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JTWC/29W/#06/10-09 12Z Prognostic Reasoning
WDPN31 PGTW 091500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 29W (NAKRI) WARNING NR
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
INITIAL POSITION: 24.7N 134.3E
INITIAL INTENSITY: 35 KTS
GEOGRAPHIC REFERENCE: 368 NM EAST-SOUTHEAST OF KADENA AB
MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 08 KTS
SIGNIFICANT WAVE HEIGHT: 17 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY INDICATES THAT
TROPICAL STORM (TS) 29W HAS SHOWN A SLIGHT ORGANIZATIONAL
IMPROVEMENT OVER THE PAST SIX HOURS. THE CONVECTION THAT WAS
DISPLACED TO THE SOUTHWEST AT 0600Z HAS PERSISTED, ORGANIZED INTO A
SMALL CDO FEATURE AND DRIFTED NORTHEASTWARD UNDER MORE FAVORABLE
SHEAR CONDITIONS. THE CIRRUS ASSOCIATED WITH THE CDO HAS OBSCURED
THE LOW LEVEL CIRCULATION CENTER (LLCC) AND THE EIR SHOWS DEEP
CONVECTION CONTINUING TO DEVELOP IN CLOSE PROXIMITY TO THE ASSESSED
LLCC, ACCOMPANIED BY AN EXTREME BURST IN LIGHTNING ACTIVITY. A
091149Z ASCAT-C BULLSEYE SHOWS AN ASYMMETRIC WIND FIELD, WITH
RELATIVELY LIGHT WINDS OF 20-25 KNOTS ACROSS THE NORTHERN PORTION
OF THE CIRCULATION AND 35 KNOT WINDS MOSTLY CONFINED TO THE
SOUTHERN HALF OF THE SYSTEM. A FEW 40 KNOT WIND BARBS WERE VISIBLE
IN THE SOUTHWESTERN QUADRANT BUT THESE WERE RAIN FLAGGED AND BEING
UNDER THE DEEPEST CONVECTION, WERE DEEMED OF QUESTIONABLE ACCURACY.
THE INITIAL POSITION WAS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
ASCAT DATA. SIMILARLY, THE INITIAL INTENSITY WAS ALSO SET WITH HIGH
CONFIDENCE USING THE ASCAT DATA, WHICH IS SUPPORTED BY MULTIPLE
T2.5 FIXES, AND A BLEND OF THE AVAILABLE OBJECTIVE DATA NOTED
BELOW. ENVIRONMENTAL CONDITIONS ARE MARGINAL THOUGH THEY ARE
IMPROVING. SSTS ARE WARM AND SHEAR HAS LOWERED SIGNIFICANTLY, DOWN
TO JUST 10 KNOTS AS OF 1200Z. THE MAIN HINDERANCE CURRENTLY IS THE
LACK OF A DECENT OUTFLOW MECHANISM AND THE PRESENCE OF DEEP POCKETS
OF DRY AIR, PARTICULARLY TO THE WEST AND NORTH OF THE LLCC, AS
INDICATED BY THE 1200Z MINAMIDAITOJIMA SOUNDING WHICH CONTINUES TO
SHOW A VERY DRY AIR MASS ABOVE 750MB AND LIGHT EASTERLY WINDS ABOVE
300MB.
INITIAL WIND RADII BASIS: SCATTEROMETER DATA
CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE NORTHEAST.
AGENCY DVORAK AND AUTOMATED FIXES:
PGTW: T2.5 - 35 KTS
RJTD: T2.0 - 30 KTS
RCTP: T3.0 - 45 KTS
KNES: T2.5 - 35 KTS
CIMSS ADT: 37 KTS AT 091140Z
CIMSS AIDT: 38 KTS AT 091140Z
CIMSS D-MINT: 27 KTS AT 090847Z
CIMSS D-PRINT: 29 KTS AT 091230Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
VWS: 5-10 KTS
SST: 28-29 CELSIUS
OUTFLOW: WEAK WESTWARD
ANALYSIS CONFIDENCE:
INITIAL POSITION: HIGH
INITIAL INTENSITY: HIGH
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) 29W HAS CONTINUED TO TRACK
NORTHWESTWARD OVER THE LAST SIX HOURS, THOUGH FORWARD SPEED HAS
SLOWED BY HALF, DOWN FROM 15 KTS AT 0600Z TO EIGHT KNOTS AT 1200Z.
THE REASON FOR THE SLOWDOWN IS LIKELY TWO-FOLD. FIRST THE STEERING
GRADIENT HAS WEAKENED SLIGHTLY AND SECOND, THE SYSTEM HAS WOBBLED A
BIT OR UNDERGONE SOME VORTEX PRECESSION AS IT TUCKED UNDER THE DEEP
CONVECTION TO THE SOUTH. HOWEVER, THERE HAS BEEN NO CHANGE TO THE
UNDERLYING STEERING PATTERN, WITH A DEEP STR TO THE NORTHEAST, A
SECONDARY RIDGE CENTERED OVER COASTAL CHINA SOUTH OF SHANGHAI AND A
COL REGION DUE NORTH OF TS 29W. OVER THE NEXT 24 HOURS, THE STR TO
THE EAST PUSHES WEST TO A POSITION SOUTH OF TOKYO, WHILE THE
SECONDARY RIDGE REMAINS QUASI-STATIONARY. THIS WILL PUT A SQUEEZE
ON TS 29W, AND THE SYSTEM WILL SQUIRT OUT TO THE NORTHWEST INTO THE
COL REGION TO THE NORTH. TRACK SPEEDS WILL REMAIN RELATIVELY STEADY
THROUGH TAU 24 AS THE GRADIENT TIGHTENS UP A BIT AS THE RIDGES
SQUEEZE TS 29W FROM BOTH SIDES. TS 29W WILL SLOW DOWN SHORTLY AFTER
TAU 24 AS IT NEARS THE AXIS OF THE RIDGE AND MOVES INTO THE COL
REGION. THERE REMAINS A POSSIBILITY OF A SHORT PERIOD OF NEARLY
QUASI-STATIONARY MOTION BETWEEN TAU 24 AND TAU 36 AS THE SYSTEM
MOVES THROUGH THE COL REGION AND ROUNDS THE RIDGE AXIS. AFTER TAU
36, BOTH OF THE RIDGE CENTERS REMAIN STATIONARY, BUT THE GLOBAL
MODELS INDICATE THE EASTERN RIDGE BUILDS AND COMES TO DOMINATE THE
STEERING INFLUENCE FOR TS 29W WITH THE TIGHTEST GRADIENT PLACED ON
THE SOUTHEASTERN SIDE OF THE SYSTEM. BY TAU 48, TS 29W WILL HAVE
ROUNDED THE RIDGE, AND ACCELERATE TOWARDS THE NORTHEAST ALONG THE
NORTHWESTERN SIDE OF THE STR. THE TRACK FLATTENS TO MORE OF AN
EASTERLY HEADING AFTER TAU 72 AS THE RIDGE MOVES TO THE WEST AND
ITSELF FLATTENS TO A MORE EAST-WEST ORIENTATION. IN TERMS OF
INTENSITY, THE RECENT BURST OF CONVECTION WILL LIKELY RESULT IN A
SMALL INCREASE IN INTENSITY, THOUGH THE NONE OF THE AVAILABLE MODEL
GUIDANCE DEPICTS THE CONVECTION PERSISTING PAST CONVECTIVE MAX. AS
THE SYSTEM MOVES UNDER THE 200MB ANTICYCLONE TO THE NORTH, IT WILL
BE CHOKED OFF FROM ANY SIGNIFICANT OUTFLOW MECHANISM AND IS THUS
NOT EXPECTED TO INTENSIFY MUCH THROUGH TAU 24. AS THE SYSTEM
REACHES AND ROUNDS THE RIDGE AXIS, IT WILL BEGIN TO TAP INTO A
DIVERGENT POLEWARD OUTFLOW CHANNEL, WHICH COMBINED WITH AN INCREASE
IN DEEP MOISTURE, WILL ALLOW FOR A PERIOD OF NEAR-RAPID
INTENSIFICATION BETWEEN TAU 48 AND TAU 72. THE ACTUAL PEAK
INTENSITY OF 80-85 KNOTS WILL LIKELY OCCUR AROUND TAU 84 AND IS NOT
CAPTURED IN THE FORECAST POINTS DIRECTLY. BY TAU 72, THE SYSTEM
WILL BEGIN EXTRATROPICAL TRANSITION AND BEGIN TO FEEL THE EFFECTS
OF RAPIDLY DECREASING SSTS, INCREASING SHEAR AND DRY AIR
ENTRAINMENT, AND START TO RAPIDLY WEAKEN, CONTINUING TO WEAKEN
THROUGH THE END OF THE FORECAST.
MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN RELATIVELY
GOOD AGREEMENT IN THE SHORT-TERM, WITH A 70NM CROSS-TRACK SPREAD AT
TAU 36, BETWEEN THE GEFS MEAN ON THE LEFT OF THE PACKAGE AND NAVGEM
ON THE RIGHT. FOR THE MOST PART, WITH THE EXCEPTION OF NAVGEM, THE
REMAINDER OF THE GUIDANCE REMAINS IN FAIRLY TIGHT CROSS-TRACK
AGREEMENT THROUGH THE REMAINDER OF THE FORECAST PERIOD, WITH A
105NM SPREAD AT TAU 120. HOWEVER, ALONG-TRACK SPREAD INCREASES
SHARPLY AFTER TAU 72. BY TAU 120, THE ALONG-TRACK SPREAD BETWEEN
THE BULK OF THE GUIDANCE IS 240NM, WITH THE ECMWF BEING THE
SLOWEST, AND REMAINDER OF THE CONSENSUS MEMBERS (EXCEPT GALWEM)
BEING CLUSTERED OUT AHEAD OF IT. GALWEM IS A SIGNIFICANT OUTLIER IN
THIS REGARD, BEING 540NM AHEAD OF THE ECMWF AT TAU 120. THE JTWC
FORECAST IS POSITIONED CLOSE TO THE ECMWF AND EC-AIFS, JUST WEST OF
THE CONSENSUS MEAN, THROUGH TAU 48 AND JUST NORTH AND SLIGHTLY
SLOWER THAN THE CONSENSUS MEAN THROUGH THE REMAINDER OF THE
FORECAST. CONFIDENCE IN THE NEAR TERM IS MEDIUM, TRENDING TO LOW IN
THE LONG-TERM DUE TO THE UNCERTAINTY ASSOCIATED WITH THE TRACK
MOTION THROUGH THE COL AND RIDGE AXIS. INTENSITY GUIDANCE REMAINS
MIXED, WITH THE SHIPS GUIDANCE CONTINUING ITS BEARISH TREND,
SHOWING MINIMAL INTENSIFICATION THROUGH THE FORECAST PERIOD. THE
MESOSCALE MODELS ON THE OTHER HAND REMAIN QUITE BULLISH, CONTINUING
TO CLUSTER BETWEEN 80-90 KNOTS FOR THE PEAK AROUND TAU 84. THE JTWC
FORECAST IS CONSISTENT WITH THE CONSENSUS MEAN THROUGH TAU 48, THEN
IS PLACED CLOSE TO THE HAFS-A THEREAFTER, WITH MEDIUM CONFIDENCE.
FORECAST CONFIDENCE:
TRACK 00-72 HR: MEDIUM
TRACK 72-120 HR: LOW
INTENSITY 00-72 HR: MEDIUM
INTENSITY 72-120 HR: MEDIUM//
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