Advisories

WDPN33 PGTW 090300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 14W (CHAN-HOM) WARNING 
NR 009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 32.4N 151.0E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 589 NM EAST-SOUTHEAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM (TS) 14W (CHAN-HOM) WITH A FULLY EXPOSED, SLIGHTLY ELONGATED
LOW LEVEL CIRCULATION CENTER (LLCC) AND NO PERSISTENT CONVECTION
BEYOND THE WEAK CLOUD BANDS WRAPPING INTO THE CENTER. ENVIRONMENTAL
ANALYSIS REVEALS MARGINALLY FAVORABLE CONDITIONS WITH WARM (27-28
C) SEA SURFACE TEMPERATURES AND LOW (10-15 KTS) VERTICAL WIND SHEAR
(10-15 KTS), OFFSET BY DRY AIR WRAPPING INTO THE CORE AS WELL AS AN
UPPER LEVEL LOW POSITIONED JUST TO THE NORTHEAST, RESULTING IN WEAK
OUTFLOW ALOFT. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE
BASED ON THE EXPOSED LLCC IN SATELLITE IMAGERY. THE INITIAL
INTENSITY IS ASSESSED WITH HIGH CONFIDENCE BASED ON A 082236Z
METOP-B ASCAT IMAGE DEPICTING THE EASTERN HALF OF THE SYSTEM WITH A
SWATH OF 40 KTS, ACCOUNTING FOR THE HISTORICALLY OBSERVED LOW BIAS
OF ASCAT SENSORS. OF NOTE, THE INITIAL INTENSITY IS ALSO HIGHER
THAN ALL CIMSS OBJECTIVE ESTIMATES AND DVORAK AGENCY FIXES LISTED
BELOW DUE TO THE LACK OF CENTRAL CONVECTION OF 14W, WHICH THE
DVORAK METHOD RELIES ON FOR THE HIGHEST ACCURACY IN FIX INTENSITY.

INITIAL WIND RADII BASIS: WIND RADII ADJUSTED BASED ON 082236Z
METOP-B ASCAT IMAGE

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT CAUSED DUE TO
A MID-LATITUDE TROUGH PASSING TO THE NORTH AND A BUILDING
SUBTROPICAL (STR) RIDGE CENTERED TO THE NORTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.5 - 35 KTS
   CIMSS ADT: 25 KTS AT 082340Z
   CIMSS AIDT: 31 KTS AT 082340Z
   CIMSS D-MINT: 37 KTS AT 090015Z
   CIMSS D-PRINT: 36 KTS AT 082340Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: WEAK EQUATORWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT AND STRONG UPPER LEVEL LOW
CENTERED CLOSELY TO THE NORTHEAST

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: ANIMATED SATELLITE IMAGERY DEPICTS 14W
BEGINNING ITS FORECAST SOUTHWESTWARD JOG AS IT REMAINS IN A WEAK
STEERING ENVIRONMENT BETWEEN A MID-LATITUDE TROUGH PASSING TO THE
NORTH AND A SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTHEAST. TS
14W IS EXPECTED TO BEGIN ITS POLEWARD TURN BEFORE TAU 12 AS THE
TROUGH PASSES TO THE NORTH AND TS 14W BECOMES PRIMARILY STEERED BY
THE STR, TRACKING IT NORTHWESTWARD TOWARDS LANDFALL OVER JAPAN JUST
PRIOR TO TAU 60. TS 14W IS EXPECTED TO TRACK OVER CENTRAL HONSHU
QUICKLY, REEMERGING OVER THE SEA OF JAPAN BEFORE TAU 72. THIS
COINCIDES WITH A MORE DIRECTLY WESTWARD TRACK AS THE STR CENTERED
TO THE NORTHEAST BUILDS AN EXTENSION WESTWARD, MAINTAINING STEERING
INFLUENCE OVER TS 14W. IN REGARDS TO INTENSITY, TS 14W IS FORECAST
TO UNDERGO A MODERATE INTENSIFICATION PERIOD OUT TO TAU 48 AS THE
UPPER LEVEL LOW BECOMES POSITIONED MORE NORTH OF THE SYSTEM,
ALLOWING FOR IMPROVED OUTFLOW ALOFT AND INCREASED MOISTURE TO THE
CORE OF THE SYSTEM. AFTER THIS POINT, TS 14W WILL ENCOUNTER
INCREASED VWS FOLLOWED BY TERRAIN INTERACTION AND INCREASED DRY AIR
ONCE IT REACHES THE SEA OF JAPAN. SUBTROPICAL TRANSITION (STT) IS
FORECAST TO BEGIN BY TAU 72 AS TS 14W INCREASES IN TRACK SPEED AND
DECOUPLES DUE TO THE INCREASED VWS AND DRY AIR WITHIN THE SEA OF
JAPAN, COMPLETING STT BY TAU 96.

MODEL DISCUSSION: TRACK MODEL GUIDANCE IS IN FAIR AGREEMENT
THROUGHOUT THE FORECAST PERIOD WITH NAVGEM BEING AN OUTLIER FAR TO
THE SOUTH AND JGSM AN OUTLIER TO THE NORTH. REGARDING ENSEMBLES,
ECMWF AND THE EC AI BASED ENSEMBLE MEMBERS DEPICT THE LARGEST
SPREAD OF TRACK GUIDANCE, WHILE GOOGLE DEEPMIND AND TO A LESSER
EXTENT, GEFS, ARE IN TIGHTER AGREEMENT OF THE LOCATION OF THE
LANDFALL OF TS 14W. THE OFFICIAL JTWC FORECAST IS PLACED WITH
MEDIUM CONFIDENCE FOR THE REMAINDER OF THE FORECAST PERIOD, CLOSELY
IN LINE WITH THE MULTI-MODEL CONSENSUS EXCLUDING NAVGEM AND THE
EXPERIMENTAL GOOGLE DEEPMIND. REGARDING INTENSITY, MODEL GUIDANCE
IS IN FAIR AGREEMENT WITH THE EXCEPTIONS OF COAMPS-TC BOTH GFS AND
NAVGEM BASED WHICH DEPICT A STEEPER DROP IN INTENSITY, DISSIPATING
TS 14W BY TAU 60. THE REMAINDER OF MODEL GUIDANCE AGREES ON THE
SHORT TERM INTENSIFICATION FOLLOWED BY A STEEPER DECREASE IN
INTENSITY AS TS 14W MAKES LANDFALL OVER JAPAN AND BEGINS STT. 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: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
NNNN