Tropical Storm ROKE Advisory Mi, 28.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) WARNING
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 21.9N 132.2E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 366 NM SOUTHEAST OF KADENA AB
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 14 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL DEPRESSION 20W (TWENTY) HAS QUICKLY CONSOLIDATED OVER THE
PREVIOUS 12 HOURS WHILE STEADILY INTENSIFYING. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS DEEP FLARING
CONVECTION FIRING PREDOMINANTLY IN THE NORTHERN SECTORS OF THE
CIRCULATION, THOUGH IT REMAINS DISORGANIZED IS NOT EVIDENCED TO BE
WRAPPING INTO THE LOW LEVEL CIRCULATION CENTER (LLCC) AS OF YET.
DUE TO A LACK OF RECENT MICROWAVE IMAGERY, THE INITIAL POSITION IS
ASSESSED WITH ONLY MEDIUM CONFIDENCE USING SPIRAL BAND ANALYSIS OF
THE OUTER LOW-LEVEL CLOUD LINES SEEN TO BE WRAPPING INTO THE
PERIPHERY OF THE SYSTEM. THE INITIAL INTENSITY IS ASSESSED AT 30
KNOTS WITH HIGH CONFIDENCE, HEDGED SLIGHTLY ABOVE THE AGENCY FIX
INTENSITIES BASED ON AN EARLIER 280124Z ASCAT-B PARTIAL PASS WHICH
SHOWED A LARGE REGION OF 30 KNOT WINDS IN THE NORTHERN PERIPHERY OF
THE SYSTEM. ADDITIONALLY A 280300Z SHIP OBSERVATION, JUST 20NM FROM
THE 0600Z POSITION, REGISTERED A 1003MB PRESSURE AND WINDS OF 32
KNOTS. THE SYSTEM LIES IN A FAVORABLE ENVIRONMENT WITH LOW (0-5
KNOTS) VWS, MODERATE RADIAL OUTFLOW WITH A POINT SOURCE OVER TOP OF
THE SYSTEM AND WARM SSTS. THE ONLY INHIBITING FACTOR AT PRESENT IS
THE DISORGANIZED CORE STRUCTURE OF THE SYSTEM.
INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).
CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED TO THE EAST.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T1.5 - 25 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 0-5 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE RADIAL
ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.
FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 20W WILL CONTINUE TO
TRACK NORTH-NORTHWESTWARD THROUGH THE NEXT 12 HOURS, REACH THE AXIS
OF THE STEERING RIDGE SHORTLY THEREAFTER AND THEN TURN SHARPLY
NORTHEAST AND THE STR FLATTENS AND ELONGATES TO A MORE EAST-WEST
ORIENTATION. ONCE THE SYSTEM CAN CONSOLIDATE A SOLID CORE, A PERIOD
OF INTENSIFICATION IS EXPECTED AS THE FAVORABLE CONDITIONS OF WARM
SSTS, LOW SHEAR AND MODERATE RADIAL OUTFLOW ARE FORECAST TO
CONTINUE THROUGH THE NEXT 36 HOURS AND THE SYSTEM SHOULD REACH PEAK
INTENSITY OF 45 KNOTS BY TAU 36. BY TAU 48, A 500MB MINOR SHORTWAVE
TROUGH IS FORECAST TO MOVE ACROSS THE JAPANESE ISLANDS AND LEAD TO
CYCLOGENESIS OF A WEAK LOW PRESSURE AREA EAST OF TOKYO, ALONG THE
TAIL END OF A COLD FRONTAL SYSTEM LEFT IN THE WAKE OF WHAT IS NOW
TY 19W. THE UPPER TROUGH AND BAROCLINIC LOW WILL BE FAIRLY WEAK AND
QUICKLY SLIDE TO THE EAST-NORTHEAST ON A RELATIVELY FLAT
TRAJECTORY, LEAVING THE FRONTAL SYSTEM STALLED TO NORTH OF TD 20W.
THE ONSET OF INCREASING SHEAR AROUND TAU 48 WILL MARK THE BEGINNING
OF AN INITIALLY SLOW WEAKENING TREND. BY TAU 72 HOWEVER THE
UPPER-LEVEL ANTICYCLONE CURRENTLY PROVIDING GOOD RADIAL OUTFLOW
WILL SLIDE TO THE SOUTHWEST OF THE SYSTEM, AND BEGIN TO IMPART
STRONGLY CONVERGENT FLOW ALOFT OVER THE TOP OF THE SYSTEM, LEADING
TO A MORE PRONOUNCED WEAKENING PHASE. AS A NOW SIGNIFICANTLY WEAKER
AND SHALLOWER SYSTEM, TD 20W WILL BUTT HEADS WITH MODERATELY STRONG
NORTHERLY FLOW BEHIND THE COLD FRONT WILL EFFECTIVELY BLOCK FURTHER
POLEWARD MOVEMENT. AS THE BAROCLINIC HIGH SLIDES EASTWARD THE LOW
LEVEL WIND FLOW BECOMES EASTERLY, AND WILL PUSH TD 20W OFF THE
WEST-NORTHWEST BY THE END OF THE FORECAST PERIOD.
MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN FAIRLY GOOD
AGREEMENT THOUGH TAU 48 WITH A 110NM SPREAD BETWEEN THE NAVGEM ON
THE WEST AND GFS ON THE EAST. AFTER THIS POINT HOWEVER, THINGS GET
ROUGH. AT THE TAU 72 POINT, THE MODELS BEGIN TO SEPARATE SHARPLY. AT
TAU 72, THE ALONG-TRACK SPREAD BETWEEN THE NAVGEM AND JGSM TO THE
WEST AND THE EGRR AND GALWEM TO THE EAST EXCEEDS 350NM. BY TAU 96
THE BULK OF THE GUIDANCE, WITH THE EXCEPTION OF GALWEM AND EGRR
STARTS TO SHOW THE TURN TO THE WEST, CONTINUING THROUGH TAU 120.
HOWEVER, THE MODELS DISAGREE ON THE SHARPNESS AND STARTING POINT OF
THE TURN, LEADING TO A VERY LARGE SPREAD IN MODEL SOLUTIONS BAY TAU
120. THE JGSM AND HWRF LIE ON THE FAR WEST OF THE ENVELOPE, WHILE
THE GFS ENSEMBLE MARKS THE EASTERN SIDE OF THE MODELS THAT SUPPORT
THE TURN, WITH OVER 450NM OF SEPARATION BETWEEN THESE TWO OUTLIERS.
THE REMAINDER OF THE GFS, JGSM, NAVGEM, COAMPS-TC, ECMWF ENSEMBLE
AND THE CONSENSUS MEAN LIE EQUALLY SPREAD WITHIN THIS 450NM
ENVELOPE. THE JTWC FORECAST TRACK LIES TO THE INSIDE AND SLIGHTLY
FASTER THAN THE CONSENSUS MEAN TO TAU 72, THEN MOST CLOSELY TRACKS
THE GFS ENSEMBLE MEAN TO THE END OF THE FORECAST. CONFIDENCE IS
MEDIUM UP TO TAU 72 AND LOW THEREAFTER DUE TO THE UNCERTAINTIES
ASSOCIATED WITH THE TIMING AND STRENGTH OF THE COLD FRONT AND
TRANSITORY HIGH PRESSURE AREA AND THE ULTIMATE IMPACT THESE WILL
HAVE ON THE SYSTEM. INTENSITY GUIDANCE IS IN GOOD AGREEMENT THAT
THE SYSTEM WILL INTENSIFY THROUGH TAU 36 TO 48, THEN WEAKEN SLOWLY
THEREAFTER. CTCX IS THE MOST AGGRESSIVE MODEL, PEAKING THE SYSTEM
AT 52 KNOTS AT TAU 36, WHILE THE REMAINDER OF THE CONSENSUS MEMBERS
LIE IN A TIGHT ENVELOPE WITH A PEAK BETWEEN 40-45 KNOTS. THE JTWC
FORECAST LIES ALONG THE MEAN, WITH A SLIGHTLY HIGHER PEAK, THEN
FOLLOWS THE MEAN DURING THE WEAKENING PHASE. CONFIDENCE IS LOW AS
IF THE SYSTEM CAN QUICKLY CONSOLIDATE, A FASTER PACE OF
INTENSIFICATION AND HIGHER PEAK IS CERTAINLY A POSSIBILITY.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: LOW
   INTENSITY 72-120 HR: LOW//
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