000 AXPZ20 KNHC 270253 TWDEP TROPICAL WEATHER DISCUSSION NWS NATIONAL HURRICANE CENTER MIAMI FL 0405 UTC THU SEP 27 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 0000 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 0200 UTC. ...SPECIAL FEATURES... TROPICAL STORM MIRIAM WAS CENTERED NEAR 21.5N 115.7W 999 MB AT 0300 UTC SEP 27 MOVING NNW OR 345 DEG AT 6 KT. MAXIMUM SUSTAINED WINDS 40 KT GUSTS 50 KT. ESTIMATED MINIMUM CENTRAL PRESSURE WAS 997 MB. CONVECTION NEAR MIRIAM CONTINUES TO BE DISRUPTED BY VERTICAL WIND SHEAR AND INGESTION OF VERY DRY MID LEVEL AIR. THIS COMBINATION HAS ERODED AWAY NEARLY ALL CLOUD COVER WITHIN 75 NM OF THE CENTER ACROSS THE SW QUADRANT...WHILE NO SIGNIFICANT CONVECTION WAS OCCURRING WITHIN 90 NM OF THE CENTER. THIS LEFT ONLY SCATTERED MODERATE CONVECTION OCCURRING FROM 90 TO 270 NM ACROSS THE NE SEMICIRCLE OF CENTER. GRADUAL WEAKENING IS EXPECTED FOR MIRIAM AND IT MOVES N-NW TONIGHT THEN MORE NW THU AND FRI. COOLER WATERS...CONTINUED VERTICAL WIND SHEAR...AND DRY AIR ENTERING THE SYSTEM ON THE W SIDE ARE EXPECTED TO BRING ABOUT THIS GRADUAL DEMISE. SWELLS GENERATED BY MIRIAM WILL CONTINUE TO AFFECT THE PACIFIC COASTS OF SOUTH AND SOUTH CENTRAL BAJA PENINSULA DURING THE NEXT COUPLE OF DAYS. THESE SWELLS WILL PRODUCE STRONG SURF AND LEAD TO DANGEROUS AND POTENTIALLY LIFE-THREATENING RIP CURRENT CONDITIONS ACROSS EXPOSED COASTLINES THROUGH THU. SEE LATEST NHC FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS MIATCMEP3/WTPZ23 KNHC FOR MORE DETAILS. A DEVELOPING TROPICAL LOW PRESSURE CENTER WAS ESTIMATED NEAR 13.5N106W AT 1007 MB...AND EMBEDDED WITHIN A LARGE LOWER TROPOSPHERIC CYCLONIC CIRCULATION...ROUGHLY BETWEEN 101W AND 110W. THE SURFACE LOW HAS DEVELOPED ALONG THE MONSOON TROUGH...AND ABUNDANT TROPICAL MOISTURE HAS CONVERGED WITHIN AND AROUND THE PERIPHERY OF THIS CIRCULATION. SCATTERED TO NUMEROUS MODERATE TO STRONG CONVECTION DESCRIBED BELOW IS OCCURRING ASSOCIATED WITH THIS FEATURE...AND WILL CONTINUE WITHIN THE BROAD CIRCULATION AS THE FEATURE IS IN A FAVORABLE ENVIRONMENT...A HIGH PRES RIDGE ALOFT...FOR SUSTAINED CONVECTION. VISIBLE SATELLITE IMAGERY THIS AFTERNOON SHOWED A LARGE LOW LEVEL WIND SURGE OF 20-25 KT MOVING FROM THE DEEP TROPICS INTO THE EASTERN SEMICIRCLE OF THIS SYSTEM...AND LIKELY BUILDING SEAS TO 8 TO 10 FT THERE. THIS SYSTEM IS EXPECTED TO CONTINUE TO SHIFT NW AROUND 10 KT THE NEXT FEW DAYS...AND HAS A MODERATE CHANCE OF DEVELOPING INTO A TROPICAL CYCLONE DURING THAT TIME. ASSOCIATED MOISTURE WILL SPREAD INTO S PORTIONS OF THE BAJA CALIFORNIA PENINSULA...THE SOUTHERN GULF OF CALIFORNIA...AND ADJACENT COASTAL MEXICO BY THU AFTERNOON...AND LEAD TO SHOWERS AND THUNDERSTORMS WITH POTENTIAL FOR HEAVY RAINS. ...INTERTROPICAL CONVERGENCE ZONE/MONSOON TROUGH... MONSOON TROUGH EXTENDS FROM 08N76W TO 10N86W TO LOW PRES NEAR 09.5N92W 1008 MB TO LOW PRES NEAR 13.5N106W 1007 MB WHERE IT FRACTURES...THEN RESUMES FROM S OF MIRIAM NEAR 14N115W TO 10N138W...WHERE IT TRANSITIONS TO ITCZ...CONTINUING ON TO BEYOND 11N140W. SCATTERED MODERATE CONVECTION NOTED WITHIN 210 NM S OF TROUGH BETWEEN 86W AND 97W. SCATTERED TO LOCALLY NUMEROUS MODERATE TO STRONG CONVECTION NOTED FROM 10N TO 16.5N BETWEEN 101W AND 112W. SCATTERED MODERATE ISOLATED STRONG CONVECTION NOTED WITHIN 150 NM S OF TROUGH BETWEEN 115W AND 128W. ...DISCUSSION... UPPER LEVEL RIDGING EXTENDING E TO W BETWEEN 10N AND 22N HAS BEEN DISSECTED BY A DIGGING MIDDLE TO UPPER LEVEL TROUGH JUST TO THE W OF MIRIAM DURING THE PAST 24 HOURS. THE TROUGH WAS NEARLY N TO S ALONG 117W-118W N OF 15N...AND HAS BECOME INVOLVED WITH MIRIAM...INDUCING 20-25 KT OF VERTICAL SWLY WIND SHEAR...AND INTRODUCING DRY AND SUBSIDENT AIR INTO W PORTIONS OF THE BROADER CIRCULATION OF MIRIAM. THE TROUGH WILL DRIFT E THROUGH THU AND THEN WEAKEN CONSIDERABLY. TO THE W AND E OF THIS TROUGH RESIDE UPPER RIDGES...CENTERED ON UPPER ANTICYCLONES NEAR 18N136W AND 18N102W. THIS GENERAL RIDGE ALIGNMENT IS EXPECTED TO REMAIN IN PLACE THROUGH FRI...WITH THE EASTERN RIDGE HELPING TO MAINTAIN A FAVORABLE ENVIRONMENT FOR ACTIVE CONVECTION ALONG THE NEAR THE MONSOON TROUGH. A 1025 HIGH CENTERED NEAR 34N142W AND EXTENDS A RIDGE SE AND ACROSS N PORTIONS TO NEAR 28N118W. A SHRINKING AREA OF FRESH TRADE WINDS ON THE SOUTHERN PERIPHERY OF THE RIDGE WAS IDENTIFIED BY RECENT SCATTEROMETER PASSES FROM 13N TO 15N W OF 134W...WHERE SEAS ARE RUNNING 8-10 FT IN MIXED NW SWELL AND BUILDING NE WIND WAVES. THE AERIAL EXTENT OF THE FRESH TRADES WILL CONTINUE TO DECREASE DURING THE NEXT 24 HOURS AS HIGH PRES WEAKENS. ELSEWHERE E OF 110W...LIGHT TO MODERATE MONSOONAL FLOW IS MAINTAINING A MOIST AND UNSTABLE REGIME S OF THE TROUGH...AND WILL REMAIN ACTIVE CONVECTIVELY DURING THE NEXT FEW DAYS AS THE UPPER RIDGE REMAINS IN PLACE. TWO SWELL EVENTS MOVING THROUGH THE AREA WATERS...PLUS SEAS GENERATED BY T.S MIRIAM WILL CONTINUE TO IMPACT THE REGIONAL WATERS THROUGH THU. MODERATE TO LARGE LONG PERIOD SWELLS FROM THE NW ARE SPREADING ACROSS THE NW WATERS...RAISING SEAS TO 7-9 FT TO THE W AND NW OF MIRIAM. MEANWHILE...A NEW PULSE OF CROSS EQUATORIAL SW SWELLS HAS MOVED THROUGH SE PORTIONS AND REACHED THE COAST OF MEXICO AND CENTRAL AMERICA WITH ANOTHER STRONG PULSE TO FOLLOW TONIGHT THROUGH THU. THESE SWELLS ARE ALREADY MERGING WITH SEAS OF 8 TO 11 FT AFFECTING MOST OF THE FORECAST WATERS W OF 105W. SWELLS GENERATED BY HURRICANE MIRIAM WILL CONTINUE TO REACH THE COASTS OF MEXICO TO PRODUCE STRONG AND LARGE SURF AND DANGEROUS RIP CURRENTS WITHIN THE SURF ZONE FROM THE SOUTH HALF OF BAJA CALIFORNIA TO NEAR MANZANILLO. SMALL SOUTHERLY SWELL FROM MIRIAM HAS MOVED THROUGH THE ENTRANCE INTO THE GULF OF CALIFORNIA AND WILL CREATE SMALL BUT STRONG SURF ALONG EXPOSED COASTLINES S OF 27N THROUGH THU MORNING. $$ STRIPLING