Advisories

WDPN32 PGTW 210900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (EIGHTEEN) 
WARNING NR 003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.6N 108.6E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 153 NM EAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS FLARING
CONVECTION NEAR AN ELONGATED LOW-LEVEL CIRCULATION CENTER (LLCC).
THUS FAR, CONVECTION HAS BEEN ABSENT OVER THE SOUTHERN AND EASTERN
PERIPHERY, INHIBITED BY A MASS OF DRY AIR STREAMING IN FROM THE
SOUTH. A 210146Z GMI 37 GHZ IMAGE REVEALED SMALL BANDING FEATURES
WRAPPING INTO THE SYSTEM CENTER. THE ENVIRONMENT IS LARGELY
UNFAVORABLE FOR DEVELOPMENT, CHARACTERIZED BY VERY WARM SEA SURFACE
TEMPERATURES (SST) MORE THAN OFFSET BY MODERATE TO HIGH VERTICAL
WIND SHEAR (VWS) AND DRY AIR WRAPPING IN FROM THE SOUTH. THE
INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON RADAR
IMAGERY. THE INITIAL INTENSITY HAS BEEN HELD AT 30 KTS WITH MEDIUM
CONFIDENCE BASED ON THE SUBJECTIVE DVORAK FIXES OF T2.0 FROM PGTW
AND KNES, WHICH IS A BIT LOWER THAN THE OBJECTIVE ESTIMATES FROM
CIMSS.

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

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 35 KTS AT 210710Z
   CIMSS ADT: 34 KTS AT 210610Z
   CIMSS AIDT: 33 KTS AT 210610Z
   CIMSS D-MINT: 35 KTS AT 210636Z
   CIMSS D-PRINT: 37 KTS AT 210610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK RADIAL

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN SHORTENED TO 72
HOURS.

FORECAST DISCUSSION: AFTER MOVING WESTWARD OVER HAINAN, TROPICAL
DEPRESSION (TD) 18W TURNED TO THE NORTH AND ENTERED THE GULF OF
TONKIN. THE CIRCULATION IS LOCATED IN AN AREA OF WEAK STEERING
CURRENTS. THE DEEP-LAYER SOUTHWESTERLY FLOW SOUTH OF THE
CIRCULATION IS COUNTERACTED BY A LOW- TO MID-LEVEL SUBTROPICAL
RIDGE (STR) TO THE NORTH. THE RIDGE WILL WEAKEN OVER THE NEXT 24
HOURS, ALLOWING FOR THE SOUTHWESTERLY FLOW TO GUIDE THE SYSTEM
EASTWARD. THEREFORE, THE JTWC FORECAST TURNS TD 18W NORTHEASTWARD
AT TAU 24 AND NEARLY EASTWARD THEREAFTER. WHILE THE WATERS WITHIN
THE GULF OF TONKIN ARE VERY WARM, THE HIGH VWS WILL PERSIST AND
INHIBIT DEVELOPMENT. THUS, THE INTENSITY FORECAST KEEPS 18W A TD
WHILE THE SYSTEM REMAINS OVER WATER. HOWEVER, SLIGHT DEVELOPMENT TO
A LOW-END TROPICAL STORM IS POSSIBLE BEFORE LANDFALL. AFTER MAKING
LANDFALL TOWARDS TAU 60, THE SYSTEM WILL DISSIPATE OVER LAND.

MODEL DISCUSSION: THE TRACK GUIDANCE SHOWS SIGNIFICANT SPREAD
BECAUSE OF THE WEAK STEERING ENVIRONMENT. THE AI MODELS TEND TO
TURN THE SYSTEM WESTWARD FOR THE FIRST 24 HOURS UNDER THE
DOMINATING STEERING INFLUENCE OF THE STR. UNDER THIS SCENARIO,
AFTER TAU 24, THE RIDGE WEAKENS AND THE DEEP-LAYER SOUTHWESTERLIES
BEGIN TO TURN THE SYSTEM EASTWARD. THE HAFS-A, ECMWF, AND GFS
MODELS TAKE THE SYSTEM NORTHWARD FOR THE FIRST 12-24 HOURS, BEFORE
DEPICTING AN EASTWARD TURN. THE JTWC FORECAST CURRENTLY ALIGNS WITH
THIS PHILOSOPHY. HOWEVER, IT IS POSSIBLE THAT THE RIDGE WILL
PREVAIL AND TURN THE SYSTEM WESTWARD FOR 12-24 HOURS, WHICH WOULD
DELAY THE INEVITABLE TERMINAL EASTWARD TRACK INTO SOUTHERN CHINA.
IN THAT CASE, SUBSEQUENT FORECASTS MAY NEED TO BE EXTENDED OUT IN
TIME. THE GFS AND HAFS-A SOLUTIONS KEEP THE INTENSITY STEADY
THROUGH THE FORECAST PERIOD, WHILE THE EXPERIMENTAL GOOGLE DEEPMIND
DEVELOPS TD 18W INTO A 35 KT TROPICAL STORM. NEARLY ALL OF THE
INTENSITY AIDS SHOW WEAKENING AFTER TAU 48. THE INTENSITY FORECAST
FAVORS THE GFS AND HAFS-A SOLUTIONS AND KEEPS MAXIMUM WIND SPEED
UNCHANGED WHILE THE SYSTEM IS OVER WATER.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: LOW
   TRACK 72-120 HR: ---
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: ---//
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