Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6
000
FXCA20 KWBC 211056
PMDCA

TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
655 AM EDT FRI JUL 21 2017

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JUL 21/06
UTC: AS IT WAS EXPECTED...AN EASTERLY WAVE IS MOVING ACROSS THE
FORECAST AREA EARLY THIS MORNING. HOWEVER...UNDER NEGATIVE
INFLUENCE OF MID LEVEL RIDGE TO THE NORTH AND SAHARAN AIR LAYER
ENVELOPING THE BASIN...THE WAVE ONLY TRIGGERED WIDELY ISOLATED
CONVECTION ACROSS THE FORECAST AREA WHILE GENERALLY FAVORING LIGHT
RAINFALL AMOUNTS IN BRIEF RAIN SHOWERS DURING THE DAY TODAY. THIS
IS TO ALSO HELP TO DISSIPATE THE SAHARAN AIR LAYER AND SET THE
STAGE FOR THE NEXT TROPICAL WAVE. ON SATURDAY WE THEN EXPECT
SCATTERED CONVECTION TO CLUSTER ACROSS WESTERN INTERIOR PORTIONS
OF PUERTO RICO...WHERE HIGHER RESOLUTION MODEL GUIDANCE IS SHOWING
MAXIMA OF A COUPLE INCHES.

THE TROPICAL WAVE IS TO ENTER THE ISLAND CHAIN ON SATURDAY
MORNING...REACHING THE VIRGIN ISLES LATER IN THE DAY. THE WAVE
CROSSES PUERTO RICO DURING THE EARLY MORNING HOURS ON SUNDAY...AND
BY DAYBREAK IT SHOULD BE MOVING ACROSS THE MONA PASSAGE. AS THE
SAHARAN AIR LAYER DILUTES...MODELS SHOWING PWAT CONTENT OF NEARLY
TWO INCHES TO PRECEDE/ACCOMPANY THIS WAVE AS IT ENTERS THE EASTERN
CARIBBEAN/FORECAST AREA ON SATURDAY...WITH HIGH MOISTURE CONTENT
TO PERSIST THROUGH SUNDAY EVENING. ALTHOUGH THE MOISTURE CONTENT
WILL BE HIGH...UNDER THE NEGATIVE INFLUENCE OF MID LEVEL RIDGE
PATTERN TO THE NORTH...THE GFS GDI FORECAST IS ONLY SHOWING ENOUGH
CONVECTIVE INSTABILITY TO MOSTLY FAVOR RAIN SHOWERS IN ASSOCIATION
WITH THIS FEATURE. STARTING ON SATURDAY AFTERNOON EXPECTING
SCATTERED SHOWERS TO REACH THE VIRGIN ISLES-EASTERN PUERTO
RICO...WITH GLOBAL MODELS AGREEING ON RAINFALL MAXIMA PEAKING AT
LESS THAN AN INCH. AS IT SPREADS ACROSS PUERTO RICO THIS WILL THEN
COMBINE WITH DIURNAL HEATING ON SUNDAY TO SUSTAIN MODERATE TO
HEAVY RAINFALL AMOUNTS ACROSS WESTERN INTERIOR PORTIONS OF THE
ISLAND. IN THIS AREA THERE WILL BE ENOUGH FORCING/INSTABILITY TO
LIKELY FAVOR GENERATION OF A FEW THUNDERSTORMS. SOME COULD BE VERY
INTENSE AS THEY OVERCOME THE UPPER LEVEL RIDGE NEGATIVE CINH.
DURING THAT PERIOD LOCALIZED MAXIMA IS EXPECTED TO PEAK AT 3-5
INCHES.

A WEAKER SAL IS TO FOLLOW THIS WAVE...ENOUGH TO SUSTAIN A DECREASE
IN DIURNAL CONVECTION ACROSS THE VIRGIN ISLES-EASTERN PUERTO RICO
ON MONDAY-TUESDAY. MEASURABLE AMOUNTS...HOWEVER...ARE TO CONTINUE
CLUSTERING OVER WESTERN INTERIOR PORTIONS OF PUERTO RICO IN
AFTERNOON CONVECTION.

GAJADHAR...SLMS (SAINT LUCIA)
DAVISON...WPC (USA)
$$




USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.