WDPN31 PGTW 170300 
MSGID/GENADMIN/NAVPACMETOCCEN PEARL//JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W WARNING NR 
01//
RMKS/
1. FOR METEOROLOGISTS.
2. PROGNOSTIC REASONING FOR 170000Z8 TO 200000Z2 SEP 01.
   A. TROPICAL DEPRESSION (TD) 21W HAS DEVELOPED NORTHWEST OF THE 
NORTHERN MARIANA ISLANDS. TROPICAL DEPRESSION (TD) 21W HAS DRIFTED 
NORTHWARD AT 03 KNOTS DURING THE PAST 6 HOURS. THE WARNING POSITION 
IS BASED ON 162330Z5 VISIBLE SATELLITE IMAGERY. THE INITIAL 
INTENSITY IS BASED ON CURRENT INTENSITY ESTIMATES OF 25 KNOTS. 
VISIBLE ANIMATION DEPICTS DISTINCT LOW-CLOUD LINES OUTLINING A LOW-
LEVEL CIRCULATION CENTER SITUATED BENEATH THE NORTHERN EDGE OF THE 
CONVECTION.
   B. TD 21W IS CURRENTLY SITUATED IN A WEAKENED RIDGE STEERING FLOW 
PATTERN. WATER VAPOR IMAGERY ANIMATION DEPICTS A MID/UPPER LEVEL LOW 
MOVING OFF OF KYUSHU. THIS SHOULD TEMPORARILY PREVENT THE 
SUBTROPICAL RIDGE FROM BUILDING IN UNTIL MID PERIOD. THE LIMITED 
DYNAMICAL AIDS (NOGAPS AND THE AVN) SHOW A DISPARITY IN TRACK 
ORIENTATION. NOGAPS AND THE BETA ADVECTION MODELS TRACK 
NORTHWESTWARD WHILE THE AVN RUN TRACKS IT TO THE NORTHEAST. 
EVALUATION OF THE AVN RUN REVEALS THAT THE (WEAK) SYSTEM IS TRACKING 
WITH THE 850 MB FLOW. NOGAPS INDICATES A MORE REALISTIC TRACK WITH 
SOME DEVELOPMENT. OUR FORECAST IS BASED ON PERSISTENCE, A BLEND OF 
NOGAPS AND THE BAMS, AND THE AFOREMENTIONED SYNOPTIC PROGGED 
PATTERN.
   C. TD 21W SHOULD INTENSIFY THROUGH THE PERIOD AS IT TRACKS 
BENEATH A COL IN THE UPPER RIDGE AND IS INFLUENCED NEAR MID PERIOD 
BY THE MID/UPPER LOW MOVING OUT OF KYUSHU.  
   D. WIND RADII ARE BASED ON CLIMATOLOGY FOR AN AVERAGE SIZE 
TROPICAL CYCLONE.  
FORECAST TEAM: ROBERTS/BALDINGER/HARRISON//