Advisories

WDPN31 PGTW 200900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 17W (SAUDEL) WARNING NR 
008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 11.9N 150.9E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 367 NM EAST-SOUTHEAST OF ANDERSEN AFB
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN ELONGATED
LOW-LEVEL CIRCULATION CENTER (LLCC), WHICH IS PARTIALLY OBSCURED BY
PERSISTENT DEEP CONVECTION ALONG THE SOUTHERN PERIPHERY. MORE
RECENTLY, NEW CONVECTION HAS FLARED UP OVER THE NE QUADRANT, A SIGN
THAT THE SYSTEM WILL BEGIN TO BECOME MORE SYMMETRIC. TROPICAL STORM
(TS) 17W HAS BEEN STRUGGLING TO INTENSIFY LARGELY DUE TO ITS HIGHLY
ASYMMETRIC STRUCTURE AND THE PRESENCE OF LIGHT TO MODERATE
NORTHEASTERLY MID-LEVEL SHEAR. A 200348Z AMSR2 37 GHZ IMAGE
CONFIRMS THIS POOR ORGANIZATION, REVEALING A LACK OF AN INNER CORE
AND DISORGANIZED CONVECTIVE BANDING. THE ENVIRONMENT IS FAVORABLE
FOR DEVELOPMENT, CHARACTERIZED BY VERY WARM SEA SURFACE
TEMPERATURES (SST) OF 29-30 DEGREES CELSIUS AND EXPANDING
UPPER-LEVEL OUTFLOW. THESE FACTORS ARE OFFSET BY THE MODEST
VERTICAL WIND SHEAR (VWS). THE INITIAL POSITION IS PLACED WITH
MEDIUM CONFIDENCE BASED ON THE PARTLY EXPOSED LLCC VISIBLE IN THE
MSI. REFLECTING THE SYSTEM'S CONTINUED POOR ORGANIZATION, THE
INITIAL INTENSITY IS HELD AT 35 KTS WITH MEDIUM CONFIDENCE, WHICH
ALIGNS WITH THE T2.5 SUBJECTIVE DVORAK FIXES FROM PGTW AND RJTD AND
IS CONSISTENT WITH THE OBJECTIVE ESTIMATES FROM CIMSS RANGING FROM
30 KTS TO 39 KTS.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A LOW- TO
MID-LEVEL RIDGE CENTERED AT 35N 172E

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 39 KTS AT 200540Z
   CIMSS ADT: 33 KTS AT 200610Z
   CIMSS AIDT: 35 KTS AT 200610Z
   CIMSS D-MINT: 30 KTS AT 200330Z
   CIMSS D-PRINT: 30 KTS AT 200540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EASTWARD

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: AFTER A WESTWARD JOG BETWEEN 191800Z AND
200000Z, TS 17W HAS RESUMED A NORTHWESTWARD TRAJECTORY ALONG THE
SOUTHWESTERN PERIPHERY OF A LOW- TO MID-LEVEL RIDGE TO ITS
NORTHEAST. THIS WESTWARD SHIFT HAS BROUGHT THE GALE-FORCE WIND
RADII CLOSER TO SAIPAN AND TINIAN. A DEEP-LAYER RIDGE, ALSO
CENTERED TO THE NORTHEAST, WILL BECOME THE DOMINANT STEERING
INFLUENCE AFTER TAU 24 AS THE STORM MATURES. THIS RIDGE WILL THEN
BEGIN TO SHIFT WESTWARD BY TAU 48, FORCING THE STORM ONTO A MORE
WEST-NORTHWESTWARD TRACK. SIGNIFICANT FORECAST UNCERTAINTY ARISES
DUE TO POTENTIAL INTERACTION WITH A MONSOON DEPRESSION, DESIGNATED
AS 94W, IN THE EXTENDED PERIOD. IN ONE SCENARIO, 94W SIGNIFICANTLY
WEAKENS THE SUBTROPICAL RIDGE (STR) OVER SOUTHERN HONSHU, ALLOWING
TS 17W TO CURVE POLEWARD BY TAU 120. IF THE STR REMAINS INTACT,
THEN THE SYSTEM WILL CONTINUE ITS NORTHWESTWARD TRAJECTORY TOWARD
THE RYUKYU ISLANDS. THE LATTER SCENARIO IS FAVORED BY THE JTWC
FORECAST. THE SYSTEM IS BEGINNING TO SHOW SIGNS OF ORGANIZATION,
WITH NEW CONVECTION FIRING UP AND WRAPPING INTO THE LLCC. THE
ENVIRONMENT AHEAD WILL BE HIGHLY FAVORABLE FOR INTENSIFICATION.
WITHIN THE NEXT 12 HOURS, THE VWS WILL EASE TO BELOW 10 KTS.
CONCURRENTLY, THE SYSTEM WILL TRAVERSE OVER INCREASING LEVELS OF
HIGH OCEAN HEAT CONTENT (OHC) AND BEGIN TO TAP INTO A POLEWARD
OUTFLOW CHANNEL INDUCED BY AN UPPER-LEVEL TROUGH. THESE FACTORS
WILL SUPPORT STEADY DEVELOPMENT IN THE NEAR TERM AS THE CIRCULATION
UNDERGOES AXISYMMETERIZATION, FOLLOWED BY A PERIOD OF RAPID
INTENSIFICATION AFTER TAU 12 TO A PEAK OF 120 KTS AT TAU 72. IN THE
EXTENDED PERIOD, AS THE STORM APPROACHES THE LARGE CIRCULATION OF
94W, IT WILL ENCOUNTER INCREASING NORTHWESTERLY VWS. THIS, COMBINED
WITH LOWER OHC, WILL LEAD TO STEADY WEAKENING TOWARD TAU 120. TO
FURTHER COMPLICATE THE INTENSITY OUTLOOK, AS THE SYSTEM APPROACHES
PEAK INTENSITY, IT MAY UNDERGO EYEWALL REPLACEMENT CYCLES (ERC),
WHICH WOULD INTRODUCE LESS PREDICTABLE FLUCTUATIONS IN STRENGTH AND
LEAD TO AN EXPANDED WIND FIELD.

MODEL DISCUSSION: MODELS ARE IN TIGHT AGREEMENT FOR THE FIRST HALF
OF THE FORECAST PERIOD WITH A CROSS-TRACK SPREAD OF 60 NM AT TAU
48. HOWEVER, THE FORECAST CONFIDENCE PLUMMETS IN THE EXTENDED
PERIOD, WITH A CROSS-TRACK SPREAD THAT INCREASES TO 250 NM AT TAU
96 AND 470 NM AT TAU 120. THIS LARGE DIVERGENCE IS DRIVEN BY TWO
DISTINCT MODEL CAMPS. THE ECMWF AND AI MODELS FAVOR A MORE ZONAL
TRACK TOWARD THE RYUKYU ISLANDS, WHILE THE HAFS-A, GFS, JGSM,
NAVGEM, AND GALWEM MODELS LEAN TOWARD A MORE POLEWARD SOLUTION.
NOTABLY, UNCERTAINTY ENVELOPE COVERS THE AREA FROM OKINAWA TO
SHIKOKU. THE JTWC FORECAST TRACK HAS CHANGED LITTLE FROM 06 HOURS
AGO, POSITIONED NEAR THE JTWC MULTI-MODEL CONSENSUS (CONW) FOR THE
FIRST 48 HOURS AND NEAR THE AI CONSENSUS THEREAFTER. VIRTUALLY ALL
INTENSITY AIDS SHOW SIGNIFICANT INTENSIFICATION. IN FACT, THE SHIP
RI INDEX GIVES A 57 PERCENT PROBABILITY OF A 65 KT INCREASE IN WIND
SPEED OVER THE NEXT 72 HOURS. THE JTWC INTENSITY FORECAST IS
ALIGNED CLOSELY WITH THE EXPERIMENTAL GOOGLE DEEPMIND SOLUTION,
PLACING IT ON THE UPPER END OF THE GUIDANCE ENVELOPE. MOST OF THE
RELIABLE AIDS DEPICT WEAKENING BEGINNING AFTER TAU 72, A TREND THAT
IS REFLECTED IN THE JTWC FORECAST.

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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