Advisories

WDIO31 PGTW 222100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 01B (ONE) WARNING NR 
001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.6N 84.8E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 362 NM SOUTHWEST OF KOLKATA, INDIA
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS PERSISTENT 
DEEP CONVECTION OVER A CONSOLIDATING, PARTIALLY EXPOSED LOW-LEVEL 
CIRCULATION CENTER (LLCC). CLOUD TOP TEMPERATURES HAVE BEEN COLDER 
THAN -80 DEGREES CELSIUS, AND NEW CONVECTION IS DEVELOPING ON THE 
UPSHEAR SIDE OF THE LLCC IN THE PRESENCE OF MODERATE NORTHEASTERLY 
VERTICAL WIND SHEAR (VWS). THE ENVIRONMENT IS GENERALLY FAVORABLE, 
CHARACTERIZED BY VERY WARM SEA SURFACE TEMPERATURES (SST) OF 29-30 
DEGREES CELSIUS AND RICH DEEP-LAYER MOISTURE, ALTHOUGH THESE FAVORABLE 
FACTORS ARE PARTIALLY OFFSET BY AROUND 20 KTS OF VWS. AFTER TRACKING 
GENERALLY WESTWARD OVER THE PAST 12 HOURS, THE SYSTEM HAS SLOWED TO 04 
KTS. ASCAT PASSES FROM 221444Z AND 221506Z REVEALED A WELL-DEFINED, 
THOUGH ASYMMETRIC, LLCC, WITH GALE-FORCE WINDS CONCENTRATED IN THE 
SOUTHERN SEMICIRCLE. BASED ON THE ASCAT DATA, 95B HAS BEEN UPGRADED TO 
TROPICAL CYCLONE (TC) 01B, AND THE INITIAL INTENSITY IS SET AT 35 KTS 
WITH HIGH CONFIDENCE. THE INITIAL POSITION IS PLACED WITH HIGH 
CONFIDENCE BASED ON THE PARTIALLY EXPOSED LLCC AND THE ASCAT PASSES.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA

CURRENT STEERING MECHANISM: A WEAK STEERING ENVIRONMENT DEFINED BY
A LOW TO MID-LEVEL SUBTROPICAL RIDGE (STR) OVER NORTHWESTERN INDIA
AND A STR OVER THE FAR NORTHERN PORTION OF THE BAY OF BENGAL

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   DEMS: T2.0 - 30 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE WESTWARD
   OTHER FACTORS: LAND INTERACTION

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TC 01B IS DRIFTING WEST-NORTHWESTWARD WITHIN A 
WEAK STEERING ENVIRONMENT BETWEEN A STR OVER NORTHWESTERN INDIA AND 
ANOTHER STR OVER THE FAR NORTHERN BAY OF BENGAL. THIS SYNOPTIC-SCALE 
PATTERN WILL CHANGE LITTLE THROUGH THE FORECAST PERIOD. THEREFORE, THE 
COMBINATION OF BETA DRIFT AND A SLIGHTLY STRONGER STEERING INFLUENCE 
OF THE STR TO THE NORTHEAST WILL DRIVE THE SYSTEM NORTHWESTWARD AT 03-
05 KTS INTO INDIA BETWEEN TAU 12 AND TAU 24. THIS MOTION WILL CONTINUE 
UNTIL DISSIPATION BY TAU 36. THE HIGH-MOISTURE ENVIRONMENT AND VERY 
WARM SST WILL PARTIALLY OFFSET THE DETRIMENTAL EFFECTS OF VWS WHILE 
THE SYSTEM REMAINS OVER WATER. THEREFORE, THE FORECAST CALLS FOR 
SLIGHT INTENSIFICATION TO 40 KTS AT TAU 12. A BRIEF PEAK SLIGHTLY 
ABOVE 40 KTS IS POSSIBLE PRIOR TO LANDFALL. TC 01B WILL DISSIPATE OVER 
LAND BY TAU 36. THE 30-KT INTENSITY AT TAU 36 REFLECTS ENHANCED 
SOUTHWESTERLY FLOW OFF THE COAST OF INDIA WHILE THE INLAND VORTEX 
WEAKENS AND BROADENS.

MODEL DISCUSSION: THE TRACK GUIDANCE ENVELOPE IS WIDE, WITH THE CROSS-
TRACK SPREAD INCREASING FROM 60 NM AT TAU 12 TO 125 NM AT TAU 24. 
GALWEM LIES ALONG THE EQUATORWARD EDGE OF THE ENVELOPE, WHILE NAVGEM 
IS THE POLEWARD OUTLIER. THE GFS, ECMWF, AND THE EXPERIMENTAL GOOGLE 
DEEPMIND SOLUTIONS ARE NEAR THE CENTER OF THE ENVELOPE AND CLOSEST TO 
THE CONSENSUS (CONW). THE JTWC FORECAST TRACK IS POSITIONED SLIGHTLY 
SOUTH OF CONW AND HEDGED TOWARD DEEPMIND AT TAU 12 AND TAU 24 WITH 
MEDIUM CONFIDENCE. THE FORECAST DEPICTS A LANDFALL BETWEEN SRIKAKULAM 
AND VISAKHAPATNAM. THE INTENSITY GUIDANCE IS MUCH TIGHTER, WITH A 05-
KT SPREAD THROUGH TAU 36. THE JTWC INTENSITY FORECAST IS ASSESSED WITH 
HIGH CONFIDENCE, ALIGNING CLOSELY WITH HAFS-A, DEEPMIND, GFS, AND 
COAMPS-TC AT TAU 24 AND TAU 36. THE 40-KT FORECAST INTENSITY AT TAU 12 
IS ON THE HIGHER SIDE OF THE ENVELOPE.

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