

WDPN31 PGTW 060900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SHANSHAN) WARNING NR 15//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
TYPHOON 17W (SHANSHAN), LOCATED APPROXIMATELY 209 NM EAST OF
CHICHI JIMA, HAS TRACKED NORTHWARD AT 10 KNOTS OVER THE PAST SIX
HOURS. THE INITIAL POSITION IS PLACED WITH GOOD CONFIDENCE BASED ON
ANIMATED MULTISPECTRAL SATELLITE IMAGERY SHOWING A FORMATIVE EYE. A
060302Z AMSR2 89GHZ IMAGE SHOWS CONVECTION IS DEEPER IN THE SOUTHERN
PORTION OF THE EYEWALL. THE INITIAL INTENSITY OF 80 KTS IS BASED ON
PGTW, KNES, AND RJTD DVORAK CURRENT INTENSITY ESTIMATES OF T4.5 (77
KTS), AND IS CLOSE TO THE 060301Z SATCON ESTIMATE OF 81 KTS, BUT
ABOVE THE LATER 060538Z SATCON ESTIMATE OF 67 KTS. DRY AIR WRAPPING
IN FROM THE WEST IS PREVENTING FURTHER INTENSIFICATION DESPITE 17W
BEING IN AN OTHERWISE FAVORABLE ENVIRONMENT WITH LOW (10-15 KT)
VERTICAL WIND SHEAR, GOOD EQUATORWARD AND POLEWARD OUTFLOW, AND WARM
SEA SURFACE TEMPERATURES (28-29 C). TY 17W CONTINUES TO TRACK ALONG
THE WESTERN PERIPHERY OF A NORTH-SOUTH ORIENTED SUBTROPICAL RIDGE
(STR) TO THE EAST.
3. FORECAST REASONING.
A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
B. THROUGH TAU 72, TY 17W WILL CONTINUE A NORTH-NORTHWESTWARD TO
NORTHWESTWARD TRACK ALONG THE WESTERN PERIPHERY OF THE STR. THE DRY
AIR AND WANING OUTFLOW ARE EXPECTED TO CAUSE TY 17W TO GRADUALLY
WEAKEN AS IT TRACKS TOWARD JAPAN, FALLING TO 65 KTS BY TAU 72. AS
THE STR TO THE EAST RECEDES, THERE WILL BE A BRIEF OPPORTUNITY FOR
INCREASED POLEWARD OUTFLOW THAT WILL ALLOW THE INTENSITY TO REMAIN
STEADY BETWEEN TAU 36-48. THE JTWC INTENSITY FORECAST FOLLOWS THE
INTENSITY CONSENSUS. THE TRACK FORECAST FOLLOWS THE MULTI-MODEL
CONSENSUS THROUGH TAU 72. MOST MEMBERS SHOW A BRIEF JOG TO THE
NORTHWEST AROUND TAU 36-48 AS THE STR BRIEFLY STRENGTHENS TO THE
NORTH OF 17W. BETWEEN TAU 48-72, ALL MEMBERS PORTRAY 17W RECURVING
AROUND THE STR. THE GFS TRACK IS THE MOST WESTERN OUTLIER, WHILE
ECMWF IS CLOSE TO AND SLIGHTLY INSIDE OF THE MULTI-MODEL CONSENSUS.
BASED ON SPREAD IN THE MODEL GUIDANCE OF 360 NM BY TAU 72,
CONFIDENCE IN THE JTWC FORECAST TRACK IS LOW FOR THIS PORTION OF THE
FORECAST.
C. AFTER ROUNDING THE STR AXIS AROUND TAU 72, TY 17W IS FORECAST
TO TRACK NORTHEASTWARD, ACCELERATE, AND BEGIN GRADUALLY
TRANSITIONING INTO AN EXTRATROPICAL CYCLONE. THE TIMING OF THE TURN
AND THE FORWARD SPEED AFTER THE TURN VARY SIGNIFICANTLY AMONG THE
GUIDANCE MEMBERS, WITH ECMWF FASTER THAN THE MULTI-MODEL CONSENSUS.
THE JTWC FORECAST TRACK CLOSELY FOLLOWS THE MULTI-MODEL CONSENSUS
THROUGHOUT THIS PERIOD. AS 17W APPROACHES 40 DEGREES NORTH LATITUDE
AROUND TAU 96 AND ENCOUNTERS COOLER SSTS AND A PASSING MIDLATITUDE
TROUGH, IT WILL BEGIN EXTRATROPICAL TRANSITION. THE GRADUAL
WEAKENING TREND CONTINUES THROUGHOUT THIS PORTION OF THE FORECAST
AND, ALTHOUGH THE WIND FIELD IS EXPECTED TO EXPAND, THE INTENSITY IS
EXPECTED TO FALL TO 50 KTS BY THE END OF TAU 120. CONFIDENCE IN THE
FORECAST TRACK REMAINS LOW DUE TO HIGH UNCERTAINTY IN THE TIMING OF
THE RECURVATURE AND THE FORWARD SPEED OF THE CYCLONE AS IT UNDERGOES
EXTRATROPICAL TRANSITION.//
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