Advisories

WDPN32 PGTW 301500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (ETAU) WARNING 
NR 015//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 31.1N 166.0E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 709 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
COMPACT, PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH
MEAGER CONVECTIVE ACTIVITY FLARING ALONG THE SOUTHEAST PERIPHERY.
THIS TINY SURFACE CIRCULATION WAS CAPTURED BY A 300941Z ASCAT-C
PASS, WHICH MEASURED WINDS NEAR 30 KTS IN THE SOUTHEAST QUADRANT.
AN UPPER-LEVEL LOW PRESENTLY SITS OVERHEAD, SEVERELY RESTRICTING
OUTFLOW ALOFT. THE SYSTEM IS ALSO TRAVERSING MARGINALLY WARM SEA
SURFACE TEMPERATURES (SST) OF 26-27 DEGREES CELSIUS AND IS
SURROUNDED BY DRY AIR. THESE ENVIRONMENTAL FACTORS ARE UNFAVORABLE
FOR TROPICAL CYCLONE DEVELOPMENT. CONSEQUENTLY, THE CONVECTIVE
ACTIVITY HAS BEEN MINIMAL, LEAVING THE LLCC INCREASINGLY FRAGILE.
THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
PARTIALLY EXPOSED LLCC IN THE SHORTWAVE-INFRARED IMAGERY. THE
INITIAL INTENSITY HAS BEEN HELD AT 30 KTS WITH HIGH CONFIDENCE
BASED ON THE SCATTEROMETER DATA, SUPPORTED BY A BLEND OF AGENCY
SUBJECTIVE DVORAK FIXES AND OBJECTIVE ESTIMATES FROM CIMSS.

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

CURRENT STEERING MECHANISM: BINARY INTERACTION WITH TROPICAL STORM
(TS) 20W AND AN EXTENSIVE RIDGE POSITIONED OVER THE CENTRAL NORTH
PACIFIC

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T1.5 - 25 KTS
   CIMSS ADT: 29 KTS AT 301210Z
   CIMSS AIDT: 25 KTS AT 301210Z
   CIMSS D-MINT: 35 KTS AT 301038Z
   CIMSS D-PRINT: 30 KTS AT 301210Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 10-15 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: WEAK POLEWARD
   OTHER FACTORS: UPPER-LEVEL LOW OVERHEAD AND SURROUNDING DRY AIR

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: HAVING COMPLETED A 180-DEGREE SOUTHWARD LOOP
INDUCED BY THE FUJIWHARA EFFECT, TD 21W HAS BEGUN TO TURN POLEWARD
AS ITS DISTANCE FROM TS 20W INCREASES. THIS POLEWARD MOTION WILL
NOW BE DRIVEN BY THE STEERING INFLUENCE OF A LARGE ANTICYCLONE
POSITIONED OVER THE CENTRAL NORTH PACIFIC. ALTHOUGH THE LLCC IS
UNRAVELING, THE FORECAST HOLDS THE INTENSITY AT 30 KTS FOR THE NEXT
36 HOURS. THE CIRCULATION WILL BECOME EMBEDDED IN 25-30 KTS OF
SOUTHERLY LOW-LEVEL FLOW. THE EVENTUAL DEMISE OF THE TROPICAL
SYSTEM WILL BE DRIVEN BY TWO COMPETING, THOUGH NOT MUTUALLY
EXCLUSIVE, FACTORS. FIRST, THE POLEWARD TRACK WILL BRING THE SYSTEM
OVER PROGRESSIVELY COOLER SST AND INTO A MORE BAROCLINIC
ENVIRONMENT, CAUSING IT TO GRADUALLY ACQUIRE SUBTROPICAL
CHARACTERISTICS. THE SECOND FACTOR INVOLVES THE SURVIVABILITY OF
THE LLCC. THE FRAGILE CIRCULATION COULD DISSIPATE BEFORE
SUBTROPICAL TRANSITION IS COMPLETE. THE CURRENT FORECAST OF 36
HOURS IN DURATION MAY BE GENEROUS, AND IT IS QUITE POSSIBLE THAT
THE CIRCULATION COULD DEVOLVE INTO AN OPEN WAVE BY TAU 24 OR
SOONER.

MODEL DISCUSSION: FOR THE FIRST TIME, NUMEROUS MODELS LOSE TRACK OF
THIS WEAK, COMPACT CIRCULATION AND INADVERTENTLY LATCH ONTO THE
STRONG VORTEX OF TS 20W AFTER TAU 12. THEREFORE, THE JTWC FORECAST
TRACK IS PLACED ALONG A BLEND OF THE RELIABLE TRACKERS, HEDGED
TOWARD THE PREVIOUS FORECAST TRACK. AVAILABLE INTENSITY GUIDANCE IS
SIMILARLY SPARSE. THE JTWC INTENSITY FORECAST ALIGNS WITH THE
ESTABLISHED FORECAST PHILOSOPHY, POSITIONED CLOSEST TO THE SHIPS
STATISTICAL-DYNAMICAL MODEL AND THE COAMPS-TC SOLUTION INITIALIZED
WITH NAVGEM BOUNDARY CONDITIONS.

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