WDPN31 PGTW 240300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (NESAT) WARNING NR 
03//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TROPICAL STORM (TS) 20W (NESAT), LOCATED APPROXIMATELY 290 NM 
NORTH-NORTHWEST OF YAP, MICRONESIA, HAS TRACKED WEST-NORTHWESTWARD 
AT 08 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL 
SATELLITE IMAGERY (MSI) DOES NOT SHOW AN INCREASE IN CONVECTION OVER 
THE PAST SIX HOURS, BUT DOES INDICATE TIGHTENING ORGANIZATION AND 
CONSOLIDATION AROUND THE LOW LEVEL CIRCULATION CENTER. THE MOST 
VIGOROUS CONVECTION ASSOCIATED WITH THE STORM IS NEAR THE CORE OF 
THE SYSTEM, AND A 232128Z SSMIS IMAGE SHOWS EXTENSIVE CURVED BANDING 
IN THE LOW LEVELS AS WELL AS TIGHTER ORGANIZATION RELATIVE TO 
PREVIOUS PASSES. AMSU CROSS SECTIONS REVEAL AN EXPANDING MID-LEVEL 
WARM ANOMALY. ANIMATED WATER VAPOR IMAGERY SHOWS TS 20W HAS WELL-
ESTABLISHED RADIAL OUTFLOW. THE 231200Z PGTW UPPER LEVEL STREAMLINE 
ANALYSIS SHOWS THAT TS NESAT EXISTS IN A REGION OF UPPER LEVEL 
DIFFLUENCE, WITH LESS THAN 10 KNOTS OF VERTICAL WIND SHEAR. SEA 
SURFACE TEMPERATURES ARE 30 DEGREES. THE CURRENT INTENSITY IS BASED 
ON A COMBINATION OF AGENCY DVORAK ESTIMATES AND OBSERVED 
CONSOLIDATION IN THE LOW LEVELS ON BOTH MSI AND MICROWAVE IMAGERY. 
TS 20W IS BEING STEERED ON A STEADY WEST-NORTWESTWARD COURSE BY THE 
SUBTROPICAL RIDGE, WHICH IS ROOTED IN AN ANTICYCLONE NEAR 35N AND 
THE DATELINE. A FRAGMENTED TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT) 
EXTENDS ALONG THE 20TH LATITUDE, BUT HAS NOT AS OF YET PRODUCED ANY 
IMPEDIMENTS TO OUTFLOW. 
3. FORECAST REASONING.
   A. THE FORECAST REASONING HAS NOT CHANGED SINCE THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
   B. THE SYNOPTIC PATTERN DICTATES THAT TS 20W WILL BE A STRAIGHT 
RUNNER.
THE SUBTROPICAL RIDGE WILL GUIDE TS 20W ALONG A STEADY WEST-
NORTHWESTWARD COURSE, ALLOWING THE SYSTEM TO REMAIN IN A LOW SHEAR 
ENVIRONMENT WITH WARM SEA SURFACE TEMPERATURES. PERIODIC VENTING 
INTO THE TUTT WILL PROVIDE INTERMITTENT BOOSTS TO OUTFLOW AND TS 20W 
WILL INTENSIFY TO TYPHOON STRENGTH NEAR TAU 24. TRACK GUIDANCE 
BEGINS TO DIVERGE BEGINNING AT TAU 36, BUT DEVIATIONS ARE A MATTER 
OF DEGREE AND NOT OF BASIC TRACK. ALL MODEL GUIDANCE INDICATES A 
STEADY WEST-NORTHWESTWARD TRACK WITH AN INCREASING POLEWARD BEND AS 
THE STORM APPROACHES THE PHILIPPINES. THE JTWC TRACK FORECAST STAYS 
VERY CLOSE TO CONSENSUS. THE INTENSITY FORECAST EXCEEDS GUIDANCE, 
BUT WITH EACH SUCCESSIVE RUN, INTENSITY AIDS SHOW A FASTER RAMP-UP 
AND A HIGHER TOP-END. THE INTENSITY FORECAST IS BASED ON A SIMPLE 
CLIMATOLOGICAL RATE OF DEVELOPMENT FOR A SYSTEM OF THIS TYPE, TRACK, 
REGION, AND SEASON. TS 20W IS EXPECTED TO REACH SUPER TYPHOON 
STRENGTH NORTHEAST OF LUZON, NEAR TAU 72. 
 C. THE RUGGED TERRAIN OF NORTHERN LUZON WILL WEAKEN TS 20W AS IT 
TRACKS OVER THE NORTHEASTERN TIP OF THE ISLAND OR THROUGH THE LUZON 
STRAIT. THE GREATEST UNCERTAINTY IN THE FORECAST IS THE DEGREE OF 
DEGRADATION TS 20W WILL UNDERGO AS IT CROSSES LUZON. THAT WILL 
DEPEND ON A MORE PRECISE KNOWLEDGE OF THE ENTRY AND EXIT POINTS, AND 
THE DURATION AND SEVERITY OF LAND INTERACTION. SOME MODELS INDICATE 
THAT TS 20W COULD TRACK NORTH OF LUZON, THUS REMAINING OVER WATER 
FOR LONGER AND BEING LESS IMPACTED BY LAND INTERACTION. GFS AND GFDN 
SHOW TS 20W MOVING THROUGH THE PAMPANGAN VALLEY AND OUT THE LINGAYEN 
GULF, WHICH WOULD SEVERELY DETERIORATE THE STORM. CONSENSUS TAKES A 
TRACK OVER THE NORTHEASTERN TIP OF LUZON AND TOWARD THE SOUTHWESTERN 
END OF THE LUZON STRAIT, WHICH WOULD RESULT IN LESS THAN 12 HOURS 
OVER LAND. NONE OF THE GUIDANCE INDICATES A RECURVATURE SCENARIO. 
VERTICAL WIND SHEAR WILL INCREASE TO 20 TO 30 KNOTS OVER THE LUZON 
STRAIT; BUT SEA SURFACE TEMPERATURES WILL REMAIN FAVORABLE FOR RE-
INTENSIFICATION.//

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