WDPN31 PGTW 300300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (CHABA) WARNING 
NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.4N 115.2E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 340 NM WEST OF MANILA, PHILIPPINES
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY
EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) LOCATED UNDER THE
NORTHERN AREA OF BUILDING CONVECTION IN THE SOUTHERN SEMI-CIRCLE.
THE LATEST VISIBLE SATELLITE LOOP INDICATES BUILDING CONVECTION IN
THE NORTHEAST QUADRANT, HOWEVER THERE IS EVIDENCE OF SHEAR FROM THE
NORTHEAST AS WELL. THE INITIAL POSITION IS PLACED WITH MEDIUM
CONFIDENCE BASED OFF THE PGTW FIX AND AFOREMENTIONED VISIBLE
SATELLITE LOOP. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH
HIGH CONFIDENCE BASED ON STRUCTURE AND TWO SHIP WEATHER
OBSERVATIONS OF 34 KTS AND 35 KTS, BOTH LOCATED SOUTHWEST OF THE 
LLCC.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE
NORTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   CIMSS ADT: 2.3 KTS AT 301543Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE WESTWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: INCREASING MAX INTENSITY TO 60 KTS
BEGINNING AT TAU 36 THROUGH TAU 48, JUST BEFORE LANDFALL OVER
EASTERN HAINAN.

FORECAST DISCUSSION: TS CHABA WILL TRACK NORTH-NORTHWESTWARD AND
STRENGTHEN TO 55 KTS OVER THE NEXT 24 HOURS. BY TAU 36, THE STR TO
THE NORTHEAST WILL BUILD IN CAUSING THE SYSTEM TO SHIFT TO A MORE
NORTHWESTWARD TRACK. AT THIS TIME INTENSITY WILL INCREASE TO 60 KTS
DUE TO TS CHABA PASSING UNDER AN AREA OF DECREASED VERTICAL WIND
SHEAR (5-10KTS) AND KEEP INTENSITY AT 60 KTS BY TAU 48, JUST
BEFORE PASSING OVER THE EASTERN PORTION OF THE ISLAND OF HAINAN. BY
TAU 72, TS CHABA WILL SUCCUMB TO THE AFFECTS OF LAND INTERACTION
BETWEEN HAINAN AND MAINLAND CHINA AND WILL DECREASE IN INTENSITY TO
35 KTS. TAUS 96 AND 120 TAKE THE SYSTEM OVER LAND IN SOUTHERN CHINA
WHERE DISSIPATION WILL OCCUR.

MODEL DISCUSSION: THE JTWC MODEL CONSENSUS ENVELOPE REMAINS
RELATIVELY TIGHT, WITH THE NAVGEM TRACKER BEING THE OUTLIER PLACED
OFF TO THE WEST OF CONSENSUS, THEN CROSSING OVER OFF TO THE EAST OF
THE ENVELOPE BY TAU 36. ALSO AT TAU 36, THE JGSM TRACKER SHOOTS OFF
TO A MORE NORTHWARD DIRECTION. BECAUSE OF THIS, THE JTWC TRACK IS
LAID TO THE LEFT OF CONSENSUS AFTER TAU 48. THERE IS A 25 NM MODEL
SPREAD IN TRACK CONSENSUS AT TAU 12, INCREASING TO 50 NM BY TAU 48.
AFTER TS CHABA INTERACTS WITH LAND BY TAU 72, THE MODEL SPREAD
INCREASES TO 120 NM, LOWERING THE CONFIDENCE IN THE TRACK FORECAST
DURING THE LATER PORTION OF THE FORECAST PERIOD. INTENSITY GUIDANCE
ALSO IS RELATIVELY UNIFORM. THE JTWC FORECAST INTENSITY IS PLACED
WITH MEDIUM CONFIDENCE. ALL AGREE ON INCREASING INTENSITY UP TO TAU
48. AFTERWARDS, THE JTWC INTENSITY IS SET BELOW CONSENSUS TO
ACCOUNT FOR A GREAT DEAL OF LAND INTERACTION WITH HAINAN AND
MAINLAND CHINA.

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