Tropical Storm VONGFONG Advisory 5月13日

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 01W (VONGFONG) WARNING
NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 01W (VONGFONG), LOCATED APPROXIMATELY
505 NM EAST-SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED
NORTHWESTWARD AT 04 KNOTS OVER THE PAST SIX HOURS. ANIMATED HIGH
RESOLUTION (1-KM) AND HIGH REFRESH RATE (2-MIN) ENHANCED INFRARED
(EIR) SATELLITE IMAGERY DEPICTS COMPACT, SYMMETRICAL AND FLARING DEEP
CONVECTION OVER THE ASSESSED LOW LEVEL CIRCULATION CENTER. OVER THE
PAST THREE HOURS, THE SYSTEM HAS SHOWN INDICATIONS OF A NASCENT EYE
FEATURE AT TIMES BUT IT HASN’T YET STUCK. HOWEVER, A 1711Z AMSR2 89
GHZ MICROWAVE IMAGE DEPICTING A CLEAR CUT MICROWAVE EYE, SO THE
DEVELOPMENT OF AN EYE FEATURE IN THE INFRARED CAN’T BE FAR OFF. THE
INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE
PREVIOUSLY MENTIONED AMSR2 89 GHZ MICROWAVE IMAGE. THE INITIAL
INTENSITY OF 50 KNOTS IS HEDGED UPWARD SLIGHTLY FROM THE PGTW CURRENT
INTENSITY ESTIMATE OF T3.0 (45 KNOTS) BASED ON AN ADVANCED DVORAK
TECHNIQUE (ADT) ESTIMATE OF T3.2 (49 KNOTS) AND THE OVERALL IMPROVED
STRUCTURE OF THE SYSTEM. TS VONGFONG IS TRAVERSING AN ENVIRONMENT
FAVORABLE FOR RAPID INTENSIFICATION WITH LOW (5-10 KTS) VWS, GOOD
EQUATORWARD AND POLEWARD OUTFLOW ALOFT AND WARM
(29-30C) SEA SURFACE TEMPERATURES (SSTS). THE SYSTEM IS TRACKING
ALONG THE WESTERN PERIPHERY OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR)
CENTERED OVER THE NORTHERN MARIANA ISLANDS THAT CONTINUES TO BUILD
WESTWARD.
3. FORECAST REASONING.
   A. WHILE THE OVERALL TRACK PHILOSOPHY HAS NOT CHANGED, THE PREVIOUS
BEST TRACK POSITION FROM 1200Z HAS BEEN ADJUSTED SIGNIFICANTLY
EASTWARD (~50 NM) BASED ON A REANALYSIS OF 1216Z ASCAT-A AND 1309Z
ASCAT-C DATA. THIS CHANGE HAS RESULTED IN A TRACK FORECAST THAT IS
FURTHER NORTH AND EAST THAN THE PREVIOUS FORECAST AND KEEPS THE SYSTEM
OVER WATER LONGER, AND SIGNIFICANTLY CHANGES THE TIMING AND LOCATION
OF LANDFALL. THE UPDATED TRACK ALSO RESULTS IN A SIGNIFICANT INCREASE
IN THE PEAK INTENSITY, AS THE SYSTEM REMAINS OVER WATER LONGER AND
WILL BE LESS IMPACTED BY TERRAIN EFFECTS.
   B. OVER THE NEXT 12 HOURS TS VONGFONG WILL TRACK GENERALLY
NORTHWESTWARD ALONG THE WESTERN PERIPHERY OF A DEEP-LAYERED
STR CENTERED NORTH OF GUAM, WHICH IS CURRENTLY MOVING TOWARDS THE
WEST. AFTER TAU 12, THE STR WILL RECENTER NEAR 20N 130E, AND PUSH TS
VONGFONG ONTO A MORE WESTERLY TRACK THROUGH 36. THEREAFTER, THE STR
REORIENTS AND MOVES A BIT FURTHER TO THE EAST, ALLOWING FOR THE SYSTEM
TO BEGIN TO MOVE NORTHWESTWARD BY TAU 72. THE ENVIRONMENT WILL REMAIN
HIGHLY FAVORABLE FOR RAPID INTENSIFICATION THROUGH TAU 48 WITH LOW VWS
(5-10 KNOTS), HIGH SSTS (29-30 CELSIUS) AND STRONG RADIAL OUTFLOW
PROVIDED BY AN UPPER-LEVEL POINT SOURCE OVER TOP OF THE SYSTEM. THE
SYSTEM IS EXPECTED TO REACH PEAK INTENSITY OF 95 KNOTS BY TAU 48 AS IT
APPROACHES THE SOUTHEAST TIP OF LUZON. NUMERICAL MODEL GUIDANCE ARE IN
FAIR AGREEMENT THROUGH TAU 72 WITH THE MAJORITY (INCLUDING ECMWF, GFS,
NAVGEM AND JGSM) ALL FAVORING A TRACK THAT REMAINS OFFSHORE OR JUST
BRUSHES THE CENTRAL PHILIPPINE ISLANDS, WHILE THE UKMET AND ITS
ENSEMBLE ARE THE SOLE SOLUTIONS THAT FAVOR A MORE SOUTHERLY TRACK
ACROSS THE CENTRAL PHILIPPINES AND INTO THE SOUTH CHINA SEA. MODEL
SPREAD AT TAU 72 IS 195NM BETWEEN OUTLIERS (EGRR AND NAVGEM). THE JTWC
FORECAST TRACK THROUGH TAU 72 FAVORS THE AFOREMENTIONED CLUSTER OF
MODEL SOLUTIONS WHICH LIE ON THE NORTHERN SIDE OF THE TRACK ENVELOPE
WITH MODERATE CONFIDENCE.
   C. AFTER TAU 72 TS VONGFONG WILL MOVE FIRST NORTHWESTWARD, THEN
NORTH AND FINALLY NORTHEASTWARD AS IT RECURVES AROUND THE RIDGE AXIS.
TRACK SPEED WILL SLOW AS IT REACHES THE RIDGE AXIS, SIMULTANEOUS TO
MOVING OVER NORTHERN LUZON, AND THEN ACCELERATE INTO THE PHILIPPINE
SEA AFTER ROUNDING THE AXIS AND EMERGING OVER WATER AROUND TAU 100. A
SECOND LANDFALL IS EXPECTED IN CENTRAL LUZON AROUND TAU 84. AS THE
SYSTEM SKIRTS SOUTHEAST LUZON, THE INTENSITY IS FORECAST TO REMAIN
STEADY AS IT TRACKS OVER COASTAL WATERS AND FEELS SOME EFFECTS OF THE
TERRAIN OFFSET BY THE CONTINUED STRONG OUTFLOW, PARTICULARLY POLEWARD
AS THE OUTFLOW PATTERN BEGINS TO TAP INTO THE STRONG WESTERLY FLOW TO
THE NORTH. AS THE SYSTEM TRACKS OVER CENTRAL LUZON BY TAU 96 IT WILL
UNDERGO SIGNIFICANT WEAKENING DUE PRIMARILY TO INTERACTION WITH THE
MOUNTAINOUS TERRAIN. ONCE BACK OVER OPEN WATERS, INCREASING VWS AND
COOLER WATERS WILL SERVE TO OFFSET STRONG, DIVERGENT OUTFLOW RESULTING
IN IT MAINTAINING STRONG TROPICAL STORM STRENGTH AS IT RAPIDLY MOVES
NORTHEASTWARD EAST OF TAIWAN. MODEL GUIDANCE BEYOND TAU 72 DIVERGES
SIGNIFICANTLY, WITH TWO BASIC SCENARIOS. THE EGRR AND ECMWF FAVOR
TAKING THE SYSTEM INTO THE SOUTH CHINA SEA AND KEEPING IT BELOW 20N
LATITUDE THROUGH TAU 120. THE OTHER SOLUTION FAVORED BY THE REMAINDER
OF AVAILABLE MODEL TRACKS, RECURVES THE SYSTEM AROUND THE RIDGE AXIS
AND TRACKS IT NORTHEAST PARALLEL THE RYUKU ISLANDS, VARYING IN THE
SPEED OF ADVANCE BUT GENERALLY AGREEING ON THE TRACK. THE SPREAD IN
THE GUIDANCE AT TAU 120 IS 965 NM WITH MOST OF THIS COMPRISING ALONG-
TRACK SPREAD. THE JTWC FORECAST LIES NEAR THE MULTI-MODEL CONSENSUS
TRACK BUT SLOWER THAN THE CONSENSUS TRACK SPEED.  THE HIGH DEGREE OF
UNCERTAINTY IN THE TRACK SPREAD AND SHARPNESS OF THE RECURVE LENDS LOW
CONFIDENCE IN THIS PORTION OF THE JTWC FORECAST TRACK.//
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