WDPN31 PGTW 090300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 03W (THREE) 
WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 10.7N 126.8E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 230 NM SOUTHEAST OF LEGAZPI, PHILIPPINES
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS RAGGED
BANDING DEVELOPING TO THE NORTHWEST AND EAST OF A LOW-LEVEL
CIRCULATION CENTER (LLCC) THAT HAS CONSOLIDATED DURING THE PAST 12
HOURS. A 090050Z ASCAT-B SCATTEROMETER PASS INDICATED A SYMMETRIC
CIRCULATION WITH 25 KT WINDS ABOUT 40 NM TO THE WEST AND SOUTH OF
THE CENTER. A SHIP ON THE NORTH SIDE OF THE LLCC REPORTED A 26 KT
WIND AT AN UNKNOWN ELEVATION AT 082300Z, AND THE ISLAND OF GUIUAN
NEAR THE LLCC HAS REPORTED WINDS AS HIGH AS 19 KT AROUND 082300Z,
WITH A SIX-HOUR MINIMUM PRESSURE OF 1004 MB. THE SYSTEM IS THUS
BEING CLASSIFIED AS A TROPICAL DEPRESSION WITH MAXIMUM SUSTAINED
WINDS OF 25 KT. LIGHT TO MODERATE EASTERLY SHEAR IS PRESENT NEAR
THE LLCC, EVIDENCED BY UPPER-LEVEL CIRRUS APPROACHING FROM THE EAST
IN THE SOUTHEAST QUADRANT.

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

CURRENT STEERING MECHANISM: WEAK STEERING FLOW WITHIN BROAD,
ZONALLY ORIENTED TROUGH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 03W IS CURRENTLY
LOCATED WITHIN A BROAD, ZONALLY-ORIENTED DEEP-LAYER TROUGH THAT
EXTENDS FROM THE PHILIPPINES EASTWARD TOWARD TROPICAL STORM 02W
(MALAKAS). STEERING CURRENTS ARE THUS WEAK, AND THE TRACK FORECAST
IS CHALLENGING. THE PRIMARY FORCING MECHANISM FOR MOVEMENT WILL BE
THE APPROACH OF 02W (MALAKAS) FROM THE EAST OVER THE NEXT 2-3 DAYS,
WHICH WILL EVENTUALLY DRAG THE BELT OF WESTERLY FLOW NORTHWARD INTO
THE VICINITY OF TD 03W AS MALAKAS TRACKS NORTHWESTWARD. THIS BELT
OF WESTERLIES SHOULD EVENTUALLY KICK 03W EASTWARD, CAUSING IT TO
GET ENTRAINED INTO THE LARGE CIRCULATION OF 02W (MALAKAS). THE
TIMING OF THIS KICK IS HIGHLY UNCERTAIN IN THE MODEL GUIDANCE,
WHICH IS UNSURPRISING GIVEN THAT BINARY INTERACTION OF TWO TROPICAL
CYCLONES TYPICALLY INTRODUCES ELEVATED FORECAST SENSITIVITY. THE
GFS MODEL ACCELERATES 03W EASTWARD AFTER 48 HOURS, WHILE THE ECMWF
MODEL LINGERS 03W NEAR THE PHILIPPINE COASTLINE UNTIL ABOUT 84
HOURS BEFORE FINALLY ACCELERATING EASTWARD. GIVEN THE BETTER
REPRESENTATION OF THE INITIAL VORTEX IN THE GFS, THE JTWC FORECAST
IS CLOSER TO THE GFS TIMELINE, SHOWING SLOW NORTHWARD AND THEN
NORTHEASTWARD DRIFT THROUGH 48 HOURS, FOLLOWED BY QUICK
ACCELERATION NORTHEASTWARD THEREAFTER. THIS WILL RESULT IN THE
ABSORPTION OF 03W'S CIRCULATION INTO THE LARGER CIRCULATION OF 02W
(MALAKAS) BETWEEN 72 AND 96 HOURS, WHICH THE JTWC FORECAST WILL
CONSIDER SYNONYMOUS WITH DISSIPATION. THE INTENSITY FORECAST IS
ALSO CHALLENGING. IN THE NEAR TERN, INTENSITY WILL BE GOVERNED
PRIMARILY BY THE INFLUENCE OF UPPER-LEVEL OUTFLOW FROM 02W
(MALAKAS), WHICH IS EXPECTED TO BEGIN ENHANCING EASTERLY SHEAR OVER
TD 03W WITHIN ABOUT 24 HOURS. THIS SHOULD LIMIT TD 03W TO GRADUAL
INTENSIFICATION, AND THE JTWC FORECAST SHOWS A SLOW INCREASE TO 40
KT BY 36 HOURS. THEREAFTER, ALTHOUGH VERTICAL SHEAR WILL CONTINUE
INCREASING AS 02W (MALAKAS) APPROACHES, THE EMBEDDING OF 03W IN
INCREASINGLY STRONG WESTERLY SURFACE FLOW WILL KEEP MAXIMUM WINDS
ELEVATED, EVEN AS 03W BEGINS TO GET DRAWN INTO THE CIRCULATION OF
02W. THE JTWC FORECAST IS THUS FOR 03W'S INTENSITY TO REMAIN AROUND
40 KT DURING THE ABSORPTION PROCESS, WITH DISSIPATION MARKING THE
POINT IN TIME THAT 03W'S CIRCULATION OPENS UP AND DISSOLVES INTO
THE LARGER WIND FIELD AROUND 02W.

MODEL DISCUSSION: MODEL SPREAD IS VERY HIGH FOR THE TRACK FORECAST.
THE JTWC TRACK FORECAST IS SOUTH OF THE CONSENSUS, CLOSEST TO THE
GFS ENSEMBLE MEAN, BUT NOT AS FAR SOUTH AS THE GFS DETERMINISTIC
RUN. THE INTENSITY FORECAST IS CLOSE TO THE MODEL CONSENSUS DURING
THE FIRST 24 HOURS, THEN AIDED BY MANUAL ANALYSIS OF NUMERICAL
MODEL OUTPUT THEREAFTER, AS AUTOMATED MODEL TRACKERS FOR 03W BEGIN
FALSELY TRACKING 02W WHEN THE TWO STORMS DRAW CLOSE.

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