000
AXPZ20 KNHC 181518
TWDEP 

TROPICAL WEATHER DISCUSSION 
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
1605 UTC MON MAY 18 2009

TROPICAL WEATHER DISCUSSION FOR THE EASTERN PACIFIC OCEAN FROM 
THE EQUATOR TO 32N...EAST OF 140W. 

BASED ON 0600 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 
0815 UTC.

...ITCZ...

THE INTERTROPICAL CONVERGENCE ZONE IS ALONG THE LINE...06N77W TO 
07N110W TO 06N130W TO 07N140W. SCATTERED MODERATE WITH ISOLATED 
STRONG CONVECTION IS WITHIN 75 NM OF THE POINT 05N88W AND ALSO 
THE POINT 08.5N112W. SCATTERED MODERATE WITH ISOLATED STRONG 
CONVECTION IS WITHIN 40 NM OF THE POINT 06N123W.

...DISCUSSION...

UPPER LEVELS
THE LARGE MIDDLE AND UPPER TROUGH/CYCLONIC CIRCULATION THAT HAS 
BEEN OVER MOST OF THE AREA WEST OF 115W IS BEGINNING TO CONTRACT 
NORTHWARD AND DRIFT EAST. AT THIS TIME THE CYCLONIC AREA IS EAST 
OF 135W TO MAINLAND MEXICO NORTH OF 20N AND EAST OF 130W TO A 
LINE FROM 20N110W TO THE ITCZ NEAR 125W SOUTH OF 20N. MOST OF 
THIS AREA IS DRY AND CLOUD FREE IN THE MIDDLE AND UPPER LEVELS 
INDICATING MODERATE TO STRONG MIDDLE AND UPPER LEVEL SUBSIDENCE. 
AN EXCEPTION IS NORTH OF 20N EAST OF 118W TO BAJA CALIFORNIA AND 
THE SOUTHERN HALF OF THE GULF OF CALIFORNIA WHERE UPPER LEVEL 
DIFFLUENCE IS CAUSING MULTI LEVEL CLOUDINESS WITH SOME EMBEDDED 
CONVECTION. AT THIS TINE THE CONVECTION IS SOUTH OF 22N. THE 
SYSTEM IS FORECAST TO CONTINUE TO CONTRACT NORTHWARD AND MOVE 
EAST MOVING OVER NORTHERN MEXICO AND THE SOUTHWEST UNITED STATES 
TUESDAY AND WEDNESDAY. A WEAK RIDGE WILL BUILD OVER THE AREA 
BEHIND THE TROUGH.

ANOTHER DEEP LAYER TROUGH IS OVER THE EASTERN PART OF THE AREA 
EAST OF 85W. THE AREA IS UNSTABLE WITH ENHANCED CONVECTION 
WITHIN THE ITCZ. AT THIS TIME THERE IS NO CONVECTION OVER PANAMA 
OR COSTA RICA BUT DAYTIME HEATING SHOULD PRODUCE CONVECTION 
OVER THESE LAND AREAS TODAY. THE TROUGH IS FORECAST TO MOVE EAST 
OVER NORTHERN SOUTH AMERICA TUESDAY.

BETWEEN THE TWO TROUGHS A DEEP LAYER RIDGE IS OBSERVED. THE 
RIDGE EXTENDS NORTH OVER MEXICO TO ABOUT 28N. THIS AREA IS 
DIFFLUENT ALOFT WITH AREAS OF MULTI LEVEL CLOUDS AND EMBEDDED 
CONVECTION MAINLY OVER THE OCEAN AT THIS TIME BUT CONVECTION 
WILL DEVELOP OVER THE MAINLAND DURING THE AFTERNOON AND EVENING 
HOURS BEFORE DIMINISHING DURING THE NIGHT.   

SURFACE
WEAK TROUGH IS ALONG A LINE FROM 31N125W TO 20N129W. THIS 
FEATURE IS FORECAST TO REMAIN NEARLY STATIONARY AND SLOWLY 
WEAKEN BECOMING DIFFUSE WEDNESDAY NIGHT. BESIDES WEAK AND 
VARIABLE DIRECTION SURFACE WINDS ASSOCIATED WITH THE FEATURE NO 
OTHER WEATHER IS ASSOCIATED WITH THE TROUGH. ELSEWHERE NORTH OF 
A LINE FROM SOUTHERN BAJA CALIFORNIA TO 12N140W A WEAK RIDGE IS 
ANALYZED. REMAINDER OF THE AREA IS ANALYZED AS WEAK TROUGH. 
SEVERAL TROUGHS ARE EMBEDDED WITHIN THE ITCZ ONE ALONG A LINE 
FROM 13N100W TO 05N102W. ANOTHER IS ALONG LINE FROM 16N115W TO 
05N119W. ANOTHER FROM 11N138W TO 06N141W. THESE TROUGH ARE 
MOVING LITTLE OR DRIFTING SLIGHTLY WESTWARD.
 
GAP WINDS...THE BROAD TROUGH OVER THE WESTERN CARIBBEAN AND THE 
EASTERN GULF OF MEXICO COUPLED WITH A RIDGE OVER MEXICO IS 
RESULTING IN N WINDS 20 TO 30 KT ACROSS THE ISTHMUS OF 
TEHUANTEPEC INTO THE GULF OF TEHUANTEPEC. THESE WINDS WILL 
CONTINUE THROUGH TUESDAY NIGHT THEN DIMINISH TO LESS THAN 20 KT 
BY THURSDAY.

$$
LL