000
AXPZ20 KNHC 200945
TWDEP 

TROPICAL WEATHER DISCUSSION
NWS NATIONAL HURRICANE CENTER MIAMI FL
1005 UTC THU SEP 19 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 
0945 UTC.

...SPECIAL FEATURES...

.A SURFACE LOW PRES HAS DEVELOPED ALONG THE MONSOON TROUGH NEAR 
12N100W 1010 MB. THE LOW IS EXPECTED TO MOVE NW AND INTENSIFY 
REACHING NEAR 12N103W TONIGHT AND NEAR 12N104W ON FRI NIGHT. 
TROPICAL CYCLONE FORMATION IS POSSIBLE.

...INTERTROPICAL CONVERGENCE ZONE/MONSOON TROUGH...

THE MONSOON TROUGH EXTENDS WESTWARD ACROSS THE SOUTHERN GULF OF 
PANAMA TURNING NW ALONG THE PACIFIC COAST OF NORTHWESTERN PANAMA 
AND ALONG COSTA RICA TO THE PACIFIC NEAR 12N94W...THEN TURNS SW 
THROUGH AN EMBEDDED 1008 MB LOW PRES AT 11N101W...THEN CONTINUES 
W TO 09N120W TO 09N130W WHERE LOW LEVEL WINDS INDICATE A 
TRANSITION TO AN ITCZ. THE ITCZ CONTINUES SW TO BEYOND 06N140W. 
ISOLATED MODERATE TO STRONG CONVECTION IS OBSERVED WITHIN 90 NM 
OF COAST OF COLOMBIA AND EASTERN PANAMA FROM 04N TO 07N...WITHIN 
60 NM OF THE CENTRAL AMERICAN COAST FROM 10N TO 13N AND FROM 15N 
TO 17N...AND A SMALL CLUSTER IS NOTED ALONG THE MEXICAN COAST AT 
21N. SCATTERED MODERATE ISOLATED STRONG CONVECTION IS OBSERVED 
WITHIN 90 NM EITHER SIDE OF A LINE FROM 11N100W TO 09N111W AND 
FROM 06N TO 12N BETWEEN 123W AND 134W. 
 
...DISCUSSION...

A LONGWAVE RIDGE DOMINATES THE EASTERN N PACIFIC AND THE WESTERN 
CONUS WITH AN UPPER ANTICYCLONE STATIONARY AT 24N117W...RIDGING 
N BEYOND 32N118W TO A CREST OVER S CENTRAL OREGON. AN UPPER 
RIDGE EXTENDS SE TO A CREST AT 22N107W AND A RIDGE ALSO EXTENDS 
SW TO A SHARP CREST AT 11N141W. SOME THIS UPPER MOISTURE IS 
SPREADING NE ACROSS THE SW COAST OF CALIFORNIA AND SOME IS 
SPREADING E ACROSS THE SOUTHERN HALF OF THE BAJA PENINSULA. A 
120 NM WIDE MOISTURE PLUME ORIGINATING FROM ITCZ CONVECTION W OF 
140W...IS SPREADING NE OVER THE DEEP TROPICAL PORTION FROM 
16N140W TO 25N133W. 

AN UPPER CYCLONE IS NOTED AT 17N120W WITH VERY DRY AIR NOTED 
WITHIN 240 NM OF THE CYCLONE CENTER. WEAK UPPER DIFFLUENCE S OF 
OF THE LARGE RIDGE IS SUPPORTING THE CONVECTION ALONG THE 
MONSOON TROUGH AND ITCZ BETWEEN 115W AND 135W AS PREVIOUSLY 
DESCRIBED. THE RESULTANT DEBRIS MOISTURE IS EVAPORATING NEAR THE 
CONVECTIVE CLUSTERS AS THE UPPER LEVELS ARE DRY FROM 22N TO 
14N BETWEEN 120W AND 137W.

A LONGWAVE TROUGH IS SHIFTING ACROSS EASTERN N AMERICA WITH AN 
EMBEDDED SHORTWAVE TROUGH MOVING THROUGH THE CENTRAL GULF OF 
MEXICO TO BASE AT 21N96W. AN UPPER ANTICYCLONE IS NOTED JUST S 
OF THIS UPPER TROUGH AT 15N105W WITH THE ENVELOPE OF UPPER 
ANTICYCLONIC FLOW DOMINATING THE TROPICS BETWEEN 85W AND 112W. 
CONVECTION IS ENHANCED UNDER THIS RIDGE AS PREVIOUSLY 
MENTIONED...WITH THE DEBRIS MOISTURE TRAPPED FOR THE MOST PART 
WITHIN THE RIDGE...EXCEPT A PLUME IS NOW SPREADING NE ACROSS THE 
GULF OF MEXICO AHEAD OF THE SHORTWAVE TROUGH. 
 
AN UPPER CYCLONE OVER THE W CARIBBEAN NEAR THE CAYMAN ISLANDS 
HAS AN UPPER TROUGH EXTENDING S TO OVER COSTA RICA. VERY DRY 
UPPER AIR IS INDICATED TO THE S OF 03N ACROSS THE ENTIRE 
DISCUSSION AREA EXTENDING NORTHWARD ACROSS THE EQUATOR FROM THE 
SOUTH PACIFIC WHICH APPEARS VERY DRY AS FAR S AS 13S TO THE E OF 
140W.   

ELSEWHERE AT THE LOW LEVELS THE REMNANTS OF TROPICAL CYCLONE 
LANE HAS WEAKENED TO AN OPEN TROUGH EXTENDING FROM NE TO SW FROM 
22N129W TO 18N133W WITH THE GRADIENT W OF THE TROUGH STILL 
MAINTAINING NE 20 TO 25 KT WINDS AND SEAS TO 10 FT. EXPECT THIS 
TROUGH TO GRADUALLY DISSIPATE OVER THE NEXT 48 HOURS WITH HIGH 
PRESSURE BUILDING FROM THE NW MAINTAINING AT LEAST A 20 KT NE 
TRADE FROM 12N TO 22N W OF 135W INTO THE WEEKEND.  

GAP WINDS...BUILDING HIGH PRESSURE IN THE WAKE OF A SURFACE 
FRONT THAT HAS STALLED DISSECTING THE EASTERN GULF OF MEXICO 
FROM NE-SW...IS SUPPORTING N WINDS AT 20 KT OVER THE BAY OF 
CAMPECHE. THESE WINDS ARE EXPECTED TO SURGE AT 20 TO 25 KT 
THROUGH THE GULF OF TEHUANTEPEC THIS MORNING...AND INCREASE TO 
20 TO 30 KT TONIGHT...AND THEN PULSE THROUGH THE WEEKEND WITH 
THE STRONGEST WINDS OCCURRING LATE AT NIGHT TILL A FEW HOURS 
AFTER SUNRISE EACH DAY.
 
$$
NELSON