Tropical Storm NANMADOL Advisory Mo, 19.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 16W (NANMADOL) WARNING NR
026//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 34.1N 131.1E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 51 NM WEST-SOUTHWEST OF IWAKUNI, JAPAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 28 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
NOW DOWNGRADED TO A STRONG TROPICAL STORM, NANMADOL HAS REACHED ITS
FURTHEST NORTHWARD POINT, JUST WEST OF SHIMONOSEKI AND HAS NOW
FINALLY MADE THE LONG-EXPECTED HARD RIGHT TURN TOWARDS THE
EAST-NORTHEAST. TRACK MOTION OVER THE PAST SIX HOURS IS 025 DEGREES
BUT ACTUALLY THE SYSTEM HAS TRACKED ALMOST DUE EAST IN THE LAST
THREE HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS
AN INCREASINGLY DISORGANIZED SYSTEM, WITH THE LOW-LEVEL CLOUD LINES
ON THE WESTERN SIDE STARTING TO PEAK OUT FROM UNDER THE UPPER-LEVEL
CIRRUS SHIELD. THE LOW LEVEL CIRCULATION CENTER (LLCC) IS BECOMING
INCREASINGLY DIFFICULT TO TEASE OUT, EVEN IN THE ANIMATED RADAR
DATA AND THE POSITIONING IS NOW MORE RELIANT UPON THE HIGH-DENSITY
SURFACE WIND OBSERVATIONS. THE COMBINATION OF THESE FACTORS LENT
HIGH CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY IS
ASSESSED WITH SOMEWHAT LOWER CONFIDENCE BASED ON A T3.5 RJTD DVORAK
INTENSITY ESTIMATE AND SURFACE PRESSURE READINGS OF 975 MB FROM
MULTIPLE SITES NEAR THE CENTER, WHICH EQUATES TO ABOUT 62 KNOTS
USING THE KNAFF-ZEHR WIND PRESSURE RELATIONSHIP. THE SYSTEM IS
CLEARLY BEING IMPACTED BY STRONG WESTERLY FLOW THROUGH THE COLUMN
AS WELL AS MID TO UPPER-LEVEL DRY AIR, AS INDICATED ON THE 0000Z
FUKUOKA SOUNDING, WHICH IS ERODING THE WESTERN HALF OF THE SYSTEM
AND PUSHING THE CONVECTION TO THE NORTHEAST OF THE CENTER. AND
WHILE THE SYSTEM IS BENEFITING FROM A BURST OF ROBUST POLEWARD
OUTFLOW, THE INCREASED SHEAR, AND TERRAIN INTERACTION IS LEADING TO
RAPID WEAKENING.
INITIAL WIND RADII BASIS: PERSISTENCE
CURRENT STEERING MECHANISM: NORTHWEST SIDE OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE EAST
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: NO CURRENT ESTIMATE
   RJTD: T3.5 - 55 KTS
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 15-20 KTS
   SST: 25-26 CELSIUS
   OUTFLOW: STRONG POLEWARD
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.
FORECAST DISCUSSION: BASED ON THE SATELLITE AND RADAR DATA, TS 16W
HAS ALREADY STARTED THE DECAPITATION PROCESS, WITH THE LOWER AND
UPPER-LEVELS ALREADY BECOMING DECOUPLED. MEANWHILE, THE FRONTAL
SYSTEM PUSHING OUT OF NORTHEAST ASIA HAS BEEN MOVING SOUTHWARD AT A
RATHER LEISURELY RATE, AND THIS ALLOWED TS 16W TO PUSH A BIT
FURTHER NORTHWARD BEFORE IT FINALLY TURNED TO THE EAST. BUT A BURST
OF COLD AIR PUSHING OUT OF SIBERIA IS PUSHING THE FRONT FORWARD MORE
QUICKLY NOW AND THE SOUTHERN EDGE OF THE FRONT SHOULD START TO
IMPINGE ON THE WESTERN SIDE OF THE TS 16WS CIRCULATION WITHIN THE
NEXT SIX HOURS OR SO. WHEN THIS OCCURS, HIGH-RESOLUTION MODELING
SUGGESTS THE DEVELOPMENT OF VERY STRONG, NEAR TYPHOON STRENGTH
WINDS, ALONG THE VERY TIGHT GRADIENT TO THE NORTH AND NORTHWEST OF
THE CENTER AS THE SYSTEM SLIDES TO THE NORTHEAST ALONG THE COAST OF
NORTHERN HONSHU. THIS BURST OF HIGH WINDS SHOULD BE RELATIVELY
SHORT-LIVED, AS THE GRADIENT WILL WEAKEN AS TS 16W ALSO WEAKENS AND
BY TAU 24, THE INTENSITY WILL BE DOWN TO 40 KNOTS AS THE SYSTEM
MAKES ANOTHER LANDFALL IN NORTHWEST HONSHU. THE SYSTEM WILL
DRAMATICALLY PICK UP SPEED TO ABOUT 30 KNOTS, OR ROUGHLY 50 PERCENT
OF THE 500MB FLOW, DRIVING NORTHEAST ALONG THE FRONTAL BOUNDARY AS
IT PUSHES TO THE EAST. THE SYSTEM WILL BEGIN EXTRATROPICAL
TRANSITION (ETT) WITHIN THE NEXT 12 HOURS AND WILL TRANSITION TO A
GALE-FORCE, COLD-CORE LOW EAST OF HOKKAIDO BY TAU 36.
MODEL DISCUSSION: THERE HAS BEEN NO CHANGE IN THE MODEL GUIDANCE
OVER THE PAST SEVERAL MODEL RUNS, WITH TWO DISTINCT GROUPINGS OF
MODELS CONTINUING TO BE REFLECTED IN THE PACKAGE, ONE FAST AND ONE
SLOW. THE ECMWF REMAINS THE EXTREME OUTLIER, INSISTING THAT THE
SYSTEM WILL DISSIPATE AND THE REMNANT CIRCULATION SLIDE TO THE
SOUTH OF TOKYO. THE FAST GROUP OF THE UKMET BASED MODELS MOVE THE
SYSTEM RAPIDLY NORTHEAST WITH A TRACK THAT REMAINS OVER THE SEA OF
JAPAN FOR THE BULK OF THE FORECAST PERIOD. AND THE SLOW GROUP OF
GFS, GFS ENSEMBLE, ECMWF ENSEMBLE AND NAVGEM DEVIATE SOUTH AFTER
TAU 12, AND TRACK THE CIRC CLOSER TO TOKYO THROUGH TAU 24, BEFORE
ENDING IN ROUGHLY THE SAME SPOT BY TAU 36. CONTINUING THE PREVIOUS
FORECAST TREND, THE JTWC FORECAST LIES ON THE SOUTHERN EDGE OF THE
FAST GROUP IN TERMS OF THE CROSS-TRACK POSITIONING, BUT MUCH SLOWER
THAN THE FAST GROUP SPEED OF ADVANCE. INTENSITY GUIDANCE IS IN
AGREEMENT ON RAPID WEAKENING AND THE JTWC FORECAST LIES AMONGST THE
PACK. HOWEVER, THE HWRF IS PICKING UP ON THE ENHANCED WINDS BROUGHT
ON BY THE FRONTAL BOUNDARY INTERACTION AND SHOWING A SLIGHT
INTENSIFICATION IN THE FIRST 12 HOURS. THIS IS CERTAINLY A
POSSIBILITY, WITH WINDS UP TO 60 KNOTS POSSIBLE OVER WATER TO THE
NORTHWEST OF THE CENTER IN THE NEXT 12 TO 18 HOURS.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM//
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