Advisories

WDPN31 PGTW 190300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SEVENTEEN) 
WARNING NR 003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 8.0N 154.2E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 140 NM EAST-NORTHEAST OF CHUUK
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
DEPRESSION (TD) 17W CONTINUING TO CONSOLIDATE WITH PERSISTENT
CONVECTION OBSCURING THE LOW LEVEL CIRCULATION CENTER (LLCC).
ENVIRONMENTAL ANALYSIS REVEALS MARGINALLY FAVORABLE CONDITIONS WITH
WARM (30-31 C) SEA SURFACE TEMPERATURES (SSTS) AND STRONG RADIAL
OUTFLOW ALOFT BEING OFFSET BY 10-15 KTS OF VERTICAL WIND SHEAR
(VWS) AND AN AREA OF DRY AIR IN THE NORTHERN SEMICIRCLE, AS THE LOW
LEVEL CLOUD LINES WRAPPING IN FROM THE NORTH REMAIN EXPOSED IN MSI.
BOTH THE 89 GHZ AND 37 GHZ 182224Z GMI MICROWAVE IMAGES FURTHER
ILLUSTRATE LOWER LEVEL CLOUD LINES WRAPPING INTO AN AREA SITUATED
ON THE NORTHEASTERN PERIPHERY OF THE CONVECTION. ADDITIONALLY, AN
182300Z METOP-C ASCAT PASS DEPICTS AN ELONGATED LLCC AND A FIELD OF
25 KTS ON THE SOUTHERN SEMICIRCLE, WITH A SMALL AREA OF 30 KTS
EMBEDDED WITHIN THE SOUTHEAST QUADRANT. THE INITIAL POSITION IS
PLACED WITH MEDIUM CONFIDENCE BASED ON THE MICROWAVE IMAGERY, ASCAT
DATA, AND OBSCURED LLCC. THE INITIAL INTENSITY IS ASSESSED WITH
HIGH CONFIDENCE BASED ON THE ASCAT DATA, AGENCY DVORAK FIXES, AND
CIMSS OBJECTIVE ESTIMATES LISTED BELOW.

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

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT COMPRISED OF
NEAR-EQUATORIAL RIDGE (NER) CENTERED TO THE SOUTHEAST AND ELONGATED
LOW-LEVEL TROUGH THAT 17W IS EMBEDDED WITHIN.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   CIMSS ADT: 32 KTS AT 180010Z
   CIMSS AIDT: 32 KTS AT 180010Z
   CIMSS D-MINT: 29 KTS AT 182221Z
   CIMSS D-PRINT: 25 KTS AT 190010Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG RADIAL
   OTHER FACTORS: AREA OF DRY AIR TO THE NORTH

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TD 17W IS FORECAST TO TRACK GENERALLY
NORTH-NORTHWESTWARD THROUGHOUT THE REMAINDER OF THE FORECAST
PERIOD. HOWEVER, AS THE VORTEX CONTINUES TO CONSOLIDATE IN THE
SHORT-TERM, IT IS POSSIBLE FOR THE TRACK TO MAKE SMALL DEVIATIONS
FROM THE OVERALL MEAN MOTION VECTOR. THE AREA OF HIGHEST WINDS
LOCATED WITHIN THE SOUTHEASTERN QUADRANT ARE EXPECTED TO CONTINUE
WRAPPING CYCLONICALLY, LIKELY CAUSING THE INDUCED SOUTHWESTWARD
MOVEMENT. THE QUASI-STATIONARY MOVEMENT IS ALSO LIKELY DUE TO THE
WEAK STEERING ENVIRONMENT OF 17W, AS THE SHALLOW SYSTEM IS EMBEDDED
WITHIN AN ELONGATED TROUGH CAUSING WEAK WESTWARD MOTION, WHILE ALSO
BEING INFLUENCED BY A NEAR-EQUATORIAL RIDGE (NER) CENTERED TO
SOUTHEAST THAT IS EXPECTED TO BUILD AND CAUSE MORE POLEWARD MOTION
BY TAU 12. AFTER THIS POINT, 17W IS EXPECTED TO CONTINUE MOVING
POLEWARD TOWARDS A WEAKNESS IN STEERING AS THE EASTWARD EXTENSION
OF A SUBTROPICAL RIDGE (STR) CENTERED OVER SOUTHERN JAPAN WEAKENS
AND A WESTWARD EXTENSION OF AN STR CENTERED TO THE NORTHEAST
BUILDS, ULTIMATELY CAUSING A WESTWARD TURN BETWEEN TAUS 60 AND 72,
COINCIDING WITH THE TIME 17W IS FORECAST TO CROSS THE NORTHERN
MARIANAS. REGARDING INTENSITY, TD 17W IS FORECAST TO STEADILY
INTENSIFY AT ROUGHLY THE CLIMATOLOGICAL RATE TO TAU 36 AS IT
GRADUALLY MOVES INTO BETTER ENVIRONMENTAL CONDITIONS OF LOWER VWS
AND ENHANCED OUTFLOW ALOFT, WHILE UNDERGOING VORTEX SYMMETRIZATION.
RAPID INTENSIFICATION (RI) IS FORECAST TO OCCUR FROM TAU 36 TO 72,
WITH INTENSITY INCREASING AT LEAST 55 KTS DURING THIS TIME. AFTER
CROSSING THE NORTHERN MARIANAS ISLAND CHAIN, 17W IS EXPECTED TO
INTENSIFY MORE GRADUALLY AND THEN PLATEAU JUST BELOW SUPER TYPHOON
THRESHOLD BY THE END OF THE FORECAST PERIOD.

MODEL DISCUSSION: TRACK MODEL GUIDANCE IS IN GOOD AGREEMENT ON THE
OVERALL NORTH-NORTHWESTWARD TRACK FOR THE ENTIRETY OF THE FORECAST
PERIOD. THE JGSM AND ECMWF DETERMINISTIC TRACKS ARE OUTLIERS TO THE
EAST, PREDICTING A NORTHEASTWARD TRACK OUT TO TAU 12 BEFORE TURNING
TO TRACK NORTHWESTWARD. ENSEMBLE MEMBERS DEPICT HIGHER CROSS TRACK
SPREAD THAN THE DETERMINISTIC OR ENSEMBLE MEAN TRACKERS, WITH
ECENS, ECAIE, AND GEFS ALL CONTAINING MEMBERS THAT TAKE 17W AS FAR
SOUTH AS ROTA. THE OFFICIAL JTWC FORECAST TRACK IS PLACED WITH
MEDIUM CONFIDENCE FOR THE FORECAST PERIOD IN BETWEEN THE
MULTI-MODEL CONSENSUS AND EXPERIMENTAL GOOGLE DEEPMIND, WITH SOME
UNCERTAINTY DUE TO THE POSITION OF THE CONSOLIDATING LLCC AFFECTING
MODEL TRACKS IN THE LONG TERM OUT TO WHEN 17W IS EXPECTED TO PASS
OVER THE MARIANAS. MODEL INTENSITY GUIDANCE IS IN GOOD AGREEMENT ON
THE SHORT-TERM GRADUAL INTENSIFICATION AND SUBSEQUENT PERIOD OF RI
BEGINNING BY TAU 36. THE OFFICIAL JTWC INTENSITY FORECAST IS
ASSESSED WITH HIGH CONFIDENCE OUT TO TAU 72 CLOSELY IN LINE WITH
THE MULTI-MODEL CONSENSUS AND MEDIUM CONFIDENCE THEREAFTER,
DEVIATING ABOVE THE CONSENSUS.

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