000 AXNT20 KNHC 131800 TWDAT TROPICAL WEATHER DISCUSSION NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 205 PM EDT MON OCT 13 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... NANA WEAKENS TO A DEPRESSION. AT 13/1500 UTC NANA IS CENTERED NEAR 16.8N 39.8W OR ABOUT 915 NM...1685 KM...WEST OF THE CAPE VERDE ISLANDS MOVING WNW OR 285 DEG AT 6 KT. ESTIMATED MINIMUM CENTRAL PRESSURE IS 1007 MB. MAXIMUM SUSTAINED WIND SPEED IS 30 KT WITH GUSTS TO 40 KT. PLEASE SEE LATEST NHC PUBLIC ADVISORY UNDER AWIPS/WMO HEADERS MIATCPAT4/WTNT34 KNHC AND THE FULL FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS MIATCMAT4/WTNT24 KNHC FOR MORE DETAILS. CONVECTION ASSOCIATED WITH NANA HAS DECREASED CONSIDERABLY SINCE YESTERDAY AND THE CENTER REMAINS EXPOSED DUE TO WESTERLY SHEAR. THE SHEAR IS NOT EXPECTED TO DECREASE DURING THE NEXT COUPLE OF DAYS AND NANA IS FORECAST TO DEGENERATE INTO A REMNANT LOW WITHIN A DAY OR SO. THE AREA OF LOW PRESSURE OVER THE EASTERN CARIBBEAN SEA HAS DEVELOPED ENOUGH CIRCULATION AND DEEP CONVECTION THIS MORNING. AS A RESULT...IT IS UPGRADED TO A TROPICAL DEPRESSION. AT 13/1500 UTC T.D. FIFTEEN IS CENTERED NEAR 14.8N 69.6W OR ABOUT 295 NM...545 KM...SOUTHWEST OF SAN JUAN PUERTO RICO AND ABOUT 150 NM...285 KM...N-NW OF CURACAO MOVING NW VERY SLOW AT 2 KT. LITTLE SIGNIFICANT MOTION IS EXPECTED DURING THE NEXT 24 TO 36 HOURS. HOWEVER...A NORTHEASTWARD MOTION IS FORECAST TO BEGIN ON WEDNESDAY. THIS TRACK SHOULD BRING THE CYCLONE VERY CLOSE TO PUERTO RICO BETWEEN 48 AND 72 HOURS AND OVER THE OPEN ATLANTIC THEREAFTER. ESTIMATED MINIMUM CENTRAL PRESSURE IS 1005 MB. MAXIMUM SUSTAINED WIND SPEED IS 30 KT WITH GUSTS TO 40 KT. SOME STRENGTHENING IS FORECAST AND THE DEPRESSION COULD BECOME A TROPICAL STORM LATER TODAY. AN AIR FORCE PLANE IS SCHEDULED TO REACH THE DEPRESSION LATER TODAY. PLEASE SEE LATEST NHC PUBLIC ADVISORY UNDER AWIPS/WMO HEADERS MIATCPAT5/WTNT35 KNHC AND THE FULL FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS MIATCMAT5/WTNT25 KNHC FOR MORE DETAILS. A LARGE AREA OF MODERATE TO STRONG CONVECTION IS OVER THE EASTERN CARIBBEAN FROM 11N-16N BETWEEN 63W-70W. THE OUTER FRINGES OF THE DEPRESSION ARE ALREADY AFFECTING THE NORTHERN LEEWARD ISLANDS AND THE US/UK VIRGIN ISLANDS. THE SAN JUAN DOPPLER RADAR SHOWS A BAND OF SHOWERS WITH EMBEDDED TSTMS APPROACHING THE SE PORTION OF PUERTO RICO. MORE SHOWER/TSTM ACTIVITY IS OVER THE EAST PART OF THE ISLAND. A TROPICAL STORM WATCH COULD BE REQUIRED FOR PUERTO RICO AND THE U.S. VIRGIN ISLANDS LATER TODAY. A BROAD AREA OF LOW PRESSURE...ANALYZED 1006 MB...HAS FORMED IN THE SOUTHWESTERN CARIBBEAN SEA NEAR 13N82W OR ABOUT 90 NM EAST-SOUTHEAST OF PUERTO CABEZAS NICARAGUA. THIS LOW IS ACCOMPANIED BY A LARGE AREA OF DISTURBED WEATHER OR MODERATE TO STRONG CONVECTION THAT COVERS FROM FROM 11N-16N BETWEEN 79W-84W. CONDITIONS APPEAR FAVORABLE FOR ADDITIONAL DEVELOPMENT...AND THE SYSTEM COULD BECOME A TROPICAL DEPRESSION DURING THE NEXT COUPLE OF DAYS AS IT MOVES SLOWLY NORTH OR NORTHWESTWARD. AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT IS SCHEDULED TO INVESTIGATE THE SYSTEM ON TUESDAY...IF NECESSARY. INTERESTS IN NICARAGUA... HONDURAS...AND THE NORTHWESTERN CARIBBEAN SEA SHOULD MONITOR THE PROGRESS OF THIS DISTURBANCE. BANDING FEATURES ARE BECOMING MORE EVIDENT TO THE N AND SE ON VIS SATELLITE IMAGERY. ...TROPICAL WAVES... A WEAK TROPICAL WAVE IS ALONG 50W S OF 20N MOVING W 10-15 KT. THE WAVE IS EMBEDDED IN AN AREA OF MOSTLY DRY AIR WITH CONVECTION CONFINED TO THE ITCZ. VIS SATELLITE IMAGERY DEPICTS A WEAK INVERTED-V PATTERN IN LOW LEVEL CLOUDS ALONG THE WAVE AXIS. A TROPICAL WAVE IS JUST WEST OF BARBADOS ALONG 59W/60W SOUTH OF 19N MOVING WEST NEAR 10N. THE VERTICAL SOUNDING FROM THIS ISLAND INDICATES E TO SE WINDS AT LOW LEVELS INDICATING THE PASSAGE OF THE WAVE AXIS. A WEAK MAXIMUM OF DEEP LAYER MOISTURE IS SEEN ON THE MIMIC TOTAL PRECIPITABLE WATER PRODUCT FROM CIMSS. ISOLATED SHOWERS ARE BETWEEN BARBADOS AND TRINIDAD. SFC WINDS ACROSS THE LESSER ANTILLES ARE MAINLY FROM THE SE DUE TO THE PRESENCE OF T.D. FIFTEEN OVER THE EASTERN CARIBBEAN. THIS TROPICAL WAVE WILL LIKELY BE ABSORBED BY THE TROPICAL CYCLONE LOCATED OVER THE EASTERN CARIBBEAN. THE TROPICAL WAVE THAT WAS PREVIOUSLY ALONG 72W/73W IS NO LONGER DISCERNIBLE. IT APPEARS THAT THIS SYSTEM WAS ABSORBED BY T.D. FIFTEEN. ...THE ITCZ... ITCZ AXIS IS CENTERED ALONG 10N15W 9N25W 13N41W 10N50W 9N80W. A 1008 MB LOW IS EMBEDDED WITHIN THE ITCZ NEAR 13N42W. CONVECTION HAS SLIGHTLY INCREASED WITHIN 70 NM EAST AND SE OF THE LOW CENTER. THIS LOW IS LOCATED WITHIN AROUND 260 NM SW OF NANA. SCATTERED MODERATE TO ISOLATED STRONG CONVECTION IS WITHIN 90-110 NM N OF AXIS BETWEEN 16W-33W. SIMILAR CONVECTION IS FROM 10N-14N BETWEEN 38W-43W...INCLUDING THE ABOVE MENTIONED SFC LOW. ...DISCUSSION... GULF OF MEXICO... AN UPPER HIGH IS CENTERED OVER SE TEXAS NEAR 29N98W PRODUCING . DRY NORTHERLY UPPER FLOW AND STABLE ATMOSPHERIC CONDITIONS NW OF A LINE FROM SE LOUISIANA TO VERACRUZ MEXICO. AN UPPER-LEVEL LOW IS MOVING WWD ACROSS THE SW PORTION OF THE GULF. IT IS CENTERED NEAR 21N92W. WEAK INSTABILITY AROUND THIS FEATURE IS HELPING TO INDUCE ISOLATED TO SCATTERED SHOWERS OVER THE YUCATAN PENINSULA AND THE EASTERN HALF OF THE GULF. A WEAK SFC TROUGH...LIKELY A REFLECTION OF THIS UPPER-LEVEL FEATURE LIES ALONG 94W SOUTH OF 23N. HIGH PRES CENTER OVER THE MID ATLANTIC STATES DOMINATES THE SE CONUS...THE STATE OF FLORIDA AND THE NORTHERN GULF WATERS. PRES GRADIENT BETWEEN THIS HIGH AND A BROAD AREA OF LOW PRESSURE THAT HAS FORMED IN THE SOUTHWESTERN CARIBBEAN SEA IS GENERATING MODERATE TO STRONG ELY WINDS OVER THE EASTERN GULF. WINDS ARE EXPECTED TO INCREASE UP TO 25 KT OVER THE EAST AND MIDDLE GULF TONIGHT AND TUE WITH BUILDING SEAS UP TO 10 FT...PARTICULARLY OVER THE CENTRAL GULF. CARIBBEAN SEA... THERE ARE TWO AREAS OF INTEREST ACROSS THE CARIBBEAN SEA TODAY. ONE IS T.D. FIFTEEN FORECAST TO STRENGTHEN AND MOVE ACROSS PUERTO RICO...AND THE SECOND ONE IS THE BROAD AREA OF LOW PRESSURE THAT HAS FORMED IN THE SOUTHWESTERN CARIBBEAN THIS MORNING. SEE SPECIAL FEATURES FOR MORE DETAILS. PRES GRADIENT BETWEEN HIGH PRES TO THE NORTH AND LOWER PRES OVER THE SW CARIBBEAN IS PRODUCING NE WINDS OF 15-20 KT OVER THE NW CARIBBEAN N OF 15N. MOIST SOUTHWESTERLY FLOW ALOFT IS E OF THE UPPER LOW IN THE SW GULF OF MEXICO AND COVERS MUCH OF THE CENTRAL AND W CARIBBEAN. THESE WINDS ARE ENHANCING CONVECTION OVER THE NW CARIBBEAN. TO THE E...A WEAK UPPER RIDGE EXTENDS FROM N VENEZUELA TO PUERTO RICO. DEEP LAYERED MOISTURE ACROSS THE CARIBBEAN IS RESULTING IN CLOUDINESS ACROSS MUCH OF THE BASIN. ATLANTIC OCEAN... NANA...CURRENTLY AS A TROPICAL DEPRESSION...CONTINUES TO MOVE ACROSS THE OPEN WATERS OF THE TROPICAL ATLANTIC. IT IS FORECAST TO WEAKEN TO A REMNANT LOW IN ABOUT 24 HOURS. SEE SPECIAL FEATURES. SCATTERED SHOWERS AND TSTMS ARE DEVELOPING OVER THE WESTERN ATLC JUST N OF THE NW BAHAMAS. THIS CONVECTIVE ACTIVITY IS MOVING WWD UNDER AN ELY WIND FLOW AROUND A HIGH PRES SYSTEM LOCATED OVER THE MID ATLC STATES. DOPPLER RADAR DEPICTS SCATTERED TO NUMEROUS SHOWERS AND ISOLATED TSTMS OVER SOUTH-CENTRAL FLORIDA. A WEAK SFC TROUGH RUNS FROM 28N59W TO 23N61W. SCATTRED SHOWERS AND ISOLATED TSTMS ARE WITHIN AROUND 200 NM AHEAD OF THE TROUGH AXIS. A DEEP LAYERED TROUGH IS OVER THE WEST-CENTRAL ATLC PRODUCING MAINLY UPPER-LEVEL WLY FLOW OVER MUCH OF THE FORECAST AREA. THIS TROUGH IS FORECAST BY THE COMPUTER MODELS TO DIG INTO THE WESTERN ATLC N OF HISPANIOLA AND PUERTO RICO TOWARD MID-WEEK. THE TROPICAL CYCLONE OVER THE EASTERN CARIBBEAN IS EXPECTED TO BECOME EMBEDDED WITHIN THE SWLY FLOW AHEAD OF THIS TROUGH AND RECURVE. A RIDGE ANCHORED ON AN UPPER-LEVEL HIGH NEAR 11N29W DOMINATES THE TROPICS AND IS ENHANCING CONVECTION ALONG THE ITCZ AXIS. FURTHER NORTH...THERE IS ANOTHER UPPER-LEVEL HIGH SITUATED BETWEEN THE AZORES AND THE MADEIRA ISLANDS. $$ GR