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JTWC/09W/#06/07-19 18Z Prognostic Reasoning
WDPN31 PGTW 192100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 09W (WIPHA) WARNING NR
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
INITIAL POSITION: 21.5N 116.0E
INITIAL INTENSITY: 55 KTS
GEOGRAPHIC REFERENCE: 111 NM EAST-SOUTHEAST OF HONG KONG
MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 17 KTS
SIGNIFICANT WAVE HEIGHT: 27 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
LOPSIDED SYSTEM, WITH PRONOUNCED CONVECTIVE AREAS DISPLACED SOUTH OF A
PARTIALLY OBSCURED LOW LEVEL CIRCULATION CENTER (LLCC). MINIMAL DEEP
CONVECTIVE ACTIVITY IS OBSERVED ACROSS THE NORTHERN HEMISPHERE OF THE
CIRCULATION. VORTICAL HOT TOWERS CONTINUE TO DEVELOP THE SOUTHERN
PERIPHERY OF THE STORM, ROUGHLY ALONG THE RADIUS OF MAXIMUM WINDS
(RMW). HOWEVER, DUE TO THE PERSISTENT MODERATE NORTHEASTERLY SHEAR,
THESE VHTS HAVE BEEN UNABLE TO SUSTAIN UP-SHEAR PROPAGATION ON A
PERSISTENT BASIS. A 191819Z NOAA-20 ATMS 89GHZ MICROWAVE IMAGE
REVEALED SHALLOW BANDING FEATURES ACROSS THE NORTHERN PORTION OF
THE ROTATION, WITH MORE SIGNIFICANT CONVECTIVE BANDING FEATURES TO
THE SOUTH AND SOUTHEAST. THE INITIAL POSITION IS ASSESSED WITH HIGH
CONFIDENCE BASED ON ANALYSIS OF THE ANIMATED EIR, THE NOTED
MICROWAVE IMAGE AND ANIMATED RADAR DATA FROM HONG KONG. RADAR IMAGERY
SUGGESTS A DUE WEST TRACK SINCE APPROXIMATELY 1700Z, WITH THE LATEST
FRAMES INDICATING THE POTENTIAL FORMATION OF A SMALL EYE. THE
INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, ROUGHLY AT
THE AVERAGE OF THE SUBJECTIVE DVORAK ESTIMATES FROM MULTIPLE
AGENCIES, AS WELL AS THE OBJECTIVE ESTIMATES NOTED ABOVE.
ENVIRONMENTAL CONDITIONS, WITH THE EXCEPTION OF SHEAR, REMAIN
FAVORABLE. HOWEVER, PERSISTENT NORTHEASTERLY SHEAR (CURRENTLY
ESTIMATED BY CIMSS AT 18 KNOTS) IS IMPEDING SIGNIFICANT
INTENSIFICATION AND DEGRADING THE OVERALL ENVIRONMENTAL ASSESSMENT.
OTHERWISE, SEA SURFACE TEMPERATURES (SSTS) ARE QUITE WARM, AND OUTFLOW
IS STRONG TO THE SOUTHWEST.
INITIAL WIND RADII BASIS: SCATTEROMETER DATA
CURRENT STEERING MECHANISM: WESTERN 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: T3.0 - 45 KTS
RCTP: T3.5 - 55 KTS
KNES: T4.0 - 65 KTS
DEMS: T3.5 - 55 KTS
CIMSS SATCON: 60 KTS AT 191731Z
CIMSS ADT: 61 KTS AT 191730Z
CIMSS AIDT: 52 KTS AT 191800Z
CIMSS D-MINT: 55 KTS AT 191731Z
CIMSS D-PRINT: 48 KTS AT 191830Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
VWS: 15-20 KTS
SST: 29-30 CELSIUS
OUTFLOW: STRONG EQUATORWARD
ANALYSIS CONFIDENCE:
INITIAL POSITION: HIGH
INITIAL INTENSITY: MEDIUM
INITIAL WIND RADII: HIGH
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.
FORECAST DISCUSSION: TROPICAL STORM (TS) 09W (WIPHA) IS FORECAST TO
RAPIDLY SHIFT TO A MORE WESTWARD TRACK AND PASS SOUTH OF HONG KONG
OVER THE NEXT 12 HOURS. RECENT RADAR DATA FROM HONG KONG INDICATES
THIS TRACK SHIFT IS ALREADY UNDERWAY. THE SYSTEM IS EXPECTED TO MAKE
LANDFALL TO THE WEST OF MACAU, NEAR THE MOUTH OF THE HUANGMAO RIVER BY
TAU 12. SUBSEQUENTLY, TS 09W IS FORECAST TO TURN WEST-SOUTHWEST,
SKIRTING THE CHINESE COAST AND REACHING THE VICINITY OF ZHANJIANG,
CHINA BY TAU 24. THE SYSTEM WILL BEGIN TO SLOW AFTER THE INITIAL
LANDFALL DUE TO FRICTIONAL EFFECTS. THIS DECELERATION WILL BE MORE
PRONOUNCED AFTER TAU 24 AS THE EXTENSION OF THE RIDGE BREAKS DOWN TO
THE NORTH DUE TO PASSAGE OF AN UPPER-LEVEL TROF. TS 09W IS FORECAST TO
MOVE BACK OVER WATER, ENTERING THE NORTHERN GULF OF TONKIN BY TAU 36,
BUT DUE TO THE WEAKENED STEERING PATTERN, TRACK SPEEDS WILL BE
SIGNIFICANTLY SLOWER ONCE IT ENTERS THE GULF. THE PRIMARY STEERING
INFLUENCE JUMPS FROM THE RIDGE TO THE EAST TO A DEEP-LAYER RIDGE OVER
THE TIBETAN PLATEAU AFTER TAU 48, STRENGTHENING THE STEERING GRADIENT,
PROVIDING AN IMPETUS FOR TS 09W TO ACCELERATE WESTWARD. A SECOND
LANDFALL IS FORECAST AROUND TAU 60, NEAR THE RED RIVER DELTA SOUTHEAST
OF HANOI, VIETNAM. AFTER LANDFALL, THE SYSTEM WILL CONTINUE TRACKING
INLAND THROUGH THE END OF THE FORECAST PERIOD. IN TERMS OF INTENSITY,
TS 09W IS FORECAST TO INTENSIFY SLIGHTLY OVER THE NEXT 12 HOURS, WITH
A PEAK INTENSITY OF 60 KNOTS EXPECTED AS THE SYSTEM MAKES LANDFALL.
DUE TO FRICTIONAL EFFECTS AND TERRAIN INTERACTIONS, THE SYSTEM WILL
WEAKEN SHARPLY THROUGH TAU 24 AS IT SKIRTS THE COASTLINE. AS IT MOVES
BACK OVER WATER IN THE GULF OF TONKIN, THE SLOWER SPEEDS AND SLIGHTLY
IMPROVED UPPER-LEVEL CONDITIONS (LOWER SHEAR AND INCREASED EQUATORWARD
OUTFLOW), WILL FACILITATE RAPID INTENSIFICATION, CAPITALIZING ON THE
VERY WARM (30-31C) SSTS IN THE GULF OF TONKIN. THE FORECAST CALLS FOR
A PEAK OF 55 KNOTS AT TAU 48, HOWEVER IT IS POSSIBLE THE ACTUAL PEAK
COULD BE SLIGHTLY HIGHER, OCCURRING IMMEDIATELY PRIOR TO LANDFALL,
EXPECTED NEAR TAU 60. REGARDLESS, ONCE THE SYSTEM MAKES LANDFALL, IT
WILL RAPIDLY WEAKEN AS IT TRAVERSES FURTHER INLAND, ULTIMATELY
DISSIPATING OVER NORTHERN LAOS NO LATER THAN TAU 96.
MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE EXHIBITS STRONG
AGREEMENT, PARTICULARLY IN THE CROSS-TRACK REALM, THROUGH THE
ENTIRE FORECAST PERIOD. FAIRLY LARGE ALONG-TRACK DIFFERENCES BEGIN
TO APPEAR AROUND TAU 36, AND ARE PARTICULARLY EVIDENT BETWEEN THE
GFS-GEFS AND THE ECMWF-ECEPS-ECAIFS COMBINATIONS. THE GFS-GEFS IS
CONSIDERABLY FASTER, INDICATING LANDFALL IN VIETNAM AT OR BEFORE TAU
48, WHILE THE ECMWF SERIES INDICATE A MUCH SLOWER TRACK ACROSS THE
GULF OF TONKIN. ADDITIONALLY, AFTER TAU 48, THE GFS-GEFS COMBO SHOW
THE SYSTEM TURNING SHARPLY BACK TO THE SOUTHEAST AND MOVING BACK OVER
HAINAN ISLAND. NO OTHER MODEL EXHIBITS THIS BEHAVIOR, THUS INTRODUCING
SIGNIFICANT UNCERTAINTY IN THE GFS-GEFS SOLUTIONS. THE JTWC FORECAST
IS CONSISTENT WITH THE ECMWF-ECEPS-ECAIFS SOLUTIONS THROUGH THE
FORECAST PERIOD, WITH HIGH CONFIDENCE. INTENSITY GUIDANCE IS IN
OVERALL GOOD AGREEMENT, ALTHOUGH THE JTWC FORECAST DEVIATES SLIGHTLY
HIGHER THROUGH TAU 12, THEN REJOINS THE PACK CONSENSUS BY TAU 12. ALL
CONSENSUS MEMBERS INDICATE THE SYSTEM WILL MAINTAIN OR INTENSIFY
BETWEEN TAU 24 AND 48, FOLLOWED BY RAPID WEAKENING. THE JTWC FORECAST
IS ABOUT 10 KNOTS HIGHER THAN THE HAFS-A AND ABOUT 5 KNOTS LOWER THAN
THE HWRF, THOUGH ABOUT 20 KNOTS HIGHER THAN THE CONSENSUS MEAN AT TAU
48 DUE TO THE INFLUENCE OF THE MUCH LOWER DECAY-SHIPS OUTPUT.
INTENSITIES WILL BE LARGELY DRIVEN BY HOW FAR INLAND THE SYSTEM TRACK
THROUGH TAU 24 AND HOW MUCH IT SLOWS AFTER TAU 24, SO CONFIDENCE IS
SET TO LOW THOUGH THE FORECAST.
FORECAST CONFIDENCE:
TRACK 0 - 72 HR: HIGH
TRACK 72-120 HR: MEDIUM
INTENSITY 0 - 72 HR: LOW
INTENSITY 72-120 HR: LOW//
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