Advisories

WDPN33 PGTW 072100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 14W (CHAN-HOM) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 32.0N 158.1E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 959 NM EAST-SOUTHEAST OF MISAWA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 16 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A MOSTLY
EXPOSED LOW-LEVEL CIRCULATION CENTER WITH FLARING CONVECTION
LIMITED TO THE NORTHERN EDGE NEAR THE LLCC. THE LLCC AND WEAK
LOW-LEVEL CLOUD BANDS SURROUNDING IT HAVE SEPARATED FROM THE LARGER
EXTENSION OF CONVECTIVE ACTIVITY WHICH IS NOW DISPLACED WELL TO THE
SOUTHEAST. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE
BASED ON THE EXPOSED LLCC IN IR IMAGERY. THE INITIAL INTENSITY OF
30 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED PRIMARILY ON THE
CIMSS OBJECTIVE INTENSITY ESTIMATES AND THE PERSISTENT STRUCTURE
OBSERVED IN EIR. THE ENVIRONMENT IS CHARACTERIZED AS MARGINALLY
UNFAVORABLE WITH MODERATE VERTICAL WIND SHEAR (VWS), 26-27 C SEA
SURFACE TEMPERATURES, AND POOR OUTFLOW.

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

CURRENT STEERING MECHANISM: SOUTHERN FLANK OF A WESTWARD EXTENSION
OF THE SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   RCTP: T2.0 - 30 KTS
   CIMSS SATCON: 31 KTS AT 071810Z
   CIMSS ADT: 32 KTS AT 071810Z
   CIMSS AIDT: 32 KTS AT 071810Z
   CIMSS D-MINT: 29 KTS AT 071554Z
   CIMSS D-PRINT: 29 KTS AT 071830Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 15-20 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: WEAK POLEWARD
   OTHER FACTORS: SIGNIFICANT DRY AIR ENTRAINMENT

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   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: TROPICAL DEPRESSION (TD) 14W (CHAN-HOM) HAS
TRACKED WEST-SOUTHWESTWARD OVER THE PAST 6 HOURS AT A HEALTHY 16 KT
TRANSLATION SPEED UNDER THE STEERING INFLUENCE OF THE RIDGE
EXTENSION TO THE NORTHEAST. THE STEERING RIDGE WILL BEGIN WEAKENING
AND RETREAT TO THE EAST AROUND TAU 12 AS A SEPARATE STR BUILDS IN
FROM THE WEST. THE STEERING ENVIRONMENT WILL BECOME WEAK AS A
RESULT AND INDUCE A SLOWER NORTHWESTWARD TRACK SPEED BETWEEN TAU
24-48. AFTER TAU 48, THE RIDGE TO THE NORTHEAST IS EXPECTED TO
BUILD BACK WESTWARD AND REGAIN CONTROL OF THE STEERING ENVIRONMENT,
RESULTING IN A WEST-NORTHWESTWARD TRACK AS THE SYSTEM ACCELERATES
TOWARD HONSHU. THE SYSTEM WILL HAVE AN OPPORTUNITY TO CONSOLIDATE
AND INTENSIFY WHILE IT REMAINS IN THE WEAK STEERING ENVIRONMENT AND
REACH A PEAK OF BETWEEN TAU 60-72. AS 14W APPROACHES HONSHU IT
WILL ENCOUNTER COOLER SEA SURFACE TEMPERATURES AND A COMPLEX
MIDLATITUDE SYNOPTIC ENVIRONMENT THAT FAVORS SUBTROPIC TRANSITION
BEGINNING AROUND TAU 96 AND COMPLETING BY TAU 120.

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN OVERALL GOOD AGREEMENT
ABOUT THE WEST-NORTHWESTWARD TRAJECTORY AND MODULATIONS INDUCED BY
THE EVOLVING STEERING ENVIRONMENT. THE CROSS-TRACK SPREAD OF 125 NM
AT TAU 72 IS ATTRIBUTABLE TO THE COMPLEX STEERING ENVIRONMENT AND
MIDLATITUDE ENVIRONMENTAL INFLUENCE ON THE SYSTEM STRUCTURE.
CROSS-TRACK SPREAD INCREASES TO 320 NM BY TAU 120 FOR THE SAME
REASONS. THE JTWC TRACK FORECAST IS PLACED CLOSE TO THE MULTI-MODEL
CONSENSUS WITH MEDIUM CONFIDENCE THROUGH TAU 72 AND LOW CONFIDENCE
THEREAFTER DUE TO UNCERTAINTY ABOUT THE SUBTROPICAL TRANSITION
SCENARIO. MODEL INTENSITY GUIDANCE IS IN GOOD AGREEMENT ABOUT
GRADUAL INTENSIFICATION FOLLOWED BY A WEAKENING TREND AFTER TAU 72.
THE JTWC INTENSITY FORECAST IS PLACED NEAR THE CONSENSUS WITH
MEDIUM CONFIDENCE THROUGHOUT THE FORECAST PERIOD. 

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