Tropical Storm VONGFONG Advisory Wed Oct 08

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 19W (VONGFONG) WARNING NR 19//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON 19W (VONGFONG), LOCATED APPROXIMATELY 498 NM NORTH-
NORTHWEST OF YAP, HAS TRACKED WEST-NORTHWESTWARD AT 13 KNOTS OVER
THE PAST SIX HOURS. THE CURRENT POSITION ESTIMATE IS BASED, WITH
HIGH CONFIDENCE, ON A RECENT PGTW SATELLITE EYE FIX AND A 070518Z
SSMI IMAGE. THE CURRENT INTENSITY OF 120 KNOTS IS CONSISTENT WITH
DVORAK T-NUMBER ESTIMATES RANGING FROM 5.5 TO 6.5 AND WITH RECENT
AUTOMATED DVORAK INTENSITY ESTIMATES. TY 19W HAS CONTINUED TO
RAPIDLY INTENSIFY OVER THE PAST SIX HOURS, EVIDENCED BY SYMMETRIC
BANDING AND A 27 NM DIAMETER EYE EVIDENT IN RECENT SATELLITE
IMAGERY. UPPER-LEVEL ANALYSIS INDICATES LOW VERTICAL WIND SHEAR AND
MULTIPLE OUTFLOW MECHANISMS, INCLUDING A TROPICAL UPPER-TROPOSPHERIC
TROUGH (TUTT) CELL POSITIONED TO THE EAST. TY 19W CONTINUES TRACKING
GENERALLY WESTWARD ALONG THE SOUTHERN PERIPHERY OF A DEEP-LAYERED
SUBTROPICAL STEERING RIDGE.
3. FORECAST REASONING.
   A. NEAR-TERM FORECAST INTENSITIES HAVE INCREASED GIVEN THE RECENT
RAPID INTENSIFICATION TREND AND THE TAU 120 FORECAST POSITION HAS
BEEN ADJUSTED SLIGHTLY WESTWARD FROM THE PREVIOUS FORECAST.
HOWEVER, THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY
FROM THE PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TY 19W WILL CONTINUE TRACKING WEST-NORTHWESTWARD UNDER THE
STEERING INFLUENCE OF THE CURRENT STEERING RIDGE THROUGH TAU 36.
THEREAFTER, A MID-LATITUDE TROUGH PASSING TO THE NORTH IS EXPECTED
TO WEAKEN THE STEERING RIDGE, CAUSING THE CYCLONE TO MAKE A SHARP
POLEWARD TURN. THE CYCLONE SHOULD CONTINUE TRACKING GENERALLY
POLEWARD ALONG THE STEERING RIDGE PERIPHERY FROM TAU 36 THROUGH TAU
72. THE DYNAMIC CONSENSUS MODEL GUIDANCE IS IN GENERALLY GOOD
AGREEMENT THROUGH TAU 72, LENDING HIGH CONFIDENCE TO THIS PORTION OF
THE FORECAST TRACK. FAVORABLE UPPER-LEVEL CONDITIONS AND PASSAGE
OVER VERY WARM WATER SHOULD ALLOW THE SYSTEM TO REACH PEAK INTENSITY
DURING THIS PERIOD.
   C. TY 19W WILL CONTINUE POLEWARD ALONG THE WESTERN EDGE OF THE
STEERING RIDGE DURING THE EXTENDED FORECAST PERIOD. A TRANSIENT
RIDGE IS EXPECTED TO BUILD TO THE NORTH OF THE SYSTEM LATE IN THE
FORECAST PERIOD, INDUCING A SLIGHT WESTWARD TRACK DEFLECTION BETWEEN
TAU 96 AND TAU 120. DYNAMIC MODEL GUIDANCE SUPPORTS THIS FORECAST
SCENARIO, BUT DEPICTIONS OF THE WESTWARD TRACK DEVIATION VARY.
GIVEN THIS UNCERTAINTY, THE CURRENT EXTENDED FORECAST TRACK LIES IN
THE MIDDLE OF THE FORECAST GUIDANCE NEAR THE MULTI-MODEL CONSENSUS.
TY 19W IS EXPECTED TO WEAKEN SLIGHTLY IN THE EXTENDED PERIOD AS
UPPER-LEVEL OUTFLOW BECOMES LESS FAVORABLE AND ALONG-TRACK OCEAN
HEAT CONTENT DROPS OFF.//
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