Tropical Storm TWENTYONE Advisory 10月13日

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE)
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 15.1N 152.0E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 359 NM EAST OF SAIPAN
   MOVEMENT PAST 6 HOURS: EASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS A COMPACT
SYSTEM THAT IS ESSENTIALLY AN IRREGULAR BALL OF CONVECTION INUNDATED
BY STRONG NORTHWESTERLY VERTICAL WIND SHEAR, AS INDICATED BY
TRANSVERSE BANDING AROUND THE EDGES. THE EIR LOOP ALSO SHOWS A TUTT
CELL TO THE NORTH THAT IS SUPPRESSING CONVECTIVE ACTIVITY ALONG THE
NORTHERN PERIPHERY OF THE SYSTEM. THE INITIAL POSITION IS PLACED WITH
MEDIUM CONFIDENCE BASED ON A PARTIAL LOW LEVEL CIRCULATION FEATURE IN
THE 121602Z AMSR-2 89GHZ IMAGE. THE INITIAL INTENSITY OF 25KTS IS
BASED ON AGENCY DVORAK ESTIMATES OF T1.5 AND IS CONSISTENT WITH THE
SUSTAINED 6-HR EIR CONVECTIVE SIGNATURE.
INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).
CURRENT STEERING MECHANISM: WEAK LOW-LAYER NEAR EQUATORIAL RIDGE TO
THE SOUTH.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS
   CIMSS SATCON: 29 KTS AT 121532Z
   CIMSS ADT: 27 KTS AT 121740Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EASTWARD
ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.
FORECAST DISCUSSION: TD 21W WILL BEGIN TO TRACK POLEWARD AS A
SUBTROPICAL RIDGE (STR) BUILDS TO THE EAST AND STEERS IT
NORTHEASTWARD THEN NORTHWARD. AFTER TAU 48, IT WILL CREST THE STR
AXIS AND ACCELERATE NORTHEASTWARD AS THE STR RECEDES, WEAKENED BY A
MID-LATITUDE TROUGH APPROACHING FROM THE NORTHWEST. THE MARGINALLY
FAVORABLE ENVIRONMENT WILL FUEL A SLOW INTENSIFICATION TO A PEAK OF
45KTS BY TAU 36. AFTERWARD, INCREASING VWS AND COOLING SST WILL
TEMPER ANY FURTHER INTENSIFICATION. BY TAU 72, THE CYCLONE WILL ENTER
THE BAROCLINIC ZONE AND BEGIN EXTRA-TROPICAL TRANSITION AND BY TAU
96, WILL TRANSFORM INTO A GALE-FORCE COLD CORE LOW.
MODEL DISCUSSION: NUMERICAL MODELS ARE IN OVERALL GOOD AGREEMENT
WITH A GRADUAL SPREAD TO APPROXIMATELY 145NM BY TAU 48, THEN
SIGNIFICANTLY DIVERGE ALONG- AND ACROSS-TRACK AFTERWARD. IN VIEW OF
THIS, THERE IS MODERATE CONFIDENCE IN THE JTWC TRACK FORECAST UP TO
TAU 72, THEN LOW AFTERWARD.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
NNNN
NNNN