WDPN31 PGTW 162100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 16W (SIXTEEN) 
WARNING NR 026//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.9N 146.3E
   INITIAL INTENSITY: 20 KTS
   GEOGRAPHIC REFERENCE: 91 NM EAST-SOUTHEAST OF HAGATNA, GUAM
   MOVEMENT PAST 6 HOURS: WESTWARD AT 17 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED SATELLITE IMAGERY AND RADAR IMAGERY FROM
GUAM SHOW THAT THE CIRCULATION OF TROPICAL DEPRESSION 16W IS
ILL-DEFINED. LOW-LEVEL EASTERLY FLOW EXISTS NORTH OF THE LATITUDE
OF GUAM, BUT SOUTH OF THIS LINE, THE FLOW CAN BE CHARACTERIZED AS
LIGHT AND VARIABLE, WITH OCCASIONAL HINTS OF WESTERLY CLOUD
MOTIONS. THE CENTER IS PLACED WITH LOW CONFIDENCE AT THE CENTER OF
BROAD ROTATION ON RADAR IMAGERY, TO THE SOUTH OF THE BELT OF
SIGNIFICANT EASTERLY FLOW. THIS POSITION IS SLIGHTLY SOUTH OF THE
EXPECTATION FROM THE PREVIOUS FORECAST. THE INTENSITY IS HELD AT 20
KT IN THE ABSENCE OF DATA JUSTIFYING A HIGHER ESTIMATE OF SUSTAINED
WINDS. WHILE A CLUSTER OF DEEP CONVECTION IS OBSERVED NORTH AND
EAST OF GUAM IN SATELLITE IMAGERY, RADAR DATA INDICATES A
SUBSTANTIAL AMOUNT OF STRATIFORM PRECIPITATION. A LARGE GUST FRONT
ROLLED THROUGH GUAM, PRODUCING A GUST OF 27 KT AT GUAM
INTERNATIONAL AIRPORT DURING ITS PASSAGE AT 161654Z. THESE
OBSERVATIONS INDICATE THE PRESENCE OF LARGE-SCALE DOWNDRAFTS WITHIN
THE CURRENT CONVECTIVE CLUSTER, EVIDENCE OF A THERMODYNAMIC
ENVIRONMENT NOT YET SUPPORTIVE OF THE SUSTAINED UPWARD MASS FLUX
NECESSARY FOR FURTHER ORGANIZATION OF THE SYSTEM. WIND SHEAR IS LOW
AT 5-10 KT OVER THE CENTER, THOUGH PROXIMITY TO A TUTT CELL TO THE
NORTHEAST IS RESULTING IN HIGH SHEAR VALUES LOCATED JUST TO THE
EAST OF THE CENTER. THIS IS RESTRICTING UPPER-LEVEL OUTFLOW IN THE
NORTHEAST QUADRANT OF THE CIRCULATION, BUT OUTFLOW IS CURRENTLY
EXPANDING IN ALL OTHER QUADRANTS.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE TO THE NORTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE
   CIMSS ADT: 31 KTS AT 161750Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE RADIAL

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 16W CONTINUES TO
STRUGGLE, SEEMINGLY DUE TO AN ENVIRONMENT THAT DOES NOT YET SUPPORT
SUSTAINED, BOTTOM-HEAVY CONVECTION THAT CAN SUPPORT VORTEX SPIN-UP.
IN SUCH A THERMODYNAMICALLY  MARGINAL ENVIRONMENT, THE FUTURE OF
16W IS UNCERTAIN. GLOBAL DYNAMICAL MODELS SUCH AS GFS AND ECMWF
SHOW LITTLE DEVELOPMENT OR DISSIPATION DURING THE NEXT SEVERAL
DAYS, WHILE THE HIGH-RESOLUTION MODELS HWRF AND COAMPS-TC, AS WELL
AS THE STATISTICAL SHIPS MODEL, SHOW MARKED INTENSIFICATION TO
TYPHOON STRENGTH. A TUTT CELL TO THE NORTHEAST IS EXPECTED TO
FOLLOW 16W WESTWARD AS THE STORM TRACKS SOUTH OF THE SUBTROPICAL
RIDGE. THIS COULD IMPART MODERATE VERTICAL SHEAR ON THE SYSTEM
PERIODICALLY DURING THE NEXT 48 HOURS. THE TUTT CELL SHOULD
GRADUALLY WEAKEN AS THEY TYPICALLY DO, AND MODEL GUIDANCE SUGGESTS
THAT ANY ASSOCIATED SHEAR WILL RELAX BEYOND 48 HOURS. AT THAT
POINT, THERMODYNAMIC FAVORABILITY WILL DETERMINE WHETHER THE VORTEX
IS ABLE TO INTENSIFY. THE JTWC FORECAST ASSUMES THAT AN ENVIRONMENT
OF WARM SEA SURFACE TEMPERATURES AND RELATIVELY DEEP MOISTURE WILL
EVENTUALLY LEAD TO THE DEVELOPMENT OF SUSTAINED CONVECTION,
KICKSTARTING SUBSTANTIAL INTENSIFICATION AFTER 48 HOURS. TD 16W IS
FORECAST TO REACH TYPHOON INTENSITY BY 96 HOURS. THERE IS LOWER
THAN AVERAGE CONFIDENCE IN THIS FORECAST GIVEN THE LARGE MODEL
SPREAD. THE TRACK IS EXPECTED TO GRADUALLY BEND MORE NORTHWARD
DURING THE 72-120 HOUR PERIOD AS THE STORM NEARS THE WESTERN
PERIPHERY OF THE SUBTROPICAL RIDGE.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE AND THE JTWC TRACK
FORECAST HAVE SHIFTED SLIGHTLY SOUTHWARD IN THE SHORT TERM, LARGELY
DUE TO A MORE SOUTHERLY ESTIMATED CURRENT POSITION. THE JTWC TRACK
FORECAST SHIFTED SLIGHTLY NORTHWARD BEYOND 72 HOURS, STAYING IN THE
MIDDLE OF A FAIRLY WIDE MODEL TRACK ENVELOPE. THE JTWC INTENSITY
FORECAST STAYS BELOW THE STRONGEST COAMPS-TC AND SHIPS FORECASTS,
BUT IS WELL ABOVE THE DYNAMICAL GLOBAL MODELS.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: LOW
   INTENSITY 72-120 HR: LOW//
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