Advisories

WDPN33 PGTW 270300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.7N 161.4E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 335 NM WEST-SOUTHWEST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A COMPACT AND 
FULLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) WHICH HAS 
UNEXPECTEDLY MOVED SOUTHEASTWARD OVER THE PAST SIX HOURS. DEEP 
CONVECTION HAS WANED OVER THE PAST SIX HOURS, AND BEEN PERSISTENTLY 
DISPLACED TO THE NORTH DUE TO MODERATE SOUTHERLY MID-LEVEL SHEAR. 
HOWEVER, IN THE LAST HOUR, DEEP CONVECTION HAS ONCE AGAIN BEGUN TO 
FORM VERY NEAR THE LLCC, PARTICULARLY FOCUSED ON THE NORTHERN FLANK. 
CIMSS DEEP-LAYER SHEAR ANALYSIS SHOWS LIGHT SOUTHERLY SHEAR, BUT MORE 
MODERATE MID-LEVEL SHEAR, WHICH SUPPORTS THE SATELLITE DEPICTION. THE 
INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE EXPOSED 
LLCC IN THE MSI. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM 
CONFIDENCE, BASED ON A BLEND OF THE OBJECTIVE AND SUBJECTIVE INTENSITY 
ESTIMATES NOTED BELOW. ENVIRONMENTAL CONDITIONS ARE MARGINAL, 
CHARACTERIZED BY WEAK OUTFLOW ALOFT, LOW DEEP-LAYER AND MODERATE MID-
LEVEL SHEAR AND WARM SSTS.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: WEAK STEERING PATTERN. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   PHFO: T2.0 - 30 KTS
   CIMSS ADT: 35 KTS AT 262340Z
   CIMSS AIDT: 36 KTS AT 262340Z
   CIMSS D-MINT: 24 KTS AT 261958Z
   CIMSS D-PRINT: 26 KTS AT 262340Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK POLEWARD
   OTHER FACTORS: BINARY INTERACTION WITH TD 21W. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN TRUNCATED TO 72
HOURS AND NOW CALLS FOR THE SYSTEM TO DISSIPATE BY MEANS OF ABSORPTION 
INTO TD 21W. 

FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 20W UNEXPECTEDLY TRACKED 
SOUTHEASTWARD OVER THE PAST SIX HOURS. WHILE THE SPECIFIC FORECAST DID 
NOT ANTICIPATE THIS TRACK VECTOR, THE 12Z AND 18Z ECMWF MODEL DID A 
GOOD JOB PICKING IT UP AND APPEARS TO HAVE INITIALIZED THE BEST OF ALL 
THE GLOBAL MODELS. GOING FORWARD, THE STEERING GRADIENT IS EXPECTED TO 
RAPIDLY SHIFT FROM THE WEST SIDE OF THE CIRCULATION TO THE SOUTHERN 
FLANK, ALONG THE NORTHERN EDGE OF A BAND OF STRONG SOUTHWESTERLY 
MONSOONAL WINDS WHICH EXTENDS FROM GUAM TO EAST OF TD 21W. TD 20W IS 
ALREADY STARTING TO TURN EASTWARD BASED ON THE LATEST MSI FRAMES, AND 
THIS TREND IS EXPECTED TO CONTINUE IN THE SHORT-TERM, THOUGH SOME 
ADDITIONAL ERRATIC MOTION IS LIKELY OVER THE NEXT FEW HOURS, 
ESPECIALLY IF THE RECENTLY DEVELOPED CONVECTION CAN STICK AND THE LLCC 
ATTEMPTS TO ALIGN. TD 20W IS FORECAST TO TURN GRADUALLY NORTHEAST, 
THEN NORTH AFTER TAU 24 AS IT RIDES UP THE WEST SIDE OF AN EXTENSION 
OF THE STR WHICH PUSHES SOUTHWARD ALONG THE DATELINE. AT THIS POINT, 
THE SYSTEM WILL BEGIN DIRECT BINARY INTERACTION WITH TD 21W, AND IS 
EXPECTED TO BE FULLY CAPTURED BY 21W BETWEEN TAU 36 AND TAU 48. ONCE 
CAPTURED, TD 20W WILL FOLLOW IN THE LEE OF 21W AND BEGIN TO EMBED 
WITHIN ITS WIND FIELD. REGARDING INTENSITY, 20W IS CURRENTLY 415NM 
SOUTHWEST OF TD 21W, FAR ENOUGH DISTANT THAT THE OUTFLOW ASSOCIATED 
WITH 21W IS NOT DIRECTLY IMPACTING 20W YET. MODEL FIELDS SUGGEST A 
REDUCTION IN SHEAR IN THE NEAR-TERM, WHICH SHOULD ALLOW FOR A PERIOD 
OF INTENSIFICATION AFTER TAU 12. TD 20W WILL HOWEVER STRUGGLE WITH 
LIMITED OUTFLOW ALOFT AND THE INFLUENCE OF DRY AIR AS WELL AS BURSTS 
OF OUTFLOW INDUCED SHEAR FROM 21W, WHICH WILL INHIBIT DEVELOPMENT. THE 
PEAK OF 45 KNOTS IS EXPECTED TO OCCUR AROUND TAU 48, BY WHICH POINT 
20W WILL BEGIN TO BE ABSORBED INTO 21W. THE SYSTEM QUOTE DISSIPATES 
UNQUOTE BY TAU 72 AS IT IS FULLY INTEGRATED INTO THE WIND FIELD OF 21W 
THOUGH IT IS EXPECTED TO RETAIN AN ASYMMETRIC WIND FIELD OF AT LEAST 
30 KNOTS AS IT DOES SO.


MODEL DISCUSSION: TRACK GUIDANCE IS IN POOR AGREEMENT, THOUGH THE 
EXACT VALUES OF CROSS-TRACK SPREAD BECOME DIFFICULT TO ASCERTAIN AFTER 
TAU 24, AS THE MODELS ARE BEGINNING TO JUMP CIRCULATIONS BETWEEN 20W 
AND 21W AS THEY UNDERGO BINARY INTERACTION, CAPTURE AND MERGER. IN 
GENERAL HOWEVER, MODEL SPREAD BETWEEN THE GFS AND NAVGEM OPENS UP TO 
AT LEAST 500NM BY TAU 72, WITH EVEN HIGHER SPREAD BETWEEN THE NAVGEM 
AND THE EC-AIFS. THE BULK OF THE GUIDANCE HOWEVER SUPPORTS A GRADUAL 
TURN NORTHWARD IN THE WAKE OF TD 21W, INCLUDING THE ECMWF, WHICH 
NOTABLY SHIFTED TRACK POLEWARD THOUGH THIS MAY BE AN ARTIFACT OF IT 
JUMPING CIRCULATIONS. THE JTWC FORECAST IS PLACED NEAR THE MULTI-MODEL 
CONSENSUS MEAN, WITH VERY LOW CONFIDENCE. INTENSITY GUIDANCE IS 
SUFFERING FROM THE SAME ISSUES OF CIRCULATION JUMPING AS THE TRACK 
GUIDANCE, WHICH RESULTS IN THE MODELS DEPICTING A SHARP 
INTENSIFICATION TREND, WHICH IS EXTREMELY UNLIKELY. THE HAFS-A SEEMS 
TO BE THE ONLY MODEL WITH A GOOD HANDLE ON THE OVERALL SITUATION, 
DEPICTING SOME INTENSIFICATION FOLLOWED BY A GRADUAL WEAKENING BEFORE 
IT TOO SUCCUMBS TO THE JUMPING TREND. THE JTWC FORECAST IS PLACED NEAR 
THE HAFS-A SOLUTION THROUGH TAU 48 THEN DEVIATES SHARPLY LOWER THAN 
ALL THE GUIDANCE MEMBERS, WITH LOW CONFIDENCE. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: LOW
   INTENSITY 00-72 HR: LOW//
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