Tropical Storm MERBOK Advisory Mo, 12.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 15W (MERBOK)
WARNING NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 21.4N 161.4E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 318 NM WEST-NORTHWEST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
UPPER-LEVEL CONDITIONS ARE COMPLEX: AN ANTICYCLONE IS CENTERED NEAR
18N 159E WITH A NARROW TUTT POSITIONED ALONG THE NORTHERN PERIPHERY
OF THE SYSTEM WHICH IS PRODUCING VIGOROUS WEST-SOUTHWESTERLY FLOW
OVER THE NORTHERN HALF OF THE SYSTEM. STRONG DIFFLUENCE OVER THE
SOUTHERN HALF OF THE SYSTEM IS MAINTAINING PERSISTENT CURVED
CONVECTIVE BANDING BUT LIMITING OVERALL DEVELOPMENT. ADDITIONALLY,
ROBUST POLEWARD OUTFLOW INTO THE TUTT CELL TO THE NORTH IS IMPROVING
QUICKLY. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS
FRAGMENTED BANDING WITH A CLUSTER OF FLARING CONVECTION OVER THE
EASTERN QUADRANT. THE INITIAL POSITION IS PLACED WITH MEDIUM
CONFIDENCE BASED ON AN 112329Z ASCAT-C BULLSEYE IMAGE SHOWING AN
ELONGATED CENTER WITH A SWATH OF 30-35 KNOT WINDS OVER THE NORTHERN
SEMICIRCLE. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH HIGH
CONFIDENCE BASED ON THE PGTW DVORAK ESTIMATE AND RECENT ASCAT DATA.
INITIAL WIND RADII BASIS: SCATTEROMETER DATA
CURRENT STEERING MECHANISM: STEERING IS CURRENTLY TRANSITIONING
FROM THE NEAR-EQUATORIAL RIDGE TO THE SOUTHEAST TO THE NORTH-SOUTH
ORIENTED STR POSITIONED TO THE EAST
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T1.5 - 25 KTS
   CIMSS SATCON: 42 KTS AT 112234Z
   CIMSS ADT: 32 KTS AT 112330Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: LOW
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.
FORECAST DISCUSSION: AS FORECAST WELL BY THE NUMERICAL MODELS, THE
STR HAS RE-ALIGNED IN A MORE NORTH-SOUTH CONFIGURATION WITH A
STRONG TUTT CELL POSITIONED TO THE NORTH NEAR 31N 164E. CONSEQUENTLY,
TS 15W IS BEGINNING TO TURN MORE POLEWARD AND IS FORECAST TO TRACK
NORTHWARD THROUGH TAU 60 THEN NORTH-NORTHEASTWARD THROUGH TAU 72 AS
IT BEGINS TO APPROACH THE STR AXIS. TS 15W IS EXPECTED TO INTENSIFY
SLOWLY THROUGH TAU 24 UNDER MARGINALLY-FAVORABLE CONDITIONS THEN AT A
FASTER RATE OF INTENSIFICATION AFTER TAU 24 AS VERTICAL WIND SHEAR
DECREASES TO LOW LEVELS AIDED BY STRONG POLEWARD OUTFLOW. THE PEAK
INTENSITY OF 85 KNOTS IS EXPECTED TO OCCUR NEAR TAU 72. NEAR TAU 72,
SST WILL GRADUALLY COOL WITH EXTRA-TROPICAL TRANSITION (ETT) EXPECTED
TO COMMENCE AS THE SYSTEM BEGINS TO INTERACT WITH THE MIDLATITUDE
WESTERLIES AND BAROCLINIC ZONE. TS 15W WILL COMPLETE ETT BY TAU 96
AS IT GAINS FRONTAL CHARACTERISTICS AND ACCELERATES NORTHEASTWARD
WITHIN THE STRONG MIDLATITUDE SOUTHWESTERLY FLOW.
MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 72 WITH A 120NM SPREAD IN SOLUTIONS AT TAU 72. THE
111800Z ECMWF ENSEMBLE (EPS) AND GFS ENSEMBLE (GEFS) ARE ALSO IN
TIGHT AGREEMENT LENDING HIGH CONFIDENCE TO THE JTWC FORECAST
TRACK. AFTER TAU 72, THE DETERMINISTIC GUIDANCE DIVERGES, MAINLY
ALONG-TRACK, WITH A SIGNIFICANT DIFFERENCE IN TRACK SPEEDS THUS
LOW CONFIDENCE. RELIABLE INTENSITY GUIDANCE IS IN FAIR AGREEMENT AND
INDICATES RAPID INTENSIFICATION (RI): COAMPS-TC (GFS VERSION)
INDICATES RI WITH A PEAK INTENSITY OF 98 KNOTS AT TAU 36 WHILE
HWRF INDICATES RI AND A PEAK INTENSITY OF 87 KNOTS AT TAU 48.
WHILE RI IS FORECASTED AFTER TAU 12, IT IS NOT LIKELY TO BEGIN NOW
DUE TO THE MARGINALLY-FAVORABLE UPPER-LEVEL CONDITIONS THUS THE JTWC
FORECAST IS SET AT A LOWER PEAK OF 85 KNOTS AT TAU 48. THE COAMPS-TC
ENSEMBLE INDICATES A 80 TO 100 PERCENT PROBABILITY OF RI IN THE TAU
12 TO TAU 30 TIMEFRAME.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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