Advisories

WDPN33 PGTW 102100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 14W (CHAN-HOM) WARNING NR 
016//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 36.8N 145.9E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 274 NM EAST-NORTHEAST OF NARITA AIRPORT
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 19 KTS
   SIGNIFICANT WAVE HEIGHT: 25 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
VIGOROUS LOW-LEVEL CIRCULATION ACCELERATING AND BEGINNING TO BEND
ITS TRAJECTORY MORE WESTWARD AS THE STORM PINWHEELS AROUND A DEEP
UPPER-LEVEL LOW CENTERED TO THE SOUTH. THIS UPPER LOW IS ALSO
TRANSPORTING DEEP-LAYER DRY AIR INTO THE SOUTHERN AND EASTERN
SEMICIRCLES OF THE CIRCULATION AND IMPOSING INCREASINGLY STRONG
EASTERLY VERTICAL SHEAR, NOW 20-25 KT. ACCORDINGLY, EROSION OF DEEP
CONVECTIVE COVERAGE HAS BEEN OBSERVED DURING THE PAST SIX HOURS,
EXACERBATED BY PASSAGE OVER COLDER WATERS OF 24 TO 25 DEGREES
CELSIUS SINCE AROUND 101300Z AFTER THE STORM MOVED NORTH OF THE
KUROSHIO CURRENT. THE REMAINING DEEP CONVECTION IS ORGANIZED INTO A
LARGE CURVED BAND ALONG THE WESTERN PERIPHERY OF THE CIRCULATION
JUST OFFSHORE OF HONSHU. UPPER-LEVEL OUTFLOW IS CONFINED PRIMARILY
TO THE WESTWARD DIRECTION. THE INITIAL INTENSITY IS SET AT 45 KT
BASED ON A BLEND OF AGENCY DVORAK AND OBJECTIVE SATELLITE-BASED
INTENSITY ESTIMATES AND EARLIER ASCAT DATA.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T3.0 - 45 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T2.5 - 35 KTS
   CIMSS D-MINT: 41 KTS AT 101631Z
   CIMSS D-PRINT: 40 KTS AT 101730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 24-25 CELSIUS
   OUTFLOW: MODERATE WESTWARD
   OTHER FACTORS: DEEP-LAYER DRY AIR MASS WRAPPING AROUND THE
SOUTHERN AND EASTERN SEMICIRCLES

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST NOW ANTICIPATES
DISSIPATION BY TAU 60 INSTEAD OF SUBTROPICAL TRANSITION.

FORECAST DISCUSSION: TROPICAL STORM 14W (CHAN-HOM) IS BEGINNING ITS
FINAL APPROACH LEG TOWARD MAINLAND JAPAN, ACCELERATING AND
BEGINNING TO TURN WESTWARD AROUND A DEEP UPPER-LEVEL LOW CENTERED
TO THE SOUTH. A WEAK UPPER-LEVEL TROUGH OVER THE SEA OF OKHOTSK IS
PULLING OUT TOWARDS THE NORTHEAST, ALLOWING THE SUBTROPICAL RIDGE
CURRENTLY CENTERED NORTHEAST OF 14W TO BUILD WESTWARD ACROSS THE
KURIL ISLANDS AND CONNECT WITH ANOTHER RIDGE OVER THE SEA OF JAPAN.
THIS WILL CEMENT A TURN WESTWARD INTO CENTRAL HONSHU BETWEEN TAU 12
AND 24, CONTINUING GENERALLY WESTWARD INTO THE SOUTHERN SEA OF
JAPAN BETWEEN TAU 36 AND 60. THE REMAINING UNCERTAINTY REGARDS HOW
HARD OF A PIVOT THE STORM WILL TAKE WHILE ROUNDING THE UPPER LOW TO
ITS SOUTH. ENSEMBLE GUIDANCE FROM THE ECMWF, AIFS, AND EXPERIMENTAL
GOOGLE DEEPMIND SYSTEMS CONTINUE TO DEPICT A WEST-SOUTHWESTWARD
MOTION IN THE FINAL HOURS BEFORE LANDFALL, WITH ABOUT HALF OF
MEMBERS ANGLING CHAN-HOM INTO CHIBA PREFECTURE. OTHER MODELS,
PARTICULARLY THE GFS AND GEFS, SHOW A FLATTER DUE WEST TRACK THAT
MAKES LANDFALL FARTHER NORTH IN IBARAKI PREFECTURE. THE OVERALL
GUIDANCE PACKAGE HAS SHIFTED SLIGHTLY NORTHWARD DUE TO THE STORM'S
STILL NORTHWESTWARD MOTION, AND THE JTWC FORECAST IS ABOUT 25 NM
NORTH OF THE PREVIOUS FORECAST AT THE POINT OF LANDFALL. THE
INTENSITY FORECAST IS DOMINATED BY OCEANIC EDDIES AND INTERACTION
OF 14W WITH THE UPPER LOW TO THE SOUTH. THE STORM HAS BEEN
TRAVERSING A COLD EDDY SINCE AROUND 101300Z, CONTRIBUTING TO
SHORT-TERM DEGRADATION OF DEEP CONVECTION. THE STORM WILL PASS OVER
A MUCH WARMER EDDY OF THE KUROSHIO CURRENT WITH SEA SURFACE
TEMPERATURES OF 27-28 DEGREES CELSIUS OVER THE NEXT SIX HOURS OR
SO, WHICH MAY ALLOW DEEP CONVECTION TO RECOVER SOMEWHAT. HOWEVER,
SIMULTANEOUSLY INCREASING VERTICAL SHEAR AND DRY ENTRAINMENT CAUSED
BY THE NEARBY UPPER LOW WILL LIKELY LIMIT OR PREVENT
INTENSIFICATION. DYNAMICAL MODEL GUIDANCE IS IN GOOD AGREEMENT ON
THIS POINT, SO WHILE SHORT-TERM FLUCTUATIONS IN INTENSITY ARE
POSSIBLE, THE JTWC FORECAST SHOWS NO SIGNIFICANT CHANGE IN
INTENSITY PRIOR TO LANDFALL IN JAPAN. THE STORM'S TRANSIT OVER THE
HIGH TERRAIN OF CENTRAL HONSHU WILL CAUSE 14W TO QUICKLY WEAKEN
BELOW TROPICAL STORM INTENSITY UPON EMERGENCE IN THE SOUTHERN SEA
OF JAPAN. STRONG SHEAR AND DRY AIR ASSOCIATED WITH THE UPPER LOW
SOUTHEAST OF THE STORM AT THIS POINT ARE EXPECTED TO BE HOSTILE TO
ANY REORGANIZATION OVER THE SEA OF JAPAN. MODEL GUIDANCE IS IN GOOD
AGREEMENT THAT 14W WILL BECOME ILL-DEFINED OVER TIME, ULTIMATELY
DISSIPATING BY AROUND TAU 60 SOMEWHERE EAST OF SOUTH KOREA.

MODEL DISCUSSION: THE JTWC TRACK FORECAST IS PLACED JUST SOUTH OF
AND SLIGHTLY SLOWER THAN THE MULTI-MODEL CONSENSUS, AND IS ABOUT 25
NM NORTH OF THE PREVIOUS FORECAST AT THE LANDFALL POINT IN HONSHU.
MODEL SPREAD GROWS BEYOND TAU 24 IN THE SOUTHERN SEA OF JAPAN AS
THE STORM BECOMES LESS WELL-DEFINED IN MOST MODEL PROJECTIONS. THE
JTWC INTENSITY FORECAST IS CLOSE TO THE CONSENSUS OF THE SUITE OF
STATISTICAL AND DYNAMICAL INTENSITY AIDS, WHICH ARE IN CLOSE
AGREEMENT AND LEND HIGH CONFIDENCE TO THE INTENSITY FORECAST.

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