WARMER 3 TO 5 DEGREES WITH DEW POINTS 3 TO 8
DEGREES HIGHER WHICH SHOULD ALLOW FOR A FEW DEGREES OF WARMING THIS
AFTERNOON. THE ADDED HEAT AND HIGHER DEW POINTS TO ALLOW FOR HEAT
INDICES TO REAR THEIR UGLY HEAD EXCEEDING 105 DEGREES IN SEVERAL
LOCATIONS EAST OF INTERSTATE 69C.

TONIGHT ENDS UP WARMER WITH A MORE STICKY HUMID FEEL WITH MOISTURE
TRANSPORT SLOWLY INCREASING OVERNIGHT AS THE 925-850MB WINDS TURN TO
THE SOUTH. LOW LEVEL CLOUDS TO INCREASE WITH THE POSSIBILITY OF A
WIDESPREAD 3000 FOOT STRATUS DECK BY SUNRISE. A FEW STREAMER SHOWERS
TO DEVELOP OFFSHORE MAY APPROACH THE COAST AND THE LOWER
RIO GRANDE VALLEY OVERNIGHT.

MONDAY A MUCH SHARPER INCREASE IN MOISTURE WITH MODEL PROJECTIONS
INDICATING PWATS APPROACHING 2 INCHES AND THE DRY LAYER ALL BUT
ERODED AS MEAN RH IN THE MID LEVELS EXCEEDS 50 PERCENT AFTER 18Z.
NOT ANTICIPATING AN ALL OUT RAIN DAY WITH A LIMITED LIFT AVAILABLE
INDICATED AT THIS TIME. SUFFICIENT HEATING AND THE APPROACH OF THE
MID/UPPER LOW THOUGH SHOULD GET ISOLATED SHOWER AND STORMS GOING
MAYBE AS EARLY IN THE MORNING BUT MORE LIKELY THROUGH THE LATE
MORNING AND AFTERNOON. NAM/GFS MODEL GUIDANCE ALSO HOLDING DOWN ON
THE POP CHANCES KEEPING PROBABILITIES BELOW 30 PERCENT AT THIS TIME.
IN CONTRAST THE WET BIAS ECMWF IS NOTING 30-35 PERCENT CHANCES.
A BLEND OF THE MODELS REVERTS TO A 20-30 PERCENT CHANCE WITH THE
HIGHER VALUES ACROSS THE HIGHER EASTERN HALF. TEMPERATURES TO
AVERAGE NEAR NORMAL WITH THE ADDED MOISTURE BUT HEAT INDICES
CONTINUE TO NEAR 105 DEGREES IN MOST LOCATIONS.

LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...BETTER AGREEMENT IN
THE MODELS TO BEGIN THE LONG TERM PERIOD WITH A TROUGH AXIS
STRETCHING ACROSS OUR AREA MONDAY NIGHT THROUGH WEDNESDAY
MORNING. THIS WILL PROVIDE THE UPPER SUPPORT AND LIFT FOR
SCATTERED CONVECTION ACROSS THE EASTERN HALF OF THE CWA. TIME
HEIGHTS AND POINT SOUNDINGS INDICATE GOOD COLUMN MOISTURE MON
NIGHT THROUGH WEDNESDAY MORNING. MOS GUIDANCE REFLECTS THIS WITH
HIGHER POP CHANCES WITH THE BEST CHANCES ACROSS THE GULF WATERS.
BEGAN THE UPSWING IN POP CHANCES LATE MONDAY NIGHT WITH CHANCE
POPS EXTENDING EAST OF HBV-RGC LINE. MY POPS ARE IN LINE WITH MEX
MOS AND LOWER THAN SUPERBLEND AS ECMWF SHOWS 70-90 AND SKEWS
SUPERBLEND HIGHER. IF MODELS CONTINUE THIS TREND WILL NEED TO
CONTINUE TO RAISE POPS DURING THIS TIMEFRAME.

DIGGING TROUGH OFF THE PAC COAST WILL CAUSE HIGH PRESSURE ACROSS
NW MEXICO TO AMPLIFY AND SHIFT EAST. THIS WILL IN TURN AID TO
SHOVE THE TROUGH EASTWARD TO WHERE OUR CWA WILL BE ON THE
SUBSIDENT WEST SIDE OF THE TROUGH BEGINNING WEDNESDAY AND THE
ADVECTION OF MID LEVEL DRY AIR INTO THE COLUMN. THUS COLUMN
MOISTURE ISNT AS GREAT AS EARLY IN THIS PERIOD AND PWATS BEGIN TO
DECREASE. MOS NUMBERS DECREASE AS WELL AND HAVE RESERVED CHANCE
POPS TO EAST OF HWY 77 WITH A CONTINUED DECREASING TREND HEADED
INTO THE WEEKEND. NOT TOTALLY CONFIDENT IN PULLING POPS
ALTOGETHER FRI-SAT AS THERE APPEARS TO BE ENOUGH SFC-650MB
MOISTURE TO WARRANT ISOLD CONVECTION. BESIDES...THERE IS
DISAGREEMENT IN THE MODELS ON HOW TO HANDLE THE TROUGH LATE IN
THE WORK WEEK WITH THE GFS INDICATING A HIGH OVER LOW BLOCK ACROSS
THE EASTERN THIRD OF THE US BY FRIDAY WHEREAS THE ECMWF KEEPS THAT
HIGH OVER WESTERN TEXAS AND THE UPPER LOW PROGRESSING INTO
THE SOUTHEAST CONUS.

MARINE...NOW THROUGH MONDAY...WEAK SURFACE HIGH PRESSURE REMAINS
OVER THE GULF WITH AN UPPER LEVEL LOW PRESSURE TROUGH TO MOVE OVER
SOUTH TEXAS LATER TODAY AND MONDAY. THE PRESSURE GRADIENT IS
EXPECTED TO REMAIN RATHER WEAK MAINTAIN A LIGHT SOUTHEAST TO SOUTH
WIND AND A SLIGHT SEA.

MONDAY NIGHT THROUGH FRIDAY...HIGH PRESSURE OVER THE WESTERN GULF
WILL MAINTAIN LIGHT TO MODERATE SOUTHEAST WINDS AND SEAS OF 3
FEET OR LESS ACROSS THE LOWER TEXAS COASTAL WATERS THROUGH THE
FORECAST PERIOD. NO EXERCISE CAUTION OR SMALL CRAFT ADVISORY
CONDITIONS EXPECTED THROUGH FRIDAY.

&&

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

$$

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