000
AXPZ20 KNHC 230928
TWDEP 

TROPICAL WEATHER DISCUSSION
NWS NATIONAL HURRICANE CENTER MIAMI FL
1005 UTC SUN SEP 23 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 CENTERED NEAR 15.3N 108.8W AT 23/0900 UTC 
MOVING NW OR 310 DEGREES AT 7 KT. ESTIMATED MINIMUM CENTRAL 
PRESSURE IS 1000 MB. MAXIMUM SUSTAINED WIND SPEED IS 45 KT GUSTS 
TO 55 KT. THE ATMOSPHERIC AND OCEANIC CONDITIONS ARE EXPECTED TO 
BE QUITE CONDUCIVE FOR MIRIAM TO GAIN MORE STRENGTH DURING THE 
NEXT COUPLE OF DAYS. THE OFFICIAL INTENSITY FORECAST CALLS FOR 
MIRIAM TO BECOME A HURRICANE BY TONIGHT OR EARLY MONDAY. THE 
STORM SHOULD CONTINUE MOVING NORTHWESTWARD FOR THE NEXT COUPLE 
OF DAYS AS IT REMAINS STEERED BY A MID-LEVEL RIDGE TO ITS 
NORTHEAST. MIRIAM HAS BECOME BETTER ORGANIZED OVERNIGHT WITH 
SMALL CENTRAL DENSE OVERCAST AND BANDING FEATURES MORE DISTINCT 
TO THE NORTHEAST AND SOUTHWEST OF THE CENTER. NUMEROUS MODERATE 
TO STRONG CONVECTION IS WITHIN 45 NM OF CENTER. NUMEROUS 
MODERATE TO SCATTERED STRONG CONVECTION CAN BE FOUND IN BANDS 
FROM 14N TO 17N BETWEEN 107W AND 111W. SEE LATEST NHC 
FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS MIATCMEP3/WTPZ23 KNHC 
FOR MORE DETAILS.

...TROPICAL WAVES...
A TROPICAL WAVE EXTENDS ALONG THE ISTHMUS OF TEHUANTEPEC INTO 
THE EPAC WATERS TO NEAR 10N94W. THIS FEATURE IS MOVING W AT 
10-15 KT. A WEAK LOW PRES OR UNDULATION IN THE MONSOON TROUGH IS 
FORECAST TO DEVELOP IN ABOUT 24-48 HOURS DUE TO THE INTERACTION 
BETWEEN THE WAVE AND THE TROUGH.  

...INTERTROPICAL CONVERGENCE ZONE/MONSOON TROUGH...
MONSOON TROUGH EXTENDS FROM 10N86W TO 09N95W TO 12N101W THEN 
RESUMES FROM 12N112W TO 09N130W. THE ITCZ CONTINUES FROM 09N130W 
TO BEYOND 09N140W. SCATTERED MODERATE CONVECTION IS NOTED WITHIN 
90 NM S OF ITCZ AXIS W OF 132W. A CLUSTER OF MODERATE TO STRONG 
CONVECTION COVERS THE AREA FROM 11N TO 13.5N BETWEEN 89W AND 
92W. ISOLATED MODERATE TO STRONG CONVECTION IS N OF 14N TO THE 
MEXICAN COAST BETWEEN 94W AND 102W.

...DISCUSSION...
ALOFT...AN UPPER TROUGH EXTENDS FROM THE CENTRAL CALIFORNIA 
COAST TO A CUT OFF LOW NEAR 26N142W...WITH SWLY WINDS OF 40-60 
KT TRANSPORTING ABUNDANT MID TO UPPER LEVEL MOISTURE ACROSS THE 
NW CORNER OF THE FORECAST AREA. A BROAD AND PERSISTENT UPPER 
TROUGH EXTENDS FROM SE LOUISIANA TO NORTHERN MEXICO...WITH BROAD 
RIDGING GENERALLY PREVAILING TO ITS S...EXTENDING E TO W BETWEEN 
90W AND 120W. THIS FLOW CONTINUES TO YIELD A LOW SHEAR AND 
DIVERGENT ENVIRONMENT...AND WILL REMAIN FAVORABLE FOR DEEP 
CONVECTION ACROSS THIS REGION TODAY. A NARROW RIDGE EXTENDS FROM 
AN ANTICYCLONE OVER THE ITCZ NEAR 09N150W TO NEAR 16N134W. AN 
INVERTED TROUGH ALONG 120W/130W S OF 18N CAN BE SEEN SEPARATING 
THE TWO RIDGES.  

AT LOW LEVELS...A 1025 MB HIGH PRES CENTERED N OF AREA NEAR 
34N136W EXTENDS A RIDGE ACROSS THE N WATERS TO NEAR 24N115W. AN 
AREA OF FRESH TRADE WINDS PERSISTS ON THE SOUTHERN PERIPHERY OF 
THE RIDGE FROM 10N TO 17N W OF 136W. A WINDSAT PASS CONFIRMED 
THE PRESENCE OF THESE WIND SPEEDS. TRADES WILL DIMINISH TO 20 KT 
OR LESS IN ABOUT 24 HOURS AS HIGH PRES SLIGHTLY WEAKENS AND A 
COLD FRONT APPROACHES AND THEN STALLS TO THE NW.  HOWEVER...NW 
SWELL GENERATED BEHIND THE FRONT WILL IMPACT THE NW WATERS TODAY 
AND MON...BUILDING SEAS TO AROUND 11 FT ACROSS FAR NW PORTIONS.  

GAP WINDS...A MODEST HIGH PRES RIDGE ACROSS EASTERN MEXICO AND 
THE W GULF OF MEXICO CONTINUES TO SUPPORT NLY GAP WINDS THROUGH 
THE GULF OF TEHUANTEPEC. COMPUTER MODELS SUGGEST N TO NE WINDS 
20-25 KT TO CONTINUE THROUGH AND DOWNWIND OF THE GULF DURING THE 
NEXT 48 HOURS...WITH SEAS BUILDING TO 9 FT LATE TONIGHT. 

$$
GR