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The Weather Prediction Center
 
Tropical Discussion
 
(Latest Discussion - Issued 1753Z Jul 20, 2018)
 
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TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
153 PM EDT FRI JUL 20 2018

TROPICAL DISCUSSION FROM JUL 20/12 UTC: GENERALLY DRY CONDITIONS
ARE EXPECTED ACROSS MOST OF THE FORECAST AREA IN ASSOCIATION WITH
THE MID-SUMMER DROUGHT...RECURRENT SAHARAN AIR LAYERS ARRIVING
FROM THE ATLANTIC...AND PERSISTENT LARGE SCALE UPPER CONVERGENT
PATTERN. MOST OF THE HEAVIEST PRECIPITATION THROUGH THE CYCLE WILL
ASSOCIATE WITH ITCZ CONVECTION IN PANAMA/COSTA RICA/COLOMBIA.

IN THE NORTH OF THE DOMAIN...A ROBUST UPPER RIDGE CONTINUES
CENTERING ON A HIGH OVER WESTERN TEXAS AND EXTENDS ACROSS THE
SOUTHWESTERN UNITED STATES INTO WESTERN CUBA/NORTHWEST BAHAMAS.
THE UPPER RIDGE IS FORECAST TO HOLD THROUGH THE CYCLE TO VENTILATE
CONVECTION ALONG ITS PERIPHERY. DRY AIR IS PRESENT ABOVE THE TRADE
WIND INVERSION OVER MOST OF THE GULF OF MEXICO IN ASSOCIATION WITH
A WEAKENING SAHARAN AIR LAYER. UNDER THE PERSISTENT PRESENCE OF
MID-LEVEL EASTERLIES...THIS DRY AIR MASS WILL ENTER MEXICO AND
LIMIT CONVECTION ACROSS MOST OF THE COUNTRY. AS A RESULT
DEVELOPMENT WILL BE LIMITED TO THE SIERRA MADRE OCCIDENTAL AND
SIERRA MADRE DEL SUR THROUGH THE CYCLE. INITIALLY...EXPECTING
RAINFALL AMOUNTS OF 10-15MM/DAY AND MAXIMA OF 20-30MM. THROUGH
SUNDAY-MONDAY THIS DECREASES TO 05-10MM/DAY AND MAXIMA OF 15MM.
TRANSIENT ENHANCEMENT IS EXPECTED IN NAYARIT/JALISCO/COLIMA ON
FRIDAY-SATURDAY IN ASSOCIATION WITH A TROPICAL WAVE EXITING INTO
THE PACIFIC.

TO THE EAST OF THE AFOREMENTIONED UPPER RIDGE...A ROBUST TROUGH IS
MEANDERING WESTWARD ACROSS THE TROPICAL NORTH ATLANTIC. THE TROUGH
IS EXTENDING AN AXIS INTO THE NORTHERN CARIBBEAN ACROSS
HISPANIOLA-JAMAICA INTO CENTRAL AMERICA...WHICH IS FORECAST TO
MEANDER OVER THE REGION THROUGH THE CYCLE. ALTHOUGH THE ATMOSPHERE
ABOVE THE INVERSION WILL CONTINUE QUITE DRY IN ASSOCIATION WITH
RECURRENT DUSTY SAHARAN AIR LAYERS STREAMING THROUGH THE
BASIN...DESTABILIZATION WILL BE SUFFICIENT TO SUSTAIN ISOLATED
DIURNAL CONVECTION. ON SUNDAY-MONDAY...A PREFRONTAL TROUGH IS
EXPECTED TO ENTER THE NORTHWESTERN CARIBBEAN AND FURTHER
DESTABILIZE THE ENVIRONMENT WHILE ADVECTING MOISTURE. AS A
RESULT...ALONG WITH THE PERSISTENT UPPER TROUGH PATTERN AND
CROSSING OF THE TUTT-INDUCED PERTURBATION TO THE NORTH (SEE
BELOW)...EXPECTING A GRADUAL INCREASING TREND FROM SUNDAY INTO
EARLY NEXT WEEK IN THE BAHAMAS-TURKS...CUBA-JAMAICA AND HAITI.
ACROSS THE NORTHWEST BAHAMAS...INITIALLY EXPECTING RAINFALL
AMOUNTS OF 05-10MM/DAY AND MAXIMA OF 15-20MM ON FRIDAY...WITH A
SLIGHT DROP TO MAXIMA OF 15MM ON SATURDAY. ON SUNDAY-MONDAY...
CONVECTION SPREADS TO THE CENTRAL BAHAMAS WITH RAINFALL
ACCUMULATIONS TO INCREASE TO 10-15MM/DAY AND MAXIMA OF 20-35MM. A
FURTHER INCREASE IS EXPECTED ON MONDAY. IN HISPANIOLA...UNDER
REMNANT MOISTURE FROM THE EXITING TROPICAL WAVE...MAXIMA WILL
APPROACH 20-30MM/DAY ON FRIDAY...DECREASING ON SATURDAY. AS THE
TUTT INDUCED TROUGH APPROACHES...EXPECTING A NEW INCREASE ON
SUNDAY-MONDAY TO 10-15MM/DAY AND MAXIMA OF 20-35MM. IN
SOUTHEASTERN CUBA AND JAMAICA...TROPICAL WAVE CONVECTION WILL
SUSTAIN 05-10MM/DAY AND MAXIMA OF 15-20MM ON FRIDAY-EARLY
SATURDAY. A NEW INCREASE IS EXPECTED BY SUNDAY-MONDAY...TO PRODUCE
MAXIMA OF 20-35MM/DAY IN SOUTHEASTERN CUBA...AND MAXIMA OF 15-25MM
IN JAMAICA.

CENTRAL AND NORTHERN PORTIONS OF CENTRAL AMERICA CONTINUE UNDER
THE INFLUENCE OF RECURRENT SAHARAN AIR LAYERS AND ASSOCIATED DRY
AIR MASSES...AS WELL AS UNDER THE INFLUENCE OF THE
CARIBBEAN-PAPAGAYO/NICARAGUA LOW-LEVEL JET. THIS WILL LEAD TO
LIMITED PRECIPITATION AMOUNTS IN AREAS ALONG AND NORTH OF THE JET.
SOME TRANSIENT ENHANCEMENT IS EXPECTED IN ASSOCIATION WITH A
PASSING TROPICAL WAVE (SEE BELOW FOR AMOUNTS). OTHERWISE EXPECTING
ISOLATED CONVECTION AND LIGHT ACCUMULATIONS. TO THE SOUTH OF THE
JET...HOWEVER...CONVECTION WILL REMAIN ENHANCED THROUGH THE CYCLE
IN ASSOCIATION WITH ENHANCED CYCLONIC VORTICITY ALONG THE SOUTHERN
FLANK OF THE LOW-LEVEL JET...AND INTERACTION WITH THE ITCZ AND
PANAMANIAN LOW PATTERN. THIS WILL MAINTAIN ACTIVE CONVECTION IN
PANAMA...COSTA RICA AND WESTERN COLOMBIA THROUGH THE CYCLE. IN
WESTERN PANAMA/COSTA RICA/SOUTHEASTERN NICARAGUA...INITIALLY
EXPECTING ACCUMULATIONS OF 15-20MM/DAY AND MAXIMA OF 25-50MM.
THROUGH SUNDAY-MONDAY THE MAXIMA INCREASES TO 35-70MM/DAY. IN
WESTERN COLOMBIA...AS WESTERLY LOW-LEVEL FLOW INCREASES IN SPEED
THROUGH THE CYCLE...EXPECTING A RESULTING INCREASE IN AMOUNTS FROM
10-15MM/DAY AND MAXIMA OF 20-45MM THROUGH EARLY SATURDAY...TO
15-20MM/DAY AND MAXIMA OF 30-60MM ON SATURDAY THROUGH MONDAY.

IN NORTHERN SOUTH AMERICA...ACTIVITY WILL CLUSTER MOSTLY ALONG THE
NET. LARGEST ACCUMULATIONS ARE EXPECTED IN EASTERN COLOMBIA AND
SOUTHERN VENEZUELA...WHERE EXPECTING 15-20MM/DAY AND MAXIMA 0F
35-70MM ON FRIDAY-SATURDAY...DECREASING TO 15-20MM/DAY AND MAXIMA
PF 25-50MM BY SATURDAY-SUNDAY. THIS WILL FURTHER DECREASE TO
10-15MM/DAY AND MAXIMA OF 20-35MM BY SUNDAY-MONDAY. EAST ACROSS
THE GUIANAS/NORTHEASTERN VENEZUELA...AMOUNTS WILL ALSO CLUSTER
NEAR THE ITCZ/NET. YET...UNDER THE INFLUENCE OF PERSISTENT SAHARAN
AIR LAYER INCURSIONS FROM THE ATLANTIC...ACCUMULATIONS ARE
FORECAST TO LIMIT TO 05-10MM/DAY AND MAXIMA OF 15-20MM THROUGH
EARLY SUNDAY...DECREASING AFTER.

TROPICAL/EASTERLY WAVES/TRADE WIND SURGES FROM 00 UTC:
INITIAL 24    36    48    60    72    84    96    TYPE      SOF
23W     27W   31W   34W   38W   42W   46W   50W    TW       20N
50W     53W   56W   59W   62W   65W   68W   71W TUTT INDCD  31N
77W     81W   85W   89W   92W   95W   98W  101W    TW       20N
100W   104W  108W  112W   DISSIPATES               TW       18N

A TROPICAL WAVE IS INITIALIZED ALONG 23W AND SOUTH OF 20N. THIS IS
TO CONFINE TO THE ATLANTIC ITCZ DURING THE NEXT FOUR DAYS. THIS
WAVE IS ALSO ASSOCIATED WITH A NEW SAHARAN AIR LAYER INCURSION
INTO THE ATLANTIC...AND IS FORECAST TO ARRIVE INTO THE BASIN WITH
LIMITED MOISTURE.

A TUTT-INDUCED TROUGH IS INITIALIZED ALONG 50W AND SOUTH OF 31N.
THIS IS FORECAST TO REMAIN BETTER DEFINED TO THE NORTH OF THE
ISLANDS THROUGH TO EARLY NEXT WEEK WHEN IT WILL BEGIN TO INTERACT
WITH A PREFRONTAL TROUGH TO ITS WEST. THE SOUTHERN END OF THE WAVE
WILL EXERT VERY LIMITED EFFECTS ON THE GREATER ANTILLES AS IT
MOVES JUST TO THEIR NORTH.

A TROPICAL WAVE ALONG 77W AND SOUTH OF 20N IS IN PHASE WITH THE
AFOREMENTIONED TUTT OVER THE NORTHERN CARIBBEAN. THIS IS FORECAST
TO ENHANCE CONVECTIVE DEVELOPMENT ACROSS EASTERN CUBA-JAMAICA
TODAY TO FAVOR RAINFALL AMOUNTS OF 05-10MM/DAY AND MAXIMA OF
15-20MM. OVER WESTERN COLOMBIA...AS IT INTERACTS WITH THE
PANAMANIAN LOW/TROUGH TO THE SOUTH...THIS WILL SUSTAIN RAINFALL
AMOUNTS OF 10-15MM/DAY AND MAXIMA OF 20-45MM THROUGH EARLY
SATURDAY...WITH LESSER AMOUNTS OVER PANAMA WHERE THE MAXIMA IS TO
REACH 20-30MM. AS THE WAVE ENTERS CENTRAL AMERICA ON SATURDAY...
IT IS TO TRIGGER RAINFALL AMOUNTS OF 15-20MM/DAY AND MAXIMA OF
25-50MM OVER WESTERN PANAMA-COSTA RICA. ACROSS EASTERN
NICARAGUA/NORTHEAST HONDURAS...IT IS FORECAST TO SUSTAIN RAINFALL
AMOUNTS OF 05-10MM/DAY AND MAXIMA OF 15-25MM. ACROSS WESTERN
HONDURAS-GUATEMALA-BELIZE THIS IS TO THEN TRIGGER RAINFALL AMOUNTS
OF 10-15MM/DAY AND MAXIMA OF 20-30MM. OVER SOUTHERN MEXICO THIS IS
TO THEN TRIGGER LIGHT ACCUMULATIONS IN ISOLATED CONVECTION. THE
METEOSAT DUST PRODUCT SHOWS A DRY AND DUSTY SAHARAN AIR LAYER
(SAL) TRAILING THIS WAVE. THIS IS TO ENTRAIN ACROSS THE
EASTERN-CENTRAL CARIBBEAN DURING THE NEXT COUPLE OF DAYS TO
REINFORCE CURRENT FAIR WEATHER PATTERN.

A TROPICAL WAVE ALONG 100W AND TO THE SOUTH OF 18N IS TO CONFINE
TO THE EASTERN PACIFIC ITCZ...YET IT WILL MARGINALLY ENHANCE
ACTIVITY IN SOUTHWESTERN MEXICO ON FRIDAY-SATURDAY TO FAVOR
10-15MM/DAY AND MAXIMA OF 20-30MM BETWEEN COLIMA/JALISCO AND
NAYARIT/SOUTHERN SINALOA.

HUDSON...MS (JAMAICA)
HERNANDEZ...IMTA (MEXICO)
WARD-FORBES...BDM (THE BAHAMAS)
GALVEZ...WPC (USA)