Advisories

WDPN32 PGTW 222100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (NARRA) WARNING 
NR 009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.0N 108.3E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 134 NM EAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
DEPRESSION (TD) 18W (NARRA) WITH PERSISTENT CIRRUS CLOUD COVER OVER
THE ENTIRETY OF THE SYSTEM AND A LARGE AREA OF FLARING CONVECTION
CENTERED TO THE NORTHWEST OF THE LOW LEVEL CIRCULATION (LLC), OVER
THE COAST OF SOUTHERN CHINA AND NORTHEASTERN VIETNAM. RADAR IMAGERY
INDICATES THE CIRCULATION IS BROAD AND ELONGATED, WITH THE
STRONGEST BAND OF CONVECTION BEING POSITIONED OVER LAND IN VIETNAM
AND WRAPPING AROUND THE WESTERN FLANK OF 18W. CURRENT ENVIRONMENTAL
CONDITIONS ARE ASSESSED AS MARGINAL, AS THE VERY WARM (30-31 C) SEA
SURFACE TEMPERATURES (SSTS) ARE OFFSET BY MODERATE (20 KTS)
VERTICAL WIND SHEAR, WEAK POLEWARD OUTFLOW ALOFT, AND AN AREA OF
DRIER AIR TO THE SOUTH OF THE SYSTEM. THE INITIAL POSITION IS
PLACED WITH LOW CONFIDENCE BASED ON A COMBINATION OF RADAR IMAGERY
AND AGENCY FIXES. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON AGENCY DVORAK FIXES, CIMSS OBJECTIVE ESTIMATES,
AND A SURFACE OBSERVATION FROM BACH LONG VY TO THE SOUTH OF 21 KTS
WESTERLY AT 221800Z.

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

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT COMPRISED OF
A NEAR-EQUATORIAL RIDGE (NER) CENTERED TO THE SOUTHWEST AND A
SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.0 - 30 KTS
   KNES: T2.5 - 35 KTS
   CIMSS D-MINT: 37 KTS AT 221752Z
   CIMSS D-PRINT: 36 KTS AT 221810Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: WEAK EQUATORWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TD 18W IS FORECAST TO CONTINUE MEANDERING,
GENERALLY COUNTERCLOCKWISE LOOP WITHIN THE GULF OF TONKIN FOR THE
REMAINDER OF THE FORECAST PERIOD BEFORE MAKING LANDFALL AND
DISSIPATING IN THE VICINITY OF BEIHAI BY TAU 96. AN ERRATIC TRACK
IN THE SHORT-TERM IS EXPECTED AS 18W REMAINS IN A WEAK STEERING
ENVIRONMENT BETWEEN THE INFLUENCE OF A NEAR-EQUATORIAL RIDGE (NER)
CENTERED TO THE SOUTHWEST AND A SUBTROPICAL RIDGE (STR) CENTERED TO
THE NORTH OF 18W. CURRENTLY, THE NER IS ASSESSED AS BEING A MORE
DOMINANT FACTOR, CAUSING THE INITIAL SOUTHWARD MOVEMENT IN TRACK
UNTIL TAU 48, AT WHICH POINT THE NER RETREATS TO THE SOUTHWEST AND
THE STR CENTERED TO THE NORTH BUILDS AND CONNECTS WITH ANOTHER STR
CENTERED OVER KYUSHU, CAUSING A POLEWARD TURN BY TAU 60 AND
LANDFALL TO THE EAST OF THE LEIZHOU PENINSULA. REGARDING INTENSITY,
TD 18W IS EXPECTED TO HAVE A SHORT WINDOW FOR INTENSIFICATION IN
THE SHORT TERM AS THE VWS MOMENTARILY ABATES AND OUTFLOW ALOFT
IMPROVES. THIS IS EXPECTED TO BE SHORT-LIVED, AND BY TAU 48 VWS
WILL SHARPLY INCREASE TO OVER 25 KTS, INTRODUCING MID-LEVEL DRIER
AIR, BEGINNING DISSIPATION OF TD 18W OVER WATER BY TAU 72 BEFORE IT
MAKES LANDFALL AND DISSIPATES BY TAU 96.

MODEL DISCUSSION: TRACK MODEL GUIDANCE IS IN POOR OVERALL AGREEMENT
AS THERE CONTINUES TO BE A WIDE SPREAD OF EXPECTED DIRECTIONS AND
TRACK SPEED, ALTHOUGH THERE REMAINS AGREEMENT ON THE GENERAL
SOUTHWARD MOTION OF TD 18W OUT TO TAU 48. GFS IS THE WESTERNMOST
SOLUTION AND MAINTAINS 18W OVER WATER FOR THE ENTIRETY OF THE
FORECAST PERIOD. CONVERSELY, GALWEM AND THE EXPERIMENTAL GOOGLE
DEEPMIND TAKE 18W THE FARTHEST EAST, DEPICTING POSSIBLE LANDFALL
OVER NORTHERN HAINAN AND THE SOUTHERN TIP OF THE LEIZHOU PENINSULA
BEFORE CONTINUING OUT INTO THE SOUTH CHINA SEA. THE OFFICIAL JTWC
FORECAST TRACK IS PLACED WITH LOW CONFIDENCE, CLOSELY IN LINE WITH
THE MULTI-MODEL CONSENSUS EXCLUDING NAVGEM, ALSO CLOSELY IN LINE
WITH THE TRACK OF THE AI CONSENSUS, WHICH BOTH FALL WITHIN THE
CENTER OF THE GUIDANCE PACKAGE. REGARDING INTENSITY, THE GFS AND
ECMWF GLOBAL MODELS DEPICT DIFFERENT ENVIRONMENTAL OUTLOOKS IN THE
SHORT-TERM, LENDING SOME UNCERTAINTY TO THE INTENSITY FORECAST.
ECMWF PREDICTS MORE FAVORABLE VWS FROM TAUS 12 TO 36, ALLOWING FOR
TD 18W TO INTENSIFY, WHILE GFS IS MORE CONSERVATIVE, MAINTAINING
VWS ABOVE 15 KTS DURING THIS PERIOD AND THUS LESS CHANCE FOR
INTENSIFICATION. THE OFFICIAL JTWC INTENSITY FORECAST IS ASSESSED
WITH LOW CONFIDENCE, CLOSELY IN LINE WITH THE DETERMINISTIC GFS
SOLUTION.

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