Tropical Storm PHANFONE Advisory Tue Sep 30

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (PHANFONE) WARNING
NR 06//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 18W (PHANFONE), LOCATED APPROXIMATELY 145 NM EAST
OF SAIPAN, HAS TRACKED NORTHWESTWARD AT 13 KNOTS OVER THE PAST SIX
HOURS. THE CURRENT POSITION ESTIMATE IS BASED ON A RECENT SATELLITE
FIX FROM PGTW AND A 292310Z AMSU PASS. THE INTENSITY ESTIMATE OF
45 KNOTS REPRESENTS AN AVERAGE OF DVORAK T-NUMBERS FROM PGTW, KNES,
AND RJTD AND A CIMSS 292023Z AUTOMATED SATELLITE INTENSITY CONSENSUS
ESTIMATE OF 50 KNOTS. DEEP CONVECTION HAS WANED A BIT OVER THE PAST
SIX HOURS, BUT THE LOW-LEVEL CIRCULATION CENTER (LLCC) REMAINS
WELL-DEFINED IN MICROWAVE AND MULTISPECTRAL SATELLITE DATA. UPPER-
LEVEL ANALYSIS INDICATES FAVORABLE CONDITIONS FOR FURTHER
DEVELOPMENT, INCLUDING RIDGING ALOFT, LOW VERTICAL WIND SHEAR, AND
A TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT) TO THE WEST ENHANCING
OUTFLOW. THE SYSTEM CONTINUES TO TRACK ALONG THE SOUTHERN PERIPHERY
OF AN EXTENSIVE SUBTROPICAL STEERING RIDGE.
3. FORECAST REASONING.
   A. TS 17W HAS TRACKED A BIT MORE POLEWARD AND FASTER THAN
PREVIOUSLY EXPECTED OVER THE PAST SIX HOURS. HOWEVER, STORM MOTION
IS EXPECTED TO SETTLE BACK TOWARD PREVIOUSLY-FORECASTED MOTION.
THEREFORE, THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TS 18W IS EXPECTED 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, LENDING HIGH CONFIDENCE TO THE JTWC FORECAST TRACK
THROUGH TAU 72. TS 18W IS EXPECTED TO STEADILY INTENSIFY THROUGH
TAU 72 AS IT PASSES OVER VERY WARM WATER AND UPPER-LEVEL CONDITIONS
REMAIN FAVORABLE.
   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 HAVE COME INTO
BETTER AGREEMENT DURING THE LATEST FORECAST CYCLE, WITH ONLY THE
GFDN MODEL DEPICTING AN OUTLIER SCENARIO TO THE WEST OF THE
CONSENSUS GROUPING. THE CURRENT TRACK FORECAST IS CONSISTENT WITH
THE MULTI-MODEL CONSENSUS. GIVEN INCREASING AGREEMENT REGARDING
TIMING AND LOCATION OF THE POLEWARD TURN, CONFIDENCE IN THE
EXTENDED TRACK FORECAST IS HIGH. 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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