WDPN31 PGTW 170300
MSGID/GENADMIN/NAVPACMETOCCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 08W WARNING NR 12//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TYPHOON (TY) 08W HAS CONTINUED TRACKING NORTH-NORTHWESTWARD
ALONG THE SOUTHEASTERN PERIPHERY OF A SUBTROPICAL STEERING RIDGE
DURING THE PAST 12 HOURS. SLOW AND STEADY INTENSIFICATION HAS GIVEN
WAY TO A PERIOD OF RAPID DEEPENING, WITH THE ESTIMATED MAXIMUM SUS-
TAINED WIND SPEED JUMPING FROM 60 KNOTS TO 80 KNOTS OVER THE PAST 06
HOURS IN RESPONSE TO STRENGTHENING UPPER LEVEL OUTFLOW AND HIGH SEA
SURFACE TEMPERATURES. THE LOW LEVEL CIRCULATION OF TY 08W IS SHROUDED
BY AN IMPRESSIVE CENTRAL DENSE OVERCAST FEATURE. THE CYCLONE HAS
EXHIBITED A DEVELOPING EYE FEATURE IN RECENT MICROWAVE SATELLITE
IMAGERY, INCLUDING A 170021Z SSMIS PASS.
   B. TS 08W IS TRACKING POLEWARD AND WESTWARD UNDER THE INFLUENCE OF
A SUBTROPICAL STEERING RIDGE TO THE EAST AND NORTHEAST. AN EXTENSION
OF THIS RIDGE ON THE POLEWARD SIDE OF TY 08W IS BEGINNING TO PRESS THE
STORM TRACK SLIGHTLY WESTWARD. THE CURRENT INTENSITY ESTIMATE IS BASED
ON 170000Z DVORAK T-NUMBERS OF 4.5 FROM RJTD AND 5.0 FROM PGTW. ANI-
MATED WATER VAPOR IMAGERY AND SATELLITE CLOUD DRIFT WIND OBSERVATIONS
INDICATE THAT EQUATORWARD OUTFLOW REMAINS STRONG AND POLEWARD OUTFLOW
HAS IMPROVED MARKEDLY. A WEAK UPPER LEVEL LOW TO THE WEST OF TY 08W IS
PROVIDING AN ADDITIONAL WESTWARD VENTING MECHANISM. 
3. FORECAST REASONING
   A. TAU 12 TO TAU 48 FORECAST POSITIONS HAVE SHIFTED WESTWARD AND
EXTENDED FORECAST TRACK SPEEDS HAVE INCREASED SINCE THE PREVIOUS
ISSUANCE OF THE PROGNOSTIC REASONING MESSAGE. ADDITIONALLY, FORECAST
INTENSITIES HAVE BEEN ADJUSTED UPWARD IN RESPONSE TO THE RECENT RAPID
INTENSIFICATION TREND.
   B. TY 08W IS NOW FIRMLY EMBEDDED IN A STRONG POLEWARD TO NORTHWEST-
WARD STEERING ENVIRONMENT DOMINATED BY SUBTROPICAL RIDGING TO THE
NORTH AND EAST. THE TYPHOON IS EXPECTED TO PROGRESS GENERALLY NORTH-
WESTWARD THROUGH TAU 24 AS IT APPROACHES THE AXIS OF THE RIDGE EXTEN-
SION TO THE NORTH. AN APPROACHING SHORTWAVE TROUGH CURRENTLY ANALYZED
OVER NORTH CENTRAL CHINA IS EXPECTED TO WEAKEN THE WESTERN PERIPHERY
OF THIS RIDGE EXTENSION, INDUCING RECURVATURE OF TY 08W ALONG THE
EASTERN COAST OF CHINA TO THE SOUTH OF SHANGHAI BETWEEN TAU 24 AND
TAU 36. HOWEVER, SOME ERRATIC MOTION NOT REFLECTED IN THE CURRENT
TRACK FORECAST MAY OCCUR IF THE STORM MOVES FURTHER WESTWARD AND
INTERACTS SIGNIFICANTLY WITH THE ISLAND OF TAIWAN. THE FORECAST TRACK
IS SUPPORTED BY THE AVAILABLE NUMERICAL MODEL TRACK GUIDANCE, WHICH
SHOWS GOOD AGREEMENT WITH THE EXCEPTION OF TCLAPS, THE EASTERN OUTLIER.
TY 08W SHOULD CONTINUE TO SIGNIFICANTLY INTENSIFY UNDER THE INFLUENCE
OF GOOD UPPER LEVEL OUTFLOW AND HIGH SEA SURFACE TEMPERATURES OVER
THE NEXT 12 HOURS. THEREAFTER, LAND INTERACTION WILL LIMIT FURTHER
INTENSIFICATION, PARATICULARLY IF THE STORM FOLLOWS A TRACK ACROSS
THE ISLAND OF TAIWAN RATHER THAN ACROSS THE EXTREME NORTHERN TIP AS
CURRENTLY FORECAST. LANDFALL ALONG THE EAST COAST OF CHINA AROUND
TAU 36 WILL INDUCE A STEADY WEAKENING TREND UNTIL TAU 48. TY 08W WILL
THEN REEMERGE OVER WATER, BUT INCREASING VERTICAL WIND SHEAR NORTH OF
THE STEERING RIDGE AXIS AND PASSAGE OVER COOLER WATER WILL LEAD TO
FURTHER WEAKENING BY TAU 72. ADDITIONALLY, THE WEAKENED TROPICAL
CYCLONE WILL ENCOUNTER A DEVELOPING MIDLATITUDE TROUGH BETWEEN
TAU 48 AND TAU 72, BEGINNING THE PROCESS OF EXTRATROPICAL TRANSITION.
   C. A WEAKENED TROPICAL STORM 08W WILL TRANSITION INTO A STRONG
EXTRATROPICAL LOW AS IT JOINS THE MIDLATITUDE WESTERLY FLOW PATTERN
IN THE EXTENDED PERIOD. IT APPEARS AT THIS POINT THAT THE TRANSITION-
ING SYSTEM WILL CROSS THE KOREAN PENINSULA AS A FORMIDABLE SYSTEM
DURING THIS PERIOD, ALTHOUGH A WESTWARD SHIFT IN THE TRACK PRIOR TO
RECURVATURE COULD INDUCE VERY SIGNIFICANT WEAKENING PRIOR TO TAU 96.
FORECAST TEAM: BRAVO//
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