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JTWC/09W/#09/07-20 12Z Prognostic Reasoning
WDPN31 PGTW 201500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 09W (WIPHA) WARNING NR
009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
INITIAL POSITION: 21.7N 111.7E
INITIAL INTENSITY: 60 KTS
GEOGRAPHIC REFERENCE: 327 NM EAST OF HANOI, VIETNAM
MOVEMENT PAST 6 HOURS: WESTWARD AT 14 KTS
SIGNIFICANT WAVE HEIGHT: 24 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY REVEALS A FULLY
OBSCURED LOW-LEVEL CIRCULATION CENTER (LLCC) ASSOCIATED WITH
TROPICAL STORM (TS) 09W (WIPHA). THE INITIAL POSITION IS CURRENTLY
ASSESSED TO BE OVER LAND WITH MEDIUM CONFIDENCE. RADAR IMAGERY, AS
WELL AS A 201046Z 37 GHZ F17 SSMIS MICROWAVE IMAGE SUITE OFFER SOME
ASSISTANCE IN ESTIMATING THE POSITION THROUGH EXTRAPOLATION. THE
INITIAL INTENSITY OF 60 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON A T3.5 DVORAK ESTIMATE FROM RJTD, OBJECTIVE ESTIMATES
LISTED BELOW, AND VISUAL DETERIORATION OF CONVECTIVE BANDING AND
STRUCTURE OF THE SYSTEM DUE TO FRICTIONAL EFFECTS OF LAND INTERACTION.
ENVIRONMENTAL CONDITIONS REMAIN FAVORABLE FOR INTENSIFICATION, IN CASE
TS 09W REEMERGES OVER WATER PRIOR TO CROSSING THE LEIZHOU PENINSULA.
WARM SEA SURFACE TEMPERATURES (SST) OF 29-30 C, HIGH ATMOSPHERIC
COLUMN MOISTURE CONTENT AND MODERATE EQUATORWARD OUTFLOW ALOFT ARE
OFFSET ONLY BY PERSISTENT LOW TO MODERATE (15-20 KTS) NORTHEASTERLY
VERTICAL WIND SHEAR (VWS).
INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK
CURRENT STEERING MECHANISM: BUILDING SUBTROPICAL RIDGE (STR) TO THE
NORTHEAST
AGENCY DVORAK AND AUTOMATED FIXES:
PGTW: NO CURRENT ESTIMATE
RJTD: T3.5 - 55 KTS
CIMSS D-MINT: 57 KTS AT 201101Z
CIMSS D-PRINT: 50 KTS AT 201210Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
VWS: 15-20 KTS
SST: 29-30 CELSIUS
OUTFLOW: MODERATE EQUATORWARD
OTHER FACTORS: LOW-LEVEL CIRCULATION CENTER IS CURRENTLY
ASSESSED TO BE OVER LAND
ANALYSIS CONFIDENCE:
INITIAL POSITION: MEDIUM
INITIAL INTENSITY: MEDIUM
INITIAL WIND RADII: LOW
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.
FORECAST DISCUSSION: TS 09W IS ANTICIPATED TO CONTINUE ON A
WEST-SOUTHWESTWARD TRACK, AS THE SYNOPTIC STEERING ENVIRONMENT IS
IN A STATE OF TRANSITION. A SUBTROPICAL RIDGE (STR) TO THE
NORTHEAST HAS WEAKENED AND IS RETROGRADING, WHILE A SECONDARY STR
TO THE NORTHEAST IS BUILDING AND IS BECOMING THE DOMINANT STEERING
INFLUENCE. AS A RESULT, TS 09W IS FORECAST TO TRACK ALONG OR JUST
INLAND OF THE SOUTHERN CHINESE COASTLINE, EVENTUALLY MOVING OVER
THE NORTHERN PORTION OF THE LEIZHOU PENINSULA AND TRANSITING OVER
WARM WATERS OF THE GULF OF TONKIN PRIOR TO MAKING LANDFALL OVER
NORTHERN VIETNAM. OVER THE NEXT 12 HOURS, THE SYSTEM WILL REMAIN IN
A FAVORABLE ENVIRONMENT ASSOCIATED WITH THIS PART OF THE SOUTH
CHINA SEA, HOWEVER PROGRESSIVE LAND INTERACTION, PARTICULARLY
DURING OVERLAND VORTEX PLACEMENT, WILL IMPEDE STRUCTURAL INTEGRITY
AND LIMIT INTENSIFICATION. AROUND TAU 12, TS 09W IS EXPECTED TO
EMERGE OVER THE GULF OF TONKIN, WHERE IT WILL ENCOUNTER HIGH OCEAN
SST OF 30-31 C, POTENTIALLY FACILITATING SHORT-TERM REINTENSIFICATION
OR STABILIZATION OF SUSTAINED WINDS. HOWEVER, A NOTABLE INCREASE IN
NORTHEASTERLY VWS TO OVER 30 KTS BY TAU 36 IS LIKELY TO CURTAIL ANY
FURTHER INTENSIFICATION. THE SYSTEM IS THEN FORECAST TO MAKE LANDFALL
EAST OF HANOI BEFORE TAU 48, WITH RAPID DECAY EXPECTED DUE TO STRONG
FRICTIONAL FORCING, INCREASED SURFACE ROUGHNESS, AND OROGRAPHIC
DISRUPTION ASSOCIATED WITH COMPLEX TERRAIN OF THE NORTHERN VIETNAM.
COMPLETE DISSIPATION OF THE LOW-LEVEL VORTEX IS ANTICIPATED BETWEEN
TAUS 72 AND 96.
MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE EXHIBITS STRONG
CROSS-TRACK CONSENSUS THROUGH LANDFALL, WITH THE MULTI-MODEL
ENVELOPE MAINTAINING A MAXIMUM SPREAD OF 50 NM PRIOR TO VIETNAM
LANDFALL, EXPANDING TO 130 NM BY THE END OF THE FORECAST PERIOD.
HOWEVER, ALONG-TRACK DISPERSION IS MORE PRONOUNCED, PRIMARILY
ATTRIBUTABLE TO DISPARATE MODEL HANDLING OF TERRAIN-INDUCED
DECELERATION. NAVGEM REPRESENTS THE SLOWEST SOLUTION, WHILE
DETERMINISTIC GFS CONTINUES TO PRESENT THE FASTEST FORWARD MOTION,
RESULTING IN A 90 NM ALONG-TRACK DISCREPANCY AT TAU 24. THIS
CONTRIBUTES TO A MEDIUM CONFIDENCE LEVEL IN THE TIMING OF THE GULF OF
TONKIN REEMERGENCE AND, BY EXTENSION, THE SUBSEQUENT FORECAST
TRACK. IN TERMS OF INTENSITY GUIDANCE, CONSENSUS INDICATES SHORT-TERM
WEAKENING AS THE SYSTEM INTERACTS WITH LAND. THEREAFTER, MOST
MODELS SUGGEST POTENTIAL FOR RE-INTENSIFICATION OVER WATER, THOUGH
MAGNITUDE VARIES CONSIDERABLY. DETERMINISTIC GFS INDICATES A MODEST 5-
10 KTS INCREASE, WHILE NAVGEM-BASED COAMPS-TC AND HAFS GUIDANCE SHOW
MORE AGGRESSIVE SCENARIOS, INDICATING PEAK INTENSITY BEYOND 90 KTS.
POST-TAU 48, RAPID WEAKENING IS PROJECTED IN MOST GUIDANCE PACKAGES,
DUE TO INCREASING VWS AND BEGINNING OF TERRAIN INTERACTION. OVERALL,
JTWC INTENSITY FORECAST CONFIDENCE IS LOW, PRIMARILY DUE TO
UNCERTAINTY IN FORWARD MOTION AND THE DEGREE AND TIMING OF LAND
INTERACTION.
FORECAST CONFIDENCE:
TRACK 0 - 72 HR: MEDIUM
TRACK 72-120 HR: MEDIUM
INTENSITY 0 - 72 HR: LOW
INTENSITY 72-120 HR: LOW////
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