686
FXCA62 TJSJ 192106
AFDSJU
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
506 PM AST WED FEB 19 2020
SYNOPSIS
SURFACE HIGH PRESSURE SPREAD ACROSS THE CENTRAL AND
NORTHEASTERN ATLANTIC WILL CONTINUE TO PRODUCE BRISK EASTERLY
WINDS THROUGH THURSDAY. THIS EASTERLY WIND FLOW WILL CONTINUE TO
PUSH PATCHES OF LOW-LEVEL MOISTURE INTO THE REGION, RESULTING IN
PASSING SHOWERS FROM TIME TO TIME. WINDS ARE EXPECTED TO DIMINISH
BY FRIDAY AND INTO THE UPCOMING WEEKEND AS THE SURFACE HIGH
PRESSURE MOVES TOWARDS THE EAST AND A SURFACE LOW PRESSURE MOVES
OVER THE WESTERN ATLANTIC. MORE UNSTABLE WEATHER CONDITIONS ARE
POSSIBLE BY LATE THIS UPCOMING WEEKEND INTO EARLY NEXT WEEK AS A
FRONTAL BOUNDARY MOVES CLOSE TO THE AREA.
SHORT TERM...TONIGHT THROUGH FRIDAY
NOT MUCH CHANGE IN THE OVERALL WEATHER PATTERN IS EXPECTED THROUGH
THE SHORT TERM PERIOD. ALOFT, THE AREA WILL CONTINUE TO REMAIN ON
THE SUBSIDENT SIDE OF A TROUGH TO THE EAST AND A RIDGE TO THE WEST.
THIS WILL CONTINUE TO PREVENT THE DEVELOPMENT OF SIGNIFICANT
SHOWERS.
AT LOWER LEVELS, A SURFACE HIGH PRESSURE SPREAD ACROSS THE CENTRAL
AND NORTHEASTERN ATLANTIC WILL CONTINUE TO PRODUCE BRISK EASTERLY
TRADE WINDS OVER THE AREA THROUGH THURSDAY. CURRENTLY, THERE IS A
PATCH OF LOW- LEVEL MOISTURE MOVING INTO THE AREA. THIS HAS
PRODUCED SOME LIGHT TO MODERATE SHOWERS ACROSS PORTIONS OF
INTERIOR AND WESTERN PUERTO RICO THIS AFTERNOON. IT IS EXPECTED
THAT THIS ACTIVITY WILL DIMINISH ACROSS THESE AREAS THROUGH THE
EARLY EVENING HOURS. ADDITIONAL LIGHT TO MODERATE BRIEF SHOWERS
ARE THEN POSSIBLE ACROSS PORTIONS OF EASTERN PUERTO RICO AND THE
USVI DURING THE OVERNIGHT AND EARLY MORNING HOURS OF THURSDAY.
LATEST MODEL GUIDANCE IS INDICATING DRIER AIR WITH PRECIPITABLE
WATER VALUES DIMINISHING TO BELOW 1 INCH AT TIMES LATER THURSDAY
AFTERNOON THROUGH FRIDAY. SAHARAN DUST, ALTHOUGH NOT SIGNIFICANT,
WILL ALSO BE MOVING OVER THE AREA, RESULTING IN SOMEWHAT HAZY SKIES.
THEREFORE, A DECREASE IN TRADE WIND SHOWER ACTIVITY IS EXPECTED
THROUGH THIS TIME FRAME. AS THE AFORMENTIONED SURFACE HIGH
PRESSURE MOVES FURTHER EASTWARD OVER THE NORTHEASTERN ATLANTIC
AND A SURFACE LOW-PRESSURE DEVELOPS OFF THE SOUTHEASTERN COAST OF
THE UNITED STATES, LOW-LEVEL WINDS WILL VEER FROM THE SOUTHEAST AS
WELL AS WEAKEN BY FRIDAY.
LONG TERM...SATURDAY THROUGH WEDNESDAY
PREV DISCUSSION
/ISSUED 453 AM AST WED FEB 19 2020/
THE RIDGE THAT DOMINATED THE AREA DURING THE WEEK AT UPPER LEVELS WILL
MOVE EAST OF THE AREA SATURDAY AND A LONG WAVE TROUGH WILL SINK
SOUTH INTO THE ATLANTIC WATERS NORTH OF THE AREA SUNDAY. BY
MONDAY IT WILL PASS TO THE EAST.
A COLD FRONT WILL BEGIN TO APPROACH THE LOCAL AREA ON SATURDAY. A
PRE-FRONTAL TROUGH WILL ALSO DEVELOP IN THE WESTERN TROPICAL
ATLANTIC WITH THE AID OF A TROUGH MOVING TOWARD THE AREA. THE WEAK
TROUGH AT 700 MB WILL PUSH INTO THE AREA SUNDAY AND MONDAY WHILE
THE FRONT APPROACHES THE NORTHWEST CORNER OF THE FORECAST AREA.
CURRENT MODELS HAVE CONSIDERABLY DRIER AIR TRAPPED BETWEEN THE
PRE-FRONTAL TROUGH AND THE FRONT AND GREATLY REDUCE THE AMOUNT OF
RAIN EXPECTED IN THE AREA DUE TO THE FRONT, SINCE A JET OF AIR
FROM THE SOUTHEAST AT LOWER LEVELS WILL TEND TO PUSH BACK AT THE
FRONT AND HOLD IT JUST NORTH OF THE AREA. BY WEDNESDAY, HOWEVER,
THE MOISTURE FROM THE TWO FEATURES MERGE OVER THE AREA AND
NORTHEASTERLY FLOW AT LOWER LEVELS CARRIES THE DAY AND ALLOWS THE
FRONT TO PASS WITH CONSIDERABLY DRIER AIR BEHIND IT. NEVERTHELESS
WOULD EXPECT AT LEAST SCATTERED TO NUMEROUS SHOWERS IN THE AREA
TUESDAY AND WEDNESDAY DUE TO THE CONVERGENCE OF THESE SYSTEMS.
BUT, BECAUSE RELATIVE HUMIDITIES ARE GENERALLY LESS THAN 30
PERCENT ABOVE 12 KFT FOR THE ENTIRE PERIOD RAINFALL AMOUNTS WILL
BE MODEST AND GENERALLY LESS THAN ONE INCH ON THE MOST FAVORED
SLOPES.
AVIATION
VFR CONDITIONS WILL CONTINUE BUT QUICK PASSING -SHRA AND
SHRA SHOULD NOT BE RULED OUT IN/NEAR THE LEEWARD ISLANDS AND E/N-PR
AT TIMES. SHRA WILL DEVELOP NEAR JPS/JMZ THROUGH AT LEAST 20/00Z,
LEAVING CLEAR SKIES OVERNIGHT. EASTERLY WINDS WILL BLOW AT 15 TO 20
KT WITH HIGHER GUSTS, DECREASING AROUND 10 KTS AFTER 19/23Z. SIMILAR
WEATHER PATTERN WILL CONTINUE THROUGH THE FORECAST PERIOD, BUT
SAHARAN DUST PARTICLES WILL INVADE THE LOCAL FLYING AREA TOMORROW
WHICH COULD LIMIT VIS.
MARINE
MODERATE TO FRESH EASTERLY TRADE WINDS BETWEEN 15 AND 20
KNOTS WILL CONTINUE TO GENERATE CHOPPY AND HAZARDOUS SEAS OF UP TO
7 FEET ACROSS MOST OF THE REGIONAL WATERS THROUGH LATE THURSDAY
NIGHT INTO EARLY FRIDAY, WHICH BY THEN SEAS ARE EXPECTED TO
BE BELOW SMALL CRAFT ADVISORY CRITERIA. FAIRLY TRANQUIL SEAS ARE
EXPECTED LATER FRIDAY INTO MOST OF THE UPCOMING WEEKEND AS WINDS
DIMINISH. HOWEVER, BY LATE SUNDAY INTO EARLY NEXT WEEK, A
NORTHERLY SWELL IS EXPECTED TO INVADE THE ATLANTIC WATERS AND
LOCAL PASSAGES TO ONCE AGAIN GENERATE HAZARDOUS SEAS.
PRELIMINARY POINT TEMPS/POPS
SJU 74 85 74 87 / 50 30 30 30
STT 74 85 74 84 / 30 20 20 20
SJU WATCHES/WARNINGS/ADVISORIES
PR...HIGH RIP CURRENT RISK THROUGH LATE TONIGHT FOR NORTHEAST-SAN
JUAN AND VICINITY.
HIGH RIP CURRENT RISK THROUGH LATE THURSDAY NIGHT FOR NORTH
CENTRAL-NORTHWEST.
HIGH RIP CURRENT RISK UNTIL 6 PM AST THIS EVENING FOR CULEBRA.
VI...HIGH RIP CURRENT RISK UNTIL 6 PM AST THIS EVENING FOR ST CROIX.
AM...SMALL CRAFT ADVISORY UNTIL 8 PM AST THURSDAY FOR ANEGADA PASSAGE
SOUTHWARD TO 17N-CARIBBEAN WATERS OF PUERTO RICO FROM 10 NM
TO 17N-COASTAL WATERS OF NORTHERN PUERTO RICO OUT 10 NM-
MONA PASSAGE SOUTHWARD TO 17N.
SMALL CRAFT ADVISORY UNTIL 2 PM AST FRIDAY FOR ATLANTIC WATERS
OF PUERTO RICO AND USVI FROM 10 NM TO 19.5N.
SMALL CRAFT ADVISORY UNTIL 2 AM AST FRIDAY FOR COASTAL WATERS OF
NORTHERN USVI AND CULEBRA OUT 10 NM-COASTAL WATERS OF
SOUTHERN USVI VIEQUES AND EASTERN PUERTO RICO OUT 10 NM.
SMALL CRAFT ADVISORY UNTIL 8 PM AST THIS EVENING FOR COASTAL
WATERS OF NORTHWESTERN PUERTO RICO OUT 10 NM.
SHORT TERM...RAM
LONG TERM....GL
PUBLIC DESK...TW
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