Advisories

SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 16W (NANGKA) WARNING 
NR 002//
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
RMKS/
WDPN31 PGTW 122100
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.7N 146.4E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 278 NM EAST OF IWO TO
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 22 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
DEPRESSION (TD) 16W (NANGKA) WITH A LOOSELY ORGANIZED LOW LEVEL
CIRCULATION CENTER (LLCC) OBSCURED BY FLARING CONVECTION. 
ENVIRONMENTAL ANALYSIS REVEALS MARGINALLY FAVORABLE CONDITIONS WITH
WARM (28-29 C) SEA SURFACE TEMPERATURES (SSTS), LOW TO MODERATE
(15-20 KTS) VERTICAL WIND SHEAR (VWS), STRONG POLEWARD AND EQUATORWARD 
OUTFLOW ALOFT, AND DRY AIR ENTRAINMENT FROM THE NORTHWEST. THE EFFECTS 
OF VWS AND DRY AIR ENTRAINMENT ARE EVIDENT IN EIR AS ANY PUFFS OF 
CONVECTION STRUGGLE TO REMAIN ATTACHED TO THE QUICKLY MOVING LLCC. THE 
INCREASED TRACK SPEED NORTHEASTWARD HAS RESULTED IN A RELOCATION OF 
INITIAL POSITION, WHICH IS PLACED WITH MEDIUM CONFIDENCE BASED ON A 
COMBINATION OF EIR IMAGERY AND AGENCY FIXES. THE INITIAL INTENSITY IS 
ASSESSED WITH HIGH CONFIDENCE BASED ON THE AGREEMENT BETWEEN CIMSS 
OBJECTIVE ESTIMATES AND AGENCY DVORAK FIXES LISTED BELOW.

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

CURRENT STEERING MECHANISM: LOW-LEVEL MONSOONAL FLOW AND
NEAR-EQUATORIAL RIDGE (NER) CENTERED TO THE SOUTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T1.0 - 25 KTS
   CIMSS SATCON: 32 KTS AT 121710Z
   CIMSS ADT: 30 KTS AT 121730Z
   CIMSS AIDT: 39 KTS AT 121730Z
   CIMSS D-MINT: 30 KTS AT 121652Z
   CIMSS D-PRINT: 28 KTS AT 121730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: SIGNIFICANTLY INCREASED TRACK SPEED
FROM PREVIOUS FORECAST.

FORECAST DISCUSSION: TD 16W IS FORECAST TO CONTINUE TRACKING
NORTHEASTWARD AT AN INCREASED TRACK SPEED OF OVER 20 KTS AS THE
COMBINED STEERING INFLUENCE OF THE LOW-LEVEL WESTERLY MONSOONAL
FLOW AND A NEAR-EQUATORIAL RIDGE (NER) CENTERED TO THE SOUTHEAST
ACT UPON THE SYSTEM. BETWEEN TAUS 12 AND 24, THIS INCREASED TRACK
SPEED IS EXPECTED TO CONTRIBUTE IN PROPELLING TD 16W ON AN EASTWARD
TURN, AS THE NER BECOMES THE PRIMARY STEERING INFLUENCE FOR THE
REMAINDER OF THE FORECAST PERIOD. THERE REMAINS SOME UNCERTAINTY IN
THIS OUTCOME, AS THE SYSTEM IS EXPECTED TO REMAIN WEAK AND SHALLOW, 
WHICH WOULD ALLOW FOR THE POSSIBILITY OF THE LOW-LEVEL MONSOONAL FLOW 
TO REMAIN THE PRIMARY STEERING INFLUENCE AND CAUSE A POLEWARD TURN 
BETWEEN TAUS 12 AND 24 RATHER THAN AN EASTWARD TRACK. THIS SCENARIO IS 
NOT DEPICTED IN THE FORECAST, BUT DOES HAVE A LARGE ENOUGH AMOUNT OF 
MODEL SUPPORT TO REMAIN A POSSIBILITY. IN REGARDS TO INTENSITY, TD 16W 
IS FORECAST TO MAINTAIN AN INTENSITY OF 30 KTS UNTIL TAU 24 BEFORE 
BEGINNING TO EXPERIENCE INCREASED VWS AND DRY AIR ENTRAINMENT CAUSING 
TD 16W TO ELONGATE, WEAKEN, AND ULTIMATELY OPEN UP INTO A TROUGH BY 
TAU 48.

MODEL DISCUSSION: TRACK MODEL GUIDANCE IS IN POOR AGREEMENT AS  THE
PHYSICS-BASED AND AI MODELS DISPLAY A BIFURCATION SCENARIO BEGINNING 
BY TAU 24, RESULTING IN A CROSS TRACK SPREAD OF 470 NM AT TAU 48. ALL 
PHYSICS-BASED MODELS DEPICT TD 16W FOLLOWING THE STEERING INFLUENCE OF 
THE MONSOONAL FLOW AND TURNING POLEWARD AT TAU 24, WHILE ALL AI MODELS 
DEPICT AN EASTWARD TURN FOLLOWING THE INFLUENCE OF THE NER TO THE 
SOUTHEAST. ENSEMBLE MODEL GUIDANCE HAS ALSO DECREASED IN AGREEMENT, 
WITH SEVERAL ECENS MEMBERS SHIFTING SOUTHWARD AND A PORTION DEPICTING 
THE EASTWARD TURN IN AGREEMENT WITH THE AI MEMBERS, LENDING CONFIDENCE 
TO THE FORECAST. ULTIMATELY, THE OFFICIAL JTWC FORECAST TRACK IS 
PLACED WITH LOW CONFIDENCE BASED ON THE SPREAD, FOLLOWING THE GUIDANCE 
OF AI MODELS BASED ON THE GENERAL SHIFT OF ENSEMBLE GUIDANCE TOWARDS 
THEIR DEPICTED TRACK AND OBSERVED SUCCESS IN OTHER COMPETITIVE 
STEERING ENVIRONMENTS. REGARDING INTENSITY, MODEL GUIDANCE IS IN GOOD
AGREEMENT ON A GRADUAL WEAKENING TREND. THE OFFICIAL JTWC INTENSITY 
FORECAST IS ASSESSED WITH MEDIUM CONFIDENCE, CLOSELY IN LINE WITH THE 
MULTI-MODEL CONSENSUS BUT DEVIATING TO DISSIPATE BY TAU 48 AS THE BULK 
OF MODELS CONTINUE TO PICK UP ON THE STRONG LOW-LEVEL FLOW AFTER THE 
LLCC HAS WASHED OUT.

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