WDPN31 PGTW 132100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 28W (YAMANEKO) WARNING NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 22.8N 165.9E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 214 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED PROXY VISIBLE SATELLITE IMAGERY SHOWS A COMPLETELY EXPOSED
LOW LEVEL CIRCULATION CENTER (LLCC), WHICH IS BEGINNING TO STRETCH
OUT AND BECOME INCREASINGLY RAGGED. MANUAL PHASE CLASSIFICATION
WORKSHEETS INDICATE THE SYSTEM IS WEAKLY SUBTROPICAL, WHILE
AUTOMATED CLASSIFICATIONS INDICATE THE SYSTEM IS STILL FULLY
TROPICAL. THE CURRENT ASSESSMENT IS THAT TD 28W IS A BORDERLINE
TROPICAL-SUBTROPICAL CYCLONE, HAVING CHARACTERISTICS OF BOTH TYPES
OF CYCLONES. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE
BASED ON THE EXPOSED LLCC AND A 131711Z SSMIS 91GHZ MICROWAVE
IMAGE. THE ASSOCIATED CONVECTION IS SHEARED FAR TO THE NORTHEAST
AND LIES IN A NORTHEAST TO SOUTHWEST ORIENTED BAND. THE INITIAL
INTENSITY IS LOWERED TO 30 KNOTS, A BIT BELOW THE AGENCY DVORAK
CURRENT INTENSITY ESTIMATES, BASED ON A 131420Z AMSR2 WINDSPEED
PRODUCT WHICH SHOWS NO MORE THAN 30 KNOT WINDS WITHIN 150NM OF THE
CENTER POSITION AS WELL AS THE QUICKLY DROPPING FINAL-T NUMBERS,
ALL OF WHICH HAVE COME DOWN TO THE T1.5-T2.0 RANGE. THE SYSTEM LIES
IN AN UNFAVORABLE ENVIRONMENT, WITH VERY STRONG VWS OFFSETTING
STRONG POLEWARD OUTFLOW AND SSTS AROUND 26C.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF THE LOW TO
MID-LEVEL TRANSITORY RIDGE TO THE NORTH AND THE WESTERN PERIPHERY
OF THE DEEP-LAYER SUBTROPICAL RIDGE (STR) TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T2.5 - 35 KTS
   CIMSS SATCON: 43 KTS AT 131438Z
   CIMSS ADT: 34 KTS AT 131740Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 30+ KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

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

FORECAST DISCUSSION: HAVING DRIFTED RATHER AIMLESSLY OVERNIGHT, TD
28W HAS NOW STARTED TO MOVE NORTHWARD AS THE SHALLOW TRANSITORY
RIDGE TO THE NORTH HAS NOW MOVED EAST OF THE 170E LONGITUDE. IN THE
DEEP-LAYER, THE WESTERN SIDE OF THE SUBTROPICAL RIDGE (STR) TO THE
EAST IS ORIENTED NORTH-SOUTH AND THE COMBINATION OF THESE TWO
FACTORS WILL PUSH TD 28W ONTO A NORTH-NORTHEASTWARD TRACK GOING
FORWARD. THE APPROACH OF A STRONG, NEGATIVELY TILTED MID-LATITUDE
TROUGH MOVING IN FROM THE WEST WILL STRENGTHEN THE STEERING
GRADIENT AND TD 28W WILL STEADILY ACCELERATE TO NEAR 30 KNOTS
THROUGH THE FORECAST PERIOD. AS DISCUSSED ABOVE, THE SYSTEM IS
CURRENT ASSESSED AS A BORDERLINE TROPICAL-SUBTROPICAL CYCLONE AND
IS LIKELY TO BECOME FULLY SUBTROPICAL WITHIN THE NEXT 12 HOURS AS
IT MOVES IT MOVES NORTH OF THE 26C ISOTHERM AND BEGINS TO INTERACT
WITH THE TROUGH TO THE WEST BUT RETAINS ITS WARM CORE STRUCTURE.
THE SUBTROPICAL PHASE WILL BE SHORT-LIVED HOWEVER, AND AS THE
SYSTEM RACES NORTHWARD, IT WILL COME INTO PHASE WITH THE NEGATIVELY
TILTED TROUGH MOVING IN FROM THE WEST, MOVE UNDER A STRONG 200MB
JET MAX AND MOVE OVER RAPIDLY COOLING WATER, BEGINNING
EXTRATROPICAL TRANSITION (ETT) BY TAU 24. THE TRANSITION TO
EXTRATROPICAL LOW WILL BE VERY RAPID AND DUE TO THE STRONG
BAROCLINIC FORCING, THE SYSTEM IS EXPECTED TO INTENSIFY AS IT MOVES
THROUGH ETT. ETT IS EXPECTED TO BE COMPLETE NO LATER THAN TAU 36,
POTENTIALLY A BIT EARLIER.  

MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN
VERY GOOD AGREEMENT, WITH MINIMAL SPREAD IN THE CONSENSUS MEMBERS
THROUGH THE FORECAST DURATION. ALONG TRACK SPREAD INCREASES
DRAMATICALLY AFTER TAU 24 AS THE MODELS ACCELERATE THE VORTEX AT
DIFFERING RATES WHILE UNDERGOING ETT. THE JTWC FORECAST REMAINS
CONSISTENT WITH THE PREVIOUS FORECAST AND LIES CLOSE TO THE
MULTI-MODEL CONSENSUS WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE IS
IN POOR AGREEMENT, WITH A WIDE SPREAD BETWEEN THE DECAY-SHIPS WHICH
SHOWS STEADY WEAKENING AND DISSIPATION THROUGH TAU 24, WHILE THE
MESOSCALE MODELS INCLUDING COAMPS-TC AND HWRF SHOW A DRAMATIC
INTENSIFICATION UP TO 50 KNOTS OR MORE BY THE END OF THE FORECAST
AS THE SYSTEM INTENSIFIES WHILE UNDERGOING ETT DUE TO THE VERY
STRONG BAROCLINIC FORCING. THE JTWC FORECAST IS NEAR THE CONSENSUS
MEAN BUT CONFIDENCE IS LOW DUE TO THE WIDE SPREAD IN THE GUIDANCE.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: LOW//
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