Advisories

WDPN33 PGTW 261500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) WARNING 
NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
 
SUMMARY:
   INITIAL POSITION: 17.2N 161.3E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 327 NM WEST-SOUTHWEST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET
 
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
DISORGANIZED CLUSTER OF PERSISTENT CONVECTION OBSCURING A VERY
COMPACT LOW-LEVEL CIRCULATION CENTER. THE CONVECTIVE ACTIVITY HAS
GOOD POLEWARD VENTILATION AMIDST A LOW SHEAR ENVIRONMENT. A CRUCIAL
ASCAT-C PASS AT 261101Z REVEALED WINDS OF 30 KTS ASSOCIATED WITH A
MINIATURE LLCC. THE ENVIRONMENT IS FAVORABLE, CHARACTERIZED BY
10-15 KTS OF SOUTHERLY VERTICAL WIND SHEAR (VWS) AND VERY WARM SEA
SURFACE TEMPERATURES (SST) OF 30-31 DEGREES CELSIUS. THE INITIAL
POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
AFOREMENTIONED ASCAT IMAGE. THIS IMAGE IS ALSO THE BASIS FOR
RAISING THE INITIAL INTENSITY TO 30 KTS WITH HIGH CONFIDENCE. THE
30 KT ASSESSMENT IS CONSISTENT WITH THE T2.0 DVORAK FIXES FROM KNES
AND PHFO AND THE MAJORITY OF OBJECTIVE ESTIMATES FROM CIMSS.
 
INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).
 
CURRENT STEERING MECHANISM: NORTHWESTERN FLANK OF A LOW- TO
MID-LEVEL NEAR-EQUATORIAL RIDGE (NER)
 
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T1.5 - 25 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   PHFO: T2.0 - 30 KTS
   CIMSS ADT: 29 KTS AT 261210Z
   CIMSS AIDT: 31 KTS AT 261210Z
   CIMSS D-MINT: 35 KTS AT 260749Z
   CIMSS D-PRINT: 28 KTS AT 261140Z
 
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
 
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE
 
3. FORECAST REASONING.
 
SIGNIFICANT FORECAST CHANGES: THE JTWC FORECAST TRACK HAS SHIFTED
EASTWARD BY 40-50 NM FROM THAT OF THE PREVIOUS WARNING.
 
FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 20W IS MEANDERING IN
A WEAKENED STEERING ENVIRONMENT CAUSED BY THE CIRCULATION OF 91W
LOCATION ROUGHLY 450 NM TO THE EAST-NORTHEAST. BOTH OF THESE
CIRCULATIONS WILL BE PICKED UP BY AN EXTENSIVE LOW- TO MID-LEVEL
NER, INDUCING A NORTHEASTWARD TRAJECTORY OVER THE NEXT 24 HOURS.
AFTER THAT POINT, A SUBTROPICAL RIDGE CENTERED OVER THE DATELINE
WILL DRIVE THE SYSTEM NORTHWARD. THERE REMAINS SUBSTANTIAL FORECAST
UNCERTAINTY. TWO DISTINCT SCENARIOS ARE POSSIBLE. THE FIRST IS THAT
TD 20W WILL BECOME ABSORBED BY THE LARGER CIRCULATION OF 91W, WHICH
WOULD RESULT IN A SIGNIFICANTLY SHORTER FORECAST. THE SECOND IS
THAT ENOUGH SEPARATION BETWEEN THE TWO CIRCULATIONS WILL EXIST THAT
ALLOWS BOTH TO SURVIVE AS DISTINCT SYSTEMS. THE JTWC FORECAST
CURRENTLY FOLLOWS THE LATTER SCENARIO. ASSUMING THAT TD 20W WILL
SURVIVE THE INTERACTION WITH 91W, A SLOW TO GRADUAL DEVELOPMENT
TREND IS LIKELY IN LARGE PART DUE TO LOW VWS AND WARM SST THROUGH
TAU 48. THE SYSTEM WILL INTENSIFY MORE QUICKLY BETWEEN TAU 48 AND
TAU 72 ONCE ITS INNER CORE BECOMES BETTER ESTABLISHED. INCREASING
VWS AND SIGNIFICANTLY LOWER OCEAN HEAT CONTENT WILL LEAD TO GRADUAL
WEAKENING BEYOND TAU 72. THROUGH THE FORECAST PERIOD, TD 20W WILL
MAINTAIN A VERY COMPACT WIND FIELD. THE SIZE OF THE CIRCULATION
PRESENTS AN ADDITIONAL CHALLENGE THAT IS DIFFICULT TO PREDICT IN
ADVANCE. MESOSCALE PHENOMENA LIKE OUTFLOW BOUNDARIES, SUBSIDENCE,
AND MARGINAL INCREASES IN VWS AS A RESULT OF THE NEARBY 91W CAN
SUBSTANTIALLY INFLUENCE THE DEVELOPMENT TREND OF TD 20W.
 
MODEL DISCUSSION: THE TRACKER GUIDANCE IS IN GOOD AGREEMENT FOR THE
FIRST 24 HOURS, BECOMING POOR BEYOND TAU 48 DUE TO UNCERTAINTIES
ASSOCIATED WITH THE INTERACTION OF 91W, WHICH MAY ALSO BE
STRENGTHENING. IT IS DIFFICULT TO DISCERN WHICH MODELS PREDICT
ABSORPTION BY 91W AND WHICH DO NOT, GIVEN THAT THE MAJORITY SHOWS
SIMILAR TRACKS. NONETHELESS, THE JTWC FORECAST TRACK IS POSITIONED
NEAR THE MULTI-MODEL CONSENSUS (CONW) WITH LOW CONFIDENCE.
LIKEWISE, A LARGE SPREAD EXISTS AMONG THE INTENSITY AIDS, WITH THE
STATISTICAL-DYNAMICAL SHIPS MODEL PEAKING THE SYSTEM AT 70 KTS,
HAFS-A AT 60 KTS, COAMPS-TC AT 50 KTS, AND GFS AT 45 KTS.
INTERESTINGLY, THE SHIPS RAPID INTENSIFICATION (RI) INDEX, FRIA,
RIDE, AND THE CIMSS RI-AI SHOW ELEVATED POTENTIAL FOR RAPID
DEVELOPMENT. GIVEN THE SPREAD AND MIXED SIGNALS, THE JTWC INTENSITY
FORECAST IS SET ESSENTIALLY ON THE CONSENSUS WITH LOW CONFIDENCE.
 
FORECAST CONFIDENCE:
   TRACK 00-72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: LOW
   INTENSITY 72-120 HR: LOW//
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