Tropical Storm NAMTHEUN Advisory 10月17日

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (NAMTHEUN) WARNING
NR 028//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 32.8N 166.7E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 810 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL STORM 23W, AFTER HAVING MADE A BRIEF EXCURSION TO TYPHOON
STRENGTH, IS NOW RAPIDLY WEAKENING. ANIMATED ENHANCED INFRARED
(EIR) SATELLITE IMAGERY SHOWS THAT THE DEEP CONVECTIVE STRUCTURE
HAS COLLAPSED, WITH A SHALLOW CONVECTIVE BANDS WRAPPING INTO A
FULLY-EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). A 161748Z GMI
37GHZ MICROWAVE IMAGE SHOWED A WEDGE OF DRIER AIR PUNCHING EASTWARD
ALONG THE SOUTHERN PERIPHERY OF THE SYSTEM CORE, WITH AN ELONGATED
AREA OF WEAK TO MODERATE CONVECTION ARRANGED IN A FRONTAL-LIKE
PATTERN, ALONG THE NORTHWEST SIDE OF THE SYSTEM. THE INITIAL
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC
AND THE PREVIOUSLY MENTIONED GMI MICROWAVE IMAGE. THE INITIAL
INTENSITY OF 55 KNOTS IS ASSESSED WITH MEDIUM CONFIDENCE, BASED ON
AGENCY CURRENT INTENSITY ESTIMATES, OBJECTIVE INTENSITY ESTIMATES
AS WELL AS EXTRAPOLATION OF A 161049Z ASCAT-A PASS. ENVIRONMENTAL
CONDITIONS ARE BECOMING INCREASINGLY HOSTILE TO THE SYSTEMS
SURVIVAL AS A TROPICAL CYCLONE. WHILE VWS IS CURRENTLY LOW TO
MODERATE, AND OUTFLOW REMAINS FAVORABLE, THE SYSTEM HAS MOVED INTO
AN AREA OF COOLER WATERS WITH SSTS BELOW 26C.
INITIAL WIND RADII BASIS: PERSISTENCE AND OBJECTIVE BEST TRACK
CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A NORTH-SOUTH
ORIENTED SUBTROPICAL RIDGE (STR) TO THE EAST.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T3.5 - 55 KTS
   RJTD: T3.0 - 45 KTS
   KNES: T3.5 - 55 KTS
   CIMSS SATCON: 62 KTS AT 161456Z
   CIMSS ADT: 57 KTS AT 161730Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 10-15 KTS
   SST: 24-25 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: WEAK UPPER-LEVEL LOW WEST OF THE SYSTEM IS
DISRUPTING UPPER-LEVEL OUTFLOW
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. HOWEVER, THE FORECAST
LENGTH HAS BEEN SHORTENED TO 36 HOURS.
FORECAST DISCUSSION: TROPICAL STORM 23W IS FORECAST TO TRACK NORTH
THEN NORTHEASTWARD THROUGH THE DURATION OF THE FORECAST TRACK,
ALONG THE NORTHWEST PERIPHERY OF THE DEEP STR TO THE EAST. THE
PRESENCE OF THE UPPER-LEVEL LOW JUST WEST OF THE CENTER IS INDUCING
A BIT OF UPPER-LEVEL CONVERGENCE, COMBINED WITH THE WEDGE OF DRY
AIR PUNCHING IN FROM THE WEST, HAS ALREADY STARTED TO TEAR THE
SYSTEM APART, AS EVIDENCED IN THE RAPIDLY DETERIORATING SATELLITE
DEPICTION. AS THE SYSTEM CONTINUES TO MOVE OVER COOLER WATERS,
ADDITIONAL RAPID WEAKENING IS EXPECTED. THE SYSTEM IS ALREADY
BEGINNING TO LOOK LIKE AN EXTRATROPICAL LOW, THOUGH ALL AVAILABLE
DATA, INCLUDING THERMAL CROSS-SECTIONS AND AUTOMATED AND SUBJECTIVE
PHASE-SPACE ANALYSES, INDICATE IT IS AT WORST BORDERLINE
SUBTROPICAL. AS THE SYSTEM CONTINUES TRACK NORTHWARD HOWEVER, IT
WILL QUICKLY BEGIN TO INTERACT WITH THE STRONG BAROCLINIC ZONE
ASSOCIATED WITH A VERY STRONG COLD FRONT RAPIDLY MOVING IN FROM THE
WEST, AND BEGIN EXTRA-TROPICAL TRANSITION (ETT) WITHIN THE NEXT 12
HOURS. ETT IS EXPECTED TO BE COMPLETE, WITH TS 23W TRANSITIONING TO
A GALE-FORCE MID-LATITUDE LOW, NO LATER THAN TAU 36, AND
POTENTIALLY AS EARLY AS TAU 24.
MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT FOR THE
DURATION OF THE FORECAST PERIOD, WITH AN 85NM SPREAD AT TAU 36,
LENDING HIGH CONFIDENCE TO THE JTWC FORECAST TRACK. INTENSITY
GUIDANCE IS ALSO IN GOOD AGREEMENT ON THE OVERALL WEAKENING
SCENARIO, LENDING HIGH CONFIDENCE TO THE JTWC FORECAST INTENSITY.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   INTENSITY 0 - 72 HR: HIGH//
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