000 AXNT20 KNHC 092359 TWDAT TROPICAL WEATHER DISCUSSION NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 805 PM EDT WED APR 09 2008 TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL AMERICA...THE GULF OF MEXICO...THE CARIBBEAN SEA...NORTHERN SECTIONS OF SOUTH AMERICA...AND THE ATLANTIC OCEAN TO THE AFRICAN COAST FROM THE EQUATOR TO 32N. THE FOLLOWING INFORMATION IS BASED ON SATELLITE IMAGERY...METEOROLOGICAL ANALYSIS...WEATHER OBSERVATIONS...AND RADAR. BASED ON 1800 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 2345 UTC. ...ITCZ... ITCZ AXIS IS CENTERED ALONG 6N11W 2N20W EQ28W EQ40W INTO NE BRAZIL NEAR 1N50W. NO ORGANIZED AREAS OF CONVECTION ARE NOTED ALONG THE ITCZ. ...DISCUSSION... GULF OF MEXICO... A SHORTWAVE TROUGH EXITING THE EASTERN GULF LAST NIGHT HAS LEFT BEHIND A BROAD MID- TO UPPER-LEVEL RIDGE OVER THE SOUTH CENTRAL GULF...WITH AN AXIS ORIENTED FROM THE FL PANHANDLE THROUGH THE ISTHMUS OF TEHUANTEPEC. THE RIDGE CURRENTLY DOMINATES MOST OF THE REGION WITH MODERATE TO STRONG SUBSIDENCE ALOFT...WITH THE STRONGEST DRYING NOTED IN THE VICINITY OF THE YUCATAN PENINSULA. HOWEVER...SATELLITE IMAGERY AND UPPER-AIR DATA SHOW A DEEP SHORTWAVE TROUGH DIGGING FROM THE WESTERN U.S. INTO NORTHERN MEXICO AND GRADUALLY ADVANCING E. STRONG HEIGHT FALLS AHEAD OF THIS SYSTEM AND A STRONGLY DIFFLUENT FLOW ALOFT HAVE ALLOWED SURFACE PRESSURES TO ABRUPTLY DROP OVER THE SOUTHERN PLAINS THIS EVENING...RESULTING IN A STRENGTHENING RETURN FLOW OVER THE WESTERN HALF OF THE GULF. IN FACT...SURFACE AND OTHER DATA SUGGEST A STEADY SE TO S FLOW OF 15 TO 20 KT AND SEAS BUILDING TO 4 TO 6 FT W OF 90W UNDER ESSENTIALLY CLEAR SKIES. ELSEWHERE...WEAK SURFACE RIDGING IN THE WAKE OF THE SHORTWAVE TROUGH NOW E OF THE AREA IS MAINTAINING A WEAKER BUT GRADUALLY INCREASING EASTERLY FLOW E OF 90W... WITH ISOLATED CONVECTIVE CELLS BRIEFLY ERUPTING OVER THE E CENTRAL GULF. AS THE WESTERN TROUGH ORGANIZES AND CLOSES OFF OVER THE PLAINS LATER THIS WEEK...THE SAME MID- TO UPPER-LEVEL RIDGE NOW OVER THE GULF SHOULD REMAIN LOCKED IN PLACE THROUGH AT LEAST FRI. EVENTUALLY THOUGH...HEIGHTS SHOULD FALL ENOUGH TO INTRODUCE A STRONG COLD FRONT OVER THE FAR NW WATERS LATE FRI AND EARLY SAT...WITH THE FRONT SWEEPING THROUGH THE REMAINDER OF THE GULF BY LATE SUN. SCATTERED TSTMS ALONG THE FRONT SHOULD GIVE WAY TO A STRONG NW TO N FLOW OF 20 TO PERHAPS 30 KT...WITH SEAS BUILDING TO 7 TO 11 FT. CARIBBEAN SEA... THE POSITIVELY-TILTED SHORTWAVE TROUGH WHICH EXITED THE GULF LATE YESTERDAY HAS AN EXTENSION INTO THE WESTERN CARIBBEAN ...WITH THE MAIN AXIS EXTENDING FROM 21N79W TO 14N85W THIS EVENING. CLOUD-TRACKED WINDS AND WATER VAPOR IMAGERY INDICATE A BROAD S TO SW FLOW AHEAD OF THE TROUGH AXIS IS TRANSPORTING MOISTURE- RICH AIR FROM THE SW TO THE CENTRAL CARIBBEAN. THE AMOUNT OF MOISTURE HAS INDEED BEEN ON THE RISE DURING THE LAST 24 HOURS...WHICH HAS HELPED LATE AFTERNOON CONVECTION TO FIRE UP FROM COSTA RICA AND PANAMA TO NORTHERN COLOMBIA. DESPITE THE INCREASE IN MOISTURE AND FAVORABLE DYNAMICS AHEAD OF THE TROUGH AXIS...LITTLE IN THE WAY OF ORGANIZED CONVECTION IS OBSERVED OVER THE REMAINDER OF THE CARIBBEAN EXCEPT FOR SCATTERED SEA BREEZE CONVECTION OVER CUBA AND JAMAICA. OTHERWISE...WEAK SURFACE HIGH PRESSURE NE OF THE AREA CONTINUES TO FAVOR NEAR CLIMATOLOGICAL TO BELOW NORMAL TRADES...WITH LITTLE MORE THAN 10 TO 15 KT OF E TO SE FLOW NOTED ACROSS THE BASIN. EXPECT THE SHORTWAVE TROUGH CURRENTLY OVER THE WESTERN CARIBBEAN SEA TO HEAD SLOWLY E THE NEXT FEW DAYS...DRAGGING THE PLUME OF MOISTURE OVER THE SW AND CENTRAL CARIBBEAN WITH IT. INCREASING INSTABILITY E OF THE TROUGH AXIS...HIGHER MOISTURE..AND A DIFFLUENT UPPER-LEVEL FLOW SHOULD FAVOR AT LEAST SOME MARGINAL INCREASE IN CLOUDS AND CONVECTION OVER THE CENTRAL AND THEN EASTERN CARIBBEAN SEA...WITH THE ACTIVITY ENDING FROM W TO E AS A DRIER NORTHWESTERLY FLOW SETS IN BY LATE WEEK. A REVITALIZED SURFACE RIDGE IN THE WAKE OF THE TROUGH OVER THE SW ATLANTIC SHOULD KEEP TRADES AT THEIR PRESENT LEVELS OR SLIGHTLY HIGHER THROUGH LATE WEEK...FOLLOWED BY A STRONG FRONT ARRIVING OVER THE NW CARIBBEAN LATE IN THE WEEKEND. ATLANTIC OCEAN... THE PREVIOUSLY MENTIONED SHORTWAVE TROUGH NOW EXTENDS FROM 31N60W TO 21N77W AND IS MOVING SLOWLY SE. THIS FEATURE SUPPORTS A SERIES OF WEAK SURFACE LOWS ALONG A WAVY COLD FRONT EXTENDING FROM 29N47W TO 31N60W TO 30N70W AND SE TO 26N75W. SCATTERED... MOSTLY LOW CLOUDS LINE THE FRONT...THOUGH ISOLATED CONVECTIVE CELLS ARE DEVELOPING WITHIN ABOUT 150 NM E OF IT N OF 28N. LIGHT WINDS PREVAIL EITHER SIDE OF THE FRONT...WITH A SE TO S WIND OF 10 KT E OF IT AND A NE 10 TO 15 KT FLOW BEHIND IT. OVER THE NEXT FEW DAYS...THE MAIN SURFACE WAVE ALONG THE FRONT NEAR 30N51W SHOULD HEAD E WHILE DRAGGING A WEAK COLD FRONT INTO THE CENTRAL ATLANTIC EXTENDING FROM 30N41W TO 25N49W BY FRI AFTERNOON. HIGH PRESSURE BEHIND THE FRONT SHOULD EXPAND EASTWARD...KEEPING FAIRLY TRANQUIL CONDITIONS IN PLACE OVER THE WESTERN ATLANTIC. AS THE RIDGE MIGRATES EASTWARD INTO THE WEEKEND...A STRONG COLD FRONT SHOULD SWEEP OFF THE EAST COAST OF THE U.S. LATE SAT AND EARLY SUN...WITH A STRONG S TO SW FLOW OF 20 TO 30 KT WITHIN ABOUT 240 NM E OF THE FRONT N OF 28N. ELSEWHERE...DEEP-LAYERED BROAD CYCLONIC FLOW CONTINUES TO PREVAIL OVER THE NE ATLANTIC N OF 18N E OF 60W. AT THE LEADING EDGE OF THE CYCLONIC FLOW...A WEAK COLD FRONT FROM 31N14W TO 27N30W TO 31N44W IS DIVING SE...INTRODUCING A WEAK SURFACE RIDGE LYING WELL N OF THE AREA. THE BROAD CYCLONIC FLOW ACROSS THE REGION HAS LED TO WEAKER-THAN-NORMAL TRADES OVER MUCH OF THE SUBTROPICAL AND TROPICAL ATLANTIC E OF 60W...WITH THE STRONGEST ANOMALIES NOTED N OF 20N. OVER THE NEXT SEVERAL DAYS...THE SURFACE RIDGE CURRENTLY WELL N OF THE AREA SHOULD BUILD SOUTHWARD AND BRING ABOUT A GRADUAL INCREASE IN THE TRADES ACROSS THE AREA...WITH VALUES CLOSER TO CLIMATOLOGY. FINALLY...A NARROW MID- TO UPPER-LEVEL RIDGE EXTENDS FROM NW AFRICA WSW TO NEAR 4N49W. ENHANCED DEEP CONVECTION ALONG THE ITCZ AND S OF THE RIDGE AXIS FROM THE LAST FEW DAYS HAS DRAMATICALLY DECREASED DURING THE LAST 24 TO 48 HOURS...LIKELY A RESULT OF THE MAIN FORCING...AN ATMOSPHERIC KELVIN WAVE...HEADING E INTO THE INDIAN OCEAN. $$ KIMBERLAIN