Advisories

WDPN33 PGTW 082100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 14W (CHAN-HOM) WARNING 
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 32.7N 151.7E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 617 NM EAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM 14W (CHAN-HOM) WITH A FULLY EXPOSED LOW-LEVEL CIRCULATION
CENTER (LLCC) AND A BAND OF WEAK CONVECTION ALONG THE SOUTHERN
SEMICIRCLE. THE PLACEMENT OF THE UPPER-LEVEL LOW TO THE EAST
CONTINUES TO INDUCE SUBSIDENCE IN THAT AREA, HINDERING THE
DEVELOPMENT OF CONVECTION. DRY AIR, INDICATED BY A FIELD OF
STRATOCUMULUS CLOUDS, TO THE NORTH AND WEST OF THE SYSTEM CONTINUES
TO ENTRAIN INTO THE CORE OF THE VORTEX, WHICH IS FURTHER IMPEDING
THE DEVELOPMENT OF CENTRAL CONVECTION. A 081533Z AMSR2 36 GHZ
MICROWAVE IMAGE SHOWS A PARTIAL MICROWAVE EYE FEATURE WITH A
DISTINCT WEAKNESS IN THE WESTERN SEMICIRCLE. THE INITIAL POSITION
IS PLACED WITH HIGH CONFIDENCE BASED ON THE AMSR2 IMAGE AND THE
EXPOSED LLCC IN ANIMATED EIR IMAGERY. THE INITIAL INTENSITY OF 40
KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE HIGH SIDE OF
THE AGENCY DVORAK AND CIMSS INTENSITY ESTIMATES LISTED BELOW. THE
LACK OF CENTRAL CONVECTION IS LIKELY CAUSING THE CURRENT INTENSITY
TO BE UNDERESTIMATED BY THE DVORAK-BASED INTENSITY ESTIMATES.
ENVIRONMENTAL ANALYSIS INDICATES THAT 14W IS IN A MARGINALLY
FAVORABLE ENVIRONMENT CHARACTERIZED BY MODERATE EQUATORWARD OUTFLOW
ALOFT, LOW (10-15 KTS) VERTICAL WIND SHEAR, AND WARM (27-28 C) SEA
SURFACE TEMPERATURES OFFSET BY SIGNIFICANT DRY AIR ENTRAINMENT.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT WITH RIDGING
OVER SOUTHERN HONSHU EXTENDING EASTWARD AND A SUBTROPICAL RIDGE
(STR) POSITIONED TO THE NORTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 48 KTS AT 081730Z
   CIMSS ADT: 34 KTS AT 081730Z
   CIMSS AIDT: 32 KTS AT 081730Z
   CIMSS D-MINT: 37 KTS AT 081534Z
   CIMSS D-PRINT: 35 KTS AT 081730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT WITH A STRONG UPPER-LEVEL LOW
IN CLOSE PROXIMITY TO THE EAST

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 IS FORECAST TO TRACK SLOWLY WESTWARD
OVER THE NEXT 12 HOURS AS IT CONTINUES TO BE PLACED WITHIN A WEAK
STEERING ENVIRONMENT. AFTER TAU 12, THE TROUGH TO THE NORTH IS
EXPECTED TO PROPAGATE NORTHEASTWARD, AWAY FROM 14W, AND THE STR TO
THE NORTHEAST WILL BUILD BACK IN. BY TAU 36, THE STEERING PATTERN
WILL BECOME MORE DEFINED AS THE SYSTEM TRACKS ALONG THE
SOUTHWESTERN PERIPHERY OF THE STR WITH INCREASING TRACK SPEEDS.
AROUND TAU 60, RIDGING WILL EXTEND TO THE NORTH OF THE SYSTEM,
CAUSING THE TRAJECTORY TO FLATTEN TO A MORE WESTWARD TRACK AS IT
APPROACHES THE COAST OF HONSHU. LANDFALL IS FORECAST TO OCCUR JUST
AFTER TAU 60 NEAR SENDAI. 14W WILL THEN QUICKLY TRAVERSE OVER
HONSHU AND EMERGE WITHIN THE SEA OF JAPAN AROUND TAU 72 AND
CONTINUE ON A WESTWARD TRACK. SUBTROPICAL TRANSITION IS FORECAST TO
BEGIN ONCE THE SYSTEM EMERGES BACK OVER WATER AS A STRONG
UPPER-LEVEL CYCLONE ANCHORS OVER SOUTHERN HONSHU, INDUCING
SIGNIFICANT VERTICAL WIND SHEAR. BY TAU 96, 14W IS EXPECTED TO HAVE
SHALLOWED OUT AND DECOUPLED UNDER THE EFFECTS OF THE HOSTILE
UPPER-LEVEL ENVIRONMENT, MARKING THE COMPLETION OF SUBTROPICAL
TRANSITION. REGARDING INTENSITY, 14W IS FORECAST TO STEADILY
INTENSIFY OVER THE NEXT 36-48 HOURS AS POLEWARD OUTFLOW ALOFT
MARKEDLY IMPROVES AND THE CORE MOISTENS. NEAR TAU 60, JUST BEFORE
LANDFALL, 14W WILL EXPERIENCE ANOTHER BOUT OF DRY AIR ENTRAINMENT
ACCOMPANIED BY INCREASING EASTERLY VERTICAL WIND SHEAR. THIS IS
EXPECTED TO CAUSE THE SYSTEM TO SLIGHTLY WEAKEN RIGHT BEFORE
LANDFALL OCCURS. AFTER TAU 60, TERRAIN INTERACTION FOLLOWED BY
SHEAR AND DRY AIR IN THE SEA OF JAPAN WILL LEAD TO FURTHER
WEAKENING AS THE SHALLOW VORTEX TRACKS TOWARD THE EASTERN COAST OF
THE KOREAN PENINSULA.

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN MODERATE
AGREEMENT THROUGHOUT THE FORECAST PERIOD WITH A CROSS-TRACK SPREAD
OF 180 NM AT TAU 72 AND PERSISTS THROUGH TAU 96. JGSM MAKES UP THE
NORTHERNMOST TRACKER WHILE THE ECMWF ENSEMBLE MEAN IS THE
SOUTHERNMOST. ENSEMBLES CONTINUE TO SHOW QUITE A BIT OF SPREAD
BETWEEN INDIVIDUAL MEMBERS, HOWEVER, THE ENSEMBLE MEANS REMAIN
CONSISTENT WITH THE REST OF GUIDANCE. THE JTWC TRACK FORECAST IS
PLACED CLOSE TO THE MULTI-MODEL CONSENSUS WITH OVERALL MEDIUM
CONFIDENCE. INTENSITY GUIDANCE GENERALLY AGREES ON STEADY
INTENSIFICATION THROUGH TAU 48 FOLLOWED BY WEAKENING THEREAFTER
WITH NO SIGNIFICANT OUTLIERS PRESENT. AS A RESULT, THE JTWC
INTENSITY FORECAST FOLLOWS CLOSELY TO THE CONSENSUS WITH MEDIUM
CONFIDENCE. 

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