Advisories

WDPN31 PGTW 211500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SAUDEL) WARNING NR 013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.0N 148.1E
   INITIAL INTENSITY: 70 KTS
   GEOGRAPHIC REFERENCE: 141 NM EAST-NORTHEAST OF SAIPAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 28 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
NOW A TYPHOON, 17W HAS UNDERGONE RAPID INTENSIFICATION (RI), WITH
ITS INTENSITY INCREASING BY 30 KTS IN THE PAST 24 HOURS. ANIMATED
ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS AN EXPANDING
CONVECTIVE ENVELOPE AS FEEDER BANDS EXPLODE OVER THE NORTHERN AND
EASTERN PERIPHERY. TY 17W NOW HAS A RAGGED EYE AS THE CENTRAL DENSE
OVERCAST ASSOCIATED WITH THE INNER CORE HAS BECOME BETTER DEFINED.
THE INCREASING UPPER-LEVEL DIVERGENCE CAN BE LARGELY ATTRIBUTED TO
EXCELLENT POLEWARD OUTFLOW ALOFT. THE SYSTEM IS IN A HIGHLY
FAVORABLE ENVIRONMENT OF HIGH OCEAN HEAT CONTENT (OHC), LOW
VERTICAL WIND SHEAR (VWS), AND RICH, DEEP-LAYER MOISTURE. THE
INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
RAGGED EYE FEATURE AND THE TIGHT GROUPING OF AGENCY FIXES. THE
SUBJECTIVE DVORAK ESTIMATES HAVE BEEN RISING, WITH PGTW, KNES, AND
RCTP ALL REPORTING T4.0. IN FACT, APPLYING THE EIR-EYE TECHNIQUE
YIELDS A DATA T-NUMBER OF 4.5. THESE ESTIMATES, COMBINED WITH THE
IMPROVED CONVECTIVE SIGNATURE, JUSTIFY RAISING THE INITIAL
INTENSITY TO 70 KTS WITH HIGH CONFIDENCE.

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: T4.0 - 65 KTS
   RJTD: T3.5 - 55 KTS
   RCTP: T4.0 - 65 KTS
   KNES: T4.0 - 65 KTS
   CIMSS SATCON: 57 KTS AT 210510Z
   CIMSS ADT: 49 KTS AT 211210Z
   CIMSS AIDT: 45 KTS AT 211210Z
   CIMSS D-MINT: 63 KTS AT 211227Z
   CIMSS D-PRINT: 60 KTS AT 211210Z

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

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE JTWC FORECAST TRACK HAS SHIFTED
SOUTHWARD BY 20 NM AT TAU 96 AND 36 NM AT TAU 120.

FORECAST DISCUSSION: TY 17W CONTINUES TO MOVE NORTH-NORTHWESTWARD
AT AN AVERAGE SPEED OF 10-13 KTS ALONG THE SOUTHWESTERN FLANK OF AN
ELONGATED STR TO ITS NORTHEAST. THIS MOTION WILL HOLD THROUGH TAU
36. AFTER THIS POINT, THE STR WILL MIGRATE WESTWARD, WHICH WILL
FORCE THE SYSTEM ONTO A MORE WESTWARD TRACK. IN THE EXTENDED
PERIOD, LOW CONFIDENCE REMAINS DUE TO THE UNCERTAIN IMPACT OF THE
MONSOON DEPRESSION, DESIGNATED AS 94W, ON A SEPARATE STR OVER
HONSHU. A STRONGER 94W WOULD ERODE THE STR, CREATING A WEAKNESS
THAT WOULD ALLOW TY 17W TO SLOW AND TURN POLEWARD, BRINGING THE
SYSTEM CLOSER TO THE NORTHERN RYUKYU ISLANDS AND KYUSHU. ON THE
OTHER HAND, A WEAKER 94W WOULD LEAVE THE STR INTACT, CAUSING TY 17W
TO CONTINUE ON A MORE WESTWARD TRAJECTORY TOWARD THE SOUTHERN OR
CENTRAL RYUKYU ISLANDS. THE JTWC FORECAST CURRENTLY FAVORS THE
LATTER SCENARIO AND DEPICTS A MORE ZONAL TRACK AT TAU 96 AND TAU
120. RAPID INTENSIFICATION (RI) BEGAN APPROXIMATELY 24 HOURS AGO
AND WILL CONTINUE. OVER THE NEXT 36 HOURS, TY 17W WILL TRAVERSE
VERY HIGH OHC WITHIN A LOW-SHEAR ENVIRONMENT. CONSEQUENTLY, THE
SYSTEM WILL CONTINUE TO RAPIDLY DEEPEN TO A PEAK INTENSITY OF 125
KTS AT TAU 48. AFTERWARD, THE INTENSIFICATION PHASE WILL END AS THE
SYSTEM MOVES OVER PROGRESSIVELY LOWER OHC. A STEADY WEAKENING TREND
WILL COMMENCE AT TAU 96, DRIVEN BY LOWER SEA SURFACE TEMPERATURES,
UPWELLING OF COOLER WATERS CAUSED BY A SLOWING FORWARD MOTION, AND
INCREASING VWS.

MODEL DISCUSSION: THE TRACK GUIDANCE SUITE IS IN EXCELLENT
AGREEMENT THROUGH TAU 60, WITH A CROSS-TRACK SPREAD OF LESS THAN 60
NM. THE UNCERTAINTY SPREAD INCREASES SIGNIFICANTLY TO 410 NM AT TAU
120, WITH GFS ANCHORING THE POLEWARD SIDE AND ECMWF ON THE OPPOSITE
EDGE. VERY NOTABLY, THE PREVIOUS SPLIT BETWEEN THE AI AND
PHYSICS-BASED MODELS IS NO MORE. THE TWO MODEL TYPES ARE NOW WELL
INTERMINGLED NEAR THE CENTER OF THE GUIDANCE ENVELOPE. THE JTWC
CONSENSUS (CONW) IS NEARLY IDENTICAL TO THE AI CONSENSUS.
REFLECTING THIS, THE JTWC FORECAST TRACK IS POSITIONED ALONG A
BLEND OF CONW AND THE AI CONSENSUS WITH HIGH CONFIDENCE THROUGH TAU
72 AND LOW CONFIDENCE THEREAFTER. NUMEROUS RI AIDS HAVE TRIGGERED,
AND THE RELIABLE INTENSITY AIDS UNANIMOUSLY DEPICT SIGNIFICANT
STRENGTHENING. THE SHIP RI INDEX GIVES AN 85 PERCENT PROBABILITY OF
A 30-KT INCREASE IN SPEED OVER THE NEXT 24 HOURS AND A 50 PERCENT
PROBABILITY OF A 65-KT INCREASE OVER THE NEXT 72 HOURS. THE JTWC
INTENSITY FORECAST IS SET WITH MEDIUM CONFIDENCE AT THE UPPER END
OF THE GUIDANCE ENVELOPE, POSITIONED CLOSEST TO THE AGGRESSIVE
RATES OF DEVELOPMENT PREDICTED BY THE COAMPS-TC, HAFS-A, AND THE
EXPERIMENTAL GOOGLE DEEPMIND MODELS. THE FORECAST PEAK OF 125 KTS
IS BELOW THE 130 KT PEAK FROM COAMPS-TC AND THE 140 KT PEAK FROM
HAFS-A BUT IS ABOVE THE 115 KT PEAK OF DEEPMIND.

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