000 AXNT20 KNHC 241759 TWDAT TROPICAL WEATHER DISCUSSION NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 205 PM EDT WED SEP 24 2008 TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL AMERICA...GULF OF MEXICO...CARIBBEAN SEA...NORTHERN SECTIONS OF SOUTH AMERICA...AND 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 1200 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 1715 UTC. ...SPECIAL FEATURES... A BROAD AREA OF LOW PRESSURE IS OVER HISPANIOLA...PUERTO RICO...AND THE ADJACENT WATERS...WITH AN ASSOCIATED 1006 MB LOW NEAR 19N71W...OR OVER WESTERN PORTIONS OF THE DOMINICAN REPUBLIC. CONVECTION HAS DECREASED IN AREAL COVERAGE AND INTENSITY OVER THE LAST 24 HOURS. HOWEVER...SCATTERED MODERATE CONVECTION LINGERS NORTHEAST OF THE DOMINICAN REPUBLIC AND OVER THE MONA PASSAGE APPROACHING PUERTO RICO...OR FROM 16N-22N BETWEEN 66W-70W. THIS SYSTEM HAS THE POTENTIAL TO DEVELOP INTO A TROPICAL CYCLONE AS IT MOVES GENERALLY NORTHWARD...AND AWAY FROM HISPANIOLA...TODAY AND TONIGHT. THE POTENTIAL FOR HEAVY RAINFALL AND LIFE-THREATENING FLASH FLOODS AND MUD SLIDES OVER HISPANIOLA WILL CONTINUE DURING THE NEXT DAY OR SO. ...TROPICAL WAVES... AN ATLANTIC OCEAN TROPICAL WAVE IS ALONG 39W/40W S OF 17N MOVING W NEAR 15 KT. A CLUSTER OF MODERATE CONVECTION HAS DEVELOPED FROM 13N-15N BETWEEN 37W AND THE WAVE AXIS. COMPUTER MODEL GUIDANCE DEPICTS WEAK CIRCULATION OVER THE NORTHERN PORTION OF THE WAVE AXIS. A CARIBBEAN SEA TROPICAL WAVE IS ALONG 64W S OF 18N MOVING W NEAR 15 KT. CONFLUENT AND DRY FLOW ALOFT OVER THE NORTHERN PORTION OF THE WAVE AXIS IS INHIBITING CONVECTION OVER THIS PORTION OF THE WAVE. ISOLATED SHOWERS ARE WITHIN 90 NM OF THE WAVE SOUTH OF 10N...WHERE SOME WEAK CYCLONIC CURVATURE HAS BEEN NOTED FROM A RECENT ASCAT PASS. A CARIBBEAN SEA TROPICAL WAVE IS ALONG 77W S OF 16N MOVING W NEAR 15 KT. THIS WAVE HAD PREVIOUSLY PULLED WEST OF THE SYSTEM DISCUSSED IN THE SPECIAL FEATURES SECTION. THIS WAVE HAS CONSIDERABLY LOST DEFINITION...AS IT IS EMBEDDED WITHIN A MINIMUM OF DEEP LAYER MOISTURE PER MIMIC TOTAL PRECIPITABLE WATER IMAGERY...WHERE THE UPPER LEVEL FLOW IS RELATIVELY WEAK. NEVERTHELESS...ISOLATED SHOWERS ARE OCCURRING WITHIN 120 NM OF THE WAVE AXIS. THE GREATER PROPORTION OF SHOWERS IS ALONG THE WAVE SOUTH OF 12N...WHERE WEAK DIFFLUENCE IN A LOW AMPLITUDE UPPER RIDGE IS PROVIDING SOME UPPER SUPPORT. A WESTERN CARIBBEAN SEA TROPICAL WAVE IS ALONG 89W/90W S OF 18N MOVING W 10-15 KT. THE WAVE IS INLAND OVER CENTRAL AMERICA AND IN THE E PACIFIC WITH ISOLATED SHOWERS 120 NM ON EITHER SIDE OF THE WAVE AXIS N OF 12N...MAINLY AFFECTING GUATEMALA... BELIZE...AND WESTERN HONDURAS. ...THE ITCZ... ITCZ AXIS IS CENTERED ALONG 7N11W 6N24W 7N34W 9N46W 9N61W. CLUSTERS OF SCATTERED MODERATE/STRONG CONVECTION ARE FROM 4N-8N BETWEEN 12W-19W. ISOLATED MODERATE CONVECTION IS FROM 3N-10N BETWEEN 22W-31W. ...DISCUSSION... GULF OF MEXICO... AN UPPER RIDGE EXTENDS EASTWARD OVER THE SOUTHERN GULF FROM AN UPPER HIGH NEAR 22N103W...WITH NORTHWESTERLY UPPER FLOW AFFECTING MUCH OF THE GULF. WEAK DIFFLUENCE IN THIS FLOW COMBINED WITH DEEP LAYER MOISTURE IS GENERATING SCATTERED MODERATE CONVECTION WITHIN 300 NM OF THE TEXAS AND MEXICAN COASTLINE ALONG THE GULF. A STRONG UPPER TROUGH OVER THE EASTERN CONUS CONTINUES DIGGING SOUTHWARD OVER THE EASTERN GULF. AS UPPER LEVEL SUPPORT WITH THIS TROUGH INTERACTS WITH A DEEPENING SURFACE LOW OVER THE SOUTHWEST ATLC...NORTHERLY SURFACE WINDS NEAR 20 KT WILL PULL DRY AIR INTO THE NORTHEAST GULF. NORTHWESTERLY UPPER FLOW WEST OF THE TROUGH AXIS WILL CONTINUE BRINGING DRY AND STABLE AIR...RESULTING IN FAIR WEATHER CONDITIONS...OVER THE CENTRAL AND EASTERN GULF. CARIBBEAN SEA... THE MAIN CONCERN CONTINUES TO BE THE AREA OF LOW PRESSURE DESCRIBED IN THE SPECIAL FEATURES SECTION. THE UPPER RIDGE PRESENTLY OVER THE GULF OF MEXICO COVERS THE W CARIBBEAN W OF 78W...WITH A NARROW UPPER TROUGH AXIS EXTENDING FROM EASTERN CUBA TO COSTA RICA. TO THE EAST OF THIS UPPER TROUGH...AN UPPER RIDGE IS AFFECTING THE EASTERN CARIBBEAN EAST OF 73W. DEEP LAYER DRY AIR BEING PULLED INTO THE WESTERN AND CENTRAL CARIBBEAN FROM THE NORTH ARE LIMITING CONVECTION THERE...WHERE WESTERLY SURFACE FLOW IS PRESENT. THE ONLY SUBSTANTIAL CONVECTION OVER THE CARIBBEAN IS OVER THE NORTHEAST CARIBBEAN DISCUSSED IN THE SPECIAL FEATURES SECTION...WITH ISOLATED STRONG CONVECTION IN A SMALL AREA OFF THE COSTA RICA COAST FROM 9N-12N BETWEEN 81W-83W. ATLANTIC OCEAN... ONE CONCERN FOR THE SW ATLC IS THE LOW PRESSURE AREA DESCRIBED IN THE SPECIAL FEATURES ABOVE...AS IT IS FORECAST TO MOVE INTO THE SW ATLC WATERS. ISOLATED MODERATE CONVECTION IN ASSOCIATION WITH THIS SYSTEM IS AFFECTING AREAS OF THE SOUTHWEST ATLANTIC FROM 20N-23N BETWEEN 66W-70W. ANOTHER CONCERN IS THE DEEPENING SURFACE LOW NEAR 33N74W. UPPER SUPPORT FROM A DEEP UPPER TROUGH NEAR THE EASTERN US COASTLINE CONTINUES TO STRENGTHEN THE ATTENDANT SURFACE LOW. AS A RESULT...STRONG NORTHEASTERLY SURFACE FLOW CONTINUES TO WRAP DRIER AND SLIGHTLY COOLER AIR OVER THE SW ATLC AND FLORIDA PENINSULA BEHIND A COLD FRONT OVER THE SW ATLC. THE COLD FRONT EXTENDS FROM 34N71W TO 31N72W TO SOUTH FLORIDA NEAR 26N81W...WITH GALE FORCE NORTHEASTERLY SURFACE WINDS BEHIND THE COLD FRONT. BANDS OF SHOWERS AND ISOLATED THUNDERSTORMS CONTINUE TO DEVELOP ALONG THE FRONT EAST OF 76W. FARTHER EAST...A BROAD UPPER RIDGE SEPARATES THE UPPER TROUGH NEAR THE EASTERN US COASTLINE FROM A LARGE UPPER LOW CENTERED NEAR 26N49W. THIS UPPER LOW IS BRINING CLOUDINESS AND ISOLATED SHOWERS TO THE CENTRAL ATLANTIC FROM 20N-30N BETWEEN 39W-53W. ANOTHER UPPER LOW IS CENTERED NEAR 32N20W...WITH AN ATTENDANT SURFACE LOW NEAR 34N23W. A SURFACE TROUGH TRAILS SOUTHWARD FROM THIS LOW TO NEAR 28N23W TO NEAR 24N29W. INSTABILITY AND LOW LEVEL SUPPORT FROM THESE FEATURES ARE GENERATING ISOLATED SHOWERS OVER THE EASTERN ATLANTIC N OF 27N EAST OF 26W. AN UPPER RIDGE EXTENDS WESTWARD FROM AN UPPER HIGH NEAR 15N20W AND COVERS MUCH OF THE TROPICAL ATLANTIC...WITH MAINLY DRY AND STABLE AIR UNDER THE RIDGE...RESULTING IN MAINLY FAIR WEATHER. MOISTURE ON THE WESTERN PERIPHERY OF THE RIDGE IS RESULTING IN SCATTERED MODERATE CONVECTION FROM 14N-18N BETWEEN 57W-60W. $$ COHEN/DGS