Tropical Storm PHANFONE Advisory Sa, 30.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (PHANFONE) WARNING
NR 05//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 18W (PHANFONE), LOCATED APPROXIMATELY 225 NM EAST-
SOUTHEAST OF SAIPAN, HAS TRACKED WEST-NORTHWESTWARD AT 09 KNOTS
OVER THE PAST SIX HOURS. THE CURRENT POSITION ESTIMATE IS BASED ON A
RECENT SATELLITE FIX FROM PGTW AND A 291633Z AMSU IMAGE. THE
INTENSITY ESTIMATE OF 45 KNOTS IS CONSISTENT WITH DVORAK T-NUMBERS
FROM PGTW, KNES, AND RJTD AND WITH A CIMSS 291633Z AUTOMATED
SATELLITE INTENSITY CONSENSUS ESTIMATE OF 43 KNOTS. RECENT MICROWAVE
SATELLITE DATA, INCLUDING THE 211633Z AMSU PASS, INDICATE IMPROVING
LOW-LEVEL STRUCTURE DURING THE PAST SIX HOURS. DEEP CONVECTION HAS
PERSISTED OVER THE CENTER OF THE CYCLONE AND INCREASED WITHIN
CONVECTIVE BANDING ALONG THE NORTHERN AND WESTERN PERIPHERY OF THE
CIRCULATION. UPPER-LEVEL ANALYSIS SHOWS AN ANTICYCLONE DEVELOPING
OVER THE LOW-LEVEL CIRCULATION CENTER AND A TROPICAL UPPER
TROPOSPHERIC TROUGH (TUTT) CELL TO THE WEST, WHICH IS ENHANCING
OUTFLOW ALOFT. THE SYSTEM CONTINUES TO TRACK ALONG THE SOUTHERN
PERIPHERY OF AN EXTENSIVE SUBTROPICAL STEERING RIDGE.
3. FORECAST REASONING.
   A. THE EXTENDED TRACK FORECAST HAS SHIFTED POLEWARD TO INDICATE
A SLIGHTLY TIGHTER RECURVATURE THAN PREVIOUSLY ANTICIPATED.
OTHERWISE, THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TS 18W IS FORECAST TO TRACK WEST-NORTHWESTWARD ALONG THE
SOUTHERN AND SOUTHWESTERN PERIPHERY OF THE SUBTROPICAL RIDGE THROUGH
TAU 72. THE DYNAMIC MODEL GUIDANCE REMAINS IN GOOD AGREEMENT DURING
THIS PERIOD. THUS, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST
TRACK THROUGH TAU 72. TS 18W IS EXPECTED TO STEADILY INTENSIFY
THROUGH TAU 72 DUE TO PASSAGE OVER VERY WARM WATER AND FAVORABLE
UPPER-LEVEL CONDITIONS, INCLUDING LOW VERTICAL WIND SHEAR AND STRONG
OUTFLOW ALOFT. GIVEN THESE FAVORABLE CONDITIONS, A PERIOD OF RAPID
INTENSIFICATION, GREATER THAN CURRENTLY FORECAST, IS POSSIBLE.
   C. TS 18W IS EXPECTED TO TURN POLEWARD AS A DEVELOPING MID-
LATITUDE TROUGH INDUCES A WEAKNESS IN THE SUBTROPICAL STEERING
RIDGE DURING THE EXTENDED FORECAST PERIOD. CONSENSUS MODEL
DEPICTIONS OF THIS DEVELOPING MID-LATITUDE TROUGH VARY, RESULTING IN
INCREASED TRACK FORECAST UNCERTAINTY FOR TS 18W IN THE EXTENDED
PERIOD. THE GFS, GFS ENSEMBLE, AND HWRF INDICATE AN EARLIER BREAK
AND FASTER RECURVATURE, WHILE THE MAJORITY OF THE GUIDANCE SHOW THE
BREAK DEVELOPING LATER AND A SLOWER CONSEQUENT RECURVATURE. THE
CURRENT FORECAST FAVORS THE MAJORITY OF THE GUIDANCE DEPICTING A
SLOWER RECURVATURE. GIVEN THE CURRENT MODEL FORECAST UNCERTAINTY,
CONFIDENCE IN THE EXTENDED FORECAST TRACK IS LOW. TS 18W IS
EXPECTED TO REACH PEAK INTENSITY NEAR THE RIDGE AXIS AROUND
TAU 96 AS POLEWARD OUTFLOW IMPROVES AHEAD OF THE DEVELOPING
MID-LATITUDE TROUGH. THEREAFTER, SLOW WEAKENING SHOULD BEGIN AS
WESTERLY VERTICAL WIND SHEAR BEGINS TO IMPINGE ON THE SYSTEM.//
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