Advisories

WDPN32 PGTW 280300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (ETAU) WARNING NR 
005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.1N 167.4E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 291 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A HIGHLY
ASYMMETRIC AND SHEARED SYSTEM, WITH FLARING DEEP CONVECTION
ORIGINATING NEAR THE ASSESSED LOW LEVEL CIRCULATION CENTER (LLCC)
AND SHEARING OFF TO THE EAST. A PARTIAL 272309Z ASCAT-C PASS
REVEALED AN ELONGATED LLC WITH THE SMALL LLCC POSITIONED ON THE
EDGE OF THE ASCAT SWATH. THE WESTERN HALF OF THE CIRCULATION IS
COMPRISED OF LIGHT WINDS WHILE THE HIGHEST, GALE-FORCE WINDS ARE
CONFINED TO THE EASTERN FLANK OF THE SYSTEM. ANIMATED WATER VAPOR
IMAGERY DEPICTS AN UPPER-LEVEL ANTICYCLONE POSITIONED TO THE
SOUTHWEST, NEAR TS 20W, WHICH IS PRODUCING SOUTHWESTERLY FLOW AND
MODERATE TO HIGH SOUTHWESTERLY SHEAR OVER 21W WHILE ALSO SUPPORTING
STRONG POLEWARD OUTFLOW. THE INITIAL POSITION IS ASSESSED WITH HIGH
CONFIDENCE BASED ON THE PARTIALLY EXPOSED LLCC IN THE MSI AND THE
ASCAT DATA. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM
CONFIDENCE, BASED ON THE HIGHER END OF THE OBJECTIVE AND SUBJECTIVE
INTENSITY ESTIMATES OUTLINED BELOW. ENVIRONMENTAL CONDITIONS ARE
MARGINAL, WITH WARM SSTS AND STRONG OUTFLOW ALOFT OFFSET BY
MODERATE TO HIGH SOUTHWESTERLY SHEAR.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WESTERN FLANK OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED TO THE EAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T3.0 - 45 KTS
   PHFO: T3.0 - 45 KTS
   CIMSS ADT: 37 KTS AT 272340Z
   CIMSS AIDT: 42 KTS AT 272340Z
   CIMSS D-PRINT: 33 KTS AT 272340Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 25-30 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL STORM (TS) 21W (ETAU) IS TRACKING
NORTHWARD ALONG THE WESTERN FLANK OF A DEEP-LAYER SUBTROPICAL RIDGE
(STR) AND IS FORECAST TO CONTINUE THIS GENERAL MOTION FOR THE NEXT
36 HOURS. AFTER TAU 36, THE SYSTEM BEGINS TO DECELERATE AS IT MOVES
INTO A WEAKER STEERING ENVIRONMENT AS THE RIDGE TO THE NORTHEAST IS
WEAKENED DUE TO THE PASSAGE OF THE REMNANTS OF TS 01C. BY TAU 48,
THE SYSTEM WILL HAVE BECOME QUASISTATIONARY AS IT MOVES DEEPER INTO
THE WEAK AND COMPETING STEERING PATTERN AND MOVES UNDER AN
UPPER-LEVEL LOW. THE SYSTEM REMAINS QUASISTATIONARY, OR LOOPING
THROUGH TAU 72 AS IT UNDERGOES BINARY INTERACTION AND FUJIWHARA
MOTION WITH TS 20W MOVING UP RAPIDLY FROM THE SOUTH. AS TS 20W
REACHES A POSITION NORTH OF 21W BY TAU 72, TS 21W WILL MOVE OFF
SLOWLY TO THE NORTHEAST AS THE TWO SYSTEMS SLOWLY EXTRICATE
THEMSELVES FROM ONE ANOTHER. REGARDING INTENSITY, THE PERSISTENT
NEGATIVE INFLUENCE OF MODERATE WESTERLY SHEAR WILL CONTINUE TO
INHIBIT SIGNIFICANT INTENSIFICATION. A SLIGHT INCREASE IN INTENSITY
IS POSSIBLE IN THE NEXT 12 HOURS, BUT WILL BE TEMPORALLY LIMITED.
THE FORECAST CALLS FOR THE SYSTEM TO MAINTAIN ITS CURRENT INTENSITY
THROUGH TAU 36 BEFORE GRADUALLY WEAKENING THROUGH THE REMAINDER OF
THE FORECAST AS IT MOVES INTO COOLER WATERS, FACES PERSISTENT SHEAR
AND BECOMES ENVELOPED IN DRY AIR. SUBTROPICAL TRANSITION (STT)
BEGINS AT TAU 72, AND WILL BE COMPLETED NO LATER THAN TAU 120, BUT
POSSIBLE AS EARLY AS TAU 96. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN GENERALLY GOOD AGREEMENT
THROUGH TAU 48, WITH CROSS-TRACK SPREAD INCREASING TO 210NM BY THIS
POINT. CONSISTENT WITH EARLIER MODEL RUNS, THE AI MODELS ARE
STRUGGLING TO PICK UP ON THE CLIMATOLOGICALLY ATYPICAL FUJIWHARA,
AND TAKING THE SYSTEM STRAIGHT THROUGH ON A NORTHWARD THEN
NORTHEASTWARD TRACK THROUGH TAU 120. THE DETERMINISTIC MODELS ON
THE OTHER HAND BEGIN TO DEPICT ERRATIC TRACK BEHAVIOR AFTER TAU 60,
AS THE SYSTEM BEGINS TO INTERACT AND FUJIWHARA WITH TS 20W MOVING
UP FROM THE SOUTHEAST. THE RESULT IS A HIGHLY CHAOTIC TRACK SPREAD
WHICH EXTENDS ACROSS NEARLY 1000NM. THE JTWC FORECAST IS BASED ON A
BLEND OF THE AVAILABLE TRACKERS, BUT MOST CLOSELY CONFORMS TO THE
MODIFIED CONSENSUS MEAN. CONFIDENCE IS LOW THROUGHOUT THE FORECAST
PERIOD. REGARDING INTENSITY, MODEL GUIDANCE IS IN BETTER AGREEMENT,
WITH THE CONSENSUS AND TC-SPECIFIC INTENSITY GUIDANCE DEPICTING AN
INTENSITY THAT MAXES OUT AT 50 KNOTS (GFS) BUT FOR THE MOST PART IS
CONFINED TO 45 KNOTS OR LESS THROUGH TAU 36, BEFORE GRADUALLY
WEAKENING THROUGH TAU 120. THE JTWC FORECAST IS CLOSELY ALIGNED
WITH THE HAFS-A WITH MEDIUM CONFIDENCE. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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