298
FXCA62 TJSJ 130058
AFDSJU
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
858 PM AST MON APR 12 2021
UPDATE
A MIXTURE OF LOW AND UPPER LEVEL CLOUDS CONTINUED TO CROSS THE
REGION TO MAINTAIN PARTLY CLOUDY TO VARIABLY CLOUDY SKIES.
HOWEVER, NO PRECIPITATION WAS OBSERVED OR DETECTED IN AND AROUND
THE ISLANDS. RECENT TJSJ 13/00Z UPPER AIR SOUNDING SUGGESTS
OVERALL STABLE CONDITIONS WITH STRONG 2-3 DEGREE CAP INVERSION
NEAR 850 MILLIBARS AND A DOMINANT SOUTHWEST WIND FLOW ABOVE 925
MILLIBARS, THEN BECOMING WESTERLY AND INCREASING WITH HEIGHT
ABOVE 700 MILLIBARS. THAT SAID, DO NOT EXPECT SIGNIFICANT AMOUNTS
OF LOW LEVEL MOISTURE ADVECTION OR PRECIPITATION,OVERNIGHT EXCEPT
FOR OVER THE OFFSHORE CARIBBEAN WATERS WHERE MOSTLY ISOLATED
SHOWERS WILL BE POSSIBLE LATER TONIGHT AND BY EARLY MORNING.
OTHERWISE NO CHANGE TO THE INHERITED SHORT TERM FORECAST PACKAGE
AND REASONING AT THIS TIME.
AVIATION...VFR DURG PRD. SCT OCNL BKN LYRS NR FL025...FL050
FL080
OVR REGIONAL WATERS AND EN ROUTE BTW P AND THE NORTHERN LEEWARD
ISLANDS. L/LVL WNDS FM SE-S 5-10 KTS BLO FL025...BCMG FM SW-W AND
INCR W/ HT ABV. SFC WNDS LIGHT AND VARIABLE BCMG MAINLY FM SE-S
5-15 KTS EXCEPT FOR SEA BREEZE VARIATIONS AFTER 13/14Z.
MARINE UPDATE
NO CHANGE TO PREVIOUS MARINE DISCUSSION AT THIS
TIME. RECENT DATA FROM THE NE PUERTO RICO BUOY 41043 SUGGESTS
SEAS BETWEEN 6-7 FEET AT 11-12 SECS AS THE NE SWELL CONTINUED TO
SLOWLY FADE. SMALL CRAFT OPERATORS SHOULD EXERCISE CAUTION OVER
THE ATLANTIC WATERS. NEARSHORE SEAS AND LOCAL PASSAGES WERE
BETWEEN 3-5 FEET WITH LIGHT TO MODERATE SSE WINDS.
PREV DISCUSSION
/ISSUED 341 PM AST MON APR 12 2021/
SYNOPSIS...
TODAY IS THE LAST DAY OF VERY DRY CONDITIONS, AS HIGHER MOISTURE
CONTENT WILL MOVE IN OVERNIGHT AND TOMORROW, AND BE WITH US FOR
THE NEXT SEVERAL DAYS. THUS ISOLATED TO SCATTERED SHOWERS ARE
EXPECTED FOR THE REST OF THE WORKWEEK. FAIRLY SLOW LOW-LEVEL WINDS
COULD LEAD TO SLOW-MOVING SHOWERS AND THE POTENTIAL FOR DECENT
RAINFALL ACCUMULATION IN THE NEXT FEW AFTERNOONS. MARINE
CONDITIONS ARE CALMING DOWN.
SHORT TERM...REST OF TODAY THROUGH WEDNESDAY...
DRY AIR CONTINUES TO PERSIST OVER THE REGION, THOUGH THAT WILL
CHANGE DURING THE NEXT FEW DAYS. CURRENTLY, PRECIPITABLE WATER
VALUES ARE AROUND 1.1, WHICH IS HIGHER THAN THE PAST FEW DAYS BUT
STILL BELOW AVERAGE. WE HAVE HAD A RIDGE OVER US AT MID TO UPPER
LEVELS, AND THAT HAS ALSO CREATED HOSTILE CONDITIONS FOR SHOWERS.
THE SOUNDING FROM THIS MORNING MEASURED A STRONG INVERSION AT AROUND
800MB. SO EVEN THOUGH THE RIDGING IS BREAKING DOWN, WE ARE STILL
FEELING ITS EFFECTS. THUS MOSTLY TRANQUIL CONDITIONS ARE EXPECTED
FOR THE REST OF THE AFTERNOON WITH MAYBE ONLY A FEW SHOWERS OVER THE
INTERIOR AND NORTHERN PUERTO RICO.
A LARGE POLAR TROUGH THAT EXTENDS THROUGH THE FULL LENGTH OF THE
ATMOSPHERE IS MOVING IN FROM THE WEST. ITS WESTERLY WINDS WILL
COUNTERACT THE NORMAL EASTERLY TRADE WINDS, MEANING LIGHT NEAR-
SURFACE WINDS FOR US THE NEXT FEW DAYS. THERE WILL BE QUITE A BIT OF
MOISTURE TO OUR SOUTH OVER THE NEXT FEW DAYS, YET THE WESTERLY WINDS
ALOFT WILL FOR THE MOST PART HOLD THAT MOISTURE AT BAY TILL LATER IN
THE WEEK. STILL, SOME MOISTURE WILL MAKE IT INTO OUR REGION, THUS
ENHANCING THE POTENTIAL FOR SHOWERS TOMORROW AND WEDNESDAY. THIS
MOISTURE INCREASE WILL PRIMARILY BE AT LOW LEVELS, BUT STILL THE MID-
ATMOSPHERIC LEVELS WILL BE QUITE DRY, THUS KEEPING THE POTENTIAL LOW
FOR STRONG SHOWERS. GIVEN THE LIGHT WINDS THOUGH, THERE WILL BE THE
POTENTIAL FOR PERSISTENT SHOWERS OVER A GIVEN AREA THAT COULD LEAD
TO SOME AREAS OF ISOLATED FLOODING. TRANQUIL CONDITIONS ARE EXPECTED
TOMORROW MORNING, BUT IN THE AFTERNOON, SHOWERS ARE POSSIBLE ACROSS
INTERIOR AND EASTERN PUERTO RICO, AS WELL AS OVER THE USVI. SOME
AREAS OF PONDING WATER AND POSSIBLY EVEN ISOLATED FLOODING ARE
POSSIBLE FOR INTERIOR AND EASTERN PUERTO RICO. ISOLATED TO SCATTERED
SHOWERS ARE EXPECTED OVERNIGHT TUESDAY AND INTO WEDNESDAY. THEN
AGAIN ON WEDNESDAY AFTERNOON UNDER FAIRLY LIGHT LOW-LEVEL WINDS,
MORE SCATTERED AND PERSISTENT SHOWERS CAN BE EXPECTED.
LONG TERM...THURSDAY THROUGH TUESDAY...
PREV DISCUSSION... /ISSUED 454 AM AST MON APR 12 2021/
RECENT MODEL GUIDANCE CONTINUES TO SUGGEST THAT A SOMEWHAT WETTER
AND UNSTABLE WEATHER PATTERN WILL CONTINUE ON THURSDAY, BUT NOW A
BIT DRIER THAN PREVIOUSLY EXPECTED AND LASTING THROUGH LATE
FRIDAY NIGHT SINCE THE AREA OF ENHANCED MOISTURE IS EXPECTED TO
STALL OVER THE REGION. A DELAY IN THE PEAK OF MOISTURE IS ONCE
AGAIN PRESENTED, SUGGESTING HIGHEST MODEL-ESTIMATED PRECIPITABLE
WATER AROUND 1.80 INCHES BY FRIDAY EVENING. OVER LAND, THE BULK OF
THE ACTIVITY WILL FAVOR SOUTHERN AND EASTERN SECTIONS OF PUERTO
RICO, AS WELL AS THE US VIRGIN ISLANDS DURING THE OVERNIGHT AND
EARLY MORNING HOURS, SHIFTING TO THE INTERIOR AND NORTHERN
SECTIONS OF PUERTO RICO IN THE AFTERNOON, WITH THE POTENTIAL FOR
ISOLATED THUNDERSTORM DEVELOPMENT SINCE 500 MB ARE EXPECTED TO
FALL AS LOW AS -8 DEGREES CELSIUS. THIS SCENARIO, UNDER A
GENERALLY WEAK STEERING FLOW, PRESENTS THE POTENTIAL FOR PONDING
OF WATER TO LOCALIZED URBAN AND SMALL STREAM FLOODING THROUGH
LATE FRIDAY NIGHT.
THEREAFTER, GLOBAL MODELS ARE NOW IN STRONG AGREEMENT
WITH A DRYING TENDENCY DOMINATING THE LOCAL WEATHER CONDITIONS
SATURDAY INTO EARLY NEXT WEEK, WITH PEAK DRYING EXPECTED ON
MONDAY. SO FAR, THIS WILL RESULT FROM A MID-TO-UPPER RIDGE MOVING
INTO THE AREA, STRENGTHENING THE TRADE WIND CAP WHILE LIMITING
ANY CHANCE FOR ORGANIZED SHOWER DEVELOPMENT. NEVERTHELESS, LIMITED
AFTERNOON CONVECTION CANNOT BE RULED OUT. AS CONDITIONS DRY OUT
AND THE GENERAL WIND FLOW BECOMES MORE SOUTHEASTERLY, EXPECTED
WARMER TEMPERATURES THAT COULD REACH THE LOWER TO MID 90S WITH
POSSIBLE HEAT INDICES INTO THE UPPER 90S TO LOWER 100S BY FRIDAY
ONWARDS.
AVIATION...
MAINLY VFR CONDITIONS ARE EXPECTED TO PREVAIL ACROSS ALL
TERMINALS DURING THE NEXT 24 HOURS. LOW-LEVEL WINDS WILL CONTINUE
FROM THE SE AT LESS THAN 15 KTS, BECOMING LIGHT AND VARIABLE
OVERNIGHT.
MARINE...
THE WEAK NORTHERLY SWELL IS DISSIPATING ACROSS THE REGION. SEAS
WILL REMAIN SOMEWHAT CHOPPY TOMORROW FOR THE OUTER ATLANTIC
WATERS, SO SMALL CRAFT OPERATORS SHOULD EXERCISE CAUTION. OTHER
AREAS WILL BE FAIRLY TRANQUIL WITH SEAS BELOW 5 FEET. SEAS WILL
BE EVEN MORE TRANQUIL WEDNESDAY AND THURSDAY, WITH EXPECTED WAVE
HEIGHTS FOR ALL LOCAL WATERS AT OR BELOW 4 FEET. ANOTHER WEAK
NORTHERLY SWELL IS EXPECTED TO IMPACT THE ATLANTIC WATERS AND
LOCAL PASSAGES FOR THURSDAY NIGHT AND FRIDAY.
PRELIMINARY POINT TEMPS/POPS
SJU 73 88 75 87 / 10 60 40 50
STT 74 85 74 85 / 20 40 40 40
SJU WATCHES/WARNINGS/ADVISORIES
PR...HIGH RIP CURRENT RISK THROUGH TUESDAY AFTERNOON FOR CULEBRA-
NORTH CENTRAL-NORTHEAST-NORTHWEST-SAN JUAN AND VICINITY-
SOUTHEAST.
HIGH RIP CURRENT RISK UNTIL 6 AM AST TUESDAY FOR MAYAGUEZ AND
VICINITY.
VI...HIGH RIP CURRENT RISK UNTIL 6 AM AST TUESDAY FOR ST.THOMAS...ST.
JOHN...AND ADJACENT ISLANDS.
AM...NONE.
SHORT TERM...RAM
LONG TERM....MB
PUBLIC DESK...ERG
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