Advisories

WDIO31 PGTW 012100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04B (MURASU) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.6N 82.6E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 633 NM SOUTH-SOUTHWEST OF KOLKATA, INDIA
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE (TC) 04B (MURASU) WITH AN EXPANDING CENTRAL DENSE OVERCAST
(CDO) COMPRISED OF COOLING CLOUD TOPS OF -85 C, OBSCURING THE LOW
LEVEL CIRCULATION CENTER (LLCC). A 011618Z METOP-C ASCAT PASS REVEALED 
A COMPACT, SLIGHTLY ELONGATED LLCC SITUATED ON THE NORTHWESTERN SIDE 
OF THE CDO WITH AN AREA OF 50 KTS ON THE EASTERN FLANK AND AN 
EXPANDING WIND FIELD. COMPARING THIS DISPOSITION OF THE LLCC TO THE 
CDO WITH PREVIOUS MICROWAVE DATA SUGGESTS IMPROVED VERTICAL ALIGNMENT 
OF THE VORTEX, SUPPORTING THE MORE RAPID INTENSIFICATION (RI) RATE OF 
04B. ENVIRONMENTAL CONDITIONS ARE ASSESSED AS FAVORABLE WITH WARM (29-
30 C) SEA SURFACE TEMPERATURES AND ROBUST POLEWARD AND EQUATORWARD 
OUTFLOW ALOFT. DEEP LAYER VERTICAL WIND SHEAR (VWS) IS ASSESSED AS LOW 
(5-10 KTS), HOWEVER ECMWF AND GFS MODELS SUGGEST MORE ELEVATED VALUES 
OF NORTHWESTERLY MID-LEVEL SHEAR (15-20 KTS). THE INITIAL POSITION IS 
PLACED WITH MEDIUM CONFIDENCE BASED ON EXTRAPOLATION FROM THE 
AFOREMENTIONED ASCAT. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM 
CONFIDENCE BASED ON EXTRAPOLATION FROM THE ASCAT, SLIGHTLY ABOVE 
DVORAK AGENCY FIXES AND MOST CIMSS OBJECTIVE ESTIMATES AS THE 
APPEARANCE AND CONVECTIVE STRUCTURE CONTINUES TO RAPIDLY IMPROVE.

INITIAL WIND RADII BASIS: 011618Z METOP-C ASCAT DATA

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE CENTERED TO THE
NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   DEMS: T3.0 - 45 KTS
   CIMSS ADT: 55 KTS AT 011800Z
   CIMSS AIDT: 46 KTS AT 011700Z
   CIMSS D-MINT: 57 KTS AT 011715Z
   CIMSS D-PRINT: 66 KTS AT 011800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: HIGH

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TC 04B IS FORECAST TO TRACK NORTHWESTWARD FOR
THE REMAINDER OF THE FORECAST PERIOD UNDER THE INFLUENCE OF THE 
SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTHEAST. AROUND TAU 24, A
SEPARATE STR IS EXPECTED TO BUILD OVER THE WESTERN COAST OF INDIA,
CAUSING A SLOW IN TRACK SPEED AS 04B MOVES INTO THE COL BETWEEN THE 
TWO STRS. LANDFALL IS FORECAST TO OCCUR AROUND TAU 24, AND 04B WILL 
CONTINUE NORTHWESTWARD WITH A WESTERN TURN EXPECTED AT TAU 48 AS THE 
SYSTEM WEAKENS OVER LAND AND BECOMES STEERED PRIMARILY BY THE LOW-
LEVEL SYNOPTIC FLOW. REGARDING INTENSITY, 04B IS FORECAST TO INTENSIFY 
OUT TO A PEAK OF 70 KTS BETWEEN TAUS 12 AND 24 AS ENVIRONMENTAL 
CONDITIONS REMAIN FAVORABLE FOR POSSIBLE RAPID INTENSIFICATION (RI) 
AND THE COMPACT SYSTEM CONTINUES TO ALIGN. THERE IS LOWER CONFIDENCE 
IN THE VALUE OF THIS PEAK, HIGHLY DEPENDENT ON FURTHER RI AS THE 
VORTEX ALIGNS COUNTERED WITH THE POSSIBLE DETRIMENTAL EFFECTS OF THE 
AFOREMENTIONED ELEVATED MID-LEVEL VWS ON THE SYSTEM. WEAKENING WILL 
BEGIN AT TAU 24 AS 04B NEARS LAND AND CONTINUE THEREAFTER, WITH 
DISSIPATION BY TAU 48 AS TERRAIN INTERACTION DISSIPATES THE VORTEX. 

MODEL DISCUSSION: TRACK MODEL GUIDANCE IS IN GOOD AGREEMENT FOR THE
ENTIRETY OF THE FORECAST PERIOD, WITH A CROSS TRACK SPREAD OF 80 NM
AND ALONG-TRACK SPREAD OF 65 NM AT TAU 48. ALL MODELS EXCEPT GFS AND 
NAVGEM DEPICT LANDFALL BY TAU 24. THE OFFICIAL JTWC FORECAST IS THUS 
ASSESSED WITH HIGH CONFIDENCE, IN GOOD AGREEMENT WITH THE MULTI-MODEL 
CONSENSUS. MODEL INTENSITY GUIDANCE CONTINUES TO BIFURCATE WITH 
SEVERAL RI AIDS (RIDE, RI40, RI55, RI30, RI25) CONTINUING TO POPULATE 
AND BOTH DECAY SHIPS MODELS PEAKING AT 80 KTS AT TAU 24. HAFS-A, 
COAMPS-TC, AND THE MULTI-MODEL CONSENSUS SHOW A MORE CONSERVATIVE PEAK 
OF 60-65 KTS BETWEEN TAU 12 AND 24, WHILE BOTH GOOGLE DEEPMINDS AND 
GFS SHOW STEADY WEAKENING. THE OFFICIAL JTWC INTENSITY FORECAST IS 
THUS ASSESSED WITH MEDIUM CONFIDENCE, IN LINE WITH THE MULTI-MODEL 
CONSENSUS INCLUDING RI AIDS (ICNE).

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   INTENSITY 00-72 HR: MEDIUM//
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