Advisories

WDPN32 PGTW 010300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 26W (CHOI-WAN) WARNING NR 
005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.7N 147.9E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 250 NM NORTHEAST OF ANDERSEN AFB
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 16 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS PERSISTENT,
DEEP CONVECTION CONCENTRATED ALONG THE WESTERN PERIPHERY OF AN
ELONGATED LOW-LEVEL CIRCULATION CENTER (LLCC). CONVECTION CONTINUES
TO STRUGGLE TO WRAP UPSHEAR INTO THE EASTERN SEMICIRCLE DUE TO
MODERATE NORTHEASTERLY VERTICAL WIND SHEAR (VWS). UPPER-LEVEL
OUTFLOW HAS BECOME RESTRICTED OVER THE EASTERN PERIPHERY. HOWEVER,
THE VORTEX TILT IS LESSENING, WITH A SMALL, RAGGED CENTRAL DENSE
OVERCAST FORMING NEARLY OVER THE LLCC AND CURVED BANDING BECOMING
INCREASINGLY CURVED. DESPITE THE SHEAR, THE ENVIRONMENT IS
FAVORABLE, DEFINED BY VERY WARM SEA SURFACE TEMPERATURES (SST) OF
30-31 DEGREES CELSIUS AND VERY HIGH OCEAN HEAT CONTENT (OHC) OF
140-150 KJ PER CM2, MODERATE DIVERGENCE ALOFT, AND ABUNDANT
DEEP-LAYER MOISTURE. A 302246Z ASCAT-B PASS REVEALED AN ASYMMETRIC
WIND FIELD, WITH MAXIMUM SUSTAINED WINDS OF 35 KTS ON THE WEST SIDE
OF THE LLCC. THE PASS ALSO INDICATED THAT THE AREAL EXTENT OF
GALE-FORCE WINDS HAS EXPANDED. SURFACE OBSERVATIONS AT SAIPAN
INTERNATIONAL AIRPORT, LOCATED APPROXIMATELY 150 NM SOUTHWEST OF
THE LLCC, INDICATE THAT WINDS HAVE RECENTLY BEGUN TO INCREASE, WITH
GUSTS NEAR GALE FORCE. SUBSEQUENT SATELLITE IMAGERY INDICATES THAT
THE LLCC HAS CONTINUED TO CONSOLIDATE SINCE THE ASCAT PASS.
THEREFORE, THE INITIAL INTENSITY HAS BEEN RAISED TO 40 KTS WITH
HIGH CONFIDENCE, BASED ON RECENT ASCAT DATA AND THE SUBSEQUENT
IMPROVEMENT IN THE SATELLITE PRESENTATION. THE INITIAL POSITION IS
PLACED WITH HIGH CONFIDENCE BASED ON THE SCATTEROMETER DATA.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A LOW- TO
MID-LEVEL SUBTROPICAL RIDGE CENTERED FAR TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.5 - 35 KTS
   CIMSS ADT: 45 KTS AT 302040Z
   CIMSS AIDT: 38 KTS AT 302040Z
   CIMSS D-MINT: 30 KTS AT 302012Z
   CIMSS D-PRINT: 37 KTS AT 302310Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: HIGH

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE TIMING OF THE CPA TO SAIPAN HAS
SHIFTED EARLIER BY SIX HOURS.

FORECAST DISCUSSION: RECENT SCATTEROMETER DATA INDICATE THAT TS 26W
HAS ACCELERATED WEST-NORTHWESTWARD, WITH AN AVERAGE FORWARD SPEED
OF 14 KTS DURING THE PAST 12 HOURS. THE SYSTEM IS BEING STEERED BY
A LOW- TO MID-LEVEL STR CENTERED NEAR 30N 172E. TOWARD TAU 24, A
PROGRESSIVE SHORTWAVE TROUGH RACING DOWNSTREAM OF NORTHERN HONSHU
AND HOKKAIDO WILL ERODE THE STR, RESULTING IN A MARKED WEAKENING OF
THE PRIMARY STEERING INFLUENCE AND CAUSING TS 26W TO BECOME NEARLY
STATIONARY. THE DEEP-LAYER RIDGE WILL REORIENT, WHILE A
NEAR-EQUATORIAL RIDGE (NER) TO THE SOUTHEAST STRENGTHENS,
ESTABLISHING A STEERING PATTERN THAT WILL TURN TS 26W SHARPLY
NORTHWARD TO NORTH-NORTHEASTWARD. AFTER TAU 72, THE TRACK WILL
SHIFT SLIGHTLY WESTWARD AS THE STR BUILDS WESTWARD. VERY HIGH OHC
VALUES GREATER THAN 120 KJ PER CM2 AND RICH DEEP-LAYER MOISTURE LIE
IN THE STORM'S PATH FOR THE NEXT 72 HOURS. FURTHERMORE, VWS WILL
EASE AFTER TAU 24. THESE SUPPORTIVE FACTORS WILL LEAD TO
SIGNIFICANT INTENSIFICATION. THE FORECAST CALLS FOR STEADY
INTENSIFICATION THROUGH TAU 12, ALLOWING TS 26W TIME TO BECOME
BETTER ORGANIZED. ONCE THE INNER CORE BECOMES ESTABLISHED, THE
FAVORABLE ENVIRONMENT WILL SUPPORT RAPID INTENSIFICATION (RI) BY
TAU 24. LOWER OHC AND INCREASING VWS TOWARD THE END OF THE FORECAST
PERIOD WILL LEAD TO GRADUAL WEAKENING. THE CIRCULATION OF TS 26W IS
LARGE, AND AS THE STORM STRENGTHENS, IMPACTS WILL EXTEND FAR FROM
THE CENTER.

MODEL DISCUSSION: THE OBSERVED ACCELERATION OF TS 26W DURING THE
PAST 12 HOURS HAS PROMPTED A WESTWARD SHIFT OF APPROXIMATELY 80 NM
IN THE OVERALL GUIDANCE SUITE. IN RESPONSE, THE JTWC FORECAST TRACK
HAS SHIFTED WESTWARD BY A SIMILAR DISTANCE THROUGH TAU 60. THE
DYNAMICAL MODELS ARE NOW CLUSTERING WITH THE AI SUITE, INDICATING
THAT THE TRACK GUIDANCE IS COMING INTO BETTER AGREEMENT. THE JTWC
FORECAST TRACK IS POSITIONED NEAR THE MEAN OF THE AI MODELS WITH
MEDIUM CONFIDENCE. GOOGLE DEEPMIND AND ECMWF REPRESENT THE
WESTERNMOST SOLUTIONS IN THE GUIDANCE ENVELOPE, WHILE GFS AND THE
GEFS ENSEMBLE MEAN DEFINE THE EASTERN SIDE. THE OVERALL GUIDANCE
SPREAD IS 115 NM AT THE POINT OF RECURVATURE. THE INTENSITY AIDS
HAVE COME INTO SUBSTANTIALLY BETTER AGREEMENT ON THE TIMING OF
INTENSIFICATION. MULTIPLE RI AIDS HAVE BEEN TRIGGERED IN EACH
FORECAST CYCLE, INCREASING CONFIDENCE OF RI ONCE TS 26W
AXISYMMETRIZES. FOR THE FIRST TIME SINCE THE INITIAL WARNING, THE
DYNAMICAL MODELS, INCLUDING GFS, HAFS-A, AND COAMPS-TC, AGREE WITH
DEEPMIND ON PEAK INTENSITY TIMING BETWEEN TAU 72 AND TAU 96. THE
PRIMARY REMAINING UNCERTAINTY IS THE MAGNITUDE OF THE PEAK
INTENSITY. THE DEEPMIND SOLUTION IS UNCHANGED FROM THE PREVIOUS
CYCLE AT 110 KTS, WHILE GFS AND HAFS-A DEPICT PEAKS OF 95 KTS AND
125 KTS, RESPECTIVELY. THE JTWC INTENSITY FORECAST LEANS HEAVILY
TOWARD THE CONSISTENT DEEPMIND SOLUTION WITH MEDIUM CONFIDENCE.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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