Tropical Storm CHOI-WAN Advisory Mon May 31

SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (FOUR) WARNING
NR 006//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 04W (FOUR), LOCATED APPROXIMATELY 698 NM
SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED NORTHWESTWARD AT 08
KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY (MSI) SHOWS THE LOW LEVEL CIRCULATION (LLC) HAS BECOME
PARTIALLY EXPOSED, OFFSETTING AN OTHERWISE DEEPENED CENTRAL
CONVECTION WITH OVERSHOOTING CLOUD TOPS SLIGHTLY SOUTHWESTWARD. THE
INITIAL POSITION IS PLACED WITH FAIR CONFIDENCE USING LOW CLOUD
TRACING INTO THE OBSCURED PORTION OF THE LLC. THE INITIAL INTENSITY
OF 50KTS IS HEDGED HIGHER THAN AGENCY AND OBJECTIVE DVORAK ESTIMATES
RANGING FROM 30KTS TO 47KTS TO REFLECT THE SLIGHT 6-HOUR IMPROVEMENT
OF THE SYSTEM AS EVIDENCED IN THE MSI LOOP AND SUPPORTED BY THE
ONSET OF FORMATIVE EYE FEATURES IN RECENT MICROWAVE IMAGES. THE
ENVIRONMENT REMAINS OVERALL FAVORABLE WITH RADIAL OUTFLOW AND WARM
(30-31C) ALONG-TRACK SST OFFSET BY MODERATE (15-20KT) NORTHEASTERLY
VERTICAL WIND SHEAR. THE CYCLONE IS TRACKING ALONG THE SOUTHWEST
PERIPHERY OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR) TO THE
NORTHEAST.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. UNDER THE STEERING INFLUENCE OF THE STR, TS 04W WILL CONTINUE
TO TRACK NORTHWESTWARD OVER THE WARM PHILIPPINE SEA UP TO TAU 72.
THE SYSTEM IS FORECAST TO STEADILY INTENSIFY UNDER THE
AFOREMENTIONED FAVORABLE CONDITIONS AS IT APPROACHES THE RIDGE AXIS,
PEAKING AT 70KTS BY TAU 72. NUMERICAL MODELS ARE IN GOOD AGREEMENT
WITH A GRADUAL SPREAD TO 270NM AT TAU 72 AND JGSM THE SOLE OUTLIER
ON THE RIGHT MARGIN OF THE MODEL ENVELOPE, LENDING FAIR CONFIDENCE
TO THE JTWC TRACK FORECAST THAT IS LAID JUST TO THE LEFT OF THE
MODEL CONSENSUS TO OFFSET JGSM.
   C. AFTER TAU 72, TS 04W WILL CREST THE STR AXIS AND TURN
NORTHWARD THEN ACCELERATE NORTHEASTWARD. THE LOWER VWS UNDER THE STR
AXIS AND INCREASED POLEWARD OUTFLOW DUE TO THE INITIAL EXPOSURE TO
THE PREVAILING WESTERLIES MAY INTENSIFY THE CYCLONE FURTHER BY TAU
78; HOWEVER, AFTERWARD, THE INCREASING VWS AND COOLING SST WILL
GRADUALLY THEN RAPIDLY ERODE THE SYSTEM DOWN TO 40KTS BY TAU 120.
CONCURRENTLY AT TAU 120, THE CYCLONE WILL BEGIN EXTRA-TROPICAL
TRANSITION AS IT ENTERS THE COLD BAROCLINIC ZONE. NUMERICAL MODELS
REMAIN IN GOOD AGREEMENT, LENDING ALSO FAIR CONFIDENCE IN THE
EXTENDED PORTION OF THE JTWC FORECAST TRACK.//
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