Advisories

WDPN33 PGTW 070300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 14W (CHAN-HOM) WARNING NR 
001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 29.2N 160.8E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 472 NM NORTHEAST OF MINAMI TORI SHIMA
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 23 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A
WELL-DEFINED, PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC)
WITH CONVECTION FLARING OVER THE CENTER. THE CENTER IS SURROUNDED
BY AN EXPANSIVE BAND OF LOW- TO MID-LEVEL CLOUDS ALONG THE NORTHERN
AND EASTERN PERIPHERY. OVER THE LAST 24 HOURS, GALE-FORCE WINDS
HAVE FULLY ENCIRCLED THE CENTER, WITH THE STRONGEST WINDS LOCATED
IN THE NORTHERN SEMICIRCLE. THIS SYSTEM IS GRADUALLY ACQUIRING
TROPICAL CHARACTERISTICS, EVIDENCED BY THE RADIUS OF MAXIMUM WINDS
DECREASING TO BELOW 100 NM AND THE CORE BECOMING MORE MOIST.
HOWEVER, IT RETAINS SOME HYBRID TRAITS. AN UPPER-LEVEL LOW IS
POSITIONED DIRECTLY OVERHEAD, THE WIND FIELD IS ASYMMETRIC, AND DRY
AIR IS SPIN WHEELING INTO THE CENTER. THE ENVIRONMENT IS
UNFAVORABLE, CHARACTERIZED BY MODERATE TO HIGH SOUTHERLY VERTICAL
WIND SHEAR (VWS), COPIOUS AMOUNTS OF DRY AIR, AND MARGINALLY WARM
SEA SURFACE TEMPERATURES (SST). THE INITIAL POSITION IS ASSESSED
WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC. HOWEVER, THE
INITIAL INTENSITY IS SET AT 45 KTS WITH MEDIUM CONFIDENCE. THIS
ASSESSMENT IS BASED ON AN OLDER 060914Z ASCAT-B PASS AND IS
SIGNIFICANTLY HIGHER THAN THE SUBJECTIVE DVORAK ESTIMATES, WHICH
ARE CONSIDERED UNREPRESENTATIVE DUE TO THE STORM'S HYBRID
CHARACTERISTICS.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: THE SOUTHWESTERN PERIPHERY OF A LOW- TO
MID-LEVEL SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T1.5 - 25 KTS
   RCTP: T2.0 - 30 KTS
   CIMSS D-PRINT: 33 KTS AT 070000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: THE FORECAST TRACK FOR TS 14W INVOLVES A
COMPLEX, MULTI-STAGE STEERING EVOLUTION. FOR THE FIRST 24 HOURS,
TROPICAL STORM (TS) 14W WILL TRAVEL NORTHWESTWARD ALONG THE
SOUTHWESTERN PERIPHERY OF A LOW- TO MID-LEVEL STR CENTERED TO ITS
NORTHEAST. AFTER TAU 24, A LONGWAVE TROUGH WILL ERODE THE RIDGE,
CREATING A WEAKER STEERING GRADIENT. THIS WILL CAUSE THE SYSTEM TO
BRIEFLY TURN MORE TOWARD THE WEST AND DECELERATE. SIMULTANEOUSLY, A
PORTION OF THIS TROUGH WILL PINCH OFF AND MOVE EQUATORWARD, FAR TO
THE WEST OF THE STORM. AFTER TAU 60, TS 14W WILL APPROACH THE
NORTHEASTERN PERIPHERY OF THIS CUT-OFF TROUGH AND BE ACCELERATED
NORTHWESTWARD TOWARD NORTHERN HONSHU. BY THE END OF THE FORECAST
PERIOD, A TRANSIENT RIDGE BUILDING OVER THE SEA OF OKHOTSK WILL
TURN THE STORM WESTWARD FOR A SECOND TIME. AN ENVIRONMENT OF WARM
SST AND MODERATE TO HIGH SHEAR WILL PERSIST FOR THE FIRST 24-36
HOURS. BY TAU 36, THE SHEAR WILL EASE, GIVING THE SYSTEM A WINDOW
OF OPPORTUNITY FOR SOME INTENSIFICATION. HOWEVER, THE MARGINALLY
WARM SST WILL LIMIT THE AMOUNT OF DEVELOPMENT. AFTER TAU 72, A
STEADY WEAKENING TREND WILL COMMENCE DUE TO A COMBINATION OF LOWER
SST AND HIGHER VWS. AS THE SYSTEM PASSES OVER NORTHERN JAPAN AND
ENTERS THE SEA OF JAPAN IN THE EXTENDED PERIOD, IT WILL UNDERGO
SUBTROPICAL TRANSITION. IT IS POSSIBLE THAT THE CIRCULATION WILL
OPEN UP INTO A TROUGH AFTER CROSSING THE LANDMASS.

MODEL DISCUSSION: THE MODEL TRACKERS ARE IN FAIR AGREEMENT THROUGH
TAU 48. BEYOND THIS POINT, THE SPREAD INCREASES SIGNIFICANTLY,
REACHING 600 NM BY TAU 120. THIS LARGE SPREAD IS DRIVEN BY MODEL
DIFFERENCES IN RESOLVING THE COMPLEX, UNUSUAL INTERACTION BETWEEN
THE CIRCULATION OF TS 14W AND A DIGGING MID-LATITUDE TROUGH. THE
GALWEM SOLUTION IS THE MOST EQUATORWARD SOLUTION THROUGH TAU 60,
AFTER WHICH THE GFS MODEL ANCHORS THE SOUTHERN EDGE OF THE
UNCERTAINTY ENVELOPE. THE NAVGEM SOLUTION IS THE MOST POLEWARD. THE
JTWC FORECAST TRACK IS POSITIONED WITH HIGH CONFIDENCE ALONG THE
TIGHT CLUSTER OF THE MODEL TRACKERS THROUGH TAU 72. IN THE EXTENDED
PERIOD, IT HEDGES TOWARD THE AI CONSENSUS WITH LOW CONFIDENCE. THE
MAJORITY OF THE INTENSITY GUIDANCE DEPICTS LITTLE CHANGE IN
INTENSITY. THE HAFS-A SOLUTION IS THE MOST AGGRESSIVE, INITIATING
DEVELOPMENT AFTER 12 HOURS TO A PEAK OF 65 KTS BY TAU 48. THIS
SOLUTION IS DISCOUNTED DUE TO THE STORM'S BROAD CIRCULATION AND
LACK OF DEEP CONVECTION NEAR THE CENTER. THE GFS, EXPERIMENTAL AI,
AND COAMPS-TC MODELS ARE MORE CONSERVATIVE. THE JTWC INTENSITY
FORECAST ALIGNS MORE CLOSELY WITH THIS LATTER GROUP OF MODELS
THROUGH TAU 120 WITH MEDIUM CONFIDENCE.

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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