000 AXPZ20 KNHC 312225 TWDEP TROPICAL WEATHER DISCUSSION NWS NATIONAL HURRICANE CENTER MIAMI FL 2205 UTC FRI AUG 31 2012 TROPICAL WEATHER DISCUSSION FOR THE EASTERN PACIFIC OCEAN FROM THE EQUATOR TO 32N...EAST OF 140W. THE FOLLOWING INFORMATION IS BASED ON SATELLITE IMAGERY...WEATHER OBSERVATIONS...RADAR...AND METEOROLOGICAL ANALYSIS. BASED ON 1800 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 2145 UTC. ...SPECIAL FEATURES... TROPICAL STORM ILEANA...CENTERED AT 22.4N 116.1W AT 31/2100 UTC. ILEANA IS MOVING NW OR 305 DEG AT 07 KT WITH AN ESTIMATED MINIMUM CENTRAL PRESSURE OF 987 MB. MAXIMUM SUSTAINED WIND SPEED IS 60 KT WITH GUSTS TO 75 KT. SCATTERED MODERATE ISOLATED STRONG CONVECTION REMAINS CONCENTRIC WITHIN ABOUT 90 NM OF CENTER. THE CYCLONE IS MOVING OVER INCREASINGLY COOLER SEAS SURFACE WATERS AND MORE STABLE ATMOSPHERIC CONDITIONS AND IS EXPECTED TO STEADILY WEAKEN. SEE THE LATEST NHC FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS MIATCMEP4/WTPZ24 KNHC FOR MORE DETAILS. TROPICAL LOW...AN ILL-DEFINED CYCLONIC SWIRL IS NOTED WITHIN THE MONSOON TROUGH AT 12N100W. SCATTERED MODERATE ISOLATED STRONG DISORGANIZED CONVECTION IS NOTED WITHIN 180 NM OVER W SEMICIRCLE OF CENTER. GLOBAL GUIDANCE SUGGESTS THAT THIS LOW PRESSURE WILL MOVE W FOR A DAY THEN TURN NW AND DEEPEN WITH POSSIBLE TROPICAL CYCLONE DEVELOPMENT. THE GRADIENT BETWEEN STRONG HIGH PRESSURE BRIDGING SW ACROSS THE GULF OF MEXICO THROUGH THE ISTHMUS OF TEHUANTEPEC AND LOW PRESSURE WITHIN THE BROAD MONSOON TROUGH HAS ENHANCED EASTERLY WINDS TO 20 TO 25 KT WELL TO THE NE AT ABOUT 420 NM FROM THE ESTIMATED CENTER. THE LOW IS FORECAST TO BE NEAR 12N101W MIDDAY SAT THEN TURN NW TO NEAR 15N105W MIDDAY ON SUN. ...INTERTROPICAL CONVERGENCE ZONE/MONSOON TROUGH... THE MONSOON TROUGH EXTENDS W FROM NW COAST OF COLOMBIA ACROSS NORTHERN PANAMA TURNING NW ACROSS COSTA RICA TO ALONG THE PACIFIC COAST OF NICARAGUA AND EL SALVADOR TO 14N93W WHERE IT TURNS ABRUPTLY SW INTO AN EMBEDDED LOW PRESSURE NEAR 12N100W. THE TROUGH THEN CONTINUES NW TO 15N108W...THEN TURNS SW THROUGH 10N130W TO BEYOND 10N140W. SCATTERED MODERATE ISOLATED STRONG CONVECTION IS OBSERVED ELSEWHERE WITHIN 240 NM OF LINE 07N91W TO 17N108W AND IS SURROUNDING THE CONVECTION PREVIOUSLY DESCRIBED THAT IS ASSOCIATED WITH THE TROPICAL LOW. THE STRONGEST ACTIVITY...VERIFIED BY LIGHTNING DATA...EXTENDS PARALLEL TO THE COAST ABOUT 50 NM OFFSHORE GUATEMALA. SCATTERED MODERATE CONVECTION IS NOTED WITHIN 30 NM EITHER SIDE OF A LINE FROM 13.5N112W TO 10N121W AND ANOTHER LINE EXTENDING FROM 09.5N128W TO 08N134W. ...DISCUSSION... A LONG WAVE UPPER TROUGH DOMINATES THE EASTERN N PACIFIC WITH AN EMBEDDED SHORTWAVE SWINGING SE AND BECOMING NEGATIVE TILT OVER THE DISCUSSION AREA FROM 32N123W TO 24N122W. A SECOND SHORTWAVE IS NOW REINFORCING FROM 32N130W TO 24N129W. THE APPROACH OF THE INITIAL SHORTWAVE IS LIKELY RESPONSIBLE FOR THE BRIEF NORTHWEST MOTION OF TROPICAL CYCLONE ILEANA. AS THIS INITIAL TROUGH LIFTS NE THE WEAKENING TROPICAL CYCLONE ILEANA SHOULD TURN MORE WESTERLY AS THE STEERING RIDGE SHOULD ALSO BUILD W. AN UPPER LEVEL ANTICYCLONE IS CENTERED AT 21N131W WITH ITS ENVELOPE OF ANTICYCLONIC UPPER FLOW COVERING THE AREA FROM 11N TO 23N BETWEEN 127W AND 145W. THESE FEATURES COMBINE TO DRY OUT THE UPPER LEVELS TO THE W OF A LINE EXTENDING FROM ARIZONA AT 32N113W TO 07N140W WITH UPPER STABILITY CONTRIBUTING THE WEAKENING TREND OF TROPICAL CYCLONE ILEANA. THE UPPER ANTICYCLONE OVER TROPICAL CYCLONE ILEANA IS DISSIPATING WITHIN IN THE MORE PREDOMINATE UPPER RIDGE EXTENDING SW FROM A LARGE ANTICYCLONE OVER NORTHWESTERN OLD MEXICO AT 29N107W. SOME OF THE DEBRIS MOISTURE FROM ILEANA CONCENTRATES INTO A 360 NM WIDE PLUME EXTENDING SW TO NEAR 12N129W. SOME OF THE TROPICAL CYCLONE DEBRIS MOISTURE ALSO CONCENTRATES INTO A 240 NM WIDE THAT SPREAD NE AROUND THE W PERIPHERY OF THE UPPER ANTICYCLONE ACROSS THE NORTHERN BAJA CALIFORNIA PENINSULA AND GULF OF CALIFORNIA...NW OLD MEXICO AND IS NOW SPREADING OVER SE ARIZONA AND SW NEW MEXICO. AN UPPER TROUGH ORIGINATING IN THE CENTRAL N ATLANTIC CONTINUES SW ACROSS THE NW CARIBBEAN AND ACROSS THE YUCATAN PENINSULA TO OVER THE TROPICAL E PACIFIC AT 14N96W...THEN TURNS SE THROUGH 05N97W CROSSING THE EQUATOR NEAR THE GALAPAGOS ISLANDS. AN UPPER ANTICYCLONE IS OVER THE WINDWARD ISLANDS OF THE E CARIBBEAN WITH ITS RIDGE PERIPHERY BUILDING W ACROSS CENTRAL AMERICA. AN AREA OF UPPER DIFFLUENCE IS INDUCED OVER CENTRAL AMERICA...AND OVER THE EASTERN PORTIONS OF THE PACIFIC MONSOON TROUGH...AND IS SUPPORTING INTENSE CONVECTION OVER COSTA RICA...NICARAGUA AND EASTERN HONDURAS. EXPECT SOME OF THIS CONVECTION TO PERSIST AND MOVE OFFSHORE THE COASTS OF COSTA RICA AND NICARAGUA EARLY THIS EVENING...AND PERHAPS OFF THE COAST OF EL SALVADOR LATER TONIGHT. IN THE MEANTIME...THE ASSOCIATED DENSE UPPER DEBRIS MOISTURE IS ADVECTED S ACROSS THE EQUATOR BETWEEN 78W AND 110W. ELSEWHERE AT THE LOW LEVELS...SOUTHWESTERLY MONSOONAL FLOW AT 15 TO 20 KT WINDS WITH SEAS 8 TO 11 FT ARE OBSERVED FROM 10N TO 22N FROM 122W TO THE COAST OF MEXICO...MERGING WITH THE SW AND W WIND WAVE OVER THE S SEMICIRCLE OF TROPICAL CYCLONE ILEANA. LONG PERIOD CROSS EQUATORIAL SOUTHWESTERLY SWELL AT 15 TO 19 SECONDS HAS REACHED ALONG 07N BETWEEN 79W AND 85W RESULTING IN SEAS TO 9 FT...WITH THIS AREA OF SW SWELLS EXPECTED TO SUBSIDE OVER THE NEXT 48 HOURS. MIXED SOUTHERLY LONG PERIOD SWELL AT 14 TO 18 SECONDS IS NOTED FROM THE EQUATOR TO 17N BETWEEN 88W AND 122W AND IS ALSO EXPECTED TO SUBSIDE OVER THE NEXT COUPLE OF DAYS. $$ NELSON