Tropical Storm NESAT Advisory 10月17日

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 23W (NESAT) WARNING NR 009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 19.7N 117.7E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 360 NM NORTH-NORTHWEST OF MANILA,
PHILIPPINES
   MOVEMENT PAST 6 HOURS: WESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY CURRENTLY
DEPICTS A COLD DENSE OVERCAST (CDO) WITH DEEP CONVECTION ON THE
EASTERN PERIPHERY WRAPPING AROUND AN EYE FEATURE THAT IS NOW PARTLY
CLOUD-FILLED OBSCURING THE LOW LEVEL CIRCULATION CENTER (LLCC),
THEREFORE THE INTENSIFICATION TREND SEEMS TO HAVE BEEN ARRESTED FOR
NOW. HOWEVER, EIR STILL INDICATES CONVECTIVE BURSTS, WITH FLARING
CONVECTIVE CLOUD TOPS TEMPERATURES OF (-70C). AN EARLIER 161027Z
SSMIS 91 GHZ MICROWAVE IMAGE DEPICTS A 40 NM DEEP CONVECTIVE CURVED
BAND FULLY SURROUNDING A 20 NM MICROWAVE EYE. THE INITIAL POSITION
IS PLACED WITH HIGH CONFIDENCE BASED ON THE CLOUD-FILLED EYE
FEATURE IN ANIMATED EIR IMAGERY ALONG WITH THE EXTRAPOLATED SSMIS
MICROWAVE IMAGE. IN COMBINATION WITH AN EARLIER BULLSEYE 161358Z
ASCAT-C IMAGE WHICH SHOWED A DEFIND LLCC WITH THE HIGHEST WIND
SPEEDS ON THE NORTHERN PERIPHERY. THE INITIAL INTENSITY IS ALSO
PLACED WITH MEDIUM CONFIDENCE BASED ON SUBJECTIVE DVORAK
INTENSITIES FROM PGTW T4.5, RJTD T3.5, AND KNES T4.5. ADDITIONALLY,
OBJECTIVE AUTOMATED DVORAK ESTIMATES INTENSITIES ARE SLIGHTLY BELOW
SUBJECTIVE AGENCY DVORAK ESTIMATES. ANALYSIS INDICATES A FAVORABLE
ENVIRONMENT WITH WARM SST, MODERATE POLEWARD AND EQUATORWARD
OUTFLOW, AND MODERATE VWS.
INITIAL WIND RADII BASIS: PERSISTENCE
CURRENT STEERING MECHANISM:  SOUTHEASTERN SIDE OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED OVER SOUTHEAST CHINA.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T4.5 - 77 KTS
   RJTD: T3.5 - 55 KTS
   KNES: T4.5 - 77 KTS
   CIMSS SATCON: 66 KTS AT 161422Z
   CIMSS ADT: 72 KTS AT 161740Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD
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.
FORECAST DISCUSSION: TYPHOON 23W HAS TRACKED DUE WEST OVER THE PAST
SIX HOURS, AS THE STEERING MECHANISM SHIFTS TO A BUILDING
SUBTROPICAL RIDGE OVER SOUTHEAST CHINA. AS THE STR CONTINUES TO
BUILD OVER SOUTHEAST CHINA, IT WILL ALSO EXTEND EASTWARD WHICH WILL
DIRECT TY 23W ONTO A SOUTHWESTWARD TRACK THROUGH TAU 48. BY TAU
72, THE STR THEN SHIFTS EASTWARD AND BUILDS OVER THE TAIWAN STRAIT.
AS THIS OCCURS THE RIDGE WILL ALSO REORIENT TO A MORE NORTHWEST TO
SOUTHEAST AXIS, ALLOWING FOR THE SYSTEM TO TURN NORTHWESTWARD AND
TRACK JUST SOUTH OF HAINAN BY TAU 96. BEFORE ULTIMATELY MAKING
LANDFALL AND MOVING INLAND INTO NORTHERN VIETNAM BY TAU 120. TY 23W
IS CURRENTLY NOT STRENGTHENING AS THE SYSTEM BATTLES DRY AIR ON THE
EASTERN SIDE OF THE SYSTEM. HOWEVER, GFS 700-300MB RELATIVE
HUMIDITY MODEL FIELDS INDICATE THAT THE DRY AIR WILL RECEDE AND
MOISTEN OVER THE NEXT 24-36 HOURS, WHICH WILL ALLOW THE SYSTEM TO
STRENGTHEN TO A PEAK INTENSITY OF 90 KNOTS. HOWEVER, BY TAU 72 AND
BEYOND, AN INCREASE IN VERTICAL WIND SHEAR (VWS) ALONG WITH
INCREASING DRY AIR AROUND THE SYSTEM IS FORECAST TO INCREASE AND AS
A RESULT A STEADY WEAKENING TREND IS EXPECTED BEFORE THE SYSTEM
ULTIMATELY MAKES LANDFALL TON NORTHERN VIETNAM.
MODEL DISCUSSION: NUMERICAL TRACK GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 96 WITH ONLY A 70 NM CROSS-TRACK SPREAD, ASIDE FROM THE
AFUI TRACKER WHICH IS THE SOUTHERN OUTLIER. BY TAU 120, CROSS-TRACK
AND ALONG TRACK SPREAD INCREASES SUBSTANTIALLY, LIKELY DUE TO HOW
MODEL TRACKERS ARE HANDLING THE LAND INTERACTION DIFFERENTLY. THE
ECMWF ENSEMBLE (EPS) AND GFS ENSEMBLE (GEFS) SOLUTIONS ARE IN GOOD
AGREEMENT WITH SOME MEMBERS SHOWING LANDFALL INTO HAINAN, BUT
OVERALL ENSEMBLE GUIDANCE SUPPORTS THE JTWC FORECAST TRACK. THE
INTENSITY GUIDANCE REMAINS MIXED, WITH HWRF REPRESENTING THE HIGH
END OF INTENSITY GUIDANCE, WHILE AHNI, CTCI, COTI AND DECAY SHIPS
(NAVGEM AND GFS) REPRESENTS THE LOW END OF INTENSITY GUIDANCE.
ENSEMBLE INTENSITY GUIDANCE CONTINUES TO SHOW HIGH PROBABILITIES OF
MODERATE INTENSIFICATION BETWEEN TAU 24 AND TAU 48. THE JTWC
INTENSITY FORECAST IS PLACED WITH MEDIUM CONFIDENCE AND LIES CLOSE
TO THE HWRF SOLUTION AND ABOVE THE CONSENSUS MEAN.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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