Advisories

WDPN31 PGTW 191500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 17W (SAUDEL) WARNING NR 
005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 9.4N 153.7E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 156 NM NORTHEAST OF CHUUK
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM 17W (SAUDEL) WITH A MUCH MORE DISORGANIZED APPEARANCE COMPARED 
TO 6 HOURS AGO. THE DEEP CONVECTION THAT WAS PREVIOUSLY NEAR THE 
CENTER HAS DISSIPATED, AND ONVECTIVE ACTIVITY IS CONFINED TO THE FAR 
SOUTHWESTERN PERIPHERY. THE LOW-LEVEL CIRCULATION CENTER (LLCC) ALSO 
APPEARS TO BE HIGHLY ELONGATED NOW, WITH POTENTIALLY MULTIPLE AREAS OF 
ENHANCED VORTICITY WITHIN THE LARGER-SCALE CIRCULATION. A 190757Z RCM-
2 SAR PASS REVEALED A RATHER COMPACT CORE WITH WINDS AS HIGH AS 50 KTS 
WITHIN A SMALL POCKET IN THE SOUTHERN SEMICIRCLE. HOWEVER, THAT 
COMPACT SURFACE CIRCULATION MUST HAVE QUICKLY WASHED OUT AFTERWARD 
BECAUSE THE EXPOSED LOW-LEVEL CLOUDS IN ANIMATED EIR AT AROUND 191200Z 
LOOK QUITE LOOSE WITH NO CLEARLY IDENTIFIABLE CENTER. THEREFORE THE 
INITIAL POSITION IS PLACED WITH VERY LOW CONFIDENCE. THE INITIAL
INTENSITY OF 35 KTS IS ALSO ASSESSED WITH LOW CONFIDENCE BASED ON THE 
AGENCY DVORAK FIXES AND CIMSS INTENSITY ESTIMATES BELOW. DESPITE THE 
STALL IN DEVELOPMENT, THE CURRENT ENVIRONMENT IS ANALYZED TO BE 
FAVORABLE FOR FURTHER DEVELOPMENT DUE TO STRONG EQUATORWARD OUTFLOW 
ALOFT, LOW (10-15 KTS) VERTICAL WIND SHEAR, AND WARM (29-30 C) SEA 
SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: PERSISTENCE FROM 190757Z RCM-2 SAR DATA 

CURRENT STEERING MECHANISM: WEAK STEERING PATTERN WITH A COMBINED
INFLUENCE OF THE SOUTHWESTERN EXTENT OF A SUBTROPICAL RIDGE (STR)
POSITIONED FAR TO THE NORTHEAST AND A NEAR EQUATORIAL RIDGE (NER)
TO THE SOUTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T2.0 - 30 KTS
   KNES: T2.5 - 35 KTS
   CIMSS ADT: 43 KTS AT 191140Z
   CIMSS AIDT: 37 KTS AT 191140Z
   CIMSS D-MINT: 34 KTS AT 190958Z
   CIMSS D-PRINT: 33 KTS AT 191140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG EQUATORWARD
   OTHER FACTORS: SLIGHT DRY AIR ENTRAINMENT FROM THE NORTH

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS 17W (SAUDEL) IS FORECAST TO TRACK NORTH-
NORTHWESTWARD AS THE SUBTROPICAL RIDGE (STR) POSITIONED SOUTH OF 
HONSHU WEAKENS AND RETREATS NORTHWARD, LETTING 17W CONTINUE GAINING 
LATITUDE. THE STR THAT IS CURRENTLY TO THE NORTHEAST OF 17W IS 
EXPECTED TO BUILD AND PROPAGATE WESTWARD, BECOMING THE PRIMARY 
STEERING MECHANISM THROUGHOUT THE REMAINDER OF THE FORECAST PERIOD. 
THE CPA TO SAIPAN HAS SLIGHTLY INCREASED COMPARED TO THE PREVIOUS 
FORECAST AND THE TIME OF CPA IS ABOUT 6 HOURS SOONER (NOW 212200Z). 
HOWEVER, THE UNCERTAINTY IN THE INITIAL POSITION MAY CAUSE THERE TO BE 
A SHIFT IN THE TRAJECTORY RELATIVE TO THE NORTHERN MARIANA ISLANDS. 
THE TRACK IS FORECAST TO BECOME MORE WEST-NORTHWESTWARD AFTER TAU 72 
AS RIDGING EXTENDS TO THE NORTH OF 17W. THE ENVIRONMENT IS FORECAST TO 
IMPROVE OVER THE NEXT 12 HOURS AS THE DRY AIR ENTRAINMENT FROM THE 
NORTH LESSENS, ALLOWING FOR THE CORE OF THE VORTEX TO MOISTEN. THE 
TIMING OF THE AXISYMMETRIZATION OF THE WIND FIELD IS CURRENTLY THE 
LARGEST LIMITING FACTOR FOR INTENSIFICATION IN THE NEAR TERM. ONCE THE 
SYMMETRIZATION COMPLETES, 17W IS FORECAST TO STEADILY INTENSIFY 
THROUGH TAU 36 TO AROUND 65 KTS. AFTER TAU 36, 17W IS FORECAST TO 
RAPIDLY INTENSIFY THROUGH TAU 72 AS THE SYSTEM TAKES ADVANTAGE OF 
STRONG DUAL CHANNEL OUTFLOW, VERY WARM (OVER 30 C) SEA SURFACE 
TEMPERATURES, AND HIGH OCEAN HEAT CONTENT (OHC) VALUES EAST OF THE 
MARIANAS. 17W IS THEN FORECAST TO FURTHER INTENSIFY TO A PEAK OF 
AROUND 125 KTS AT TAU 96 BEFORE SLIGHTLY WEAKENING THROUGH TAU 120 AS 
THE SYSTEM BEGINS TO ENTER AN AREA OF INCREASED NORTHERLY SHEAR AND 
LOWER OHC.

MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 72 
WITH NO NOTABLE OUTLIERS PRESENT. THE CROSS-TRACK SPREAD AT TAU 72 IS 
MEASURED TO BE ABOUT 100 NM. AFTER TAU 72, MODELS BEGIN TO FAN OUT 
CONSIDERABLY. GFS AND NAVGEM MAKE UP THE NORTHERNMOST MEMBERS WHILE 
THE ECMWF AND THE ECMWF ENSEMBLE MEAN MAKE UP THE SOUTHERNMOST. AS A 
RESULT, CROSS-TRACK SPREAD AT TAU 120 INCREASES TO 330 NM. THE JTWC 
TRACK FORECAST IS PLACED NEAR THE MULTI-MODEL CONSENSUS THROUGH TAU 72 
AND THEN CLOSER TO THE ECMWF AND EXPERIMENTAL GOOGLE DEEPMIND ENSEMBLE 
MEANS THROUGH TAU 120. INTENSITY GUIDANCE CONTINUES TO BE IN RATHER 
POOR AGREEMENT. COAMPS-TC AND ITS CORRESPONDING RAPID INTENSIFICATION 
(RI) AID (CTR-1) CONTINUE TO SHOW A MUCH MORE RAPID RATE OF 
INTENSIFICATION THROUGH THE FIRST 36-48 HOURS. ADDITIONALLY, GFS NOW 
SHOWS RAPID DEVELOPMENT THROUGH TAU 36, SUGGESTING AN INTENSITY OF 
OVER 85 KTS AT THAT TIME. CONSIDERING THE CURRENT DISORGANIZED NATURE 
OF THE SYSTEM, THE JTWC INTENSITY FORECAST FOLLOWS THE HAFS-A AND HWRF
DEPICTION THROUGH TAU 72 WITH MEDIUM CONFIDENCE. AFTER TAU 72, THERE 
IS EXTREME DISAGREEMENT IN INTENSITIES BETWEEN MODELS. ON ONE HAND, 
HAFS-A AND HWRF WEAKEN THE SYSTEM EXTREMELY QUICKLY; ON THE OTHER 
HAND, COAMPS-TC AND THE GOOGLE DEEPMIND SHOW ONLY A GRADUAL DECLINE 
THROUGH TAU 120. THE JTWC INTENSITY FORECAST IS SITUATED CLOSER TO THE 
LATTER GROUPING THROUGH TAU 120 WITH LOW CONFIDENCE.

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