Tropical Storm CHOI-WAN Advisory Sat May 29

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 04W (FOUR) WARNING
NR 001//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 04W (FOUR), LOCATED APPROXIMATELY 56
NM SOUTH OF PALAU, HAS TRACKED WESTWARD AT 07 KNOTS OVER THE PAST
SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS
A RAPIDLY CONSOLIDATING SYSTEM WITH FRAGMENTED, FORMATIVE BANDS
WRAPPING INTO A DEFINED LOW LEVEL CIRCULATION (LLC). THE EIR LOOP
ALSO SHOWS OVERSHOOTING CLOUD TOPS OVER THE CENTRAL CONVECTION. A
291709Z ATMS 88.2GHZ MICROWAVE SATELLITE PASS HIGHLIGHTS THE DEEP
CENTRAL CONVECTION THAT IS OFFSET SOUTHWESTWARD OF THE LLC. THE
INITIAL POSITION IS BASED WITH FAIR CONFIDENCE ON A NOTCH FEATURE
ALONG THE NORTHEAST QUADRANT OF THE AFOREMENTIONED MICROWAVE
IMAGERY AND PLACED ADJACENT TO THE 291740Z PGTW SATELLITE FIX. THE
INITIAL INTENSITY IS SET WITH FAIR CONFIDENCE BASED ON THE PGTW AND
KNES DVORAK ESTIMATES OF T1.5(25KTS) AND EXTRAPOLATED FROM THE WIND
AND SEA LEVEL PRESSURE OBSERVATIONS FROM PALAU. TD 04W IS IN A
FAVORABLE ENVIRONMENT WITH GOOD RADIAL OUTFLOW, LOW (5-10KTS)
VERTICAL WIND SHEAR (VWS) ALOFT, AND VERY WARM (30-31C) SEA SURFACE
TEMPERATURES (SST) IN THE PHILIPPINE SEA. THE CYCLONE IS TRACKING
ALONG THE SOUTHWEST PERIPHERY OF THE SUBTROPICAL RIDGE (STR) TO THE
EAST-NORTHEAST.
3. FORECAST REASONING.
   A. THIS IS THE INITIAL PROGNOSTIC REASONING BULLETIN ON THIS
SYSTEM AND SETS THE FORECAST PHILOSOPHY.
   B. TD 04W IS FORECAST TO CONTINUE NORTHWESTWARD OFFSHORE AND
PARALLEL TO THE EAST COAST OF THE PHILIPPINES OVER THE NEXT 72 HOURS
UNDER THE STEERING INFLUENCE OF THE STR. THE SYSTEM IS FORECAST TO
RAPIDLY INTENSIFY UNDER THE AFOREMENTIONED FAVORABLE CONDITIONS,
REACHING 45KTS BY TAU 24. AFTERWARD, INTENSIFICATION WILL BE MORE
GRADUAL AS VWS BECOMES MORE MODERATE (15-20KTS), AND BY TAU 72 WILL
REACH 60KTS. THE FORECAST TRACK IS LAID SLIGHTLY TO THE LEFT AND
AHEAD OF MODEL CONSENSUS DURING THE FIRST 36 HOURS DUE TO THE UKMET
TRACKER BEING SLOW AND TO THE RIGHT OF THE MODEL ENVELOPE, SHOWING A
MORE DIRECT NORTHWARD TRANSIT. AFTER TAU 36, THE FORECAST TRACK IS
MORE ALIGNED, ALBEIT SLIGHTLY AHEAD OF CONSENSUS.
   C. AFTER TAU 72, TD 04W WILL BEGIN TO ROUND THE RIDGE AXIS THEN
ACCELERATE NORTHEASTWARD. AT THIS STAGE, IT WILL ALSO BE OVER THE
WARMEST WATERS (31-32C) OF ITS FORECAST TRACK AND VWS WILL WEAKEN TO
05-10KTS, FUELING THE INTENSITY TO TYPHOON STRENGTH OF 65KTS. BY TAU
120, THE SYSTEM WILL REACH 70KTS AS IT BECOMES EXPOSED TO INCREASED
POLEWARD OUTFLOW ASSOCIATED WITH THE PREVAILING WESTERLIES UNDER THE
POLAR FRONT JET. NUMERICAL MODELS REMAIN IN GOOD AGREEMENT, LENDING
ALSO FAIR CONFIDENCE IN THE EXTENDED PORTION OF THE JTWC FORECAST
TRACK.//
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