WDPN31 PGTW 121500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SEVENTEEN) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 7.2N 140.0E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 178 NM SOUTHEAST OF YAP
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 11 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY AS WELL AS
ANIMATED PROXY VISIBLE (PROXYVIS) IMAGERY SHOWS FLARING CONVECTION
FIRING ALONG A LINE OF CONVERGENCE TO THE NORTH OF A VERY SMALL LOW
LEVEL CIRCULATION CENTER (LLCC), FORMING A SMALL CENTRAL DENSE
OVERCAST (CDO) TYPE FEATURE. CIRRUS DEBRIS FROM THE CONVECTIVE
ACTIVITY IS OBSCURING THE LLCC, BUT SPIRAL BAND ANALYSIS OF THE
OUTER BANDS IN THE CLEAR AIR TO THE EAST LENT LOW TO MODERATE
CONFIDENCE TO THE INITIAL POSITION. A 120901Z SSMIS 91GHZ MICROWAVE
IMAGE PRESENTED A RATHER DISORGANIZED CIRCULATION, WITH CLEAR
TURNING IN THE MID-LEVEL CONVECTION, BUT ONLY WEAK TURNING EVIDENT
IN THE LOWER LEVELS, WITH A SIGNIFICANT VORTEX TILT WESTWARD WITH
HEIGHT DUE MODERATE EASTERLY SHEAR. THE LLCC IS ASSESSED TO BE
SOMEWHAT ELONGATED ALONG AN EAST-WEST AXIS WITH A PARTIAL 121202Z
ASCAT-B PASS SUGGESTING THE EASTERN EDGE OF THE CIRCULATION COULD
BE AS FAR EAST AS 143E, THOUGH TD 17W IS LOCATED IN THE WESTERN
SIDE OF THE LARGER, ELONGATED ROTATION. THE INITIAL INTENSITY IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AVERAGE OF THE AGENCY
FIXES RANGING BETWEEN T1.0 TO T2.0 AND THE OVERALL STRUCTURAL
IMPROVEMENT. THE SYSTEM SITS IN A SOMEWHAT MARGINAL ENVIRONMENT,
WITH MODERATE EASTERLY DIFFLUENT OUTFLOW ALOFT, LOW DEEP-LAYER
SHEAR BUT MODERATE TO HIGH SHEAR IN THE UPPER-LEVELS AND A FAIR
AMOUNT OF DRY AIR IN THE MID-LEVELS.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: SLOWLY WESTWARD ALONG THE SOUTHERN SIDE
OF A DEEP-LAYER SUBTROPICAL RIDGE TO THE NORTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.0 - 25 KTS
   KNES: T2.0 - 30 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE WESTWARD
   OTHER FACTORS: HIGH UPPER-LEVEL SHEAR, DRY MID-LEVEL AIR. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TD 17W (SEVENTEEN) IS EXPECTED TO SLOWLY DRIFT
WESTWARD UNDER THE INFLUENCE OF A GENERALLY WEAK STEERING PATTERN.
PASSAGE OF A LOW- TO MID-LEVEL TROUGH TO THE NORTH HAS RESULTED IN
DEVELOPMENT OF A BREAK IN THE RIDGE DUE NORTH OF THE SYSTEM,
WEAKENING THE STEERING GRADIENT SUCH THAT THE SYSTEM WILL VERY
SLOWLY MOVE TOWARDS THE WEST FOR THE FIRST 48 HOURS OF THE
FORECAST. AFTER TAU 48 THE RIDGE BUILDS IN TO THE NORTH,
STRENGTHENS THE STEERING GRADIENT, AND IN RESPONSE TD 17W
ACCELERATES TO THE WEST-NORTHWEST. AFTER TAU 72 THE SYSTEM WILL
TURN MORE TOWARDS THE NORTHWEST TOWARDS ANOTHER BREAK THAT DEVELOPS
IN THE SUBTROPICAL RIDGE TO THE NORTH. DUE TO THE RATHER
DISORGANIZED LOW-LEVEL VORTEX STRUCTURE, THE CONTINUED PRESENCE OF
DRY MID-LEVEL AIR AND MODERATE EASTERLY SHEAR, THE SYSTEM WILL
STRUGGLE TO DEVELOP THROUGH THE FIRST 72 HOURS, WITH ONLY SLIGHT
INTENSIFICATION EXPECTED AFTER TAU 24 AS THE SHEAR IS EXPECTED TO
WEAKEN A NOTCH OR TWO. AFTER TAU 48, TD 17W IS FORECAST TO REACH
TROPICAL STORM STRENGTH AS IT MOVES INTO A MORE FAVORABLE
ENVIRONMENT OF SLIGHTLY LOWER SHEAR AND ENHANCED MID-LEVEL
MOISTURE, BUT IT WILL CONTINUE TO STRUGGLE AGAINST MODERATE
EASTERLY SHEAR THROUGHOUT THE FORECAST PERIOD. 

MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN
FAIRLY GOOD AGREEMENT THAT THE SYSTEM WILL REMAIN NEARLY
QUASI-STATIONARY, OR DRIFT SLOWLY WESTWARD, THROUGH THE FIRST 48
HOURS, FOLLOWED BY A MORE RAPID TRACK TO THE NORTHWEST THEREAFTER.
BY TAU 72, THE GUIDANCE ENVELOPE EXPANDS TO JUST 100NM
ACROSS-TRACK, BUT AS MIGHT BE EXPECTED, ALONG-TRACK SPREAD IS MORE
SIGNIFICANT, INCREASING TO 150NM BY TAU 72 BETWEEN THE NAVGEM, AND
THE UKMET AND ECMWF ENSEMBLE MEANS RACING OUT AHEAD OF THE
REMAINDER OF THE GUIDANCE. BY TAU 120, ACROSS-TRACK SPREAD REMAINS
THE SAME WHILE ALONG-TRACK SPREAD INCREASES SHARPLY TO 420NM
BETWEEN THE NAVGEM AND UKMET ENSEMBLE AND THE GFS ENSEMBLE WHICH IS
MUCH SLOWER. THE JTWC FORECAST LIES CLOSE TO THE MULTI-MODEL
CONSENSUS THROUGHOUT THE FORECAST PERIOD WITH LOW CONFIDENCE DUE TO
VERY LARGE ALONG-TRACK SPREAD. INTENSITY GUIDANCE IS MIXED, WITH
THE BULK OF THE GUIDANCE KEEPING THE SYSTEM BELOW 35 KNOTS THROUGH
THE FORECAST PERIOD, WITH THE NOTABLE EXCEPTIONS OF THE COAMPS-TC
(NAVGEM AND GFS VERSIONS) WHICH PEAKS THE SYSTEM BY TAU 96 AT 100
KNOTS AND 70 KNOTS RESPECTIVELY, AND THE HAFS-B WHICH KEEPS THE
SYSTEM AT 25 KNOTS OR LOWER THROUGHOUT THE FORECAST PERIOD. THE
JTWC FORECAST LIES CLOSE TO THE CONSENSUS MEAN THROUGH THE FORECAST
PERIOD WITH LOW CONFIDENCE DUE TO THE EXTREME SPREAD IN THE
GUIDANCE. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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