WDXS31 PGTW 072100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 12S (JENNA) WARNING NR 
007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.6S 91.2E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 506 NM SOUTHWEST OF THE COCOS ISLANDS
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 23 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
DECAYING VORTEX COMPLETING A DECOUPLING PROCESS DUE TO WESTERLY
VERTICAL SHEAR OF AROUND 30 KT, PUSHING ANY REMAINING DEEP
CONVECTION WELL TO THE SOUTHEAST OF THE LOW-LEVEL CIRCULATION
CENTER (LLCC). A DEEP-LAYER DRY AIR MASS IS NOW OVERSPREADING THE
LLCC FROM THE WEST AND NORTH. AN EARLIER 171133Z SMOS PASS
INDICATED THAT THE STORM RETAINS A STRONG WIND FIELD, WITH MAXIMUM
WINDS OF AROUND 55 KT SOUTH OF THE LLCC AT THE TIME. THE INITIAL
INTENSITY IS SET AT 50 KT, ASSUMING GRADUAL SPIN-DOWN OF THE WIND
FIELD HAS CONTINUED OVER THE PAST SIX HOURS. THE LACK OF DEEP
CONVECTION IS LEADING TRADITIONAL DVORAK ESTIMATES TO RUN A BIT
LOWER.

INITIAL WIND RADII BASIS: SMOS DATA

CURRENT STEERING MECHANISM: LOW-LEVEL SUBTROPICAL RIDGE TO THE
SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   CIMSS AIDT: 51 KTS AT 071420Z
   CIMSS D-PRINT: 36 KTS AT 071720Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY UNFAVORABLE
   VWS: 30+ KTS
   SST: 26-27 CELSIUS
   OUTFLOW: WEAK POLEWARD
   OTHER FACTORS: DEEP-LAYER DRY AIR MASS OVERSPREADING THE LLCC
FROM THE WEST AND NORTH

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE 12S (JENNA) HAS NOW BEEN
DECOUPLED BY STRONG WESTERLY SHEAR, AND THE SHALLOW SURFACE VORTEX
IS NOW TURNING WESTWARD ALONG THE NORTHERN PERIPHERY OF THE
SUBTROPICAL RIDGE. WESTERLY SHEAR OF AT LEAST 30 KT IS EXPECTED TO
PERSIST OVER THE NEXT COUPLE OF DAYS AS AN ANTICYCLONIC
WAVEBREAKING EVENT OVER THE SOUTHWESTERN INDIAN OCEAN DRIVES
UPPER-LEVEL WESTERLIES EQUATORWARD, AS FAR AS THE 15-20S LATITUDE
BELT. THIS SHEAR WILL FORCE CONTINUED DECAY OF THE VORTEX AS IT
TRANSITS WESTWARD THROUGH A DEEP-LAYER DRY AIR MASS. THE INTENSITY
FORECAST IS BASED ON GRADUAL SPIN-DOWN OF THE WIND FIELD, WITH
GALES POSSIBLY LINGERING FOR AS LONG AS 36 HOURS DUE TO ENHANCED
GRADIENT FLOW IN THE SOUTHERN SEMICIRCLE OF THE STORM. THE JTWC
FORECAST TERMINATES AT 48 HOURS WHEN JENNA IS FORECAST TO DECAY
BELOW THE WARNING THRESHOLD OF 35 KT.

MODEL DISCUSSION: THE JTWC TRACK FORECAST IS CLOSE TO THE
MULTI-MODEL CONSENSUS, AND THE JTWC INTENSITY FORECAST IS CLOSE TO
A BLEND OF THE GFS AND ECMWF GLOBAL MODELS.

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