WDPN31 PGTW 280300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 21W (CHABA) WARNING NR 01//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION 21W (CHABA), LOCATED APPROXIMATELY 196 NM
EAST-NORTHEAST OF ANDERSON AFB, GUAM, HAS TRACKED WESTWARD
AT 14 KNOTS OVER THE PAST SIX HOURS. THE CURRENT POSITION IS BASED
ON ANIMATED MULTISPECTRAL SATELLITE IMAGERY, SATELLITE FIXES FROM
RJTD AND PGTW, AND A 280021Z METOP-B ASCAT PASS. THERE IS MODERATE
CONFIDENCE IN THE INITIAL POSITION GIVEN THE LACK OF A CLEAR,
DEFINITIVE CENTER FEATURE IN RECENT SATELLITE IMAGERY. THE INTENSITY
OF 30 KNOTS IS PRIMARILY BASED ON ANALYSIS OF THE AFOREMENTIONED
PARTIAL SCATTEROMETRY PASS, WHICH SHOWS A RELATIVELY BROAD AREA OF
25-30 KT WINDS TO THE NORTH AND NORTHEAST OF THE LOW LEVEL
CIRCULATION CENTER (LLCC). THE INITIAL INTENSITY IS ASSESSED
SIGNIFICANTLY BELOW THE SUBJECTIVE DVORAK INTENSITY ESTIMATES
FROM ALL REPORTING AGENCIES OF T3.0 (45 KTS), WHICH ARE LIKELY
UNREALISTICALLY HIGH DUE TO FLARING CONVECTION. TD 21W HAS CONTINUED
TO CONSOLIDATE OVER THE PAST SIX HOURS, WITH DEEP CONVECTION
MAINTAINING NEAR THE CENTER, BUT MOSTLY CONFINED TO THE SOUTH OF THE
ASSESSED LLCC. UPPER-LEVEL ANALYSIS INDICATES THAT AN UPPER LEVEL
ANTI-CYCLONE HAS DEVELOPED OVER THE PAST SIX HOURS OVER TOP OF TD
21W, PROVIDING GOOD OUTFLOW, AND LOW VERTICAL WIND SHEAR (VWS),
SUPPORTING THE INCREASED CONVECTIVE SIGNATURE.
3. FORECAST REASONING.
    A. THIS IS THE FIRST PROGNOSTIC REASONING MESSAGE FOR 21W AND
SETS THE FORECAST PHILOSOPHY
    B. TD 21W IS EXPECTED TO MOVE ON A WESTWARD TRAJECTORY OVER THE
NEXT 36 HOURS ALONG THE SOUTHERN PERIPHERY OF A DEEP-LAYER SUB-
TROPICAL (STR) RIDGE TO THE NORTH. BY TAU 48, THE STR REORIENTS
FURTHER TO THE EAST, ALLOWING TD 21W TO MAKE A SLOW TURN TO A
NORTHWESTWARD TRACK. LITTLE TO NO INTENSIFICATION IS EXPECTED IN THE
NEAR TERM AS A TUTT CELL IS EXPECTED TO DEVELOP TO THE NORTHEAST OF
THE SYSTEM OVER THE NEXT 12 HOURS, REDUCING OR ELIMINATING THE
POLEWARD OUTFLOW CHANNEL. THE TUTT WILL MOVE SLOWLY WESTWARD IN
PHASE WITH 21W, LIMITING INTENSIFICATION. BY AROUND TAU 60, THE TUTT
WILL BE NORTHWEST OF THE CENTER AND THE IMPROVING OUTFLOW, LOW TO
MODERATE VERTICAL WIND SHEAR AND PASSAGE OVER WARM WATER WILL ENABLE
A MORE RAPID INTENSIFICATION. AVAILABLE NUMERICAL MODEL TRACK
FORECAST GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 72, WITH ALL
SOLUTIONS INDICATING A TRACK NORTH OF GUAM IN THE NEAR TERM, AND A
NORTHWESTWARD TURN BY TAU 48. THE CURRENT FORECAST LIES CLOSE TO THE
MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE.
    C. IN THE EXTENDED PERIOD, TD 21W SHOULD CONTINUE TO TRACK
NORTHWESTWARD UNDER THE CONTINUED STEERING INFLUENCE OF THE DEEP-
LAYER STR. PEAK INTENSITY IS EXPECTED TO OCCUR AROUND TAU 96 AS
ENVIRONMENTAL CONDITIONS REMAIN GENERALLY FAVORABLE. AVAILABLE MODEL
GUIDANCE SHOWS A LARGE SPREAD AFTER TAU 72, WITH THE GFS AND HWRF
SOLUTIONS LEANING TOWARDS A MORE RAPID RECURVE SOUTH OF JAPAN, WHILE
THE ECMWF AND THE ECMWF ENSEMBLE SHOW A TRACK MUCH CLOSER TO TAIWAN.
THE OFFICIAL FORECAST TRACK LIES SLIGHTLY SOUTH OF THE MULTI-MODEL
CONSENSUS IN THE LATER TAUS, LENDING LOW CONFIDENCE TO THAT PORTION
OF THE TRACK FORECAST./=