Tropical Storm NEOGURI Advisory Cu, 18.10.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (TWENTYONE) WARNING
NR 006//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 21W (NEOGURI), LOCATED APPROXIMATELY 408 NM
SOUTH-SOUTHEAST OF KADENA AB, HAS TRACKED NORTH-NORTHWESTWARD AT 06
KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED
SATELLITE IMAGERY REVEALS TWO AREAS OF DEEP CONVECTION THAT ARE
OBSCURING THE LOW LEVEL CIRCULATION CENTER. THE POSITION IS PLACED
WITH LOW CONFIDENCE BETWEEN THESE TWO LOBES. THE INITIAL INTENSITY
OF 40 KTS IS SET ABOVE THE MULTI-AGENCY DVORAK ESTIMATES OF
T2.0-T2.5 (30-35 KTS), BUT BELOW THE 172010Z ADVANCED DVORAK
TECHNIQUE ESTIMATE 53 KTS. THE ENVIRONMENT IS FAVORABLE WITH
MODERATE (15-20 KTS) VERTICAL WIND SHEAR (VWS), GOOD POLEWARD
OUTFLOW, AND WARM (28-29 CELSIUS) SEA SURFACE TEMPERATURE (SST). TS
21W IS TRACKING SLOWLY NORTH-NORTHWESTWARD TOWARDS A BREAK IN A
SUBTROPICAL RIDGE (STR) WITH LIMITED STEERING.
3. FORECAST REASONING.
   A. THE FORECAST TRACK HAS BEEN SHIFTED TO THE RECURVE SCENARIO.
   B. IN THE NEAR TERM, TS 21W IS EXPECTED TO GRADUALLY SHIFT TO A
NORTHWESTERLY TRACK AS RIDGING SLOWLY BUILDS TO THE NORTHEAST AND
TAKES OVER STEERING. THERE IS SIGNIFICANT UNCERTAINTY IN THE TIMING
AND STRENGTH OF THE STR. DUE TO THIS UNCERTAINTY, THERE IS POOR
CONFIDENCE IN THE TRACK GUIDANCE MEMBERS WITH A SPREAD OF
APPROXIMATELY 400NM BY TAU 72. THE UPPER LEVEL ENVIRONMENT HAS
LIMITED THE INTENSIFICATION OF TS 21W TO THIS POINT. HOWEVER, WITH
THE APPROACHING TROUGH TO THE NORTHWEST OF TS 21W, THIS MAY
STRENGTHEN THE POLEWARD OUTFLOW AND CAUSE SIGNFICANT INTENSITY
CHANGES. TS 21W IS EXPECTED TO CROSS THE STR AXIS BY TAU 72 AND
BEGIN RECURVING TO THE NORTHEAST. ECMWF SHOWS A WESTWARD TRACK THAT
HAS YET TO MATERIALIZE. THE REMAINING CONSENSUS MEMBERS HAVE SHIFTED
TO A RECURVE SCENARIO. THE JTWC TRACK FORECAST IS PLACED TO THE
NORTHEAST OF THESE REMAINING MEMBERS AS TS 21W MAY CONTINUE TO TRACK
TOWARDS THIS BREAK IN THE STR BEFORE GAINING STEERING GUIDANCE. DUE
TO THE AFOREMENTIONED FACTORS, THERE IS OVERALL LOW TRACK AND
INTENSITY CONFIDENCE IN THIS PORTION OF THE FORECAST.
   C. BEYOND TAU 72, TS 21W IS EXPECTED TO RECURVE AS THE
APPROACHING TROUGH WEAKENS THE STR AND TS 21W ENCOUNTERS
SOUTHWESTERLY FLOW. TS 21W IS CURRENTLY EXPECTED TO ENCOUNTER
INCREASED SHEAR WHICH WILL LIKELY WEAKEN TS 21W ALONG ITS POLEWARD
TRACK. WITH SPREAD STILL EXCEEDING 500 NM BY TAU 120, THERE IS LOW
CONFIDENCE IN THE EXTENDED PORTION OF THE JTWC TRACK FORECAST.//
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