TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
845 AM EDT THU SEP 12 2024

WEEKLY DISCUSSION FOR PUERTO RICO AND THE USVI SEP 12/12UTC:

A COUPLE AREAS OF LOW PRESSURE LOCATED ALONG THE MONSOON THROUGH
ARE MOVING WEST AS THEY WEAKEN. AT THIS TIME...THE NATIONAL
HURRICANE CENTER GIVES THEM A LOW CHANCE OF TROPICAL CYCLONE
FORMATION...AND THE LATEST GUIDANCE WEAKENS THEM INTO A SFC TROUGH
AND THEN DISSIPATES THEM BY THIS WEEKEND. THAT BEING SAID...ONE OF
THE LOW PRESSURES...CURRENTLY KNOWN AS INVEST 92L WILL APPROACH
PR/USVI FROM LATE FRIDAY INTO SATURDAY...LIKELY AS A SFC TROUGH.
THIS WILL HAVE AN IMPACT IN THE AVAILABLE MOISTURE AND RAINFALL
OVER PR/USVI. AT THIS MOMENT...THE GLOBAL MODELS SUGGEST THAT THE
PRECIPITABLE WATER VALUES WILL BE FROM NEAR NORMAL TODAY INTO
FRIDAY...TO ABOVE NORMAL FRIDAY NIGHT INTO SATURDAY AND SUNDAY
WITH THE PASSAGE OF THE SFC TROUGH...THEN MOISTURE WILL DECREASE
ON MONDAY INTO THE NORMAL LEVELS. THERE IS SOME SAHARAN DUST THAT
WILL BE MOVING IN EARLY IN THE WORKWEEK...WHICH MAY CONTRIBUTE TO
THE LOWER PRECIPITABLE WATER VALUES. BY TUESDAY...THE AVAILABLE
MOISTURE IS FORECAST TO BE BELOW NORMAL...BUT PATCHES OF MOISTURE
COULD BE EMBEDDED IN THE TRADES BY MIDWEEK.

A WEAK MID LEVEL TROUGH WILL CONTINUE TO WEAKEN TODAY...BEING
REPLACED BY A MID LEVEL HIGH PRESSURE...WHICH WILL PERSIST FOR THE
NEXT SEVERAL DAYS. THE UPPER LEVELS WILL HAVE A SIMILAR
SCENARIO...WITH A WEAK UPPER TROUGH WEAKENING AND HAVING A RIDGE
PERSIST THEREAFTER. THIS WILL CAUSE SUBSIDENCE OVER THE AREA AND
SOMEWHAT LIMIT THE CONVECTION OVER PR. HOWEVER...THE LOW LEVEL
MOISTURE ADVECTION COMBINED WITH THE DIURNAL HEATING AND LOCAL
EFFECTS WILL HELP WITH THE DEVELOPMENT OF LOCALLY INDUCED SHOWERS
AND THUNDERSTORMS IN THE AFTERNOON ACROSS WESTERN
PR...PARTICULARLY THE NORTHWESTERN QUADRANT UNDER AS EAST TO
SOUTHEAST PREVAILING WIND FLOW. THE USVI AND OTHER SURROUNDING
AREAS ARE FORECAST TO OBSERVE MAINLY SCATTERED SHOWERS WITH
PERHAPS VERY ISOLATED AND BRIEF THUNDERSTORMS. DUE TO THE
APPROACHING SFC TROUGH LATE FRIDAY INTO SATURDAY... THE RAINFALL
PATTERN COULD FAVOR EASTERN PR MORE SO THAT WOULD BE THE CASE
TODAY AND FRIDAY. HOWEVER...SOME SHOWER AND THUNDERSTORM ACTIVITY
WOULD BE FORECAST FOR THE NORTHWESTERN QUADRANT OF PR. ON
SUNDAY...GIVEN THE LINGERING MOISTURE AND BEING ON THE EAST SIDE
OF THE SFC TROUGH AXIS...NORTHWESTERN PR STILL HAS A CHANCE OF
SHOWER AND THUNDERSTORM ACTIVITY IN THE AFTERNOON. THE EGDI
ALGORITHM IS CURRENTLY SUGGESTING THE CHANCE OF ISOLATED TO
SCATTERED THUNDERSTORMS EACH AFTERNOON FROM TODAY THROUGH SUNDAY.
THEN BY MONDAY AND TUESDAY...THE ALGORITHM SUGGESTS A VERY SLIGHT
CHANCE OF THUNDERSTORMS AS THE DRY AIR AND SOME SAHARAN DUST IS
FORECAST TO MOVE IN. A VERY SLIGHT CHANCE OF THUNDERSTORMS IS THEN
OBSERVED AGAIN BY MIDWEEK WITH THE PATCHES OF MOISTURE THAT COULD
MOVE IN.

OVERALL...THE RAINFALL PATTERN IS EXPECTED TO BE TYPICAL BASED FOR
THE TIME OF YEAR...WITH THE POTENTIAL OF LOCALLY INDUCED
CONVECTION ACROSS WESTERN PR IN THE AFTERNOONS...AND BRIEF
SCATTERED TO LOCALLY NUMEROUS SHOWERS ACROSS THE SURROUNDING AREAS
INCLUDING THE USVI GIVEN THE AVAILABLE MOISTURE. THE APPROACHING
SFC TROUGH COULD ENHANCE MOISTURE AND SHOWER ACTIVITY OVERALL LATE
FRIDAY INTO SATURDAY. THE AMOUNTS OF RAIN OVER THE NEXT 5 DAYS ARE
FORECAST TO BE UPWARDS OF 5 INCHES ACROSS NORTHWESTERN PR...WITH
2-3 INCHES OR SO FORECAST ELSEWHERE IN PR AND THE USVI. SOUTH AND
SOUTHWESTERN COASTAL PR HAVE THE LOWEST PRECIPITATION
FORECAST...WITH 5-DAY TOTALS OF AROUND 1 INCH OR LESS. IT MAY PAY
TO KNOW THAT THE GLOBAL MODEL GUIDANCE IS WELL BELOW THIS
FORECAST...WITH BOTH THE GFS AND ECMWF SUGGESTING A 5-DAY TOTAL
MAXING OUT AT JUST OVER ONE INCH OF RAIN ACROSS NORTHWESTERN PR.
GIVEN THE AMOUNT OF MOISTURE EXPECTED AND THE ENHANCEMENTS BY
DIURNAL HEATING AND LOCAL EFFECTS...WE HAVE DECIDED TO GO WELL
OVER THE GLOBAL MODEL GUIDANCE...SUGGESTING VALUES THAT COULD MAX
OUT NEAR 1-2 INCHES EACH DAY INTO THE WEEKEND FOR NORTHWESTERN PR.


ALAMO...WPC (USA)