Advisories

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 WITHIN 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 
AFOREMENTIONED 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 OF 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 AGRESSIVE 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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