Tropical Storm TWENTYFIVE Advisory Do, 20.10.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 25W (TWENTYFIVE)
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 21.0N 129.4E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 341 NM SOUTH-SOUTHEAST OF KADENA AB
   MOVEMENT PAST 6 HOURS: WESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
OVER THE PAST SIX HOURS TD 25W HAS INCREASED ITS FORWARD
SPEED UP TO 8 KNOTS AS THE SYSTEM TRANSITS GENERALLY WESTWARD.
CURRENTLY, ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A
CLUSTER OF PERSISTENT DEEP CONVECTION OBSCURING THE LOW-LEVEL
CIRCULATION CENTER (LLCC). AN EARLIER 192242Z 89GHZ MICROWAVE IMAGE
DEPICTS A FORMATIVE CONVECTIVE BAND ON THE SOUTHERN PERIPHERY OF THE
LLCC. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON A
PARTIAL 200033Z ASCAT-C IMAGE, WHICH SHOWS A DEFINED CENTER WITH
20-25 KNOT WINDS OVER THE NORTHERN PERIPHERY. THE INITIAL INTENSITY
OF 25 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON A COMBINATION
OF AGENCY DVORAK WITH PGTW ESTIMATING T1.5, AND KNES ESTIMATING
T3.5, ALONG WITH THE PARTIAL ASCAT DATA. ENVIRONMENTAL CONDITIONS
CONTINUE TO BE MARGINALLY FAVORABLE WITH WEAK EQUATORWARD OUTFLOW,
WARM (28-29C) SEA SURFACE TEMPERATURES (SSTS), AND LOW (05-10KTS)
VERTICAL WIND SHEAR (VWS).
INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).
CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE
NORTHWEST
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T1.5 - 25 KTS
   RJTD: T1.0 - 25 KTS
   KNES: T3.5 - 55 KTS
   CIMSS ADT: 28 KTS AT 192340Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 0-5 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: WEAK EQUATORWARD
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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 IS FORECAST TO TRACK
GENERALLY WESTWARD AT AN INCREASING TRACK SPEED ALONG THE SOUTHEASTERN
PERIPHERY OF THE SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTHWEST
OVER THE TAIWAN STRAIT THROUGH THE NEXT 48-72 HOURS. BY TAU 72, THE
SYSTEM IS FORECAST TO TRACK INTO THE SOUTH CHINA SEA (SCS) AND BEGIN
TO SLOW ITS FORWARD MOTION, AS IT REACHES A COL AREA OF WEAK COMPETING
STEERING FLOW BETWEEN TWO SUBTROPICAL RIDGES TO THE NORTH. AFTER
TAU 72, A DEEP LAYER RIDGE OVER EAST CHINA WILL THEN BECOME THE
MAIN STEERING FEATURE WHICH WILL ALSO INDUCE A WEAK NORTHEASTERLY
SURGE EVENT. THIS WILL ALLOW THE SYSTEM TO THEN BE STEERED TO THE
WEST-SOUTHWESTWARD AND THEN SOUTHWESTWARD THROUGH THE END OF THE
FORECAST. TD 25W WILL STEADILY INTENSIFY THROUGH TAU 72
HOURS AS THE OVERALL ENVIRONMENT IS ONLY MARGINALLY FAVORABLE DUE
TO WARM (28-29C) SEA SURFACE TEMPERATURES (SSTS), LOW (05-10KTS)
VERTICAL WIND SHEAR (VWS) OFFSET BY WEAK OUTFLOW ALOFT. THE OUTFLOW
WILL REMAIN LESS FAVORABLE DURING THIS TIME FRAME DUE TO THE
UPPER LEVEL WESTERLIES FORECAST TO REMAIN FURTHER NORTH OF THE
SYSTEM PROHIBITING GOOD POLEWARD OUTFLOW NEEDED FOR RAPID
INTENSIFICATION (RI). BETWEEN TAU 72 AND TAU 96, AS THE SYSTEM MOVES
SOUTHWESTWARD INTO THE SOUTH CHINA SEA WHERE SSTS ARE WARM ALONG WITH
IMPROVED OUTFLOW, THERE IS A WINDOW OF OPPORTUNITY FOR THE SYSTEM TO
EXPERIENCE A BRIEF INTENSIFICATION PERIOD UP TO 50 KNOTS. THEN BY
TAU 120, THE SYSTEM WILL TRACK NEAR COOLER UP-WELLED WATERS FROM
THE COLD WAKE 23W LEFT BEHIND ALONG WITH INCREASING VWS AND
DRY AIR FORECAST TO INCREASE AROUND THE SYSTEM. THESE UNFAVORABLE
ENVIRONMENTAL FACTORS WILL RESULT IN STEADY WEAKENING.
MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE REMAINS IS IN FAIR
AGREEMENT WITH THE EXCEPTION OF EGRI AND NAVGEM WHICH REMAIN THE
NORTHERN OUTLIERS THROUGH TAU 72. HOWEVER, BY BETWEEN TAU 96 AND
TAU 120 CROSS TRACK SPREAD INCREASES TO ROUGHLY 100-200NM. ENSEMBLE
GUIDANCE FROM BOTH GFS ENSEMBLE (GEFS) AND  ECMWF ENSEMBLE (EPS)
ARE IN GOOD AGREEMENT THROUGH TAU 72, WITH LARGE UNCERTAINLY
BETWEEN TAU 96 AND TAU 120. THE EPS SHOWS A TRACK THAT REMAINS
WESTWARD AND JUST SOUTH OF HAINAN WHILE THE GEFS SHOWS A MORE
SOUTHWESTERLY TRACK SOLUTION BY TAU 120. THEREFORE, THE JTWC TRACK
FORECAST IN PLACED WITH MEDIUM CONFIDENCE BASED ON THE SPREAD OF
SOLUTIONS AND UNCERTAINLY FOR THE END OF THE FORECAST. THE
INTENSITY FORECAST REMAINS VERY SIMILAR TO THE PREVIOUS FORECAST AS
HIGH RESOLUTION MODEL INTENSITY GUIDANCE SUCH AS HWRF AND COAMPS-TC
WAS UNAVAILABLE. ADDITIONALLY, GEFS INTENSITY GUIDANCE REMAINS
UNCHANGED FROM THE PREVIOUS FORECAST AND CONTINUES TO INDICATE HIGH
PROBABILITY OF STEADY INTENSIFICATION AND A 10-20 PERCENT CHANCE OF
MODERATE INTENSIFICATION BETWEEN TAU 48 AND TAU 96. THEREFORE, THE
JTWC FORECAST INTENSITY IS PLACED WITH MEDIUM CONFIDENCE.
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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