000 AXPZ20 KNHC 270920 TWDEP TROPICAL WEATHER DISCUSSION NWS NATIONAL HURRICANE CENTER MIAMI FL 1005 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 0600 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 0845 UTC. ...SPECIAL FEATURES... TROPICAL STORM MIRIAM IS CENTERED NEAR 21.8N 116.0W AT 27/0900 UTC MOVING NNW OR 330 DEG AT 5 KT. MAXIMUM SUSTAINED WINDS 40 KT GUSTS 50 KT. ESTIMATED MINIMUM CENTRAL PRESSURE WAS 999 MB. CONVECTION NEAR MIRIAM CONTINUES TO BE DISRUPTED BY VERTICAL WIND SHEAR AND INGESTION OF VERY DRY MID LEVEL AIR. WEAKENING IS FORECAST BY ALL GUIDANCE DUE TO COOLER WATERS...MODERATE OR STRONG WIND SHEAR...AND DRY AIR ENTERING THE SYSTEM ON THE W SIDE. AS A RESULT...MIRIAM SHOULD BECOME A REMNANT LOW IN A DAY OR SO AS IT MOVES TO THE WEST WITH A DECREASE IN FORWARD SPEED. SWELLS GENERATED BY MIRIAM WILL CONTINUE TO AFFECT THE SOUTH AND WEST COASTS OF THE SOUTHERN AND CENTRAL BAJA PENINSULA TODAY BUT WILL BEGIN TO GRADUALLY SUBSIDE ON FRIDAY. THESE SWELLS COULD CAUSE LIFE-THREATENING SURF AND RIP CURRENT CONDITIONS. SEE LATEST NHC FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS MIATCMEP3/WTPZ23 KNHC FOR MORE DETAILS. A DEVELOPING SURFACE LOW PRESSURE ALONG THE MONSOON TROUGH IS CENTERED NEAR 14.5N106W WITH AN ESTIMATED MINIMUM CENTRAL PRESSURE OF 1007 MB. SATELLITE IMAGERY INDICATES THAT CONVECTION HAS INCREASED DURING THE PAST FEW HOURS PARTICULARLY TO THE SW SEMICIRCLE. ENVIRONMENTAL CONDITIONS APPEAR CONDUCIVE FOR SOME DEVELOPMENT OF THE LOW AS IT MOVES NORTH-NORTHWESTWARD AT AROUND 10 KT DURING THE NEXT COUPLE OF DAYS. THIS SYSTEM HAS A MEDIUM CHANCE OF BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS. THE 0436 ASCAT PASS PROVIDED OBSERVATIONS OF 20-25 KT WINDS EXTENDING WITHIN 210 NM E SEMICIRCLE OF THE CENTER. NUMEROUS MODERATE TO ISOLATED STRONG CONVECTION IS SEEN WITHIN 240 NM SW SEMICIRCLE OF CENTER. ASSOCIATED MOISTURE WILL SPREAD INTO S PORTIONS OF THE BAJA CALIFORNIA PENINSULA...THE SOUTHERN GULF OF CALIFORNIA...AND ADJACENT COASTAL MEXICO WITH THE POTENTIAL OF LOCALLY HEAVY RAINS DURING THE NEXT COUPLE OF DAYS. ...INTERTROPICAL CONVERGENCE ZONE/MONSOON TROUGH... MONSOON TROUGH EXTENDS FROM 08N78W TO 09N90W TO 12N101W WHERE IT FRACTURES...THEN RESUMES FROM S OF MIRIAM NEAR 14N116W TO 11N130W TO 11N140W. ISOLATED MODERATE TO STRONG CONVECTION IS NOTED WITHIN 150 NM S OF TROUGH BETWEEN 90W AND 98W. SCATTERED MODERATE ISOLATED STRONG CONVECTION CAN BE FOUND FROM 10N TO 14N BETWEEN 110W AND 116W. SCATTERED MODERATE CONVECTION IS FROM 09N TO 11N BETWEEN 116W AND 124W. ...DISCUSSION... ALOFT...TWO ANTICYCLONIC CIRCULATIONS ARE NOTED...ONE NEAR 18N102W AND THE OTHER NEAR 18N133W. IN BETWEEN...THERE IS A TROUGH EXTENDING NEARLY N TO S ALONG 30N118W 22N117W 15N118W. THIS TROUGH 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 TODAY AND THEN WEAKEN CONSIDERABLY. TO THE W AND E OF THIS TROUGH RESIDE UPPER RIDGES ASSOCIATED WITH THE AFOREMENTIONED ANTICYCLONES. 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 MONSOON TROUGH. NE WIND FLOW DOMINATES THE WATERS E OF 110W. A 1023 HIGH PRES CENTERED NEAR 34N143W EXTENDS A RIDGE SE AND ACROSS THE NW WATERS TO NEAR 24N122W. MAINLY MODERATE TRADE WINDS ARE NOTED ON THE SOUTHERN PERIPHERY OF THE RIDGE BY RECENT SCATTEROMETER PASSES...WHERE SEAS ARE RUNNING 8-9 FT IN MIXED NW SWELL AND BUILDING NE WIND WAVES. 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 REGIONAL WATERS...PLUS SEAS GENERATED BY T.S. MIRIAM WILL CONTINUE TO IMPACT THE FORECAST WATERS TODAY. 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 TODAY. 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 THIS MORNING. ADDITIONAL PULSES OF NW SWELL WILL REACH THE NW CORNER OF THE DISCUSSION AREA FRI NIGHT INTO SAT...BUILDING SEAS TO 9 FT. $$ GR