Advisories

WDPN33 PGTW 270900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (TWENTY) WARNING NR 
005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.7N 162.3E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 291 NM WEST-SOUTHWEST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: EASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM 20W WITH A HIGHLY COMPACT PARTIALLY EXPOSED LOW-LEVEL 
CIRCULATION CENTER (LLCC) WITH BUILDING DEEP CONVECTION SLIGHTLY
OFFSET TO THE NORTH OF THE CENTER. A 270218Z AMSR2 89 GHZ MICROWAVE
IMAGE REVEALED EXPOSED LOW-LEVEL CLOUDS TIGHTLY WRAPPING INTO THE
COMPACT CORE. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE
BASED ON THE AMSR2 IMAGE AND ANIMATED MSI. THE INITIAL INTENSITY OF
35 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY
DVORAK FIXES AND CIMSS INTENSITY ESTIMATES LISTED BELOW. THE AGENCY
DVORAK FIXES OF T2.0 ARE ESTIMATED TO BE SLIGHTLY UNDERESTIMATING
THE INTENSITY DUE TO THE SMALL SIZE OF THE SYSTEM. ENVIRONMENTAL
ANALYSIS INDICATES THAT 20W IS NOW IN A FAVORABLE ENVIRONMENT
CHARACTERIZED BY MODERATE POLEWARD OUTFLOW ALOFT, LOW (10-15 KTS)
VERTICAL WIND SHEAR, AND WARM (29-30 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A NEAR EQUATORIAL
RIDGE (NER) POSITIONED TO THE SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   PHFO: T2.0 - 30 KTS
   CIMSS SATCON: 37 KTS AT 270440Z
   CIMSS ADT: 34 KTS AT 270540Z
   CIMSS AIDT: 38 KTS AT 270540Z
   CIMSS D-PRINT: 27 KTS AT 270540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: BINARY INTERACTION WITH TROPICAL STORM 21W (ETAU)

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: EXTENDED THE FORECAST TO 96 HOURS AND
NOW CALLS FOR SUBTROPICAL TRANSITION RATHER THAN DISSIPATION OVER
WATER. ALSO, THE TRACK IS FURTHER EAST THAN THE PREVIOUS FORECAST.

FORECAST DISCUSSION: TS 20W IS FORECAST TO BEGIN TURNING 
NORTHEASTWARD WITHIN THE NEXT 24 HOURS AS THE STEERING MECHANISM
TRANSITIONS TO A SUBTROPICAL RIDGE (STR) POSITIONED TO THE
NORTHEAST. NEAR TAU 36, THE SYSTEM WILL TRACK MORE NORTHWARD AS IT
TRACKS ALONG THE WESTERN PERIPHERY OF THE STR THROUGH TAU 72. AFTER
TAU 72, AN UPPER-LEVEL TROUGH IS EXPECTED TO DIG IN FROM THE NORTH
AND PICK UP 20W, PULLING IT FURTHER POLEWARD. SUBTROPICAL TRANSITION 
IS FORECAST TO COMPLETE AT AROUND TAU 96 AFTER THE VORTEX PASSES THE 
26 C SST ISOTHERM AND INTERACTS WITH THE STRONG MID-LATITUDE TROUGH. 
HOWEVER, THERE IS A SEPARATE SCENARIO THAT IS STILL POSSIBLE. THE 
OTHER SCENARIO IS THAT 20W BECOMES ABSORBED BY THE LARGER 21W TO THE 
NORTHEAST. THIS SCENARIO IS MORE LIKELY IF TWO THINGS OCCUR. THE FIRST 
BEING THAT 20W WEAKENS CONSIDERABLY OVER THE NEXT 48 HOURS, AND THE 
OTHER IS IF THE TRACK SPEEDS OF 21W ARE SLOWER THAN ANTICIPATED. 
REGARDING INTENSITY, 20W IS FORECAST TO STEADILY INTENSIFY THROUGH TAU 
48 TO A PEAK OF 60 KTS. IF THE SYSTEM IS ABLE TO STABILIZE A CORE OF 
CONVECTION, THEN A HIGHER INTENSITY IS POSSIBLE DUE TO THE COMPACT 
NATURE OF THE WIND FIELD. AFTER TAU 48, WESTERLY SHEAR AND DRY AIR IS 
EXEPCTED TO INCREASE, CAUSING 20W TO WEAKEN AS IT TRACKS NORTHWARD. 

MODEL DISCUSSION: MODEL TRACK GUIDANCE CONTINUES TO BE IN POOR 
AGREEMENT WITH A CROSS-TRACK SPREAD OF 370 NM AT TAU 72. HOWEVER, A
MORE DISCERNIBLE TRACK DIFFERENCE BETWEEN 20W AND 21W IS MUCH MORE
APPARENT THIS MODEL RUN. THE ONLY TWO TRACKERS THAT JUMP TO 21W ARE
NOW NAVGEM AND GALWEM, MAKING UP THE FAR WESTERN OUTLIERS. ON THE 
OTHER HAND, THE EXPERIMENTAL GOOGLE DEEPMIND AND THE GFS ARE THE 
EASTERNMOST TRACKERS. THE JTWC TRACK FORECAST IS PLACED CLOSER TO THE 
GOOGLE DEEPMIND ENSEMBLE MEAN WITH OVERALL LOW CONFIDENCE. INTENSITY 
GUIDANCE IS NOW IN EXTREMELY POOR AGREEMENT WITH MULTIPLE RAPID 
INTENSIFICATION (RI) AIDS TRIGGERING THIS MODEL RUN. ADDITIONALLY, THE 
COAMPS-TC (GFS-BASED) SHOWS A PEAK INTENSITY OF 105 KTS, WHICH 
ACTUALLY ALIGNS WITH THE MULTITUDE OF RI AIDS. SHIPS OUTPUT HAS A 70 
PERCENT PROBABILITY OF AN INCREASE OF 55 KTS IN 48 HOURS AS WELL. 
HAFS-A AND GOOGLE DEEPMIND SHOWS A PEAK INTENSITY OF 55-60 KTS AND THE 
JTWC INTENSITY FORECAST IS PLACED CLOSER TO THIS DEPICTION WITH VERY 
LOW CONFIDENCE. THERE COULD BE MAJOR CHANGES TO THE INTENSITY FORECAST 
FOR 20W IF RI APPEARS MORE LIKELY THAN NOT. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: LOW
   INTENSITY 72-120 HR: LOW//
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