000 AXNT20 KNHC 292340 TWDAT TROPICAL WEATHER DISCUSSION NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 805 PM EDT SAT SEP 29 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 1800 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 2315 UTC. ...SPECIAL FEATURES... TROPICAL STORM MELISSA IS CENTERED NEAR 15.6N 29.5W OR ABOUT 320 NM WEST OF THE CAPE VERDE ISLANDS AT 29/2100 UTC MOVING WNW AT 11 KT. ESTIMATED MINIMUM CENTRAL PRESSURE IS 1005 MB. MAXIMUM SUSTAINED WIND SPEED IS 35 KT WITH GUSTS TO 45 KT. SEE LATEST NHC FORECAST ADVISORY UNDER AWIPS/WMO HEADERS MIATCMAT4/WTNT24 KNHC AND THE PUBLIC ADVISORY UNDER MIATCPAT4/WTNT34 KNHC FOR MORE DETAILS. A LONG IR SATELLITE LOOP SHOWS CONVECTION CLEARLY INCREASING AFTER BEING VERY MINIMAL AROUND 15Z. A WELL ESTABLISHED BURST HAS BEEN DEEPENING AND EXPANDING NEAR AND NE OF THE CENTER. GIVEN THE MODERATE SHEAR OVER THE SYSTEM NOW (REASON FOR ITS ASYMMETRIC CONVECTIVE STRUCTURE)...FURTHER STRENGTHENING IS NOT ANTICIPATED. WEAKENING IS FORECAST BEYOND A COUPLE OF DAYS AS SHEAR INCREASES AND AS MELISSA MOVES OVER COOLER WATERS. THE LAST ADVISORY HAS BEEN ISSUED ON TROPICAL DEPRESSION KAREN. STRONG WLY SHEAR HAS TAKEN ITS TOLL ON THIS SYSTEM. AS OF 21Z...THE T.D. WAS LOCATED NEAR 17.3N 54.3W OR ABOUT 430 NM EAST OF THE LEEWARD ISLANDS MOVING NW AT 8 KT. ESTIMATED MINIMUM CENTRAL PRESSURE IS 1009 MB. MAXIMUM SUSTAINED WIND SPEED IS 30 KT WITH GUSTS TO 40 KT. SEE LATEST NHC FORECAST ADVISORY UNDER AWIPS/WMO HEADERS MIATCMAT2/WTNT22 KNHC AND THE PUBLIC ADVISORY UNDER MIATCPAT2/WTNT32 KNHC FOR MORE DETAILS AND REFERENCES. SCATTERED MODERATE CONVECTION IS PERSISTING DOWNSHEAR E OF THE ILL-DEFINED LOW-LEVEL CENTER FROM 14N-18N BETWEEN 49W-52W. ...TROPICAL WAVES... TROPICAL WAVE IS INTRODUCED ON THE 18Z ANALYSIS ALONG 18W S OF 14N ESTIMATED TO BE MOVING W 10-15 KT. THIS WAVE IS LOWER AMPLITUDE THAT MANY OF PREVIOUS ONES THIS MONTH. THERE ARE SEVERAL DATA SOURCES SUPPORTING THE EXISTENCE OF THIS NEW WAVE. THE MIMIC TPW ANIMATION SHOWS A MODEST BULGE OF MOISTENED AIR OFF THE AFRICAN COAST. SATELLITE IMAGERY AND DERIVED WINDS SHOW A BROAD AREA OF TURNING WITH SCATTERED SHOWERS AND TSTMS POSSIBLE WITHIN 180 NM OF THE AXIS. IN ADDITION...SOUNDING DATA FROM DAKAR SHOWS THE 600-800 MB FLOW VEERING TO THE SE AND ACCELERATING BEHIND THE WAVE. TROPICAL WAVE IS ALONG 39W/40W S OF 16N MOVING W 15 KT OVER THE PAST 24 HOURS. NIGHT CHANNEL VIS IMAGES SHOW A FAINT AREA OF SLIGHT CYCLONIC TURNING IN THE SURROUNDING LOW CLOUDS. THERE IS ALSO A WEAK NWD PERTURBATION IN THE ITCZ...WHICH IS WELL REFLECTED IN THE MIMIC TPW ANIMATION. SHOWER ACTIVITY REMAINS VERY MINIMAL NEAR THIS FEATURE WITH ONLY A SMALL PATCH WITHIN 30 NM OF 11N36.5W AT THE MOMENT. TROPICAL WAVE IS OVER CENTRAL AMERICA AND THE NW CARIB ALONG 85W S OF 20N MOVING W 15 KT. LOW-LEVEL CONVERGENCE NEAR THE WAVE ALONG WITH DIFFLUENCE ALOFT IS GENERATING A GOOD AMOUNT OF SHOWER AND TSTM ACTIVITY WITHIN 180 NM EITHER SIDE OF THE WAVE AXIS N OF 14N AND BETWEEN 78W-84W S OF 14N. ...THE ITCZ... ITCZ AXIS IS CENTERED ALONG 6N9W 8N36W 7N50W 8N60W. OUTSIDE OF THE SHOWER ACTIVITY ASSOCIATED WITH THE E ATLC TROPICAL WAVE...THE ITCZ IS RATHER INACTIVE THIS EVENING. ONLY SCATTERED SHOWERS AND ISOLATED TSTMS ARE WITHIN 30 NM S OF THE AXIS E OF 13W AND WITHIN 240 NM S OF THE AXIS BETWEEN 22W-34W. DISCUSSION... GULF OF MEXICO... A WEAKENING NEARLY STATIONARY FRONTAL BOUNDARY IS ANALYZED ACROSS THE GULF ALONG 28N. LATEST SFC DATA SHOWS A PRONOUNCED DEWPOINT GRADIENT WITH MID TO UPPER 50'S OVER THE SE U.S. AND MID 70'S IN THE CENTRAL GULF WATERS. WHILE THAT DIFFERENCE IS IMPRESSIVE FOR THIS TIME OF YEAR...THE BOUNDARY IS NOT EASY TO LOCATE AS TEMPS AND WINDS ARE FAIRLY UNIFORM. SPEAKING OF THE WINDS...THEY ARE QUITE BRISK TODAY OUT OF THE E DUE TO STRONG HIGH PRES CENTERED OVER THE ERN GREAT LAKES. ACTIVE WEATHER IS MOSTLY CONFINED TO THE NW CORNER OF THE REGION WHERE DOPPLER RADAR AND LIGHTNING DATA REVEAL SCATTERED SHOWERS AND TSTMS N OF 25N W OF 93W. THIS ACTIVITY APPEARS TO BE ENHANCED BY THE WEAK FRONT...A LOW-LEVEL TROUGH...AND DIFFLUENCE ALOFT NEAR THE CREST OF AN UPPER HIGH CENTERED OVER THE BAY OF CAMPECHE. SKIES ARE FAIR ACROSS THE E GULF AS MODERATE TO STRONG SUBSIDENCE...ON THE BACK SIDE OF A SHARP UPPER TROUGH...COVERS THE AREA. THE EXCEPTION IS QUICK MOVING ISOLATED SHOWERS AND TSTMS OVER S AND CENTRAL FLORIDA MOVING INTO THE EXTREME ERN WATERS STEERED BY THE LOW-LEVEL FLOW. CARIBBEAN SEA... THE COMBINATION OF AN UPPER HIGH CENTERED NEAR 15N75W AND AN UPPER LOW N OF THE NRN LEEWARD ISLANDS IS PRODUCING A BELT OF STRONG UPPER WESTERLY WINDS ACROSS THE E CARIB. THESE WINDS ARE CARRYING A PLUME OF CLOUDINESS AND ISOLATED SHOWERS. THIS IS VERY UNFAVORABLE CONDITIONS FOR TROPICAL CYCLONES AND IS THE REASON FOR THE DEMISE OF KAREN. IR IMAGERY HAS LIT UP THIS AFTERNOON AND EVENING OVER THE ISLANDS OF JAMAICA...CUBA AND HISPANIOLA. THIS ACTIVITY IS PRIMARILY INDUCED BY LOCAL FLOWS AND TOPOGRAPHIC INFLUENCES. THE MOST WIDESPREAD AREA OF UNSETTLED WEATHER IS IN THE FAR WRN CARIB AND OVER CENTRAL AMERICA... ASSOCIATED WITH A TROPICAL WAVE. FOR DETAILS REFER TO THAT SECTION ABOVE. CLUSTERS OF MODERATE TO STRONG ITCZ RELATED CONVECTION HAS BEEN BURSTING OVER NRN COLOMBIA AND VENEZUELA DURING THE PAST SEVERAL HOURS. ATLANTIC OCEAN... BROAD SFC TROUGHING IS OBSERVED ACROSS A PORTION OF THE SW ATLC. AS OF 21Z...A SFC TROUGH AXIS WAS ANALYZED ALONG 32N66W 26N74W TO THE WINDWARD PASSAGE. THE COMBINATION OF LOW-LEVEL CONVERGENCE ALONG THIS BOUNDARY AND UPPER DIFFLUENCE...BETWEEN A SHARP MID TO UPPER TROUGH ALONG AND OFF THE U.S. E COAST AND AN UPPER RIDGE TITLED FROM ERN CUBA TO BERMUDA...IS PRODUCING A RATHER LARGE AREA OF SCATTERED SHOWERS AND TSTMS BETWEEN 68W-76W. A COLD FRONT IS LINED UP TO THE W OF THE SFC TROUGH EXTENDING FROM 32N71W TO CENTRAL FLORIDA. NOTICEABLY DRIER AND MORE STABLE AIR IS FILTERING IN BEHIND THE FRONT AS INDICATED BY THE THIN LOW-LEVEL STRATOCUMULUS CLOUDS. STRONG WINDS AND HIGH SEAS ARE A BIG CONCERN BEHIND THE FRONT WITH SHIPS AND BUOYS REPORTING NELY WINDS IN THE 20-30 KT RANGE AND SEAS UP TO 11 FT AT THIS TIME. THESE MARINE CONDITIONS COULD GET EVEN MORE HAZARDOUS AS THESE CONTINUED STRONG WINDS WILL LIKELY DRIVE WIND WAVES AND SWELLS FROM THE NE. REFER TO OUR OFFSHORE WATERS FORECAST...MIAOFFNT3...AND YOUR LOCAL NWS OFFICE FOR DETAILS. FARTHER E...AN UPPER LOW IS CENTERED NEAR 25N62W. THE TIGHTENED W TO SWLY UPPER FLOW BETWEEN THIS SYSTEM AND AN UPPER HIGH IN THE TROPICAL ATLC IS PRODUCING STRONG SHEAR OVER KAREN...WHICH IS NO LONGER CONSIDERED A TROPICAL CYCLONE. REFER TO SPECIAL FEATURES FOR DETAILS. THE UPPER PATTERN ACROSS THE E ATLC IS MOSTLY GOVERNED BY TROUGHING WHICH EXTENDS TO THE S OF A VERY LARGE STRONG OCCLUDED LOW PRES SYSTEM CENTERED ROUGHLY BETWEEN THE AZORES AND THE IBERIAN PENINSULA. THE REMAINDER OF THE SFC PATTERN IS FAIRLY WEAK WITH A 1024 MB HIGH ANALYZED NEAR 35N47W AND A 1019 MB NEAR 26N24W. THIS IS ALLOWING WINDS TO BE LIGHT TO MODERATE EXCEPT NEAR THE CIRCULATIONS OF EX-KAREN...T.S. MELISSA AND NEAR THE AFRICAN COAST WHERE NLY FLOW IS TYPICALLY STRONGER. $$ CANGIALOSI