Tropical Storm NAMTHEUN Advisory Sat Oct 16

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (NAMTHEUN) WARNING
NR 025//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 29.3N 164.2E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 614 NM NORTH-NORTHWEST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 17 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TIGHTLY
CURVED BANDING WRAPPING INTO A COMPACT CENTRAL DENSE OVERCAST
FEATURE. BANDING IS PARTIALLY EXPOSED ON THE WESTERN SEMI-CIRCLE
DUE TO HIGH-LEVEL VERTICAL WIND SHEAR (VWS) AND THE PRESENCE OF DRY
AIR. DESPITE WARMING CLOUD TOPS AND DEGRADATION OF THE PRIMARY
FEEDER BAND ON THE SE SIDE, A TIMELY 152324Z ASCAT-B PASS REVEALED
WINDS OF AT LEAST 55 KNOTS IN THE SE QUADRANT. A 152233Z ASCAT-C
ULTRA-HIGH RESOLUTION PRODUCT AND A 151936Z SMAP PASS SUGGESTED
THE PRESENCE OF A VERY SMALL AREA OF POSSIBLE 60 KNOT WINDS.
SUBJECTIVE DVORAK ESTIMATES REMAINED UNCHANGED, RANGING FROM T3.0
TO T3.5 (45 TO 55 KNOTS). SATCON AND ADT HAVE ALSO REMAINED STEADY
AT 52 AND 51 KNOTS, RESPECTIVELY. BASED ON ALL AVAILABLE DATA, THE
INITIAL INTENSITY WAS ASSESSED AT 55 KNOTS, AND THE 151800Z
INTENSITY WAS REVISED UPWARDS TO 55 KNOTS GIVEN THE LATE-ARRIVING
SMAP DATA. THE INITIAL POSITION WAS PLACED WITH HIGH CONFIDENCE
BASED ON THE ASCAT-B DATA, AND INDICATES THE TRACK SPEED HAS
ACCELERATED OVER THE PAST SIX HOURS. AN UPPER-LEVEL JET TO THE
NORTHEAST IS PROVIDING ROBUST OUTFLOW SUPPORT. ADDITIONALLY, VWS
HAS BACKED OFF SLIGHTLY, AND SSTS REMAIN SUPPORTIVE.
INITIAL WIND RADII BASIS: SCATTEROMETER DATA
CURRENT STEERING MECHANISM: A BUILDING SUBTROPICAL RIDGE (STR) TO
THE EAST
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T3.0 - 45 KTS
   RJTD: T3.0 - 45 KTS
   KNES: T3.5 - 55 KTS
   CIMSS SATCON: 52 KTS AT 152036Z
   CIMSS ADT: 51 KTS AT 152340Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: A STREAM OF LOW MID-LEVEL RELATIVE HUMIDITY AIR
IS WRAPPING IN ALONG THE SOUTHWESTERN PERIPHERY OF THE SYSTEM.
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: HIGH
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING, HOWEVER, TRACK SPEEDS HAVE
BEEN ADJUSTED SLIGHTLY FASTER BASED ON RECENT ACCELERATION OF
FORWARD MOTION.
FORECAST DISCUSSION: TS 23W IS IN THE PROCESS OF SHIFTING TO A MORE
NORTHERLY TRACK AS THE STEERING RIDGE TO THE EAST CONTINUES TO
STRENGTHEN. HIGH AMPLITUDE FLOW IS ALLOWING NAMTHEUN TO MAINTAIN
TROPICAL CHARACTERISTICS AS IT TRAVELS WELL INTO THE MID-LATITUDES,
AND IT HAS SO FAR REMAINED RESILIENT TO THE EFFECTS OF VWS. SST WILL
REMAIN GENERALLY SUPPORTIVE FOR THE NEXT 12-18 HOURS. TS NAMTHEUN
SHOULD BE ABLE TO MAINTAIN STRONG TROPICAL STORM INTENSITY OVER THE
NEXT 12 HOURS GIVEN THE VERY FAVORABLE OUTFLOW SUPPORT, HOWEVER,
AFTER THAT TIME DECLINING SST AND THE EFFECTS OF DRY AIR ENTRAINMENT
WILL DRIVE A SLOW WEAKENING TREND. EXTRA-TROPICAL TRANSITION (ETT)
WILL BEGIN AROUND TAU 36, AND BE WELL UNDERWAY BY TAU 48 AS THE
SYSTEM BEGINS TO ROUND THE RIDGE. AT THE SAME TIME, TS 23W WILL
INCREASINGLY FEEL THE INFLUENCE OF INCREASING VWS DUE TO AN
APPROACHING TROUGH. TS 23W WILL COMPLETE ETT BY TAU 72, WITH THE
REMNANT GALE-FORCE LOW ACCELERATING TO THE NORTHEAST
MODEL DISCUSSION: NUMERICAL MODELS REMAIN IN VERY TIGHT AGREEMENT
ON THE TRACK SOLUTION, LENDING HIGH CONFIDENCE TO THE JTWC TRACK
FORECAST THAT IS LAID CLOSE TO THE MULTI-MODEL CONSENSUS. INTENSITY
GUIDANCE HAS CONSISTENTLY UNDERESTIMATED NAMTHEUNS RECENT
INTENSIFICATION; HOWEVER, THERE IS VERY LIMITED TIME REMAINING FOR
ANY FURTHER INTENSIFICATION. CONFIDENCE IN THE INTENSITY FORECAST
IS SET AT MEDIUM DUE TO UNCERTAINTIES ASSOCIATED WITH ETT.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: ---
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: ---//
NNNN
NNNN