Tropical Storm NANMADOL Advisory Mo, 12.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 16W (SIXTEEN)
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 21.7N 138.5E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 242 NM SOUTHWEST OF IWO TO
   MOVEMENT PAST 6 HOURS: EASTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A RAGGED
AND STILL SOMEWHAT DISORGANIZED CIRCULATION WITH POCKETS OF FLARING
CONVECTION FIRING AND DISSIPATING ON THE PERIPHERY OF A PARTIALLY
EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). THE SYSTEM IS VERY
BROAD, WITH AN OUTERMOST CLOSED ISOBAR RADIUS OF 250NM, BRINGING IT
CLOSE TO THRESHOLD OF A MONSOON DEPRESSION. EARLIER SCATTEROMETER
DATA INDICATED THE SYSTEM HAS SLID SOUTHWARD OVER THE PREVIOUS 12
HOURS AND BUT NOW HAS COME TO A HALT AND BEGUN DRIFTING BACK TO THE
EAST. A 121846Z SSMIS COLORIZED 89GHZ MICROWAVE IMAGE SHOWED
DISORGANIZED AND FRAGMENTED BANDING FEATURES, PARTICULARLY ON THE
NORTHERN SIDE OF THE LLCC, WRAPPING INTO THE ILL-DEFINED CENTER,
AND LENT MEDIUM CONFIDENCE TO THE INITIAL POSITION. MEANWHILE, THE
SSMIS WINDSPEED ESTIMATE SHOWED PATCHES OF 25-30 KNOTS WINDS,
MOSTLY LIMITED TO THE EAST AND SOUTHEAST SECTORS, WHICH ALIGNS WELL
WITH A 121130Z ASCAT-B PASS SHOWING A BROAD REGION OF 20-25 KNOT
WINDS TO THE EAST OF THE LLCC. ANALYSIS REVEALS AN OVERALL MARGINAL
ENVIRONMENT WITH FAVORABLE SHEAR AND SSTS BEING OFFSET BY WEAK
OUTFLOW CONDITIONS ALOFT. THE STEERING PATTERN IS WEAK WITH THE
SYSTEM EMBEDDED IN A LARGE UPPER-LEVEL TROUGH CONNECTING WITH TY
14W TO THE WEST, AND A LARGE SUBTROPICAL RIDGE (STR) TO THE NORTH
AND NEAR EQUATORIAL RIDGE (NER) TO THE SOUTHEAST.
INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).
CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT BETWEEN NER
TO THE SOUTHEAST AND STR TO THE NORTH
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T2.5 - 35 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T1.0 - 25 KTS
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 0-5 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK 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: THE SYSTEM CURRENTLY LIES WITHIN A COMPLEX
STEERING PATTERN DEFINED BY A NER TO THE SOUTH AND A STR TO THE
NORTH, AND ITSELF BEING EMBEDDED WITHIN A LARGE, NEARLY MONSOON
GYRE-SIZED TROUGH WHICH CONNECTS WITH TY 14W TO THE WEST. THE
STRONGEST GRADIENT HOWEVER IS CURRENTLY ON THE SOUTHERN SIDE OF THE
SYSTEM AND THUS THE PRIMARY STEERING INFLUENCE IS THE NER TO THE
SOUTH. HENCE THE SYSTEM IS FORECAST TO DRIFT SLOWLY
EAST-NORTHEASTWARD THROUGH THE NEXT 24 TO 36 HOURS. AROUND TAU 36
THE NER WILL WEAKEN AND FLATTEN OUT, REDUCING THE GRADIENT FORCE ON
THE SOUTHERN SIDE OF THE SYSTEM AND AS THE PRIMARY STEERING
MECHANISM SHIFTS TO THE STR OVER JAPAN, TD 16W WILL TURN SHARPLY
ONTO A WEST-NORTHWEST HEADING BY TAU 48. ONCE ESTABLISHED ON THE
NEW COURSE, THE SYSTEM IS EXPECTED TO SPEED UP AS IT TRACKS ALONG
THE SOUTHWESTERN PERIPHERY OF THE DEEP STR. THE APPROACH OF A
MID-LATITUDE TROUGH FROM THE WEST WILL BEGIN TO ERODE THE RIDGE BY
TAU 96, ALLOWING FOR THE SYSTEM TO BEGIN TO RECURVE POLEWARD AT
THAT TIME. WHILE SHEAR IS EXPECTED TO REMAIN LOW THROUGH MOST OF
THE FORECAST PERIOD, THIS IS PRIMARILY DUE TO THE FACT THAT THE
SYSTEM DOES NOT ESTABLISH A GOOD OUTFLOW CHANNEL FOR AT LEAST THE
FIRST 48 HOURS, WHICH WILL KEEP A LID ON THE PACE OF CONSOLIDATION
AND INTENSIFICATION THROUGH THAT POINT. BY TAU 48, MODELS SHOW THE
FIRST HINTS OF AN UPPER-LEVEL POINT SOURCE AND BY TAU 72 SHOW
ESTABLISHMENT OF A POLEWARD OUTFLOW CHANNEL, WHICH WILL ALLOW FOR
A MORE RAPID PACE OF INTENSIFICATION, TO A PEAK OF 90 KNOTS BY TAU
96. THEREAFTER THE SYSTEM STALLS OUT AT 90 KNOTS AS IT GETS CUT OFF
FROM THE POLEWARD OUTFLOW CHANNEL AND FACES CONVERGENT FLOW ALOFT
EMANATING FROM A STRONG ANTI-CYCLONE AT 200MB POSITIONED TO THE
NORTHEAST.
MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN FAIRLY GOOD
AGREEMENT OVERALL, WITH ALL CONSENSUS MEMBERS AGREEING ON THE
SHORT-TERM DRIFT TO THE EAST-NORTHEAST FOLLOWED BY THE SHARP TURN
TO THE WEST BETWEEN TAU 24 AND 48. GFS IS THE OUTLIER UP TO TAU 48,
SHOWING A WIDER MORE GRADUAL TURN AND EXTENDING EAST OUT TO NEAR
141E BEFORE TURNING WEST. THE REMAINDER OF THE PACK CONCURS ON A
TURN AROUND 140E OR A BIT FURTHER WEST, LEADING TO A SPREAD IN THE
GUIDANCE OF ABOUT 100NM AT THE TURNING POINT. ONCE FIRMLY ON THE
WEST-NORTHWEST TRACK, THE CROSS-TRACK SPREAD ACTUALLY DECREASES TO
90NM BY TAU 72, THOUGH THE GFS LAGS BEHIND THE PACK DUE TO IT'S
FURTHER EAST STARTING POINT. AFTER TAU 72, THE DETERMINISTIC
TRACKERS QUICKLY SPREAD TO 230NM WITH THE ECMWF AND GALWEM ON THE
LEFT AND GFS ENSEMBLE MEAN AND NAVGEM ON THE RIGHT OF THE ENVELOPE.
ENSEMBLE GUIDANCE SHOWS A HIGH DEGREE OF UNCERTAINTY, WITH THE
ECENS INDICATING A STRONGER RIDGE THAT FIGHTS OFF THE TROUGH
APPROACHING FROM THE WEST AND REMAINS IN PLACE TO THE NORTH, WITH A
RESULTANT FLATTER TRACK, JUST NORTH OF OKINAWA. THE GEFS MEANWHILE
ERODES THE RIDGE MUCH FASTER, ALLOWING FOR A SHARP RECURVE TOWARDS
SOUTHERN KYUSHU, THOUGH SOME MEMBERS DO AGREE WITH THE ECENS MEAN.
THE JTWC TRACK LIES JUST EAST AND NORTH OF THE CONSENSUS MEAN,
HEDGED TOWARDS THE GFS ENSEMBLE MEAN WITH LOW CONFIDENCE THROUGHOUT
THE FORECAST DUE TO THE UNCERTAINTIES IN THE NEAR-TERM ERRATIC
MOTION AND LONGER TERM ENSEMBLE SPREAD. INTENSITY GUIDANCE IS IN
GOOD AGREEMENT ON A RATHER SLOW INTENSIFICATION TREND THROUGH TAU
48 FOLLOWED BY SHARPER INTENSIFICATION THEREAFTER. HWRF AND
DECAY-SHIPS (GFS) ARE ON THE HIGHER END OF THE ENVELOPE AFTER TAU
72, AND PEAK THE SYSTEM AT 105 KNOTS. THE JTWC FORECAST LIES
SLIGHTLY BELOW THE CONSENSUS TO TAU 48 THEN MATCHES IT THROUGH THE
REMAINDER OF THE PERIOD.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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