WDPN31 PGTW 291500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 27W (TWENTYSEVEN) WARNING NR 001// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 12.3N 141.6E INITIAL INTENSITY: 30 KTS GEOGRAPHIC REFERENCE: 187 NM WEST-SOUTHWEST OF NAVSTA GUAM MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 07 KTS SIGNIFICANT WAVE HEIGHT: 10 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A CONSOLIDATING SYSTEM WITH IMPROVED CONVECTIVE BANDING WRAPPING INTO A RAGGED LOW-LEVEL CIRCULATION CENTER. UNFORTUNATELY, THERE IS NO USEFUL MICROWAVE IMAGERY OR SCATTEROMETRY IMAGERY AVAILABLE, THEREFORE, THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE. THE INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH HIGH CONFIDENCE AND IS HEDGED ABOVE THE PGTW AND KNES DVORAK ESTIMATES. WITH THE EXCEPTION OF THE EASTERN QUADRANT, OUTFLOW IS VIGOROUS WITH POLEWARD OUTFLOW ENHANCED BY STRONG WESTERLY FLOW TO THE NORTH. ADDITIONALLY, VERTICAL WIND SHEAR IS LOW. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE(STR) POSITIONED TO THE NORTH. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS RJTD: T1.0 - 25 KTS KNES: T1.5 - 25 KTS FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 5-10 KTS SST: 29-30 CELSIUS OUTFLOW: STRONG POLEWARD OTHER FACTORS: HIGH, VERY FAVORABLE OCEAN HEAT CONTENT VALUES. ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: HIGH INITIAL WIND RADII: NOT APPLICABLE 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY. FORECAST DISCUSSION: TD 27W IS FORECAST TO TRACK WEST-NORTHWESTWARD THROUGH TAU 36 UNDER THE STEERING INFLUENCE OF THE STR POSITIONED TO THE NORTH. AFTER TAU 36, A DEEP MIDLATITUDE TROUGH IS FORECAST TO DIG INTO THE EAST CHINA SEA, WHICH WILL WEAKEN THE STR AND REALIGN THE STR TO A MORE POLEWARD CONFIGURATION. AS THE SYSTEM TURNS MORE POLEWARD BETWEEN TAU 36 TO 48 IT IS EXPECTED TO SLOW AS IT ENCOUNTERS A STRONG NORTHEAST SURGE EVENT. THEN GRADUALLY ACCELERATE NORTH-NORTHEASTWARD AS THE SURGE EVENT WEAKENS BY TAU 72. RAPID INTENSIFICATION IS ANTICIPATED AFTER TAU 48 DUE TO STRONG POLEWARD OUTFLOW INTO THE AFOREMENTION MIDLATITUDE TROUGH. THE SYSTEM SHOULD PEAK AT 115 KNOTS BY TAU 72 DUE TO VERY FAVORABLE ENVIRONMENTAL CONDITIONS. AFTER TAU 72, TD 27W WILL TRACK NORTH-NORTHEASTWARD AS IT TRACKS ALONG THE WESTERN PERIPHERY OF THE STR THROUGH TAU 120. AFTER TAU 72, STEADY WEAKENING WILL OCCUR AS VERTICAL WIND SHEAR INCREASES TO 25 TO 30 KNOTS. MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT THROUGH TAU 72 WITH THE EXCEPTION OF NAVGEM, THE SOLE OUTLIER, WHICH TRACKS THE SYSTEM WESTWARD THEN SLOWLY NORTH-NORTHWESTWARD. AFTER TAU 72, THERE IS A HIGH DEGREE OF UNCERTAINTY (LOW CONFIDENCE) IN THE JTWC FORECAST TRACK. SEVERAL MODELS (GFS, GFS ENSEMBLE MEAN, UKMET ENSEMBLE MEAN) INDICATE THE SYSTEM WILL SLOW AND STALL AS A LOW-LEVEL RIDGE BUILDS TO THE NORTH AND A FRONTAL SYSTEM RAPIDLY PUSHES EASTWARD. THE GFS ENSEMBLE (GEFS) SHOWS A BIFURCATION WITH THE BULK OF THE SOLUTIONS STALLING AND TRACKING TOWARD AND OVER THE PHILIPPINES AFTER TAU 120. THE ECMWF DETERMINISTIC SOLUTION DEPICTS A RAPID NORTH-NORTHEASTWARD TRACK AND IS THE FASTEST MODEL AT THIS TIME. THE ECMWF ENSEMBLE AT 290600Z, HOWEVER, SHOWS ONLY A FEW SOLUTIONS WITH A SIMILAR FAST POLEWARD TRACK. THE BULK OF THE EPS SOLUTIONS ARE STALLING AND SLOWING BETWEEN 21N AND 24N. THE ECMWF DETERMINISTIC SOLUTION APPEARS UNLIKELY DUE TO THE BUILDING RIDGE TO THE NORTH AND LACK OF A FRONTAL SYSTEM SO THE JTWC FORECAST FAVORS THE GFS DETERMINISTIC MODEL SOLUTION AT THIS TIME. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN