Tropical Storm NANMADOL Advisory Di, 13.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 16W (SIXTEEN)
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 21.7N 138.5E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 242 NM SOUTHWEST OF IWO TO
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) CONTINUES TO DEPICT
A VERY BROAD LOW LEVEL CIRCULATION (LLC) WITH DISORGANIZED FLARING
CONVECTION SURROUNDING THE ASSESSED CENTER. A PARTIAL 122156Z SSMIS
89GHZ MICROWAVE IMAGE SHOWED WEAK, FRAGMENTED CONVECTIVE BANDS
WRAPPING INTO A FAIRLY WELL-DEFINED LLCC. THE INITIAL POSITION AND
INTENSITY ARE BOTH ASSESSED WITH HIGH CONFIDENCE ON A TIMELY
122356Z PARTIAL ASCAT-B PASS WHICH SHOWED THE LLCC JUST EAST OF AN
AREA OF CURVED CONVECTION WITH WINDS UP TO 25 KNOTS JUST TO THE
SOUTH OF THE CENTER AND EXTENDING OUT ABOUT 75NM FROM THE CENTER TO
THE EAST-SOUTHEAST. THE ENVIRONMENT REMAINS MARGINALLY FAVORABLE,
WITH LOW VWS, AND WARM SSTS PROVIDING AMPLE ENERGY FOR DEVELOPMENT.
THE TWO MITIGATING FACTORS ARE THE BROAD, MONSOON DEPRESSION-ESQUE
NATURE OF THE CIRCULATION AND THE LACK OF A GOOD OUTFLOW MECHANISM.
THE MAJORITY OF THE OUTFLOW IS RESTRICTED TO THE LOCAL REGION
AROUND TD 16W, WITH ONLY A WEAK TAP INTO AN AREA OF ENHANCED
EQUATORWARD OUTFLOW REGION TO THE EAST. THE SYSTEM IS DRIFTING
EASTWARD IN A WEAK STEERING ENVIRONMENT.
INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).
CURRENT STEERING MECHANISM: COMPETING STEERING ENVIRONMENT BETWEEN
NEAR EQUATORIAL RIDGE (NER) TO THE SOUTH AND SUBTROPICAL RIDGE
(STR) TO THE NORTH.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T1.5 - 25 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS
   CIMSS ADT: 29 KTS AT 122330Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 0-5 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK RADIAL
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.
FORECAST DISCUSSION: CURRENTLY TRAPPED IN A COMPETING STEERING
ENVIRONMENT, STUCK BETWEEN TWO DEEP-LAYER RIDGES; THE SUBTROPICAL
RIDGE (STR) SITTING OVER TOKYO AND A NEAR EQUATORIAL RIDGE (NER)
LYING TO THE SOUTH OF GUAM AND EXTENDING WEST OVER THE PHILIPPINES.
AT PRESENT, THE NER IS IN A SLIGHTLY BETTER POSITION AND WINNING
THE STEERING BATTLE, BUT ONLY JUST, PUSHING TD 16W ON A SLOW
EAST-NORTHWESTWARD DRIFT. THIS GENERAL MOTION IS EXPECTED TO
CONTINUE THROUGH TAU 24, WHEN TD 16W WILL TAKE A SHARP TURN
POLEWARD AND THEN WESTWARD AS THE NER BEGINS TO WEAKEN AND THE STR
TO THE NORTH ASSUMES THE DOMINANT STEERING ROLE. BY TAU 36 THE
SYSTEM IS FORECAST TO HAVE STEADIED UP ON A WEST-NORTHWEST TRACK
ALONG THE SOUTHWEST PERIPHERY OF THE STR TO THE NORTH, AND WILL
GRADUALLY PICK UP SPEED AS IT HEADS FOR THE NORTHERN RYUKUS. A
DYNAMIC 500MB TROUGH IS FORECAST TO PASS BY TO THE NORTH AROUND TAU
96, ERODE THE STEERING RIDGE AND ALLOW FOR TD 14W TO BEGIN ITS
RECURVE, TURNING TO THE NORTH BY TAU 120. DUE TO THE BROAD NATURE
OF THE SYSTEM, IT WILL REQUIRE A FAIR AMOUNT OF TIME TO CONSOLIDATE
AND INTENSIFY AND THIS PROCESS IS MADE ALL THE MORE CHALLENGING
BY THE LACK OF A GOOD OUTFLOW CHANNEL TO EXHAUST THE LOW LEVEL
INFLOW. IN LIGHT OF THESE LIMITATIONS, INTENSIFICATION IS EXPECTED
TO BE SLOW THROUGH THE FIRST COUPLE OF DAYS, ONLY REACHING 50 KNOTS
BY TAU 48. AFTER THIS POINT THE SYSTEM SHOULD BE WELL CONSOLIDATED,
THOUGH STILL QUITE LARGE, AND IS EXPECTED TO ESTABLISH A POINT
SOURCE OR ANTICYCLONE ALOFT, ALLOWING FOR INCREASED OUTFLOW. IT
ALSO LOOKS TO ESTABLISH A WEAK POLEWARD OUTFLOW CHANNEL AROUND TAU
72. THE COMBINATION OF CONTINUED LOW VWS, INCREASED OUTFLOW AND
WARM SSTS WILL FUEL NEAR-RAPID INTENSIFICATION BETWEEN TAU 48 AND
96, TO A PEAK OF 95 KNOTS, POSSIBLY SLIGHTLY HIGHER. THEREAFTER,
UPPER-LEVEL CONVERGENCE SLAMMING THE SYSTEM FROM THE WEST WILL
OFFSET LOW VWS AND MODEST POLEWARD OUTFLOW, LEADING TO SLIGHT
WEAKENING BY THE END OF THE FORECAST.
MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE HAS STARTED TO COME
INTO BETTER AGREEMENT WITH THIS MODEL RUN, WITH ALL OF THE CONSENSUS
MEMBERS AGREEING ON THE OVERALL TRACK SCENARIO; A SHORT EASTWARD
TRACK, SHARP TURN BACK TO THE WEST-NORTHWEST AND ULTIMATE RECURVE
BY TAU 120. THERE STILL REMAINS SOME UNCERTAINTY IN THE TIMING OF
THE OVER-THE-SHOULDER TURN, WITH NAVGEM NOW TAKING THE TROPHY OF
FURTHEST EAST PROGRESSION PRIOR TO THE TURN. AND THERE REMAINS A
MODEST AMOUNT OF CROSS-TRACK SPREAD, WHICH INCREASES STEADILY FROM
75NM AT TAU 72 TO 330NM AT TAU 120 BETWEEN THE EXTREME OUTLIERS
(AEMN AND JGSM), WHILE THE REMAINDER OF THE MODELS ARE FAIRLY
TIGHTLY PACKED IN A 130NM ENVELOPE. ALONG-TRACK SPREAD INCREASES
AFTER TAU 72 AS THE MODELS HANDLE THE TIMING OF THE RIDGE EROSION
DIFFERENTLY, AND INCREASES TO ABOUT 150NM BY TAU 120. OVERALL TRACK
CONFIDENCE HAS INCREASED SINCE THE PREVIOUS FORECAST, BUT REMAINS
MEDIUM THROUGH THE PERIOD. NO SIGNIFICANT CHANGE IN THE INTENSITY
GUIDANCE IS OBSERVED, WITH ALL TRACKERS INDICATING SLOW
INTENSIFICATION TO TAU 48, WITH A FASTER RATE OF INTENSIFICATION
THEREAFTER. PEAK INTENSITIES REMAIN BETWEEN 85-105 KNOTS. THE
COAMPS-TC ENSEMBLE IS INDICATING A MODEST PROBABILITY (40-50
PERCENT) OF RAPID INTENSIFICATION (RI) BETWEEN ROUGHLY TAUS 48 TO
96, A LARGE JUMP SINCE THE PREVIOUS RUN. THE JTWC FORECAST LIES
ALONG THE CONSENSUS MEAN THROUGH MOST OF THE FORECAST AND DEVIATES
HIGH AFTER TAU 72, THOUGH REMAINS BELOW THE MOST AGGRESSIVE OF THE
RI AIDS. CONFIDENCE IS HIGH IN THE NEAR-TERM FORECAST, TRENDING TO
LOW IN THE EXTENDING TAUS.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: HIGH
   INTENSITY 72-120 HR: LOW//
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