821
FXCA62 TJSJ 260131 AAA
AFDSJU
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE SAN JUAN PR
931 PM AST MON OCT 25 2021
UPDATE
ISOLATED SHOWERS CONTINUE VERY WIDELY SEPARATED. THE
BEST SHOWERS THIS EVENING WERE WEST OF AGUADILLA IN THE MONA
PASSAGE WITH TOPS AROUND 25 KFT, ALTHOUGH MOST TOPS OVER LAND WERE
LESS THAN 10 KFT OF WHICH THERE ARE VERY FEW. THE SOUNDING CAME
OUT CLOSE TO THE GOES TOTAL PRECIPITABLE WATER PRODUCT AT AROUND
1.48 INCHES. A BAND OF 1.8 INCHES IS APPROACHING THE NORTHEAST
CORNER OF THE FORECAST AREA, BUT CLOUDS AND SHOWERS ARE STILL ONLY
SCATTERED THERE WITH SOUTHEAST LOW LEVEL FLOW. HENCE SHOWER
ACTIVITY FOR PUERTO RICO AND THE U.S. VIRGIN ISLANDS IS EXPECTED
TO INCREASE SOMEWHAT OVERNIGHT AND DURING THE MORNING AS BETTER
MOISTURE MOVES IN FROM THE SOUTHEAST, BUT A DISTINCT AREA OF MUCH
DRIER AIR AT 850 MB REACHES SAINT CROIX BY 26/12Z AND THE SOUTH
COAST OF PUERTO RICO BY 26/15Z. THUS SHOWER ACTIVITY SHOULD END
FROM SOUTH TO NORTH DURING THE FOLLOWING 6 HOURS WITH THE
EXCEPTION OF CONVECTIVE ACTIVITY IN THE NORTHWEST CORNER OF THE
ISLAND WHERE WINDS CONVERGE AROUND THE CORNER OF THE ISLAND DURING
THE AFTERNOON. ONLY MINOR CHANGES TO THE GRIDS--MAINLY TO
TEMPERATURES.
AVIATION
VFR CONDS TO PREVAIL. LCL MTN OBSCURATIONS IN ISOLATED
SHRA TIL ARND 26/16Z. SHRA WITH MVFR AND MTN OBSCURATIONS TO DVLP
IN NW PR AFT 26/17Z AND CONT TILL BYD 26/22Z. SFC WINDS LAND
BREEZES LESS THAN 10 KT, BMCG SEA BREEZES 10-15KT WITH GUSTS TO 20
ALONG THE COASTS. MAX WINDS NNW 20-25 KT BTWN FL425-440 AND ESE AT
20 KT NR FL080.
MARINE
WINDS AND SEAS WILL PROGRESSIVELY DIMINISH DURING THE
NEXT 48 HOURS. LAND AND SEA BREEZES WILL BEGIN TO PREVAIL AGAINST
THE PREDOMINANT FLOW OF 10-15 KNOTS FROM THE SOUTHEAST. ISOLATED
SHOWERS. A POSSIBILITY OF THUNDERSTORMS RETURNS ON WEDNESDAY AS
MID LEVELS COOL SEVERAL DEGREES.
PREV DISCUSSION
/ISSUED 441 PM AST MON OCT 25 2021/
SYNOPSIS...A RIDGE ALOFT WILL PERSIST INTO TUESDAY, RESULTING IN
STABLE CONDITIONS ALOFT AND THUS INHIBITING THE DEVELOPMENT OF
WIDESPREAD, ORGANIZED CONVECTIVE ACTIVITY. THE RIDGE ALOFT IS
EXPECTED TO WEAKEN WEDNESDAY AND THURSDAY AS A SHORTWAVE TROUGH
MOVES ACROSS AND DEPENDING ON THE AMOUNT OF MOISTURE AVAILABLE,
CONVECTIVE ACTIVITY IS POSSIBLE, ESPECIALLY ACROSS NORTHERN AND
NORTHWESTERN PORTIONS OF PUERTO RICO. MORE SOUTHERLY AND LIGHTER
WINDS ARE EXPECTED FOR TUESDAY AND WILL PERSIST THROUGH THE END OF
THE WEEK.
SHORT TERM...TONIGHT THROUGH WEDNESDAY...
A RIDGE ALOFT PERSISTS OVER THE AREA, RESULTING IN THE
CONTINUATION OF STABLE WEATHER CONDITIONS. PATCHES OF LOW-LEVEL
MOISTURE PUSHING MOISTURE LEVELS TO NEAR NORMAL WILL CONTINUE TO
RESULT IN ISOLATED TO LOCALLY SCATTERED SHOWERS ACROSS THE AREA
FROM TIME TO TIME. HOWEVER, GIVEN THAT THESE SHOWERS WILL BE
BRIEF, ONLY MINOR ACCUMULATIONS ARE EXPECTED.
WEATHER CONDITIONS BEGIN TO CHANGE SOMEWHAT ON TUESDAY. A SURFACE LOW
PRESSURE SYSTEM DEVELOPING AND EXITING OFF THE MID-ATLANTIC AND
NORTHEASTERN COAST OF THE UNITED STATES WILL FORCE A SURFACE HIGH
PRESSURE CURRENTLY OVER THE WESTERN ATLANTIC FURTHER EAST INTO THE
CENTRAL ATLANTIC. THIS WILL CAUSE THE LOW-LEVEL FLOW TO BECOME
SOUTHEASTERLY AS WELL AS LIGHTER AT LESS THAN 10 KNOTS. HOWEVER,
ALOFT, THE MID-LEVEL RIDGE WILL CONTINUE TO PERSIST AND THUS
CONDITIONS WILL REMAIN UNFAVORABLE FOR ORGANIZED AND WIDESPREAD
CONVECTIVE ACTIVITY. THUS, THE ONLY AREAS THAT HAVE THE HIGHEST
PROBABILITY OF SEEING MEASURABLE RAINFALL ACTIVITY WILL BE ACROSS
NORTHWESTERN PORTIONS OF PUERTO RICO DURING THE AFTERNOON HOURS
AS SEA BREEZE CONVERGENCE WILL HELP ENHANCE ACTIVITY. BY LATE
TUESDAY INTO WEDNESDAY, A MID AND UPPER-LEVEL SHORTWAVE TROUGH IS
EXPECTED TO WEAKEN THE RIDGE. HOWEVER, DESPITE CONDITIONS BECOMING
MARGINALLY MORE UNSTABLE, MOISTURE CONTENT IS EXPECTED TO BE AT
BELOW NORMAL LEVELS AND THUS CONVECTIVE ACTIVITY WILL BE LIMITED.
GIVEN THAT THE WINDS ARE EXPECTED TO BECOME MORE SOUTHERLY
TUESDAY AND WEDNESDAY, HIGH TEMPERATURES ALONG THE NORTH COAST OF
PUERTO RICO ARE EXPECTED TO BE AT ABOVE NORMAL LEVELS, IN THE
LOWER 90S.
LONG TERM...THURSDAY THROUGH TUESDAY...
PREV DISCUSSION... /ISSUED 459 AM AST MON OCT 25 2021/
MODEL GUIDANCE CONTINUES TO BE CONSISTENT ON A MARGINALLY UNSTABLE
WEATHER PATTERN FOR MOST OF THE LONG TERM PERIOD. THURSDAY STARTS
WITH AN UPPER-LEVEL SHORT WAVE TROUGH CROSSING THE AREA AND LOW-
LEVEL WINDS SHIFTING TO SOUTHEAST THROUGH FRIDAY. DISCREPANCIES
STILL EXIST REGARDING THE AMOUNT OF MOISTURE CONVERGENCE THROUGH THE
END OF THE WORKWEEK AS MENTIONED IN PREVIOUS DISCUSSIONS. GFS
CONTINUES TO HAVE BETTER MOISTURE ADVECTION DUE TO A SOUTHERLY FLOW
THAN ECMWF. HOWEVER, WINDS ARE STILL EXPECTED TO WEAKEN AS PRESSURE
GRADIENT DECREASES OVER THE AREA. THEREFORE, UNCERTAINTY STILL
EXISTS ON HOW MUCH MOISTURE WILL CONVERGE OVER THE FORECAST AREA.
RECENT GFS MODEL RUNS ARE CONSISTENT THAT THE BULK OF THE
MOISTURE PLUME WILL STAY SOUTH OF THE REGION BUT CLOSE ENOUGH FOR
ABOVE NORMAL MOISTURE CONTENT TO REACH THE AREA. IF THIS PLAYS
OUT AND WITH ENOUGH INSTABILITY IN THE ATMOSPHERE, WIDESPREAD
CONVECTIVE DEVELOPMENT WOULD BE LIKELY WITH THE POTENTIAL FOR
ISOLATED TO SCATTERED THUNDERSTORMS ACROSS THE AREA. THE BULK OF
THE ACTIVITY COULD BE OBSERVED BY THURSDAY NIGHT WITH SHOWERS AND
THUNDERSTORMS DEVELOPING OVER THE CARIBBEAN WATERS AND BEING
ADVECTED TOWARDS THE SOUTHERN AND SOUTHEASTERN COASTAL
MUNICIPALITIES OF PUERTO RICO AND ST. CROIX. DURING THE DAY,
SURFACE HEATING AND SEA BREEZE CONVERGENCE COULD PRODUCE SHOWERS
AND SCATTERED THUNDERSTORMS OVER INTERIOR AND NORTHERN HALF OF
PUERTO RICO AND OCCASIONALLY AFFECTING THE LOCAL ISLANDS. LIGHT
STEERING WINDS WILL INCREASE THE POTENTIAL OF FLOODING THREAT
ESPECIALLY IN URBANIZED REGIONS SUCH AS THE GREATER BAYAMON AND
SAN JUAN METROPOLITAN AREA. IF THE ECMWF RUN PLAYS OUT INSTEAD,
THEN A MORE LESS WIDESPREAD AND MORE LOCALIZED ACTIVITY COULD BE
OBSERVED AS MOISTURE INTRUSION OVER THE FORECAST AREA WILL BE
GREATLY REDUCED.
THE WEEKEND ALSO LOOKS MARGINALLY UNSTABLE WITH THE NORTHEAST
CARIBBEAN UNDER THE BOUNDARY OF AN UPPER-LEVEL BUILDING RIDGE AND
TROUGH. BY SUNDAY THROUGH THE END OF THE FORECAST PERIOD, BOTH
GLOBAL MODELS ARE CONSISTENT IN THE DEVELOPMENT OF AN UPPER CUT-OFF
LOW LINGERING OVER PUERTO RICO AND U.S. VIRGIN ISLANDS BUT STRENGTH
AND POSITIONING STILL NOT COMPLETELY AGREEING WITHIN EACH MODEL
SOLUTION. MOISTURE STILL DOES NOT LOOK IMPRESSIVE, AT TIMES BOTH
MODELS SHOWING BELOW NORMAL VALUES. HENCE, CONTINUE TO EXPECT A MORE
SEASONAL WEATHER PATTERN FOR THE WEEKEND THROUGH TUESDAY WITH DEEP
CONVECTION DEVELOPING DURING EACH AFTERNOON DEPENDING ON THE
AVAILABLE MOISTURE AND THE EVOLUTION OF UNSTABLE WEATHER CONDITIONS
ALOFT. ALTHOUGH MODELS ARE CONSISTENT ON THE PROGRESS OF WEATHER
FEATURES FOR THIS WEEK AND EARLY NEXT WEEK, LOCATION DIFFERENCES OF
THESE FEATURES CONTINUE TO BRING FORECAST UNCERTAINTY. THEREFORE,
CONFIDENCE CONTINUES TO BE LOW TO MODERATE.
AVIATION...PREVAILING VFR CONDITIONS EXPECTED. ISOLATED TO SCT
SHRA ACTIVITY MAY RESULT IN VCSH AT TIMES ACROSS
TNCM/TKPK/TIST/TISX/TJSJ THROUGH THE OVERNIGHT PERIOD, HOWEVER, NO
IMPACTS TO OPERATIONS ARE EXPECTED. WINDS WILL CONTINUE FROM THE E
BETWEEN 10 AND 15 KTS WITH SEA BREEZE VARIATIONS THROUGH 25/22Z,
BECOMING LIGHT AND VARIABLE OVERNIGHT.
MARINE...SEAS ARE DIMINISHING ACROSS THE LOCAL WATERS AS THE
WINDS AND SWELL ARE SUBSIDING. MAINLY TRANQUIL MARINE CONDITIONS
WITH SEAS OF LESS THAN 5 FEET WILL PREVAIL THROUGH THURSDAY. FOR
BEACHGOERS, A MODERATE RISK OF RIP CURRENTS IS EXPECTED FOR MOST
OF THE NORTH AND EAST-FACING BEACHES OF THE ISLANDS TONIGHT AND
TUESDAY.
PRELIMINARY POINT TEMPS/POPS
SJU 77 93 76 93 / 30 30 30 30
STT 75 90 78 90 / 30 30 30 20
SJU WATCHES/WARNINGS/ADVISORIES
PR...NONE.
VI...NONE.
AM...NONE.
SHORT TERM...WS
LONG TERM....GL
PUBLIC DESK...LIS
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