Tropical Storm NAMTHEUN Advisory 17.10.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (NAMTHEUN) WARNING
NR 029//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 33.8N 167.3E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 871 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL STORM 23W HAS RAPIDLY DETERIORATED OVER THE PAST SIX
HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A
SYMMETRICAL, WELL DEFINED LOW LEVEL CIRCULATION CENTER (LLCC),
WHICH IS DEVOID OF ANY SIGNIFICANT CONVECTION. AMSU THERMAL
CROSS-SECTION ANALYSIS REVEALS A NEAR-SURFACE WARM ANOMALY AND A
STRONG COLD ANOMALY IN THE MID-TROPOSPHERE, INDICATING THAT TS 23W
IS RAPIDLY TRANSITIONING INTO A SUBTROPICAL SYSTEM. OVERALL
ENVIRONMENTAL CONDITIONS ARE UNFAVORABLE FOR TROPICAL CYCLONE
DEVELOPMENT, WITH COOL (24-25C) SSTS, CONVERGENT FLOW ALOFT IN THE
SOUTHERN PORTION OF THE CIRCULATION, AND DRY AIR ENCROACHING UPON
THE SYSTEM CORE ALONG THE SOUTHERN HALF OF THE CIRCULATION AS WELL
AS DRY AIR ABOVE 400MB. THE INITIAL POSITION IS ASSESSED WITH HIGH
CONFIDENCE BASED ON THE EXPOSED LLCC IN ANIMATED MSI. THE INITIAL
INTENSITY OF 35 KNOTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON A
162145Z AND 162214Z ASCAT-A AND ASCAT-B PASSES RESPECTIVELY, WHICH
INDICATED MAXIMUM WINDS OF 35 KNOTS, WHICH WERE ISOLATED TO THE
SOUTHEASTERN QUADRANT OF THE SYSTEM.
INITIAL WIND RADII BASIS: SCATTEROMETER DATA
CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF LOW TO MID-LEVEL
SUBTROPICAL RIDGE (STR) TO THE EAST.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T3.0 - 45 KTS
   RJTD: T2.5 - 35 KTS
   KNES: T3.0 - 45 KTS
   CIMSS SATCON: 49 KTS AT 162024Z
   CIMSS ADT: 34 KTS AT 162340Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 10-15 KTS
   SST: 24-25 CELSIUS
   OUTFLOW: WEAK EASTWARD
   OTHER FACTORS: ENTRAINMENT OF COOLER, DRIER AIR INTO THE CORE OF
THE SYSTEM. DRY AIR ALOFT AND UPPER-LEVEL CONVERGENCE IN THE
SOUTHERN PORTION OF THE CIRCULATION.
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: HIGH
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN TRUNCATED TO 24
HOURS.
FORECAST DISCUSSION: TROPICAL STORM 23W IS FORECAST TO CONTINUE
TRACKING GENERALLY NORTH-NORTHEAST ALONG THE WESTERN PERIPHERY OF A
DEEP STR TO THE SOUTHEAST THROUGH THE DURATION OF THE FORECAST
PERIOD. WHILE THE TRACK IS NOT IN QUESTION, THE DEMISE OF TROPICAL
STORM 23W HAS BECOME A RACE, TO SEE WHETHER IT TRANSITIONS TO AN
EXTRA-TROPICAL LOW OR DISSIPATES BEFORE FULLY TRANSITIONING. THROW
IN A SUBTROPICAL TRANSITION IN THE NEAR-TERM, AND THE OVERALL
SCENARIO HAS BECOME UNEXPECTEDLY COMPLEX. WHILE TS 23W HAS AT LEAST
STARTED SUBTROPICAL TRANSITION, IT IS NOT FULLY SUBTROPICAL AT
ANALYSIS TIME. IT IS POSSIBLE THAT IT WILL COMPLETE THIS TRANSITION
WITHIN THE NEXT 12 HOURS. ITS TIME AS A SUBTROPICAL SYSTEM WILL BE
SHORT-LIVED HOWEVER SINCE THE SYSTEM IS EXPECTED TO BEGIN
INTERACTING WITH THE BAROCLINIC ZONE RAPIDLY APPROACHING FROM THE
NORTH AND WEST AND THE ASSOCIATED MID-LATITUDE JET STREAM, AND WILL
ULTIMATELY TRANSITION TO AN EXTRA-TROPICAL SYSTEM NO LATER THAN TAU
24. DUE TO THE PRESENCE OF DRY AIR ALOFT, COOL SSTS AND CONVERGENT
FLOW ALOFT, THE SYSTEM IS FORECAST TO STEADILY WEAKEN THROUGH THE
FORECAST PERIOD.
MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH THE
DURATION OF THE FORECAST PERIOD, LENDING HIGH CONFIDENCE TO THE
JTWC FORECAST TRACK. INTENSITY GUIDANCE IS MIXED, WITH HWRF AND GFS
SHOWING NEAR-TERM INTENSIFICATION, WHICH IS A KNOWN ISSUE WITH
THESE MODELS AS TROPICAL CYCLONES TRANSITION TO EXTRA-TROPICAL
LOWS. THE REMAINDER OF THE INTENSITY GUIDANCE INDICATES RAPID
DISSIPATION. THE JTWC FORECAST LIES JUST ABOVE THE CONSENSUS MEAN
WITH HIGH CONFIDENCE.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   INTENSITY 0 - 72 HR: HIGH//
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