WDPN31 PGTW 181500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SEVENTEEN) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 7.9N 154.9E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 180 NM EAST OF CHUUK
   MOVEMENT PAST 6 HOURS: WESTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
DEPRESSION 17W WITH FLARING CONVECTION OBSCURING THE ASYMMETRIC AND
ELONGATED LOW-LEVEL CIRCULATION CENTER (LLCC). AN 181001Z METOP-B
ASCAT PASS CONFIRMED THE ASYMMETRIC NATURE OF THE WIND FIELD. THE
DATA SHOWED A LARGE SWATH OF 25 KT WINDS WITHIN THE SOUTHERN
SEMICIRCLE AND WEAKER (10-15 KTS) WINDS WITHIN THE NORTHERN
SEMICIRCLE. THE INITIAL POSITION AND INTENSITY ARE BOTH PLACED WITH 
HIGH CONFIDENCE BASED ON THE ASCAT PASS. CONVECTIVE ACTIVITY HAS 
REMAINED CONFINED TO THE SOUTHERN PORTION OF THE CIRCULATION DUE TO A 
TONGUE OF DRY AIR TO THE NORTH ALONG WITH MODERATE NORTHEASTERLY 
VERTICAL WIND SHEAR. THE CURRENT ENVIRONMENT IS ANALYZED TO BE 
MARGINALLY FAVORABLE, CHARACTERIZED BY MODERATE EQUATORWARD OUTFLOW 
ALOFT AND WARM (29-30 C) SEA SURFACE TEMPERATURES OFFSET BY THE DRY 
AIR AND NORTHEASTERLY SHEAR.

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

CURRENT STEERING MECHANISM: WEAK STEERING PATTERN WITH AN ELONGATED
SUBTROPICAL RIDGE (STR) CURRENTLY SITUATED TO THE NORTH-NORTHWEST,
PREVENTING POLEWARD MOVEMENT

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.0 - 25 KTS
   KNES: T1.0 - 25 KTS
   CIMSS D-PRINT: 27 KTS AT 181300Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: SLIGHT DRY AIR ENTRAINMENT FROM THE NORTH

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TD 17W IS FORECAST TO TRACK SLOWLY
NORTH-NORTHWESTWARD OVER THE NEXT 24 HOURS AS THE SYSTEM REMAINS
WITH A WEAK STEERING PATTERN. NEAR TAU 24, THE UPPER-LEVEL LOW THAT
IS CURRENTLY CENTERED NEAR 36N 157E WILL BEGIN TO FILL AND ALLOW
FOR A SUBTROPICAL RIDGE (STR) TO BUILD IN BEHIND IT. THIS STR WILL
THEN EXTEND SOUTHWESTWARD AND BECOME THE PRIMARY STEERING MECHANISM
FOR 17W THROUGH THE REMAINDER OF THE FORECAST PERIOD. TRACK SPEEDS
ARE EXPECTED TO INCREASE IN THE NORTHWESTWARD DIRECTION STARTING
AROUND TAU 36 AS 17W TRACKS ALONG THE SOUTHWESTERN PERIPHERY OF THE
AFORMENTIONED STR. 17W IS FORECAST TO PASS ABOUT 140 NM NORTH OF
SAIPAN, CLOSE TO THE ISLAND OF PAGAN, AT AROUND TAU 96. AFTER TAU
96, THE TRACK IS EXPECTED TO TURN MORE WESTWARD AS THE STR
PROPAGATES TO THE WEST AND EXTENDS TO THE NORTH OF 17W. IN TERMS OF
INTENSITY, 17W IS FORECAST TO GRADUALLY INTENSIFY THROUGH TAU 24 AS
THE VORTEX STRUGGLES WITH ASYMMETRY AND NORTHEASTERLY SHEAR. AFTER
TAU 24, THE WIND FIELD WILL BECOME MORE SYMMETRIC AND NORTHEASTERLY
SHEAR WILL ABATE, ALLOWING FOR A QUICKER RATE OF INTENSIFICATION.
NEAR TAU 48, THE SYSTEM WILL BEGIN TO TAKE ADVANTAGE OF GREATLY
ENHANCED OUTFLOW AND VERY WARM (OVER 30 C) SEA SURFACE
TEMPERATURES. THIS WILL MARK THE BEGINNING OF A PERIOD OF RAPID
INTENSIFICATION (RI) THAT IS FORECAST TO CONTINUE THROUGH TAU 96.
THE JTWC FORECAST EXPLICITLY CALLS FOR RI FROM TAU 48 TO TAU 96
WITH AN INTENSITY OF 50 KTS INCREASING TO 115 KTS. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN AGREEMENT ON
THE NORTHWESTWARD TRACK WITH A COUPLE NOTABLE OUTLIERS. THE HAFS-A
HAS DEPICTED A TRACK MUCH FURTHER SOUTH THAN THE REMAINDER OF
GUIDANCE, SUGGESTING A TRACK BETWEEN TINIAN AND ROTA. ALL OTHER
MODELS SHOW A TRACK WELL NORTH OF SAIPAN. THE ECMWF DETERMINISTIC
IS THE NORTHERNMOST MODEL, TRACKING THE VORTEX NEARLY STRAIGHT
NORTHWARD THROUGH TAU 36 BEFORE MAKING A NORTHWESTWARD TURN. THE
JTWC TRACK FORECAST IS PLACED CLOSE TO THE EXPERIMENTAL GOOGLE
DEEPMIND ENSEMBLE MEAN AND EC-AIFS WITH OVERALL MEDIUM CONFIDENCE.
INTENSITY GUIDANCE GENERALLY AGREES ON GRADUAL INTENSIFICATION
THROUGH TAU 36 FOLLOWED BY A MORE RAPID RATE OF INTENSIFICATION
THROUGH TAU 96. MESOSCALE MODELS ALL SUGGEST AN INTENSITY OF 115
KTS OR HIGHER AT TAU 96 WITH HWRF BEING THE MOST GAGRESSIVE AT 130
KTS. THE JTWC INTENSITY FORECAST IS PLACED CLOSE TO THE CONSENSUS
THROUGH TAU 60 AND THEN IN LINE WITH THE MESOSCALE MODELS THROUGH
TAU 120 WITH MEDIUM CONFIDENCE. 

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