Advisories

WDPN33 PGTW 092100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 14W (CHAN-HOM) WARNING NR 
012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 33.2N 150.2E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 535 NM EAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS THE FULLY 
EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) OF TROPICAL STORM (TS) 14W 
(CHAN-HOM) WITH ONLY A FEW SMALL PUFFS OF FLARING CONVECTION CONFINED 
TO THE NORTHEAST QUADRANT. ENVIRONMENTAL CONDITIONS REMAIN MARGINALLY 
FAVORABLE AS THE UPPER LEVEL LOW SITUATED JUST TO THE NORTHEAST OF 14W 
CONTINUES TO CAUSE CONVERGENCE ALOFT, DRY AIR ENTRAINMENT ON THE 
WESTERN AND SOUTHERN FLANKS OF 14W, AND LOW (10-15 KTS) NORTHWESTERLY 
SHEAR, ALL EVIDENT IN EIR. THESE CONDITIONS ARE OFFSET ONLY MARGINALLY 
BY SSTS OF 26-27 C. THE INITIAL POSITION IS PLACED WITH HIGH 
CONFIDENCE BASED ON THE EXPOSED LLCC. THE INITIAL INTENSITY IS 
ASSESSED WITH MEDIUM CONFIDENCE HEDGING ABOVE AGENCY DVORAK FIXES AND 
CIMSS OBJECTIVE ESTIMATES LISTED BELOW AS THE DVORAK METHOD STRUGGLES 
WITHOUT PERSISTENT CONVECTION.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SOUTHWESTERN EXTENSION OF A SUBTROPICAL
RIDGE (STR) CENTERED TO THE NORTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.0 - 30 KTS
   CIMSS SATCON: 41 KTS AT 091800Z
   CIMSS ADT: 25 KTS AT 091800Z
   CIMSS AIDT: 29 KTS AT 091800Z
   CIMSS D-MINT: 33 KTS AT 091605Z
   CIMSS D-PRINT: 30 KTS AT 091800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 10-15 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: WEAK EASTWARD
   OTHER FACTORS: UPPER LEVEL LOW POSITIONED JUST TO THE NORTH OF
THE SYSTEM AND DRY AIR ENTRAINMENT

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: MEDIUM

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS 14W HAS BEGUN ITS NORTH-NORTHWESTWARD TRACK
AFTER COMPLETING A SMALL COUNTER-CLOCKWISE LOOP EARLIER, EVIDENT IN
EIR. IT IS EXPECTED TO CONTINUE NORTHWESTWARD UNDER THE STEERING
INFLUENCE OF THE SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTHEAST
NEAR THE ALEUTIAN ISLANDS. AT TAU 36 THIS STR IS EXPECTED TO BUILD
A WESTWARD EXTENSION LEADING TO A FORECAST WESTWARD TURN OF TS 14W
IN RESPONSE. THE SYSTEM IS FORECAST TO MAKE LANDFALL OVER CENTRAL
HONSHU JUST AFTER TAU 36 AND REEMERGE INTO THE SEA OF JAPAN BY TAU 48 
AS IT INCREASES IN TRACK SPEED. THIS INCREASE IN TRACK SPEED IS DUE TO 
THE MORE DEFINED STEERING PATTERN AS TS 14W CROSSES OVER JAPAN. IN 
REGARDS TO INTENSITY, TS 14W IS FORECAST TO EXPERIENCE A BRIEF 
INTENSIFICATION BETWEEN TAUS 12 AND 36 AS THE UPPER LEVEL LOW FILLS 
AND A SEPARATE UPPER LEVEL TROUGH TO THE NORTHWEST CAUSES INCREASED 
OUTFLOW. PRIOR TO LANDFALL, TS 14W IS THEN EXPECTED TO ENCOUNTER 
INCREASED DRY AIR ENTRAINMENT AND COOLING SSTS OFF THE COAST OF 
HONSHU, LEADING TO A WEAKENING TREND BEFORE IT BEGINS TERRAIN 
INTERACTION, FURTHER DECREASING INTENSITY BEFORE IT TRACKS INTO THE 
SEA OF JAPAN. INCREASED UNFAVORABLE CONDITIONS OF INCREASED DRY AIR 
AND STRONG UPPER LEVEL NORTHERLIES IN THE SEA OF JAPAN ARE EXPECTED TO 
BEGIN SUBTROPICAL TRANSITION (STT) BY TAU 60 AND COMPLETE STT AND 
DISSIPATE OVER WATER BY TAU 72, PRIOR TO LANDFALL OVER THE KOREAN 
PENINSULA. 

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN GOOD AGREEMENT IN THE
SHORT TERM WITH A CROSS TRACK SPREAD OF 55 NM JUST PRIOR TO THE 
FORECAST LANDFALL OVER JAPAN. ONCE IN THE SEA OF JAPAN IT EXPANDS
TO 100 NM BY TAU 72 WHILE ALONG TRACK SPREAD INCREASES AS STT BEGINS, 
LEADING TO AN ALONG TRACK SPREAD OF 140 NM BY TAU 72, EXCLUDING THE 
OUTLIER OF GALWEM WHICH TRACKS TO THE SOUTH. THE OVERALL PACKAGE OF 
BOTH DETERMINISTIC AND ENSEMBLE MODELS HAS BEEN MOVING SLIGHTLY SOUTH 
OVER TIME, BUT REMAIN IN GOOD AGREEMENT ON THE RESULTING TRACK. THE 
OFFICIAL JTWC FORECAST IS PLACED WITH HIGH CONFIDENCE, SLIGHTLY FASTER 
BUT IN LINE WITH THE MULTI-MODEL CONSENSUS AND IN GOOD AGREEMENT WITH 
THE AI CONSENSUS AN EXPERIMENTAL GOOGLE DEEPMIND. MODEL INTENSITY 
GUIDANCE IS IN GOOD AGREEMENT OF A BRIEF PERIOD OF INTENSIFICATION AND 
THEN WEAKENING TREND PRIOR TO LANDFALL, FOLLOWED BY A SHARP DECREASE 
ONCE TS 14W TRACKS OVER JAPAN WITH THE MAJORITY OF MODELS DEPICTING 
DISSIPATION NO LATER THAN TAU 72. THE OFFICIAL JTWC INTENSITY FORECAST 
IS ASSESSED WITH MEDIUM CONFIDENCE FOR THE REMAINDER OF THE FORECAST
PERIOD, CLOSELY IN LINE WITH THE MULTI-MODEL CONSENSUS.

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