TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
222 PM EDT MON SEP 27 2021

THE NHC IS ISSUING ADVISORIES FOR HURRICANE SAM LOCATED IN THE
TROPICAL NORTH ATLANTIC. PLEASE REFER TO THEIR DISCUSSION FOR
OFFICIAL FORECAST GUIDANCE.

TROPICAL DISCUSSION FROM SEP 27/18 UTC: A MID/UPPER LEVEL HIGH
OVER SOUTHERN MEXICO/THE GULF EXTENDS A RIDGE NORTH INTO THE
CENTRAL USA. THE RIDGE IS BOOKENDED BY TWO TROUGHS...ONE EXTENDING
OVER THE WESTERN USA TO NORTHERN MEXICO...WHILE THE OTHER LIES
ACROSS THE BAHAMAS-CUBA TO THE NORTHWEST CARIBBEAN. THROUGH
THURSDAY MORNING THE TROUGH TO THE WEST AMPLIFIES ACROSS
SONORA/CHIHUAHUA TO THE BAJA PENINSULA/SINALOA. INITIALLY DIURNAL
CONVECTION OVER SINALOA WILL RESULT IN RAINFALL AMOUNTS OF
05-10MM/DAY AND MAXIMA OF 15-25MM...WHILE OVER COAHUILA EXPECTING
ACCUMULATION OF 05-10MM/DAY AND MAXIMA OF 15MM. AS THE TROUGH
DEEPENS...ACTIVITY OVER SINALOA WEAKENS...WHILE THE ONE OVER
COAHUILA INTENSIFIES WHILE BUILDING SOUTH INTO CENTRAL
MEXICO...TRIGGERING RAINFALL AMOUNTS OF 10-15MM/DAY AND MAXIMA OF
20-35MM IN SCATTERED DEEP CONVECTION. THIS IS TO PERSIST THROUGH
THURSDAY WHILE BUILDING EAST ACROSS TAMAULIPAS AND NORTHERN
PORTIONS OF VERACRUZ.

THE UPPER LEVEL RIDGE TO THE EAST...AS IT HOLD OVER SOUTHERN
MEXICO/THE GULF...WILL VENT DIURNAL CONVECTION OVER THE YUCATAN
AND NORTHERN CENTRAL AMERICA. OVER THE YUCATAN/BELIZE THIS WILL
INITIALLY FAVOR MODERATE RAINFALL AMOUNTS WITH ACCUMULATION OF
05-10MM/DAY AND MAXIMA OF 15MM. ON TUESDAY TO WEDNESDAY ACTIVITY
OVER THE NORTHERN YUCATAN PERSISTS...WHILE OVER BELIZE AND
SOUTHERN PORTIONS OF THE YUCATAN IT TENDS TO WEAKEN. HIGHER
RAINFALL AMOUNTS ARE EXPECTED OVER SOUTHERN GUATEMALA-CHIAPAS IN
SOUTHERN MEXICO. IN THIS AREA INITIALLY EXPECTING RAINFALL AMOUNTS
OF 10-15MM/DAY AND MAXIMA OF 20-45MM. THROUGH TUESDAY THIS WEAKENS
TO 05-10MM/DAY AND MAXIMA OF 15-25MM.

AS THE RIDGE OVER THE GULF HOLDS...A HIGHLY AMPLIFIED MID/UPPER
LEVEL TROUGH WILL EXTEND OVER THE WESTERN ATLANTIC AND ACROSS THE
BAHAMAS-CUBA TO THE NORTHWEST CARIBBEAN. AXIS REMAINS NEARLY
UNCHANGED DURING THE NEXT DAY OR SO...THEN GRADUALLY SHIFTS EAST
TOWARDS HISPANIOLA/PUERTO RICO AS A SHORT WAVE TROUGH ROUNDS THE
LONG WAVE AXIS ON WEDNESDAY. THIS VORTEX IS TO FOCUS ITS SHORT
WAVE ENERGY ACROSS THE BAHAMAS AND CUBA...AND LATER IN THE WEEK IT
IS EXPECTED TO INDUCED THE SOUTHWARD AMPLIFICATION OF THE LONG
WAVE AXIS. AT LOW LEVELS...THE TROUGH ALOFT SUSTAINS A MEANDERING
FRONT THAT TRAILS ACROSS THE WESTERN ATLANTIC TO THE NORTHWEST
BAHAMAS...WHILE SUSTAINING A PREFRONTAL SHEAR LINE OVER THE
CENTRAL BAHAMAS/CUBA. THIS PERSIST THROUGH TUESDAY...RAPIDLY
BECOMING ILL DEFINED ON WEDNESDAY. OVER THE CENTRAL BAHAMAS AND
CUBA THE SHEAR LINE WILL INITIALLY FAVOR RAINFALL AMOUNTS OF
05-10MM/DAY AND MAXIMA OF 15-25MM. ON TUESDAY TO WEDNESDAY
ACTIVITY OVER THE BAHAMAS WANES...WHILE CONVECTION ACROSS CUBA
SURGES...WITH RAINFALL AMOUNTS OF 10-15MM/DAY AND MAXIMA OF
20-35MM. OVER THE CAYMAN ISLANDS THIS WILL RESULT IN ENHANCEMENT
ON WEDNESDAY TO FAVOR RAINFALL AMOUNTS OF 05-10MM/DAY AND MAXIMA
OF 15MM. OVER JAMAICA IT WILL INCREASE FROM 05-10MM/DAY AND MAXIMA
OF 15MM ON TUESDAY TO MAXIMA OF 15-25MM ON WEDNESDAY. THIS IS TO
ALSO ENHANCE CONVECTION ACROSS HISPANIOLA...WITH RAINFALL AMOUNTS
OF 10-15MM/DAY AND MAXIMA OF 20-35MM. ON TUESDAY THIS INCREASES TO
MAXIMA OF 20-45MM. OVER PUERTO RICO ALSO EXPECTING AN
INCREASE...WITH RAINFALL AMOUNTS OF 05-10MM/DAY AND MAXIMA OF
15-25MM TODAY...GRADUALLY INCREASING TO 15-20MM/DAY AND MAXIMA OF
35-70MM ON WEDNESDAY.

UNDER INFLUENCE OF THE HIGH AMPLITUDE TROUGH OVER THE WESTERN
ATLANTIC...THE NHC IS FORECASTING HURRICANE SAM TO RECURVE TO THE
NORTH-NORTHWEST WHILE REMAINING EAST OF THE LESSER ANTILLES.
ALTHOUGH NOT EXPECTED TO HAVE A DIRECT IMPACT OVER THE
ISLANDS...OUTER BAND CONVECTION IS TO AFFECT THE LEEWARD ISLANDS
ON WEDNESDAY-THURSDAY MORNING...WITH RAINFALL AMOUNTS OF
00-05MM/DAY AND MAXIMA OF 10MM.

TROPICAL WAVES/TUTT INDUCED PERTURBATIONS/TRADE WIND SURGES
INITIALIZED AT 12 UTC:
TYPE  INIT 28/00 28/12 29/00 29/12 30/00 30/12 01/00 SOF
EW     62W   63W   65W   66W   67W   69W   71W   72W 24N
EW     72W   74W   76W   77W   79W   81W DISSIP.     18N
TW     83W   85W   87W   89W   91W   93W   95W   97W 13N

AN EASTERLY WAVE IS INITIALIZED ALONG 62W AND SOUTH OF 24N. AS IT
MOVES ACROSS PUERTO RICO...IN INTERACTION WITH THE UPPER LEVEL
TROUGH...THIS WILL SUSTAIN RAINFALL AMOUNTS OF 15-20MM/DAY AND
MAXIMA OF 30-60MM. THIS PERSISTS THROUGH WEDNESDAY WHILE THE DAILY
MAXIMA INCREASES TO 35-70MM.

AN EASTERLY WAVE ALONG 72W AND SOUTH OF 18N IS FORECAST TO
DISSIPATE AS IT ENTERS CENTRAL AMERICA LATER IN THE WEEK.
MEANWHILE...OVER NORTHERN AND WESTERN COLOMBIA THIS IS O SUSTAIN
RAINFALL AMOUNTS OF 10-15MM/DAY AND MAXIMA OF 20-45MM...WHILE OVER
EASTERN COLOMBIA/AMAZONIA TO THE SOUTH...EXPECTING ACCUMULATION OF
05-10MM/DAY AND MAXIMA OF 15-30MM. ON TUESDAY ACTIVITY ACROSS
NORTHERN COLOMBIA INCREASES TO 15-20MM/DAY AND MAXIMA OF
30-60MM...WITH SIMILAR AMOUNTS AS IT ENTERS PANAMA/COSTA RICA ON
WEDNESDAY.

A TROPICAL WAVE ALONG 83W AND SOUTH OF 13N IS TO INTERACT WITH THE
ITCZ IN SUPPORT OF SCATTERED DEEP CONVECTION ACROSS WESTERN PANAMA
TO COSTA RICA/SOUTHERN NICARAGUA...WITH ACCUMULATION OF
15-20MM/DAY AND MAXIMA OF 30-60MM. ON TUESDAY...AS THE WAVE MOVES
AWAY...THIS DECREASES TO 10-15MM/DAY AND MAXIMA OF 20-45MM. OVER
EL SALVADOR/SOUTHERN HONDURAS TO GUATEMALA THE WAVE IS TO THEN
TRIGGER RAINFALL AMOUNTS OF 05-10MM/DAY AND MAXIMA OF
15-25MM...WITH SIMILAR AMOUNTS OVER CHIAPAS IN SOUTHERN MEXICO ON
WEDNESDAY.

DAVISON / GALVEZ...WPC (USA)