000
AXPZ20 KNHC 171557
TWDEP 

TROPICAL WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
1605 UTC THU SEP 17 2009

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

BASED ON 1200 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH 
1430 UTC.

...SPECIAL FEATURE...

TROPICAL STORM MARTY WAS LOCATED NEAR 19.7N 113.1W AT 17/1500 
UTC...DRIFTING N-NW OR 335 DEG AT 2 KT. MAXIMUM SUSTAINED WIND 
SPEED WAS 40 KT WITH GUSTS TO 50 KT. ESTIMATED MINIMUM CENTRAL 
PRESSURE WAS 1003 MB. SEE LATEST NHC FORECAST/ADVISORY UNDER 
AWIPS/WMO HEADERS MIATCMEP1/WTPZ21KNHC FOR MORE DETAILS. THE 
CONVECTIVE PATTERN WITH MARTY WEAKENED OVERNIGHT...WITH CLOUD 
TOPS WARMING AND BECOMING ELONGATED IN NATURE. HOWEVER...IN THE 
PAST 3 HOURS...DEEP CONVECTION HAS BLOSSOMED OVER THE LOW LEVEL 
CENTER OF MARTY AND TOPS HAVE COOLED. MODEST SLY WIND SHEAR 
APPEARED TO HAVE IMPACTED MARTY OVERNIGHT...AND IS FORECAST TO 
INCREASE FROM THE SW WITHIN THE NEXT 24 HOURS. THIS WIND SHEAR 
AND A DRIER MORE STABLE AIRMASS LOCATED UPSTREAM IN THE 
ENVIRONMENT THAT MARTY IS EXPECTED TO MOVE INTO SHOULD INDUCE A 
WEAKENING TREND FOR THE STORM BY THIS EVENING OR TONIGHT...IF 
NOT SOONER. MARTY IS EXPECTED TO BEGIN MOVING SLOWLY 
NORTH-NORTHWESTWARD LATER TODAY AND TONIGHT AS A WEAK RIDGE 
STRENGTHENS EAST OF THE CYCLONE...AND THEN ACCELERATE AND MOVE 
MORE NW AND THEN W OVER THE NEXT SEVERAL DAYS.   

...TROPICAL WAVE...

A TROPICAL WAVE WAS ANALYZED ALONG 128W MOVING W 10 KT. THERE IS 
ONLY ISOLATED CONVECTION WAS NOTED NEAR THIS WEAK 
PERTURBATION. 

A TROPICAL WAVE WAS ANALYZED ACROSS THE FAR W CARIBBEAN ALONG 
83W AND EXTENDED S INTO THE EPAC INTO THE ITCZ THERE. ONLY 
CONVECTION WITHIN THE ITCZ WAS NOTED NEAR THIS WAVE AXIS.

...THE ITCZ...

THE INTERTROPICAL CONVERGENCE ZONE EXTENDED FROM THE SW 
CARIBBEAN ALONG 11N...W-SW ACROSS COSTA RICA TO 08N103W...THEN 
CONTINUED ANEW FROM 14N115W TO 12.5N127W TO 09N140W. ISOLATED TO 
SCATTERED MODERATE TO STRONG CONVECTION WAS WITHIN 120 NM OF THE 
AXIS E OF 97W...AND ALSO WITHIN 120 NM EITHER SIDE OF AXIS W OF 
117W. SCATTERED TO NUMEROUS MODERATE TO STRONG CONVECTION WAS 
OCCURRING ALONG AND OFFSHORE OF THE MEXICAN COAST WITHIN 200 NM
OF THE POINT 16N103W.

...DISCUSSION...

N OF 20N...A SURFACE RIDGE...ANCHORED ON A 1026 MB HIGH PRES 
NEAR 34N141W...HAS BECOME STATIONARY OVER THE FAR N AND NW 
WATERS COVERING ROUGHLY THE AREA N OF 20N W OF 120W. THIS REGION 
IS MARKED BY MODERATE TO STRONG SUBSIDENCE AND BROKEN TO 
OVERCAST STRATOCUMULUS CLOUDS WIDESPREAD ACROSS THE AREA. THIS 
RIDGE IS PRODUCING TRADE WINDS OF 20 KT AND SEAS TO 9 FT BETWEEN 
15N AND 20N W OF 125W. THE LEADING EDGE OF A COLD FRONT HAS 
STALLED ACROSS THE FAR NW CORNER OF THE AREA. A DEEP-LAYER 
TROUGH ACROSS THE FAR NE PACIFIC SUPPORTS THIS FRONTAL SYSTEM. 
THIS FRONT IS EXPECTED TO DISSIPATE E OF 130W TODAY WHILE THE W 
PORTION IS FORCED SW. WAVE HEIGHTS WILL BUILD INTO THE 8 TO 10 
FT RANGE IN W-NW SWELL TODAY OVER THESE NW WATERS.

S OF 20N...ASIDE FROM TROPICAL STORM MARTY...AND THE CONVECTION 
ALONG THE ITCZ...CONDITIONS ARE RELATIVELY QUIET OVER MOST OF 
THE AREA. A DIFFLUENT PATTERN ALOFT ASSOCIATED WITH THE UPPER 
LEVEL RIDGE EXTENDING FROM THE SRN GULF W TO MARTY WAS HELPING 
TO VENTILATE AND SUSTAIN A SEVERAL AREAS OF DEEP CONVECTION OVER 
THE EPAC WATERS S OF 20N AND E OF 105W. A CLUSTER OF MODERATE TO 
STRONG CONVECTION WAS N OF 13N AND E OF 105W TO THE PACIFIC 
COAST OF MEXICO...AND WAS ASSOCIATED WITH FRESH WLY FLOW 
CONVERGING ALONG A WEAK AND BROAD LOW LEVEL TROUGH. 

WAVE HEIGHTS ARE GENERALLY RUNNING AT 8 TO 9 FT OVER MUCH OF THE 
FORECAST AREA PRIMARILY IN NW SWELL. NEW PULSES OF S-SW LONG 
PERIOD SWELL WILL MOVE INTO THE WATERS DURING THE NEXT FEW DAYS 
AND MIX WITH W AND NW SWELL...AND RAISE WAVE HEIGHTS ABOVE 8 FT 
E OF 110W...TONIGHT THROUGH THE WEEKEND.

$$