Advisories

WDPN31 PGTW 202100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 17W (SAUDEL) WARNING NR 
010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.7N 149.6E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 238 NM EAST-SOUTHEAST OF SAIPAN
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS VIGOROUS
CENTRAL CONVECTION PERSISTING AROUND THE CENTRAL CORE OF TROPICAL
STORM (TS) 17W, AS WELL AS DEEP CONVECTIVE BANDING WRAPPING AROUND
THE EASTERN AND NORTHERN EDGES. ANIMATED WATER VAPOR IMAGERY
DEPICTS CONTINUED STRONG EQUATORWARD OUTFLOW SUPPLEMENTED BY A WEAK
BUT DEVELOPING POLEWARD OUTFLOW CHANNEL. A 201603Z AMSR-2 89 GHZ
MICROWAVE IMAGE REVEALED THAT THE LOW-LEVEL CIRCULATION HAS BECOME
MORE SYMMETRIC OVER THE PAST 6 HOURS. THE INITIAL POSITION IS
PLACED WITH MEDIUM CONFIDENCE BASED PRIMARILY ON EXTRAPOLATION FROM
THE CENTER IN THE AMSR-2 MICROWAVE IMAGE, SUPPLEMENTED BY THE
AGENCY DVORAK FIXES. THE INITIAL INTENSITY OF 40 KTS IS ASSESSED
WITH MEDIUM CONFIDENCE BASED ON T2.5-3.0 AGENCY FIXES AND THE CIMSS
OBJECTIVE INTENSITY ESTIMATES LISTED BELOW. THE ENVIRONMENT IS
CHARACTERIZED AS HIGHLY FAVORABLE FOR INTENSIFICATION WITH LOW
VERTICAL WIND SHEAR (VWS), VERY WARM SEA SURFACE TEMPERATURES, HIGH
OCEAN HEAT CONTENT (OHC), ABUNDANT DEEP-LAYER MOISTURE, AND ROBUST
OUTFLOW ALOFT.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SOUTHWESTERN FLANK OF A SUBTROPICAL
RIDGE (STR) CENTERED TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T3.0 - 45 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   CIMSS ADT: 41 KTS AT 201800Z
   CIMSS AIDT: 39 KTS AT 201800Z
   CIMSS D-MINT: 39 KTS AT 201604Z
   CIMSS D-PRINT: 35 KTS AT 201740Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS 17W (SAUDEL) HAS MAINTAINED A NORTHWESTWARD
TRACK OVER THE PAST 12 HOURS UNDER THE STEERING INFLUENCE OF THE
STR SITUATED TO THE NORTHEAST. THE NORTHWESTWARD TRAJECTORY IS
FORECAST TO CONTINUE OVER THE NEXT 36 HOURS IN THIS STEERING
ENVIRONMENT, WITH A CONSISTENT TRACK SPEED THAT HAS SETTLED AROUND
10 KTS. AFTER TAU 36, AN EXTENSION OF THE STEERING RIDGE IS
EXPECTED TO CONTINUE BUILDING WESTWARD AND INDUCE A
WEST-NORTHWESTWARD TRACK THROUGH THE REMAINDER OF THE FORECAST
PERIOD. A SECOND STR CENTERED OVER KYUSHU IS ALSO EXPECTED TO BUILD
SOUTHEASTWARD DURING THE LATER PERIOD OF THE FORECAST AND AID IN
MAINTAINING THE WEST-NORTHWESTWARD TRACK OF 17W. IT'S WORTH NOTING
THAT THERE IS SOME UNCERTAINTY ABOUT THE STRENGTH AND INFLUENCE
THAT THE RIDGE OVER KYUSHU WILL EXERT, WHICH INTRODUCES UNCERTAINTY
TO THE TRACK FORECAST IN THE LATER TAUS. REGARDING INTENSITY, TS
17W HAS ENTERED A HIGHLY FAVORABLE ENVIRONMENT AND ALREADY EXHIBITS
SIGNS OF CONSOLIDATION AND SYMMETRIZING. A LENGTHY PERIOD OF
RAPID INTENSIFICATION (RI) THROUGH TAU 36 IS EXPECTED IN THIS
ENVIRONMENT. ONCE THE SYSTEM EXITS THE REGION OF HIGH OHC THE
INTENSIFICATION RATE WILL SLOW UNTIL IT REACHES A PEAK OF 120 KTS
AROUND TAU 60. AFTER TAU 60, LOWER OHC, INCREASING VWS, AND
POTENTIAL EYEWALL REPLACEMENT CYCLES ARE EXPECTED TO CAP THE
INTENSIFICATION AND RESULT IN A STEADY GRADUAL WEAKENING PHASE.

MODEL DISCUSSION: MODEL TRACK GUIDANCE REMAINS IN GOOD AGREEMENT
THROUGH TAU 48 WITH A CROSS-TRACK SPREAD OF 45 NM, WITH THE
EXCEPTION OF JGSM AS A POLEWARD OUTLIER. AFTER TAU 48 THERE IS A
BIFURCATION OF TRACK GUIDANCE INTO TWO GROUPS. ECMWF AND MOST OF
THE EXPERIMENTAL AI TRACKERS FAVOR A CONTINUED WEST-NORTHWESTWARD
TRACK WHILE THE REST OF GUIDANCE (INCLUDING GFS, JGSM, GALWEM, AND
EXPERIMENTAL GOOGLE DEEPMIND) PREDICT VARYING DEGREES OF POLEWARD
TURNING. THE JTWC TRACK FORECAST FAVORS THE FORMER GROUPING AS THE
ECMWF GLOBAL MODEL TREATMENT OF THE STEERING RIDGE EVOLUTION
APPEARS MORE REALISTIC AT THIS TIME. THE MODEL INTENSITY GUIDANCE
IS IN GOOD AGREEMENT ABOUT THE NEAR-TERM RI SCENARIO, WITH SEVERAL
MORE RI AIDS TRIGGERING IN THE LATEST MODEL RUNS. ALL INTENSITY
GUIDANCE AGREE ON THE INTENSITY PROFILE THROUGHOUT THE FORECAST
PERIOD AND THE JTWC INTENSITY FORECAST IS PLACED ON THE UPPER END
OF THE ENVELOPE NEAR THE HAFS-A SOLUTION.

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