Advisories

WDPN31 PGTW 200300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 17W (SAUDEL) WARNING NR 
007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 11.4N 151.6E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 235 NM NORTH OF CHUUK
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS THE
PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) OF TROPICAL 
STORM (TS) 17W REGAINING SOME CENTRAL CONVECTION, WHICH IS OBSCURING 
THE CENTER OF THE VORTEX AND THE SOUTHWEST QUADRANT. WEAK BUT APPARENT
MID-LEVEL NORTHEASTERLY SHEAR HAS PREVENTED THE CONVECTION FROM
WRAPPING AROUND TO THE NORTHERN SEMICIRCLE OVER THE PAST 6 HOURS.
ANIMATED WATER VAPOR IMAGERY SHOWS A ROBUST EQUATORWARD OUTFLOW
CHANNEL AND A WEDGE OF DRY AIR TO THE NORTHWEST THAT MAY BE A
CONTRIBUTING FACTOR TO THE LIMITED CONVECTION OBSERVED. THE INITIAL
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON PARTIALLY EXPOSED
LLCC AND TIGHT AGREEMENT OF THE AGENCY DVORAK FIXES. THE INITIAL
INTENSITY OF 35 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
AGENCY DVORAK INTENSITY ESTIMATES AS THERE HAS BEEN A LACK OF
RECENT WIND SPEED IMAGERY. TS 17W IS IN A FAVORABLE ENVIRONMENT
CHARACTERIZED BY WARM SEA SURFACE TEMPERATURES AND GOOD EQUATORWARD
OUTFLOW. THE RELATIVELY LOW 5-10 KTS OF MID-LEVEL SHEAR AND WEAK
DRY AIR ENTRAINMENT APPEAR TO BE HAVING AN OUTSIZED NEGATIVE IMPACT
ON INTENSIFICATION DUE TO THE SHALLOWNESS OF THE VORTEX.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: SOUTHWESTERN EXTENSION OF A SUBTROPICAL
RIDGE CENTERED TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T2.0 - 30 KTS
   KNES: T2.5 - 35 KTS
   CIMSS ADT: 28 KTS AT 200000Z
   CIMSS AIDT: 33 KTS AT 200000Z
   CIMSS D-MINT: 29 KTS AT 192010Z
   CIMSS D-PRINT: 28 KTS AT 200040Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG EQUATORWARD
   OTHER FACTORS: WEAK DRY AIR ENTRAINMENT

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TS 17W (SAUDEL) HAS TAKEN ON A SLIGHTLY MORE
WESTWARD COMPONENT TO ITS TRACK OVER THE PAST 6 HOURS AS THE STR TO
THE NORTHEAST BEGINS TO ASSERT CONTROL OVER THE STEERING
ENVIRONMENT. THE SYSTEM IS FORECAST TO CONTINUE TRACKING
NORTHWESTWARD UNDER THIS STEERING INFLUENCE THROUGH TAU 60, THEN
SHIFT TO A WEST-NORTHWESTWARD TRACK AS THE RIDGE BUILDS AND
MIGRATES WESTWARD. THE FORECAST TRACK SPEED HAS INCREASED DUE TO
THE RECENT ACCELERATION OF TS 17W, WITH PASSAGE NORTH OF SAIPAN NOW
EXPECTED AROUND 211400Z. AS FOR INTENSITY, THE SYSTEM IS EXPECTED
TO SLOWLY INTENSIFY AS IT CONSOLIDATES OVER THE NEXT 12-24 HOURS,
THEN EXPERIENCE AN EXTENDED PERIOD OF RAPID INTENSIFICATION TO A
PEAK OF 120 KTS AROUND TAU 72. THIS PERIOD OF RI WILL BE FUELED BY
VERY WARM (30-31 C) SEA SURFACE TEMPERATURES, HIGH OCEAN HEAT
CONTENT, EXCELLENT OUTFLOW ALOFT, AND A LOW WIND SHEAR ENVIRONMENT.
AFTER TAU 72, A GRADUAL WEAKENING TREND IS EXPECTED DUE TO A
COMBINATION OF FACTORS THAT INCLUDE POTENTIAL EYEWALL REPLACEMENT
CYCLES AND COOLER SEA SURFACE TEMPERATURES. 

MODEL DISCUSSION: MODEL TRACK GUIDANCE REMAINS IN GOOD AGREEMENT
ABOUT THE NORTHWESTWARD TRACK OVER THE NEXT 72 HOURS, HOWEVER,
THERE IS SOME UNCERTAINTY IN TRACK SPEED INTRODUCED BY AN
ALONG-TRACK SPREAD OF OVER 100 NM AT THAT TIME. JGSM AND NAVGEM ARE
CONSIDERED TO BE FAST OUTLIERS WHILE THE REMAINDER OF GUIDANCE IS
MUCH CLOSER TOGETHER. AFTER TAU 72, TRACK GUIDANCE BEGINS TO SPREAD
DUE TO UNCERTAINTY IN THE TIMING OF THE SYSTEM'S APPROACH OF THE
STEERING RIDGE AXIS. THE GROUP OF EXPERIMENTAL AI MODELS ARE
CLUSTERED ON THE SOUTHERN END OF THE ENVELOPE WHILE THE
PHYSICS-BASED MODELS (AND THE EXPERIMENTAL GOOGLE DEEPMIND
SOLUTION) MAKE UP THE NORTHERN CLUSTER. THE JTWC TRACK IS PLACED
NEAR THE MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE THROUGH TAU 72,
THEN FAVORS THE LATTER GROUPING WITH MEDIUM CONFIDENCE. MODEL
INTENSITY GUIDANCE AGREES ON INTENSIFICATION, INCLUDING THE PERIOD
OF RI, WITH THE EXCEPTION OF SHIPS GUIDANCE, WHICH DEPICTS A MUCH
SLOWER RATE OF INTENSIFICATION. INTENSITY GUIDANCE SPREADS 30KT
ENVELOPE BY TAU 120, LENDING LOW CONFIDENCE TO THE LATER PORTION OF
THE INTENSITY FORECAST.

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