000 AXNT20 KNHC 301757 TWDAT TROPICAL WEATHER DISCUSSION NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 205 PM EDT SUN SEP 30 2007 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 1200 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 1715 UTC. ...SPECIAL FEATURE... MELISSA IS DOWNGRADED TO A TROPICAL DEPRESSION AT 30/1500 UTC. AT THIS TIME...IT WAS CENTERED NEAR 16.1N 32.6W OR ABOUT 495 NM WEST OF THE CAPE VERDE ISLANDS...MOVING WNW AT 11 KT. ESTIMATED MINIMUM CENTRAL PRESSURE IS 1007 MB. MAXIMUM SUSTAINED WIND SPEED IS 25 KT WITH GUSTS TO 35 KT. SEE LATEST NHC FORECAST ADVISORY UNDER AWIPS/WMO HEADERS MIATCMAT4/WTNT24 KNHC AND THE PUBLIC ADVISORY UNDER MIATCPAT4/WTNT34 KNHC FOR MORE DETAILS. AFTER MOST OF THE CONVECTION DISSIPATED ABOUT 12 HOURS AGO...THE CYCLONE HAS ONLY BEEN ABLE TO GENERATE SMALL PULSATING BURSTS OF CONVECTION. STRONG WESTERLY SHEAR PERSISTS OVER THE STORM. THE ENVIRONMENT AHEAD OF MELISSA IS EXPECTED TO REMAIN HOSTILE AS WESTERLY SHEAR IS FORECAST TO PERSIST. THE NHC INTENSITY FORECAST MAINTAINS MELISSA AS A TROPICAL DEPRESSION DURING THE NEXT DAY OR SO. PRESENTLY A SMALL CLUSTER OF MODERATE CONVECTION IS NEAR 16N32W. ...TROPICAL WAVES... TROPICAL WAVE IS OFF THE COAST OF W AFRICA ALONG 22W S OF 15N MOVING W 10-15 KT. THE MIMIC TPW ANIMATION SHOWS A MODEST SURGE OF MOISTENED AIR JUST SOUTH OF THE CAPE VERDE ISLANDS. SATELLITE DERIVED WINDS SHOW VERY WELL THE WIND SHIFT ASSOCIATED WITH THE WAVE. SCATTERED MODERATE CONVECTION IS MAINLY NEAR THE INTERSECTION OF THE WAVE AND THE ITCZ FROM 4N-7N BETWEEN 20W-24W...WITH ISOLATED SHOWERS WITHIN 100 NM AHEAD OF THE WAVE AXIS FROM 8N-11N. TROPICAL WAVE IS ALONG 40W S OF 16N MOVING W 15 KT. INVERTED-V LOW LEVEL SIGNATURE IS OBSERVED ON VIS IMAGERY. THERE IS ALSO A WEAK NWD PERTURBATION IN THE ITCZ THAT IS NOTED ON THE MIMIC TPW ANIMATION. CONVECTION IS MAINLY CONFINED TO THE ITCZ. SCATTERED SHOWERS ARE FROM 4N-7N BETWEEN 38W-42W. A WEAK TROPICAL WAVE IS OVER CENTRAL AMERICA ALONG 89W S OF 20N MOVING W 10-15 KT. THE WAVE...COUPLED WITH A DIFFLUENT PATTERN ALOFT IS GENERATING SCATTERED SHOWERS AND TSTMS OVER THE GULF OF HONDURAS. THE WAVE ALSO EXTENDS INTO THE EPAC WHERE IS PRODUCING SIMILAR SHOWER ACTIVITY. ...THE ITCZ... ITCZ AXIS IS CENTERED ALONG 7N15W 4N27W 7N37W 7N45W 8N60W. OUTSIDE OF THE CONVECTION ASSOCIATED WITH THE TROPICAL WAVES... WIDELY SCATTERED MODERATE CONVECTION IS FROM 2N-5N BETWEEN 14W-21W. SCATTERED MODERATE CONVECTION IS FROM 2N-6N BETWEEN 30W-38W. DISCUSSION... GULF OF MEXICO... A WEAKENING FRONTAL BOUNDARY MOVED ACROSS SOUTH FLORIDA AND THE SE GULF EARLY THIS MORNING A NOW IS REACHING THE CITY OF HAVANA CUBA. NUMEROUS SHOWERS AND TSTMS WERE REPORTED IN SOUTH FLORIDA WITH THE PASSAGE OF THE FRONTAL BOUNDARY. CURRENTLY...A NARROW BAND OF SHOWERS AND TSTMS IS AFFECTING WESTERN CUBA. WINDS ARE ON THE INCREASE BEHIND THE FRONTAL SYSTEM WITH THE FLORIDA KEYS ALREADY REPORTING GUSTY WINDS OF 25 KT. COMPUTER MODELS HAVE A WEAK LOW DEVELOPING ALONG THE FRONTAL BOUNDARY OVER THE BAHAMAS...THEN THE LOW IS FORECAST TO MOVE WWD ACROSS THE FLORIDA KEYS LATE MONDAY THROUGH TUESDAY. STRONG HIGH PRESSURE FOLLOWS THE FRONT WITH A 1034 MB HIGH OVER NEW ENGLAND. THIS HIGH EXTENDS A RIDGE SW COVERING THE EASTERN U.S. AND NORTHERN GULF WATERS N OF 25 N. AS A RESULT...THE NE QUADRANT OF THE GULF HAS NELY SURFACE FLOW OF 20 TO 25 KT WITH BUILDING SEAS. UNDER THE INFLUENCE OF THE SFC RIDGE MOSTLY FAIR WEATHER PREVAILS. A SFC TROUGH IS ANALYZED OVER THE BAY OF CAMPECHE AND THE SW GULF. SHOWER ACTIVITY IS MINIMAL ASSOCIATED WITH THIS TROUGH. IN THE UPPER LEVELS...AN UPPER HIGH LOCATED OVER NORTH-CENTRAL MEXICO COVERS THE WESTERN GULF...INCLUDING THE BAY OF CAMPECHE AND THE YUCATAN PENINSULA. AN UPPER TROUGH CROSSES THE STRAITS OF FLORIDA AND THE FAR SE GULF WITH A DEVELOPING UPPER LOW OVER THE NW BAHAMAS. MID/UPPER DRY DOMINATES MOST OF THE EASTERN GULF AND THE FLORIDA PENINSULA...THUS A DECREASING TREND IN SHOWER/TSTM ACTIVITY IS EXPECTED OVER SOUTH FLORIDA THIS AFTERNOON. THE MENTIONED UPPER TROUGH IS SUPPORTING THE FRONTAL SYSTEM OVER WESTERN CUBA. MOIST ONSHORE FLOW IS PRODUCING SOME SHOWER ACTIVITY OVER THE NW CORNER OF THE GULF AND THE COAST OF TEXAS. CARIBBEAN SEA... SCATTERED MODERATE TO ISOLATED STRONG CONVECTION IS OVER THE CARIBBEAN PLAINS OF NICARAGUA AND ADJACENT REGIONAL WATERS. THIS ACTIVITY IS ASSOCIATED WITH A MOIST LOW LEVEL FLOW AND UPPER DIFFLUENCE BETWEEN AN UPPER RIDGE THAT EXTENDS SE FROM AN UPPER HIGH ANCHORED OVER NORTH-CENTRAL MEXICO...AND A SHORT WAVE TROUGH OVER E CUBA/JAMAICA. SIMILAR CONVECTION IS ALSO SEEN BETWEEN HAITI AND JAMAICA ASSOCIATED WITH THE SHORT WAVE TROUGH AND A SURFACE TROUGH THAT EXTENDS FROM THE W ATLC INTO THE CARIBBEAN THROUGH THE WINDWARD PASSAGE. MORE ABOUT THIS TROUGH IN THE ATLANTIC SECTION. THE WEATHER PATTERN OVER THE EASTERN CARIBBEAN HAS NOT CHANGED DURING THE LAST FEW DAYS. NWLY WINDS ASSOCIATED WITH AN UPPER LOW LOCATED NEAR 20N60W ARE ADVECTING HIGH CLOUDS/MID TO UPPER-LEVEL MOISTURE OVER PUERTO RICO AND THE NE CARIBBEAN INTO THE LESSER ANTILLES. THESE WINDS...PAIRED WITH THE LOCAL EFFECTS ARE HELPING TO INDUCE SCATTERED SHOWERS AND TSTMS OVER THE AREA...PARTICULARLY DURING THE AFTERNOON AND EARLY EVENING HOURS. AN UPPER RIDGE EXTENDING NE FROM AN UPPER HIGH LOCATED NEAR NORTHERN COLOMBIA IS ALSO SUPPORTING SOME CONVECTIVE ACTIVITY OVER HISPANIOLA. TRADEWINDS REMAIN LIGHT TO MODERATE ACROSS THE BASIN. ATLANTIC OCEAN... THERE ARE THREE AREAS OF INTEREST IN THE ATLANTIC FORECAST AREA. FIRST...T.D. MELISSA. SEE SPECIAL FEATURE. SECOND...THE REMNANTS OF KAREN...LOCATED A COUPLE HUNDRED EAST OF LEEWARD ISLANDS... WHICH ARE INTERACTING WITH AN UPPER-LEVEL TROUGH. THIS IS PRODUCING A LARGE AREA OF DISORGANIZED SHOWERS AND TSTMS. A SFC TROUGH IS ANALYZED IN THIS AREA AND RUNS FROM THE WINDWARD ISLANDS ALL THE WAY NE TO 21N51W. A WEAK 1012 MB LOW IS ALONG THE TROUGH AXIS NEAR 16N58W. VIS SATELLITE IMAGERY SHOWS A SWIRL OF LOW CLOUDS RELATED TO THIS LOW. UPPER-LEVEL WINDS ARE CURRENTLY UNFAVORABLE FOR REGENERATION OF THIS SYSTEM...BUT THEY COULD BECOME A LITTLE MORE FAVORABLE DURING THE NEXT DAY OR TWO AS THIS SYSTEM MOVES NORTHWESTWARD AT 10 TO 15 MPH. THIRD...A LARGE AREA OF DISORGANIZED CLOUDINESS AND THUNDERSTORMS ARE LOCATED OVER THE WESTERN ATLANTIC...BAHAMA ISLANDS...AND PORTIONS OF SOUTH FLORIDA. THIS ACTIVITY IS ASSOCIATED WITH A SURFACE TROUGH AND THE ABOVE MENTIONED FRONTAL BOUNDARY. SOME SLOW DEVELOPMENT OF THIS SYSTEM IS POSSIBLE DURING THE NEXT SEVERAL DAYS AS IT MOVES SLOWLY WESTWARD TOWARD THE STRAITS OF FLORIDA AND THE FLORIDA KEYS. A COLD FRONT ALSO CLIPS THE FORECAST AREA ALONG 30N BETWEEN 25W AND 35W. A NARROW BAND OF SHOWERS IS ASSOCIATED WITH THE FRONT. A SFC RIDGE CONTROLS THE REMAINDER OF THE ATLC. IN THE UPPER LEVELS...A TROUGH IS OVER THE W ATLANTIC AND THE STRAITS OF FLORIDA. A RIDGE IS JUST E OF THE TROUGH. THESE UPPER FEATURES ARE ENHANCING CONVECTION OVER THE W ATLC AND THE BAHAMAS. A LARGE UPPER LEVEL LOW IS CENTERED NEAR 20N60W PRODUCING THE SW SHEAR OVER THE REMNANTS OF KAREN. WLY FLOW IS OVER THE E ATLANTIC N OF 20N E OF 40W. A LARGE UPPER LEVEL HIGH IS OVER THE TROPICS JUST EAST OF THE CAPE VERDE ISLANDS. $$ GR