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JTWC/09W/#07/07-20 00Z Prognostic Reasoning
WDPN31 PGTW 200300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 09W (WIPHA) WARNING NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
INITIAL POSITION: 21.6N 114.6E
INITIAL INTENSITY: 65 KTS
GEOGRAPHIC REFERENCE: 48 NM SOUTH-SOUTHEAST OF HONG KONG
MOVEMENT PAST 6 HOURS: WESTWARD AT 13 KTS
SIGNIFICANT WAVE HEIGHT: 29 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TYPHOON (TY) 09W (WIPHA) HAS RAPIDLY CONSOLIDATED AND INTENSIFIED OVER
THE PAST SIX HOURS AS IT PASSES SOUTH OF HONG KONG. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTED AN EXPLOSIVE BURST
OF DEEP CONVECTION, WHICH BEGAN JUST PRIOR TO THE 200000Z HOUR,
SUBSEQUENTLY EXPANDING AND COVERING THE ASSESSED LOW LEVEL
CIRCULATION CENTER. FORTUNATELY, THE SYSTEMS CLOSE PROXIMITY TO
HONG KONG ALLOWS RADAR TO PROVIDE VALUABLE INSIGHTS INTO THE OTHERWISE
OBSCURED INNER STRUCTURE. THE RADAR EYE HAS SIGNIFICANTLY DECREASED IN
SIZE OVER THE LAST COUPLE OF HOURS, WITH DEEP CONVECTION FORMING A
PARTIAL EYEWALL ALONG THE SOUTHERN HALF OF THE RADAR EYE. THE NORTHERN
SIDE REMAINS SIGNIFICANTLY WEAKER, LIKELY A RESULT OF THE PERSISTENT
NORTHEASTERLY SHEAR, CURRENTLY ESTIMATED AT ABOUT 16 KNOTS. OFFICIAL
SURFACE OBSERVATIONS FROM THE VICINITY OF HONG KONG INDICATE SUSTAINED
GALE TO STORM-FORCE WINDS AND PRESSURES (AT SHORE STATIONS) OF AROUND
980MB, WHILE UNOFFICIAL OBSERVATIONS SUGGEST THE POTENTIAL FOR
HIGHER WINDSPEEDS AND LOWER PRESSURES. A 192006Z GMI COLOR-ENHANCED
37GHZ MICROWAVE IMAGE DEPICTED A SOLID CYAN RING FEATURE, INDICATING
THE SYSTEM WAS PRIMED FOR POSSIBLE RAPID INTENSIFICATION. THE INITIAL
POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE RADAR EYE
FEATURE. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE,
BASED ON THE OVERALL AVERAGE OF THE OBJECTIVE ESTIMATES NOTED ABOVE
AND SLIGHTLY EXCEEDING THE BULK OF THE SUBJECTIVE, AGENCY DVORAK
CURRENT INTENSITY ESTIMATES. THE ENVIRONMENT IS, IN THE AGGREGATE,
FAVORABLE FOR FURTHER INTENSIFICATION, WITH WARM SSTS, AND GOOD
EQUATORWARD OUTFLOW, OFFSET SOMEWHAT BY THE MODERATE NORTHEASTERLY
SHEAR.
INITIAL WIND RADII BASIS: PERSISTENCE
CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF AN EXTENSION
OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED FAR TO THE
NORTHEAST.
AGENCY DVORAK AND AUTOMATED FIXES:
PGTW: T3.5 - 55 KTS
RJTD: T3.5 - 55 KTS
RCTP: T3.5 - 55 KTS
KNES: T4.0 - 65 KTS
DEMS: T3.5 - 55 KTS
CIMSS SATCON: 64 KTS AT 192230Z
CIMSS ADT: 60 KTS AT 200000Z
CIMSS AIDT: 64 KTS AT 200100Z
CIMSS D-MINT: 66 KTS AT 192022Z
CIMSS D-PRINT: 63 KTS AT 200030Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
VWS: 15-20 KTS
SST: 29-30 CELSIUS
OUTFLOW: MODERATE EQUATORWARD
ANALYSIS CONFIDENCE:
INITIAL POSITION: HIGH
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: TY 09W IS, AS PREVIOUSLY FORECAST, NOW
TRACKING DUE WEST ALONG THE SOUTHWESTERN SIDE OF AN EXTENSION OF
THE STR TO THE NORTHEAST. THE SYSTEM IS EXPECTED TO MAKE LANDFALL
IN THE VICINITY OF YANGJIANG, CHINA WITHIN THE NEXT 12 HOURS, THEN
TURN SOUTHWESTWARD, SKIRTING THE COASTLINE OF SOUTHERN CHINA. THE
SYSTEM WILL PASS IN CLOSE PROXIMITY TO ZHANJIANG AND BE MOVING BACK
OVER OPEN WATERS OF THE FAR NORTHEASTERN GULF OF TONKIN BY TAU 24.
THE FORWARD SPEED OF TY 09W IS FORECAST TO BEGIN DECELERATING AFTER
LANDFALL, DUE TO FRICTIONAL EFFECTS AS IT INTERACTS WITH THE
TERRAIN. AFTER TAU 24, THE WEAKENING OF THE RIDGE TO THE NORTH DUE
TO PASSAGE OF AN UPPER-LEVEL TROF, WILL RESULT IN A WEAKER STEERING
GRADIENT AND MORE SIGNIFICANT SLOWDOWN AS THE SYSTEM MOVES INTO THE
GULF OF TONKIN. THE SLOWDOWN WILL ONLY LAST ABOUT 24 HOURS. HOWEVER,
AS ANOTHER STRONG RIDGE OVER THE TIBETAN PLATEAU BUILDS IN FROM THE
WEST AND STRENGTHENS THE STEERING GRADIENT, TY 09W WILL ONCE AGAIN
PICK UP SPEED AFTER TAU 48 AS IT APPROACHES A SECOND LANDFALL OVER
THE RED RIVER DELTA SOUTHEAST OF HANOI, VIETNAM. IN TERMS OF
INTENSITY, THE SYSTEM HAS CLEARLY CAPITALIZED ON THE SHALLOW
WARM WATERS ALONG THE CHINESE COAST AND SUCCESSFULLY COUNTERED THE
MODERATE SHEAR. TY 09W IS LIKELY TO INTENSIFY A BIT FURTHER PRIOR TO
LANDFALL, THOUGH THE PROXIMITY TO THE COASTLINE WILL LIKELY SERVE TO
TEMPER THE INTENSIFICATION SOMEWHAT AS IT GETS CLOSER TO THE COAST.
THE FORECAST CURRENTLY CALLS FOR A PEAK OF 70 KNOTS AS THE SYSTEM HITS
THE COAST, BUT IT IS POSSIBLE IT COULD REACH A SLIGHTLY HIGHER PEAK IF
THE CONVECTION ALL REMAINS OFFSHORE FOR A BIT LONGER. AS IT SKIRTS THE
COAST, THE SYSTEM WILL WEAKEN SIGNIFICANTLY DUE TO FRICTIONAL EFFECTS
AND IS FORECAST TO STILL BE A STRONG TROPICAL STORM WHEN IT MOVES BACK
OVER THE GULF OF TONKIN. ONCE OVER WATER AGAIN, THE SYSTEM IS FORECAST
TO REINTENSIFY RATHER QUICKLY, UP TO 60-65 KNOTS AS IT MOVES ACROSS
VERY WARM (30-31C) WATERS AND MAINTAINS A RELATIVELY FAVORABLE UPPER-
LEVEL ENVIRONMENT OF MODERATE SHEAR AND GOOD EQUATORWARD OUTFLOW. ONCE
ASHORE IN VIETNAM, THE SYSTEM WILL RAPIDLY WEAKEN AND ULTIMATELY
DISSIPATE OVER THE MOUNTAINS OF NORTHERN LAOS NO LATER THAN TAU 96.
MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GOOD AGREEMENT
THROUGH MOST OF THE FORECAST PERIOD, WITH MINIMAL CROSS- OR
ALONG-TRACK UNCERTAINTY THROUGH THE EARLY PORTION OF THE FORECAST.
CROSS-TRACK UNCERTAINTY REMAINS FAIRLY CONSISTENT THROUGH THE MID- AND
EXTENDED-RANGE FORECAST, BUT ALONG-TRACK SPREAD BEGINS TO APPEAR AS
EARLY AS TAU 36, WHEN THE GFS-GEFS COMBINATION STARTS TO DIVERGE FROM
THE REMAINDER OF THE MODELS. THE GFS MODELS INDICATE A LANDFALL IN
VIETNAM AS EARLY AS TAU 48, WHILE THE REMAINDER OF THE PACKAGE ARRIVES
ABOUT 12-18 HOURS LATER. AFTER LANDFALL, THE ECMWF-ECEPS-ECAIFS
COMBINATION REJOINS THE GFS BEFORE THE MODELS
DISPERSE AS THE SYSTEM WEAKENS AND VORTEX IS LOST. THE JTWC
FORECAST REMAINS CONSISTENT WITH THE PREVIOUS TRACK AND THE ECMWF
TRACKER THROUGH THE FORECAST PERIOD, WITH HIGH CONFIDENCE.
INTENSITY GUIDANCE IS MIXED. ONLY THE DECAY-SHIPS (GFS) INDICATES
ANY SORT OF NEAR-TERM INTENSIFICATION, WHILE THE HAFS-A AND
CONSENSUS MEAN SHOW STEADY BUT RELATIVELY SLOW WEAKENING TO TAU 24.
CONVERSELY, THE GFS AND HWRF SHOW RAPID WEAKENING TO JUST 40 KNOTS BY
TAU 12, FOLLOWED BY RAPID INTENSIFICATION IMMEDIATELY AFTER. MOST OF
THE MODELS SHOW SOME DEGREE OF REINTENSIFICATION IN THE GULF OF
TONKIN, THOUGH THERE IS A SIGNIFICANT AMOUNT OF UNCERTAINTY IN THE
ACTUAL PEAK. THE JTWC FORECAST IS POSITIONED ABOUT 10 KNOTS HIGHER
THAN THE CONSENSUS MEAN THROUGH THE FORECAST PERIOD. THE EXTENT TO
WHICH THE SYSTEM PENETRATES INLAND IN THE SHORT TERM WILL HAVE A
SIGNIFICANT IMPACT ON THE ULTIMATE INTENSITY AND THUS THE CONFIDENCE
IN THE INTENSITY FORECAST IS MEDIUM TO LOW.
FORECAST CONFIDENCE:
TRACK 0 - 72 HR: HIGH
TRACK 72-120 HR: MEDIUM
INTENSITY 0 - 72 HR: MEDIUM
INTENSITY 72-120 HR: LOW////
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