WDPN32 PGTW 301500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (CHABA) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.0N 115.3E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 359 NM WEST-NORTHWEST OF MANILA,
PHILIPPINES
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 19 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS SPIRAL
BANDS OF DEEP CONVECTION WRAPPING INTO A PERSISTENT DEEP CONVECTIVE
CLOUD STRUCTURE. A 301050Z SSMIS 91 GHZ MICROWAVE IMAGE DEPICTS
FORMATIVE CONVECTIVE BANDING IN ALL QUADRANTS WITH SEVERAL
ADDITIONAL CONVECTIVE BANDS TO THE SOUTH AND WEST OF THE LOW LEVEL
CIRCULATION (LLC). THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON EIR AND SSMIS MICROWAVE IMAGERY. THE INITIAL
INTENSITY OF 40 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
IMPROVING CONVECTIVE STRUCTURE AS WELL AS A COMBINATION OF AGENCY
DVORAK INTENSITY ESTIMATES WHICH ARE SLIGHTLY LAGGED BEHIND AT T2.5
AND CIMSS ADT SHOWING 45 KNOTS.

INITIAL WIND RADII BASIS: PERSISTENCE 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   CIMSS ADT: 45 KTS AT 300840Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE WESTWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 04W (CHABA) IS FORECAST TO TRACK
NORTHWESTWARD IN A FAVORABLE
ENVIRONMENT BEFORE MAKING LANDFALL EAST OF LUICHOW PENINSULA.
ENVIRONMENTAL CONDITIONS ARE FORECAST TO REMAIN FAVORABLE,
CHARACTERIZED BY WARM SEA SURFACE TEMPERATURES (SST) (30C),
MODERATE DIVERGENCE ALOFT AND LOW-MODERATE VERTICAL WIND SHEAR (9-16 
KNOTS) WHICH WILL ALLOW CHABA TO REACH A POTENTIAL MAXIMUM INTENSITY 
OF 70 KNOTS BY TAU 36. BETWEEN TAU 36 AND TAU 48 CHABA WILL BEGIN TO
INTERACT WITH BOTH THE ISLAND OF HAINAN AND EVENTUALLY MAINLAND
CHINA SIGNIFICANTLY IMPACTING INTENSITY AS THE SYSTEM CONTINUES TO
TRACK NORTHWESTWARD. BETWEEN TAU 48 AND TAU 96 TC 04W WILL STEADY
UP ON AN APPROXIMATE NORTH-NORTHEASTWARD TRACK AS IT CONTINUES TO 
SUCCUMB TO THE OROGRAPHIC IMPACTS OF SOUTHERN CHINA BEFORE FINALLY 
DISSIPATING NEAR TAU 120.

MODEL DISCUSSION: NUMERICAL MODELS ARE IN TIGHT AGREEMENT THROUGH
TAU 48 WITH ONLY A 120NM SPREAD IN THE SOLUTIONS. PAST TAU 48 AVNI
TAKES AN UNREALISTIC ABRUPT TURN WESTWARD WHILE THE BULK OF THE
GUIDANCE CONTINUES THE SYSTEM NORTHEASTWARD AND ALONG TRACK MODEL
SPREAD INCREASES, FOR THIS REASON JTWC FORECAST TRACK IS PLACED
WITH MEDIUM CONFIDENCE. RELIABLE INTENSITY GUIDANCE IS IN GOOD
AGREEMENT WITH ALL MEMBERS SHOWING A STEADY INTENSIFICATION THROUGH
TAU 36 AFTER WHICH LAND INTERACTION AND EVENTUAL LANDFALL OCCURS
AND A SHARP DECREASE IN INTENSITY IS REFLECTED. THERE IS A
POSSIBILITY FOR A MORE DRAMATIC INTENSIFICATION TREND THROUGH TAU
36 WITH A BULK OF THE MODELS DISAGREEING ON THE EXTENT OF PEAK
INTENSITY, THUS JTWC INTENSITY FORECAST IS PLACED WITH MEDIUM
CONFIDENCE.

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