Advisories

WDIO31 PGTW 020900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04B (MURASU) WARNING 
NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.3N 80.9E
   INITIAL INTENSITY: 65 KTS
   GEOGRAPHIC REFERENCE: 656 NM SOUTHWEST OF KOLKATA, INDIA
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A COMPACT 
VORTEX CHARACTERIZED BY DEEP INNER-CORE CONVECTION AND VIGOROUS 
OVERSHOOTING CLOUD TOPS DEVELOPING OVER AND AROUND THE ASSESSED LOW-
LEVEL CIRCULATION CENTER (LLCC). A 020029Z RCM-3 SYNTHETIC APERTURE 
RADAR (SAR) PASS REVEALED AN ELLIPTICAL, ASYMMETRIC LLCC FULLY 
ENCLOSED BY GALE-FORCE RADII, WITH PEAK SURFACE WINDS OF 65-70 KTS 
CONCENTRATED IN THE NORTHEASTERN QUADRANT. THE SAR RETRIEVAL PROVIDED 
HIGH CONFIDENCE IN THE 0000Z POST-ANALYSIS POSITION, AND PROMPTED A 
REANALYSIS OF THE WIND FIELD, WHICH IS ASSESSED TO BE CONSIDERABLY 
MORE EXPANSIVE THAN PREVIOUSLY ESTIMATED. HOWEVER, THE SUBSEQUENT 
ABSENCE OF TIMELY MICROWAVE AND SCATTEROMETER MEASUREMENTS RESULTS IN 
THE 0600Z INITIAL POSITION BEING ASSESSED WITH LOW CONFIDENCE, AS 
EVEN COASTAL IMD DOPPLER RADAR REFLECTIVITY DOES NOT RESOLVE A 
DISCRETE LLCC AT THIS TIME. NEVERTHELESS, THE SYSTEM IS ASSESSED TO 
HAVE DECELERATED SIGNIFICANTLY BASED ON THE BROADER CYCLONIC ENVELOPE 
EVIDENT IN COMPOSITE RADAR IMAGERY AND ANIMATED MSI. THE INITIAL 
INTENSITY IS ASSESSED AT 65 KTS, WEIGHTED TOWARD THE UPPER BOUND OF 
AVAILABLE SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES OUTLINED BELOW 
AND SUPPORTED BY EXTRAPOLATION OF THE EARLIER SAR DATA. ENVIRONMENTAL 
CONDITIONS REMAIN MARGINALLY FAVORABLE, CHARACTERIZED BY ELEVATED SEA 
SURFACE TEMPERATURES (SSTS), LOW DEEP-LAYER VERTICAL WIND SHEAR 
(VWS), AND STRONG DUAL-CHANNEL UPPER-TROPOSPHERIC OUTFLOW ALOFT, 
TEMPERED BY PERSISTENT, MODEL-DIAGNOSED NORTHWESTERLY MID-LEVEL SHEAR 
AND INCREASING LAND INTERACTION.

INITIAL WIND RADII BASIS: SAR DATA 

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) OVER THE FAR NORTHERN BAY OF BENGAL. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   DEMS: T3.5 - 55 KTS
   CIMSS ADT: 61 KTS AT 020530Z
   CIMSS AIDT: 57 KTS AT 020530Z
   CIMSS D-MINT: 54 KTS AT 020023Z
   CIMSS D-PRINT: 66 KTS AT 020530Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 5-10 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD
   OTHER FACTORS: ELEVATED MID-LEVEL NORTHWESTERLY SHEAR, AND
PROXIMITY TO LAND.

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   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: TROPICAL CYCLONE (TC) 04B HAS DECELERATED 
ABRUPTLY OVER THE PAST SIX HOURS AS IT TRANSITIONS INTO A RAPIDLY 
EVOLVING COL REGION, INDUCED BY THE BIFURCATION OF THE STR TO THE 
EAST INTO TWO DISTINCT ANTICYCLONIC CELLS. OVER THE NEXT 12 TO 18 
HOURS, AS THIS RIDGE BIFURCATION AMPLIFIES, THE WESTERN PERIPHERY OF 
THE EASTERN ANTICYCLONIC CELL WILL REORIENT ALONG A MORE MERIDIONAL 
(NORTH-SOUTH) AXIS, STEERING TC 04B ONTO A MORE NORTH-NORTHWESTWARD 
TRAJECTORY. TRANSLATION SPEEDS WILL REMAIN QUASI-STATIONARY TO SLOW, 
AND TC 04B COULD BRIEFLY STALL IMMEDIATELY OFFSHORE BEFORE DRIFTING 
INLAND WHILE EMBEDDED WITHIN THE WEAK-STEERING COL REGION. LANDFALL 
IS CURRENTLY FORECAST TO OCCUR NEAR TAU 12, THOUGH A FURTHER 
MERIDIONAL TRACK DEFLECTION OR ADDITIONAL DECELERATION COULD DELAY 
AND SHIFT THE LANDFALL LOCATION FURTHER NORTH ALONG THE COASTLINE. 
POST-LANDFALL, TC 04B WILL ADVECT SLOWLY NORTHWARD BEFORE RECURVING 
SHARPLY WESTWARD BY THE END OF THE FORECAST PERIOD AS THE SHALLOWING 
VORTEX DECOUPLES ALOFT AND BECOMES GOVERNED BY PROGRESSIVELY LOWER-
TROPOSPHERIC STEERING FLOW. REGARDING INTENSITY, FURTHER 
INTENSIFICATION IS UNLIKELY; ALTHOUGH SYNOPTIC UPPER-LEVEL AND 
OCEANIC CONDITIONS REMAIN FAVORABLE, ENCROACHMENT ON THE INDIAN 
COASTLINE WILL INDUCE CONTINENTAL DRY AIR ENTRAINMENT WITHIN THE 
BOUNDARY LAYER ALONG THE WESTERN SEMICIRCLE OF THE INNER CORE. 
ADDITIONALLY, COASTAL DECELERATION OVER SHALLOW SHELF WATERS WILL 
INDUCE LOCALIZED OCEANIC UPWELLING OF COOLER NEARSHORE WATERS, 
DEPRIVING THE VORTEX OF THE SURFACE HEAT FLUXES REQUIRED TO SUSTAIN 
INTENSIFICATION. FOLLOWING LANDFALL, TC 04B WILL RAPIDLY SPIN DOWN 
DUE TO SURFACE FRICTIONAL DISSIPATION AND TOPOGRAPHIC DISRUPTION, 
COMPLETELY DISSIPATING OVER EASTERN INDIA BY TAU 48.

MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD OVERALL AGREEMENT, THOUGH 
THE MAJORITY OF THE MULTI-MODEL GUIDANCE SUITE FAILS TO RESOLVE THE 
SUBTLE MERIDIONAL TRAJECTORY DEFLECTION. ONLY THE DETERMINISTIC GFS, 
GOOGLE DEEPMIND (GDM), HAFS-A, AND HWRF CAPTURE THIS POLEWARD 
CURVATURE, WHEREAS THE REMAINDER OF THE GUIDANCE SUITE MAINTAINS A 
LINEAR NORTHWESTWARD TRAJECTORY PRIOR TO RECURVING WESTWARD BY TAU 
48. THE OFFICIAL JTWC TRACK FORECAST IS ALIGNED CLOSELY WITH THE GFS 
AND HAFS-A SOLUTIONS, WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE IS 
IN TIGHT AGREEMENT, WITH ONLY THE STATISTICAL-DYNAMICAL SHIPS-GFS 
INDICATING MODEST INTENSIFICATION PRIOR TO LANDFALL, WHILE THE 
REMAINDER OF THE INTENSITY GUIDANCE SUITE DEPICTS IMMEDIATE 
WEAKENING. THE OFFICIAL JTWC INTENSITY FORECAST IS POSITIONED 05 KTS 
ABOVE THE MULTI-MODEL INTENSITY CONSENSUS (ICNW) THROUGH THE FORECAST 
PERIOD WITH HIGH CONFIDENCE. 

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