WDPN31 PGTW 122100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SEVENTEEN) 
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 7.3N 139.0E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 142 NM SOUTH-SOUTHEAST OF YAP
   MOVEMENT PAST 6 HOURS: WESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A BROAD
REGION OF FLARING CONVECTION AROUND A PARTIALLY OBSCURED LOW LEVEL
CIRCULATION CENTER (LLCC). THE DEEPEST CONVECTION IS DISPLACED TO
THE WESTERN AND SOUTHERN QUADRANTS OF THE LLCC AS THE SYSTEM
CONTINUES TO BE IMPACTED BY EASTERLY OUTFLOW ALOFT. ADDITIONALLY,
MID-LEVEL DRY AIR UNDER THE SUBTROPICAL RIDGE (STR) TO THE NORTH
CONTINUES TO HINDER CONVECTION AROUND THE NORTHERN AND WESTERN
SEMI-CIRCLE. THE INITIAL POSITION IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON A 121842Z SSMIS 91 GHZ MICROWAVE IMAGE DEPICTS
FORMATIVE BANDING OVER THE WESTERN AND NORTHERN SEMICIRCLE WITH
SEVERAL SHALLOW BANDS ON THE EASTERN PERIPHERY OF THE LLCC. THE
INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
AVERAGE OF THE AGENCY DVORAK INTENSITY ESTIMATES RANGING BETWEEN
T1.5 TO T2.0. OVERALL, THE ENVIRONMENT IS ASSESSED AS MARGINAL,
CHARACTERIZED BY MODERATE EASTERLY DIFFLUENT OUTFLOW ALOFT,
MODERATE TO HIGH EASTERLY VERTICAL WIND SHEAR (VWS), WARM (30-31C)
SEA SURFACE TEMPERATURES (SSTS), AND DRY AIR IN THE MID-LEVELS.

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

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF THE DEEP-LAYER
SUBTROPICAL RIDGE TO THE NORTH-NORTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   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: OVER THE NEXT 48 HOURS, TD 17W (SEVENTEEN) IS
FORECAST TO TRACK SLOWLY WESTWARD DUE TO AN OVERALL WEAK STEERING
PATTERN DUE TO THE PRESENCE OF A MID-LEVEL TROUGH TO THE NORTH,
WHICH IS INFLUENCING THE STEERING GRADIENT OF A DEEP-LAYER RIDGE TO
THE NORTH-NORTHEAST. HOWEVER, AFTER TAU 48, THE RIDGE TO THE
NORTH-NORTHEAST IS EXPECTED TO RESTRENGTHEN, THUS, AS A RESULT, 17W
WILL ACCELERATE TO THE WEST-NORTHWEST. THE SYSTEM WILL CONTINUE TO
TRACK WEST-NORTHWEST THROUGH TAU 72, AFTER WHICH ANOTHER BREAK IN
THE STEERING RIDGE IS EXPECTED TO OCCUR, ALLOWING 17W TO START
TRACKING NORTHWESTWARD THROUGH THE END OF THE FORECAST PERIOD. 
IN TERMS OF INTENSITY, 17W WILL STRUGGLE TO DEVELOP THROUGH TAU 72
DUE TO THE OVERALL DISORGANIZED LOW-LEVEL STRUCTURE AND THE
PRESENCE OF DRY MID-LEVEL AIR AND MODERATE EASTERLY SHEAR. AFTER
TAU 72, THE SYSTEM IS EXPECTED TO EXPERIENCE GRADUAL
INTENSIFICATION AS IT MOVES INTO A MORE FAVORABLE ENVIRONMENT OF
SLIGHTLY LOWER SHEAR AND ENHANCED MID-LEVEL MOISTURE.   

MODEL DISCUSSION: GLOBAL DETERMINISTIC NUMERICAL MODELS AND
ENSEMBLES ALL SUGGEST THE SYSTEM WILL DRIFT SLOWLY WESTWARD THROUGH
TAU 48. FOLLOWED BY A MORE RAPID TRACK TO THE NORTHWEST THEREAFTER.
BY TAU 72, SIGNIFICANT CROSS TRACK SPREAD INCREASES TO 120 NM
SPREAD. THE MODELS THEN TURN THE SYSTEM TOWARD THE NORTHWEST
BETWEEN TAU 72 AND 96, BUT THE TIMING AND EXTENT OF THE TURN IS
UNCERTAIN. THE OVERALL TRACK AND INTENSITY FORECAST CONFIDENCE IS
MEDIUM THROUGH TAU 72 AND LOW BEYOND. THE INTENSITY GUIDANCE IS IN
OVERALL GOOD AGREEMENT WITH HWRF, HAFS AND GFS ALL SHOWING GRADUAL
INTENSIFICATION THROUGHOUT THE FORECAST PERIOD.  THEREFORE, THE
JTWC INTENSITY FORECAST FOLLOWS CLOSE TO THE MULTI-MODEL CONSENSUS
WITH MEDIUM CONFIDENCE.  

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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