285
FXCA62 TJSJ 210912
AFDSJU
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
512 AM AST MON OCT 21 2019
SYNOPSIS
A LIGHT TO MODERATE EAST TO NORTHEAST WIND FLOW IS
FORECAST TO HOLD THROUGH AT LEAST MIDWEEK, VEERING TO THE EAST-
SOUTHEAST BY THE END OF THE WORKWEEK INTO THE WEEKEND AS A SURFACE
HIGH PRESSURE BECOMES THE DOMINANT FEATURE. A MOIST AND
RELATIVELY UNSETTLE WEATHER PATTERN IS EXPECTED TO PREVAIL ACROSS
THE FORECAST AREA DURING THE NEXT FEW DAYS. OVERALL, SHOWERS ARE
EXPECTED TO MOVE OVER WINDWARD AREAS DURING THE OVERNIGHT AND
EARLY MORNING HOURS, FOLLOWED BY SHOWER AND THUNDERSTORM
DEVELOPMENT EACH AFTERNOON. ADDITIONAL RAINS COULD RESULT IN URBAN
AND SMALL STREAM FLOODING, WHILE LOCALLY HIGH RAINFALL
ACCUMULATIONS COULD LEAD TO FLASH FLOODING AND MUDSLIDES MAINLY
ALONG THE INTERIOR AND WESTERN SECTORS OF PUERTO RICO.
SHORT TERM...TODAY THROUGH WEDNESDAY
A TUTT EXTENDS FROM THE CENTRAL ATLANTIC TO THE SOUTHEAST CARIBBEAN
SEA WITH HIGH PRESSURE AT UPPER LEVELS OVER THE FLORIDA PENINSULA. A
SHORT-WAVE NORTH OF THE THE LOCAL AREA IS MOVING AROUND THE HIGH AND
WILL FORM A WEAK CUT-OFF LOW ON TUESDAY JUST NORTH OF OUR NORTHERN
WATERS. IT WILL CROSS INTO THE EASTERN TIP OF THE DOMINICAN REPUBLIC
ON TUESDAY NIGHT ESSENTIALLY REVERSING THE NORTHERLY UPPER LEVEL
FLOW OVER THE AREA. LOW PRESSURE WILL CONTINUE IN THIS AREA THROUGH
WEDNESDAY.
AT MID LEVELS HIGH PRESSURE CONTINUES OVER OR EAST OF FLORIDA
THROUGH THE PERIOD. A TROUGH WILL DEVELOP OVER AND SOUTH OF THE AREA
TUESDAY EVENING AND FORM A REFLECTION OF THE UPPER LEVEL LOW OVER
THE DOMINICAN REPUBLIC ON WEDNESDAY.
FLOW AT LOWER LEVELS WILL CONTINUE NORTHEAST TO EAST. A TROUGH AT
850 MB IS AT 45 WEST AND EXTENDS NORTH OF 25 NORTH. THIS WEAKENS
CONSIDERABLY AS IT MOVES TOWARDS THE AREA. IT ENTERS THE EASTERN
CARIBBEAN ON TUESDAY NIGHT AND PASSES THROUGH THE FORECAST AREA ON
WEDNESDAY. ALSO IN PLAY, HOWEVER, IS A WEAK BAND OF MOISTURE AT 700
MB ALONG A BROAD, OLD AND ILL-DEFINED FRONTAL BOUNDARY WHICH
CURRENTLY IS ALMOST 350 MILES WIDE. IT IS CURRENTLY INVADING THE
AREA AND IS PART OF THE SOURCE OF THE SHOWERS AROUND US.
PRECIPITABLE WATER VALUES FLUCTUATE SOMEWHAT AROUND 1.8 INCHES
THROUGHOUT THE PERIOD AND THIS COMBINATION OF UPPER AND LOWER
FEATURES WILL KEEP SCATTERED SHOWERS IN THE FORECAST FOR MOST OF THE
AREA, ALTHOUGH SHOWERS WILL BE MORE DEFINITE DURING THE AFTERNOONS
IN WESTERN AND SOUTHWESTERN PUERTO RICO AND EXTENDING EAST INTO THE
INTERIOR PORTIONS EACH AFTERNOON.
LONG TERM...THURSDAY THROUGH MONDAY
A FAIRLY MOIST AND UNSETTLED WEATHER PATTERN IS EXPECTED THROUGH
MOST OF THE LONG TERM FORECAST PERIOD WITH GFS MODEL-ESTIMATED
PRECIPITABLE WATER VAPOR VALUES RANGING BETWEEN 1.5 AND 2.0
INCHES FROM THURSDAY MORNING THROUGH MONDAY AFTERNOON. ON THURSDAY
AND FRIDAY, THE COMBINATION OF A WESTWARD MOVING SURFACE INDUCED
TROUGH AND A MEANDERING TUTT LOW WILL SUPPORT INCREASED MOISTURE
TRANSPORT, LOW-LEVEL CONVERGENCE AND FAVORABLE CONDITIONS ALOFT.
AS THESE FEATURES MOVES FARTHER AWAY, A SURFACE HIGH PRESSURE
MEANDERING OVER THE WESTERN ATLANTIC WILL BECOME THE DOMINANT
FEATURE, DRIVING THE REMNANTS OF A FRONTAL BOUNDARY OVER THE
NORTHEASTERN CARIBBEAN UNDER A GENERALLY EASTERLY WIND FLOW. THIS
PATTERN WILL CONTINUE THROUGH AT LEAST MONDAY AFTERNOON, BEFORE
A DRIER AIRMASS BEGINS TO FILTER IN FROM THE EAST AND
PRECIPITABLE WATER VAPOR VALUES DROP TO NEAR AN INCH BY MONDAY
NIGHT. IN THE MEANTIME, ANOTHER CUTOFF TUTT LOW IS FORECAST TO
MOVE INTO THE EASTERN CARIBBEAN WITH ITS CONVERGENT/SUBSIDENCE
SIDE SETTLING OVER THE FORECAST AREA ON SUNDAY THROUGH MONDAY.
OVERALL, OVERNIGHT AND EARLY MORNING SHOWERS ARE FORECAST TO
STREAM OVER THE LOCAL WATERS INTO WINDWARD AREAS OF PUERTO RICO
AND THE U.S. VIRGIN ISLANDS. AS EACH DAY PROGRESSES, LOCALLY
INDUCED SHOWERS AND THUNDERSTORMS ARE EXPECTED TO DEVELOP,
CLUSTERING ALONG THE INTERIOR AND WESTERN SECTORS OF PUERTO RICO
EACH AFTERNOON. STREAMER-LIKE SHOWERS AND ISOLATED THUNDERSTORMS
FROM SIERRA DE LUQUILLO AND THE LOCAL ISLANDS MOVING INTO THE SAN
JUAN METROPOLITAN AREA AND EASTERN PUERTO RICO CANNOT BE RULED OUT
EACH AFTERNOON. ON SUNDAY AND MONDAY, AREAL COVERAGE AND
INTENSITY OF SHOWERS AND THUNDERSTORMS IS EXPECTED TO DECREASE DUE
TO UNFAVORABLE CONDITIONS ALOFT AND EROSION OF AVAILABLE MOISTURE.
AVIATION
SHRA IN NE FLOW TO CONTINUE OVR LCL WATERS AND INLAND 5-
10 MILES. SHRA TO DVLP INTERIOR PR AND VCTY TIST/TISX AFT 21/15Z.
MTN OBSCURATIONS AND BRIEF MVFR PSBL TJBQ, AND IN TJMZ AFT 21/17Z.
OTHERWISE ALL TAF SITES EXPECTED TO REMAIN VFR. SFC WINDS LESS THAN
10 KT, BCMG AFT 21/14Z NE 10-20 KT WITH LCL HIR GUSTS. MAX WINDS WNW
20-25 KT BTWN FL400-465.
MARINE
TRANQUIL MARINE CONDITIONS WITH SEAS UP TO 5 FEET AND
EAST WINDS UP TO 15 KNOTS ARE EXPECTED TO PREVAIL ACROSS THE
REGIONAL WATERS DURING THE NEXT FEW DAYS. SHOWERS ARE FORECAST TO
STREAM OVER THE WATERS DURING THE OVERNIGHT AND MORNING HOURS,
ESPECIALLY ACROSS THE ATLANTIC WATERS. SHOWER AND THUNDERSTORM
ACTIVITY IS POSSIBLE OVER THE WATERS WEST OF PUERTO RICO, AS WELL
AS OVER THE WATERS BETWEEN THE U.S. VIRGIN ISLANDS AND PUERTO RICO
EACH AFTERNOON. A MODERATE RISK OF RIP CURRENTS WILL PREVAIL FOR
BEACHES ALONG THE NORTH COAST OF PUERTO RICO, INCLUDING VIEQUES
AND CULEBRA, AS WELL AS FOR SAINT THOMAS AND SAINT CROIX.
PRELIMINARY POINT TEMPS/POPS
SJU 89 78 87 77 / 40 40 40 40
STT 88 78 87 77 / 50 40 50 50
SJU WATCHES/WARNINGS/ADVISORIES
PR...NONE.
VI...NONE.
AM...NONE.
SHORT TERM...WS
LONG TERM....ICP
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