Tropical Storm NANMADOL Advisory 9月19日

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 16W (NANMADOL) WARNING NR 025//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 33.1N 130.5E
   INITIAL INTENSITY: 70 KTS
   GEOGRAPHIC REFERENCE: 40 NM EAST OF SASEBO, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TYPHOON NANMADOL MADE LANDFALL INITIALLY JUST SOUTH OF KAGOSHIMA
AROUND THE 180900Z HOUR, AS CONFIRMED BY A 180926Z RADARSAT-2
SYNTHETIC APERTURE RADAR (SAR) PASS WHICH SHOWED A SMALL BUT VERY
WELL DEFINED EYE SITTING JUST OFF THE COAST OF KAGOSHIMA CITY. THE
SYSTEM HAS THENCE TRACKED NORTHWARD TO ITS CURRENT POSITION AS
CONFIRMED BY SURFACE OBSERVATIONS AND RADAR DATA WHICH SHOWED A
RAGGED EYE FEATURE NEAR OMUTA. THE SYSTEM IS CLEARLY WEAKENING AS
EVIDENCED THE RAPIDLY DETERIORATING DEPICTION IN ANIMATED ENHANCED
INFRARED (EIR) SATELLITE IMAGERY. THE EIR HINTS AT THE START OF A
DECOUPLING BETWEEN THE LOWER AND UPPER LEVELS, WITH THE CONVECTIVE
CLOUD TOPS SHEARING OFF TO THE NORTHEAST AND STARTING TO EXPOSE THE
LOWER LEVEL CIRCULATION TO THE SOUTHWEST. THE INITIAL POSITION IS
ASSESSED WITH HIGH CONFIDENCE BASED ON THE WIND FIELD IN THE DENSE
SURFACE OBSERVATION NETWORK AND THE RADAR EYE FEATURE. THE INITIAL
INTENSITY IS ASSESSED AT A GENEROUS 70 KNOTS, PRIMARILY BASED ON AN
RJTD OVER LAND INTENSITY ESTIMATE OF T4.0, AND SEA LEVEL PRESSURE
READINGS AROUND 966MB FROM THE SURROUNDING REGION, WHICH EQUATES TO
A WIND SPEED BETWEEN 70-75 KNOTS. WHILE OVER LAND WIND REPORTS ARE
FAIRLY LIGHT, OFFSHORE WINDS ARE CRANKING, PARTICULARLY TO THE
NORTHWEST, BASED ON THE SAR AND EARLIER ASCAT DATA, REACHING OVER
65 KNOTS IN THE OVER WATER SECTION OF THE WESTERN HALF OF THE
STORM. THE ENVIRONMENT IS UNFAVORABLE AT THIS POINT. CIMSS MOTION
VECTORS SHOW UPPER-LEVEL WESTERLY WINDS AROUND 25 KNOTS AND THE
EVIDENT EROSION OF THE WESTERN SIDE OF THE SYSTEM SUGGESTS STRONGER
LOW TO MID-LEVEL WINDS ARE PUSHING IN FROM THE WEST ON THE NOSE OF
A COLD SURGE POURING OUT OF THE YELLOW SEA. THE SYSTEM HAS STARTED
TO TURN MORE NORTHEAST AS IT HAS ROUNDED THE AXIS OF THE RIDGE TO
THE EAST.
INITIAL WIND RADII BASIS: SCATTEROMETER AND SAR DATA
CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE EAST
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: NO CURRENT ESTIMATE
   RJTD: T4.0 - 65 KTS
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 10-15 KTS
   SST: 25-26 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: WEAK EQUATORWARD OUTFLOW, MID-LEVEL WESTERLY
SHEAR
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: HAVING REACHED THE RIDGE AXIS, TYPHOON
NANMADOL IS NOW STARTING TO TURN THE RUDDER OVER AND SHOULD START
TO TRACK MORE AND MORE TOWARDS THE NORTHEAST AS IT MOVES NORTH OF
THE RIDGE AXIS. THE TRACK SHOULD TURN POLEWARD EVEN MORE SHARPLY AS
A COLD FRONT, CURRENTLY DRAPED ACROSS THE CENTRAL SEA OF JAPAN,
CONTINUES TO SLIDE SOUTHWARD OVER THE NEXT 12 TO 24 HOURS. BY TAU
24 THE FRONT SHOULD BE ON TOP OF THE REMNANT CIRCULATION, AND THE
SYSTEM WILL PICK UP SPEED AS IT RACES NORTHEAST ALONG THIS
BOUNDARY, NEARING THE KANAZAWA REGION BY TAU 24. TRACK SPEEDS
INCREASE SHARPLY AND WILL EXCEED 25 KNOTS AFTER TAU 24, AS THE
SYSTEM CONTINUES MOVING NORTHEAST ALONG THE FRONTAL BOUNDARY AS IT
CROSSES HONSHU AND EMERGES BACK INTO THE PACIFIC AROUND TAU 36. THE
SYSTEM WILL BEGIN EXTRATROPICAL TRANSITION (ETT) BY TAU 24, WITH
TRANSITION TO A WEAK EXTRATROPICAL LOW NO LATER THAN TAU 48 TO THE
EAST OF HOKKAIDO. WHILE STILL CLASSIFIED AS A MINIMAL TYPHOON,
NANMADOL IS RAPIDLY WEAKENING AND WILL CONTINUE TO DO SO THROUGH
THE FORECAST PERIOD, DUE TO THE INFLUENCE OF MULTIPLE NEGATIVE
FACTORS INCLUDING INCREASING VWS, LAND INTERACTION AND THE INFLUX
OF COOL, DRY AIR ASSOCIATED WITH THE FRONTAL BOUNDARY. THE FORECAST
CALLS FOR THE SYSTEM TO BE 40 KNOTS AS IT EMERGES BACK INTO THE
PACIFIC, WITH TRANSITION TO AN EXTRATROPICAL GALE-FORCE LOW BY TAU
48. HOWEVER, IT IS POSSIBLE THE SYSTEM WILL WEAKEN EVEN FASTER THAN
THE MODELS SUGGEST AND MAY ACTUALLY DISSIPATE PRIOR TO ETT.
MODEL DISCUSSION: TRACK GUIDANCE IS IN OVERALL GOOD AGREEMENT ON
THE GENERAL SCENARIO, ESPECIALLY THROUGH TAU 12. EVEN AS EARLY AS
TAU 24 HOWEVER, THE MODELS SEPARATE INTO TWO GROUPS; FAST AND SLOW,
AS HAS BEEN SEEN OVER THE PREVIOUS DAYS. THE FAST GROUP CONTINUES
TO MADE OF THE UKMET, GALWEM AND JGSM AND ENSEMBLES, WHICH MOVE THE
SYSTEM OUT AT A VERY FAST PACE AFTER TAU 24, WITH THE TAU 36
POSITIONS AS FAR NORTH AS 43N. MEANWHILE THE REMAINDER OF THE
MODELS ARE IN GROUP 2, INCLUDING NAVGEM, GFS, ECMWF, HWRF AND THE
ENSEMBLES. THIS GROUP HANGS BACK, MOVING THE SYSTEM MUCH SLOWER
TOWARDS THE NORTHEAST AFTER TAU 12. THE JTWC FORECAST LIES AMONGST
THE SECOND GROUPING, BUT DOES EXTEND THE FORECAST POINTS DOWN TRACK
AT A FASTER PACE THAN THE EARLIER FORECAST. CONFIDENCE IS HIGH FOR
THE FIRST 12 HOURS BUT DEGRADES TO LOW THEREAFTER DUE TO
UNCERTAINTIES ON THE APPROACH SPEED OF THE FRONT TO THE WEST AND
HOW QUICKLY THE SYSTEM WILL PICK UP SPEED AHEAD OF IT. INTENSITY
GUIDANCE IS IN GOOD AGREEMENT ON THE RAPID WEAKENING, AND THE JTWC
FORECAST LIES ALONG THE HIGHER EDGE OF THE ENVELOPE WITH MEDIUM
CONFIDENCE.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM//
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