000
AXPZ20 KNHC 171000
TWDEP 

TROPICAL WEATHER DISCUSSION 
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
1005 UTC WED DEC 17 2008

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 
0900 UTC.

...ITCZ...

INTERTROPICAL CONVERGENCE ZONE...05N77W TO 05N92W TO 07N107W TO 
08N118W TO 09N125W TO 07N140W. SCATTERED MODERATE FROM 08N TO 
13N BETWEEN 106W AND 111W. SCATTERED MODERATE FROM 06N TO 15N 
BETWEEN 116W AND 134W. SCATTERED MODERATE/ISOLATED STRONG WITHIN 
90 NM S OF THE AXIS BETWEEN 137W AND 146W.

...DISCUSSION...

AMPLIFICATION OF THE LONGWAVE UPPER TROUGH OFF THE CONUS W COAST 
CONTINUES AS IT ABSORBS A SECONDARY UPPER TROUGH FURTHER SOUTH. 
THE RESULTANT PHASING OF THESE TROUGHS IS ENHANCING OVERALL 
VORTICITY IN THE BASE OF THE LONGWAVE TROUGH. THIS PROCESS...IN 
TURN...IS QUICKLY ENHANCING STRONG LIFT DOWNSTREAM OF THE TROUGH 
AXIS...WHICH CURRENTLY EXTENDS FROM 17N135W TO 27N129W TO N OF 
THE FORECAST AREA. THE INTERACTION OF THE STRONG LIFT WITH A 
PRE-EXISTING DEVELOPING FRONTAL ZONE EXTENDING W OF THE S 
CALIFORNIA BORDER IS INDUCING STRONG CYCLOGENESIS OFF THE BAJA 
COAST...WITH A 1011 MB SURFACE LOW ANALYZED AT 17/0600 UTC NEAR 
31N120W. AN ATTENDANT SURFACE COLD FRONT IS DEVELOPING IN 
RESPONSE TO INCREASING COLD AIR ADVECTION W OF THE DEVELOPING 
LOW...WITH THE FRONT EXTENDING FROM THE LOW TO 26N127W TO 
28N139W. THIS LOW IS FORECAST TO DEEPEN RATHER QUICKLY OVER THE 
NEXT DAY AS IT IS EJECTED OVER THE WESTERN CONUS...WITH A 
DEEPENING RATE OF 0.4 TO 0.7 MB PER HOUR THROUGH 18/0000 UTC. 
THIS WILL INCUR A STRONG ISALLOBARIC AND AGEOSTROPHIC COMPONENT 
TO THE LOW LEVEL FLOW IN THE VICINITY OF THE LOW RESULTING IN 
STRONG SOUTHERLY AND SOUTHWESTERLY WINDS AHEAD OF THE FRONT. 
GLOBAL AND REGIONAL MODEL GUIDANCE SUGGESTS LIKELIHOOD OF 30 KT 
WINDS IN THIS FLOW AT 10-M...WITH 35 TO 40 KT WINDS IN THE 
LOWEST 30 MB ABOVE SEA LEVEL. DEEPER MIXING RESULTING FROM 
INCREASED INSTABILITY UNDER THE UPPER TROUGH SHOULD TRANSFER THE 
FASTER WINDS ALOFT TO THE SURFACE. ADDITIONAL ACCELERATION OF 
THIS FLOW WILL OCCUR IN THE GULF OF CALIFORNIA GIVEN THE NEARLY 
PARALLEL ORIENTATION OF THE FLOW TO THE LONG AXIS OF THE GULF. 
GIVEN THESE FACTORS...AND THE OVERALL UPWARD TREND AMONG RECENT 
NWP MODEL RUNS...HIGH ENOUGH CONFIDENCE IS PRESENT TO POST A 
GALE WARNING...ALBEIT A MARGINAL GALE...FOR FAR NE PORTIONS OF 
THE FORECAST AREA NEAR THE LOW. 

NORTHERLY WINDS OF 20 TO 30 KT ARE OCCURRING IN THE WAKE 
OF THIS COLD FRONT...AND IN THE WAKE OF A DISSIPATING COLD FRONT 
FARTHER SOUTH EXTENDING FROM 29N115W TO 22N125W TO 23N138W... 
WITH THE HIGHEST WINDS N OF 25N BASED ON AN ASCAT PASS NEAR 
17/0530 UTC. THESE WINDS WILL SPREAD SOUTHWARD WITH THE 
SOUTHWARD PROGRESSION OF THE FRONTS...BUT WILL LIKELY DECREASE 
WITH THE RELAXING SURFACE PRESSURE GRADIENT FROM THE DEPARTURE 
OF THE SURFACE LOW. THE NORTHERLY WINDS ARE RESULTING IN 
NORTHERLY SWELL CONTINUING TO ENTER THE AREA FROM THE 
NORTH...WITH SEAS OF UP TO 16 FT BASED ON WAVEWATCH GUIDANCE 
AND SHIPS REPORTING OVER 10 FT SEAS. THESE HIGHER SEAS WILL 
CONTINUE TO OVERSPREAD MUCH OF THE FORECAST AREA OVER THE NEXT 
COUPLE DAYS...THOUGH MAXIMUM WAVE HEIGHTS ARE FORECAST TO 
DECREASE WITH TIME.

NORTHEASTERLY TRADE WINDS OF 20 TO 25 KT CONTINUE ACROSS A LARGE 
PART OF THE AREA AHEAD OF THE COLD FRONTS BASED ON A RECENT 
QUIKSCAT PASS...WITH INCREASING WAVE HEIGHTS RESULTING FROM THE 
APPROACH OF NORTHERLY SWELL. THE AREA OF 20+ KT WINDS IS 
EXPECTED TO SHRINK OVER THE NEXT COUPLE DAYS IN RESPONSE TO THE 
APPROACH OF THE FRONTS DESPITE THE INCREASING WAVE HEIGHTS. 
ADDITIONALLY...NE WINDS OF 20 KT ARE FUNNELING INTO THE GULF OF 
PAPAGAYO AND AREAS IMMEDIATELY DOWNWIND BASED ON AN ASCAT PASS 
NEAR 17/0400 UTC. SIMILAR WINDS ARE EXPECTED TO CONTINUE OVER 
THE NEXT COUPLE DAYS. ADDITIONALLY...THE INCREASE PRESSURE 
GRADIENT RESULTING FROM SURFACE PRESSURE RISES OVER CENTRAL 
MEXICO IS BRINGING N TO NE WINDS OF 20 TO 25 KT TO THE GULF OF 
TEHUANTEPEC.

OTHERWISE...WATER VAPOR IMAGERY DEPICTS A LARGE POOL OF UPPER 
LEVEL MOISTURE OVER THE E PACIFIC E OF 134W...MAINLY ACROSS E 
PORTIONS OF THE AMPLIFIED UPPER LONGWAVE TROUGH DISCUSSED ABOVE. 
TO THE W...DEEP LAYER DRY AIR IS BEGINNING TO WRAP AROUND THE 
TROUGH...WHILE A HIGHLY AMPLIFIED UPPER RIDGE IS BRINGING 
SUBSIDENCE IN NORTHWESTERLY FLOW ALOFT W OF 136W...WITH SOMEWHAT 
OF AN OMEGA BLOCK PATTERN OVER MUCH OF THE CENTRAL AND E PACIFIC 
REGION. FARTHER TO THE SOUTH...MAINLY ZONAL FLOW IS PRESENT OVER 
THE ITCZ AXIS...WITH A WEST-TO-EAST UPPER RIDGE S OF 19N E OF 
115W PROVIDING AMPLE UPPER LEVEL DIVERGENCE FOR CONVECTION 
ASSOCIATED WITH THE ITCZ E OF 115W. ADDITIONALLY... LOW LEVEL 
CONVERGENCE ASSOCIATED WITH SURFACE TROUGHS EXTENDING FROM 
13N106W TO 7N110W AND FROM 13N130W TO 7N132W...BOTH OF WHICH ARE 
EMBEDDED WITHIN THE ITCZ...ARE ENHANCING CONVECTION IN THE 
VICINITY OF THE ITCZ.

$$
COHEN/CANGIALOSI/COBB