Tropical Storm TWENTYFIVE Advisory Fr, 21.10.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 25W (TWENTYFIVE)
WARNING NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 20.4N 124.6E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 404 NM NORTH-NORTHEAST OF MANILA,
PHILIPPINES
   MOVEMENT PAST 6 HOURS: WESTWARD AT 14 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL DEPRESSION 25W CONTINUES TO SLOWLY INTENSIFY IN A
MARGINALLY FAVORABLE ENVIRONMENTAL OVER THE PAST 12 HOURS.
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A RAGGED
DISORGANIZED SYSTEM, BUT HAS RETAINED A SMALL CORE WITH DEEP
CONVECTION FIRING ON THE WESTERN SIDE OF THE OBSCURED LOW LEVEL
CIRCULATION CENTER (LLCC). AN EARLIER 202118Z SSMIS 91GHZ MICROWAVE
IMAGE REVEALS SHALLOW BANDING WRAPPING INTO AN ELONGATED LOW-LEVEL
CIRCULATION CENTER POSITIONED ALONG THE NORTHEASTERN EDGE OF A
CURVED CONVECTIVE BAND. THE INITIAL POSITION IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON THE SSMIS MICROWAVE IMAGE ALONG WITH
AGENCY DVORAK POSITION FIXES. THE INITIAL INTENSITY OF 30 KNOTS IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON A COMBINATION OF OBJECTIVE
AND SUBJECTIVE DVORAK INTENSITY ESTIMATES WHICH REMAIN ON THE HIGH
END OF INTENSITY ESTIMATES. ENVIRONMENTAL CONDITIONS ARE MARGINALLY
FAVORABLE WITH DRY STABLE AIR TO THE NORTH, MODERATE EQUATORWARD
OUTFLOW, WARM (29-30C) SEA SURFACE TEMPERATURES (SSTS), AND LOW
(5-10KTS) VERTICAL WIND SHEAR (VWS). THESE ENVIRONMENTAL FACTORS
FOR FORECAST TO PREVENT RAPID CONSOLIDATION AND STRENGTHENING.
INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).
CURRENT STEERING MECHANISM: DEEP-LAYER SUBTROPICAL RIDGE (STR)
CENTERED OVER TAIWAN.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   CIMSS ADT: 39 KTS AT 202340Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 5-10 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
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: TROPICAL DEPRESSION 25W'S FORECAST TRACK
REMAINS VERY SIMILAR TO PREVIOUS FORECASTS, AS THE SYSTEM GENERALLY
TRACKS WEST-NORTHWESTWARD ALONG THE SOUTHEASTERN PERIPHERY OF A DEEP-
LAYER
SUBTROPICAL RIDGE (STR) POSITIONED OVER TAIWAN. THIS GENERAL
WESTWARD TRACK WILL CONTINUE THROUGH TAU 24. BY TAU 36, THE SYSTEM
WILL THEN PASS THROUGH THE LUZON STRAIGHT AND MOVE INTO THE SOUTH
CHINA SEA WITH A SLIGHT TRACK POLEWARD AS A WEAK TROUGH PASSES TO
THE NORTH THAT WILL INDUCE A WEAKNESS WITHIN THE STEERING RIDGE. BY
TAU 48, 25W WILL THEN REACH AN INFLECTION POINT WHEREBY THE PRIMARY
STEERING MECHANISM BECOMES A DEEP LAYER RIDGE
OVER EAST CHINA. THIS STEERING MECHANISM WILL THEN STEER THE
SYSTEM GENERALLY SOUTHWESTWARD TOWARD SOUTHERN VIETNAM THROUGH THE
REMAINED OF THE FORECAST PERIOD. TD 25W WILL SLOWLY INTENSIFY
THROUGH THE NEXT 24 HOURS DUE TO THE INITIAL HIGHLY ASYMMETRICAL
LLCC ALONG WITH THE DRY STABLE AIR OVER THE NORTHERN PERIPHERY.
HOWEVER, AFTER TAU 48 THE SYSTEM MOVES INTO A MORE CONDUCIVE
ENVIRONMENTAL WITH IMPROVED UPPER-LEVEL OUTFLOW WHICH WILL ALLOW FOR
A FAST INTENSIFICATION PERIOD. AFTER TAU 72, DECREASING SEA SURFACE
TEMPERATURES (SSTS) LEFT IN THE WAKE OF TY 23W ALONG WITH UPPER-LEVEL
CONVERGENT EASTERLY FLOW WILL RESULT IN STEADY WEAKENING THROUGH TAU
120.
MODEL DISCUSSION: NUMERICAL TRACK MODELS ARE IN TIGHT AGREEMENT
THROUGH 72 WITH A LARGER SPREAD UP TO 370 NM BY TAU 120. PAST TAU
48 AFUI TAKES AN UNREALISTIC ABRUPT TURN WESTWARD WHILE THE BULK OF
THE GUIDANCE CONTINUES THE SYSTEM SOUTHWESTWARD AND  CROSS-TRACK
SPREAD INCREASE. ADDITIONALLY, THE GFS ENSEMBLE (GEFS) SOLUTIONS
SUPPORT THE JTWC FORECAST TRACK THROUGH TAU 72, WITH INCREASING
CROSS-TRACK SPREAD BY TAU 120. FOR THESE REASON THE JTWC FORECAST
TRACK IS PLACED WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE IS
UNCERTAIN, WITH AVAILABLE GUIDANCE INDICATING A SPREAD BETWEEN 30
KNOTS (COTI) AND 50 KNOTS (DECAY SHIPS), ALONG WITH HWRF NOW
SHOWING A MUCH LOW PEAK OF 40 KNOTS FROM THE PREVIOUS MODEL RUN.
THE JTWC FORECAST IS HEDGED SLIGHTLY ABOVE THE SHIPS GUIDANCE, AND
THE CONSENSUS MEAN TO MAINTAIN CONTINUITY WITH PREVIOUS INTENSITY
GUIDANCE. THEREFORE, CONFIDENCE IS LOW IN THE FORECAST INTENSITY.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: LOW
   INTENSITY 72-120 HR: LOW//
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