Tropical Storm NORU Advisory Fr, 23.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (EIGHTEEN) WARNING NR
005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 18.1N 132.6E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 607 NM NORTH-NORTHWEST OF YAP
   MOVEMENT PAST 6 HOURS: WESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN UPPER
LEVEL CONVECTIVE STRUCTURE THAT APPEARS TO BE SLIGHTLY TILTED AND
DISPLACED FROM THE NOW PARTIALLY EXPOSED LOW LEVEL CIRCULATION
CENTER (LLCC). AN EARLIER 222039Z SSMIS MICROWAVE IMAGE ALSO
CONFIRMS THE SLIGHTLY TILTED STRUCTURE OF THE SYSTEM, AS THE
HIGHEST BRIGHTNESS TEMPERATURES ARE LOCATED WITHIN THE SOUTHWESTERN
QUADRANT IN RELATION TO THE LOW LEVEL CIRCULATION CENTER (LLCC).
THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE
PARTIALLY EXPOSED LLCC, AND PLACED CLOSE TO AGENCY DVORAK FIXES.
THE INITIAL INTENSITY IS PLACED WITH LOW CONFIDENCE BASED ON A
COMBINATION OF SUBJECTIVE AND OBJECTIVE AGENCY DVORAK INTENSITY
ESTIMATES, PGTW T2.5, KNES T3.0, RJTD 2.0, ALONG WITH ADT 34 KTS,
AIDT 40 KTS, AND SATCON 40 KTS .
INITIAL WIND RADII BASIS: PERSISTENCE
CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE
NORTHEAST
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T3.0 - 45 KTS
   CIMSS SATCON: 40 KTS AT 222125Z
   CIMSS ADT: 34 KTS AT 222340Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK WESTWARD
ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: LOW
   INITIAL WIND RADII: MEDIUM
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.
FORECAST DISCUSSION: TS 18W HAS BECOME PARTIALLY EXPOSED OVER THE
LAST 6 HOURS AS THE SYSTEM IS CURRENTLY SITUATED IN A MARGINALLY
FAVORABLE ENVIRONMENT, CHARACTERIZED BY WARM SEA SURFACE
TEMPERATURES (SSTS) (29-30C), WEAK RADIAL OUTFLOW, OFFSET BY
MODERATE VERTICAL WIND SHEAR (10-15 KNOTS) AND MID LEVEL DRY AIR
SURROUNDING THE SYSTEM. OVER THE NEXT 24 HOURS, THE SYSTEM IS
FORECAST TO TRACK GENERALLY WESTWARD UNDER THE INFLUENCE OF A
DEEP-LAYER SUBTROPICAL RIDGE TO THE NORTHEAST WHICH HAS TAKEN OVER
AS THE DOMINATE STEERING FEATURE WILL CONTINUE TO STEER 18W
TOWARD THE COAST OF LUZON, BEFORE MAKING LANDFALL SHORTLY AFTER TAU
48. RAPID INTENSIFICATION GUIDANCE IS TRIGGERED, HOWEVER BECAUSE
THE SYSTEM WILL HAVE EASTERLY CONVERGENT FLOW ALOFT WITH OVERALL
WEAK UPPER-LEVEL OUTFLOW, AS WELL AS MID LEVEL DRY AIR TO CONTEND
WITH, THE FORECAST WILL MAINTAIN A GRADUAL INTENSIFICATION UP TO 65
KNOTS PRIOR TO LANDFALL INTO LUZON. THIS LAND INTERACTION WILL
LIKELY PUT A CAP ON INTENSIFICATION AND POSSIBLY DECREASE
INTENSITY, BUT ONLY SLIGHTLY GIVEN THE SHORT DURATION OVER LAND.
HOWEVER, BY TAU 72, THE SYSTEM MOVES BACK OVER THE WARM WATERS OF
THE SOUTH CHINA SEA, AS THE UPPER LEVEL ENVIRONMENT BECOMES MORE
FAVORABLE FOR INTENSIFICATION, CHARACTERIZED BY ENHANCED OUTFLOW
AND LOW VERTICAL WIND SHEAR. THUS, GIVEN THE FAVORABLE
ENVIRONMENTAL FACTORS, THE OFFICIAL JTWC FORECAST SHOWS AN INCREASE
UP TO 80 KNOTS BY TAU 96. BY TAU 120, 18W WILL CONTINUE A STRAIGHT
RUNNING WESTWARD TRACK BEFORE EVENTUALLY MAKING LANDFALL INTO
NORTHERN VIETNAM.
MODEL DISCUSSION: NUMERICAL MODELS ARE IN OVERALL TIGHT AGREEMENT
WITH THE EXCEPTION OF JGSM WHICH MAKES UP THE SOUTHERN OUTLIER,
WHILE NAVGEM IS THE NORTHERN OUTLIER. OVER THE NEXT 48 HOURS, TRACK
SPREAD WILL GRADUALLY INCREASE TO 50 NM. AFTER THE SYSTEM MOVES
PAST LUZON AND BEYOND TAU 72, GUIDANCE BEGINS TO DIVERGE EVEN
FURTHER TO A 100 NM CROSS-TRACK SPREAD, HOWEVER GUIDANCE REMAINS IN
OVERALL GOOD AGREEMENT REGARDING ALONG-TRACK SPREAD. INTENSITY
GUIDANCE INDICATES UNCERTAINTY DUE TO RAPID INTENSIFICATION
GUIDANCE BEING TRIGGERED THROUGH TAU 48, HOWEVER INTENSITY GUIDANCE
BECOMES MORE IN LINE FROM TAU 72 THROUGH TAU 96. THUS, THE JTWC
INTENSITY FORECAST IS PLACED WITH MEDIUM CONFIDENCE OVERALL.
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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