Tropical Storm NEOGURI Advisory Thu Oct 17

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) WARNING NR 001//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 21W (TWENTYONE), LOCATED APPROXIMATELY 542 NM EAST-
NORTHEAST OF MANILA, PHILIPPINES, HAS TRACKED WEST-NORTHWESTWARD AT 11 KNOTS OVER
THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS PERSISTENT
DEEP CONVECTION FLARING OVER A SLOWLY-DEVELOPING LOW-LEVEL CIRCULATION (LLC). A
160840Z SSMIS 91GHZ IMAGE DEPICTS A BROAD, DEFINED LLC WITH FRAGMENTED DEEP
CONVECTIVE BANDING OVER THE NORTHEAST QUADRANT AND A WEAKLY-DEFINED CENTER. A
161204Z ASCAT-C IMAGE SHOWS A DEFINED, ELONGATED CIRCULATION WITH WESTERLY WINDS
OVER THE SOUTHERN SEMICIRCLE AND 20-25 KNOT WINDS OVER THE EASTERN SEMICIRCLE,
WHICH SUPPORTS THE CURRENT INTENSITY ASSESSMENT OF 25 KNOTS. THIS ASCAT IMAGE
ALSO SUPPORTS THE CURRENT POSITION WITH GOOD CONFIDENCE. TD 21W IS TRACKING
ALONG THE SOUTHERN PERIPHERY OF THE LOW-LEVEL SUBTROPICAL RIDGE (STR) POSITIONED
TO THE NORTH.
3. FORECAST REASONING.
   A. THIS IS THE INITIAL PROGNOSTIC REASONING MESSAGE AND SETS THE
FORECAST PHILOSOPHY ON THIS SYSTEM.
   B. TD 21W WILL TRACK WEST-NORTHWESTWARD THEN WESTWARD UNDER THE STEERING
INFLUENCE OF THE LOW-LEVEL STR THROUGH TAU 36. AFTER TAU 36, THE SYSTEM IS EXPECTED
TO TURN WEST-SOUTHWESTWARD AS IT BEGINS TO INTERACT WITH A NORTHEAST SURGE EVENT.
DUE TO THE WEAK NATURE OF THE SYSTEM, MODEL GUIDANCE IS LIMITED TO AEMN, NVGM AND
AVNO, HOWEVER, THESE MODELS ARE IN GOOD AGREEMENT. THEREFORE, THERE IS HIGH
CONFIDENCE IN THE JTWC FORECAST TRACK. THE ECMWF MODEL TRACKER ALSO SUPPORTS
THE JTWC FORECAST TRACK INTO NORTHERN LUZON BUT REFLECTS A GENERALLY WEAK SYSTEM.
DUE TO THE MARGINAL ENVIRONMENT, TD 21W IS FORECAST TO INTENSIFY SLIGHTLY TO 30
KNOTS THEN QUICKLY DISSIPATE AS IT INTERACTS WITH THE NORTHEAST SURGE AND TRACKS
OVER LUZON.//
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