MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) WARNING NR 01// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION 20W (TWENTY), LOCATED APPROXIMATELY 370 NM EAST-NORTHEAST OF SAIPAN, HAS TRACKED NORTHWESTWARD AT 10 KNOTS OVER THE PAST SIX HOURS. TD 20W HAS CONSOLIDATED INTO A DEPRESSION DURING THE PAST SIX HOURS AS A FLARE OF DEEP CONVECTION DEVELOPED OVER THE NORTHWESTERN PORTION OF THE LLCC. THE CURRENT POSITION IS BASED ON A 141620Z SSMI PASS, WHICH SHOWS A PARTIALLY-EXPOSED LOW- LEVEL CIRCULATION CENTER (LLCC). THE INTENSITY OF 25 KNOTS IS CONSISTENT WITH DVORAK T-NUMBER ESTIMATES FROM PGTW AND KNES. THE SYSTEM IS TRACKING NORTHWESTWARD ALONG THE PERIPHERY OF A SUBTROPICAL STEERING RIDGE TO THE NORTHEAST. TD 20W LIES SLIGHTLY EQUATORWARD OF AN UPPER-LEVEL RIDGE AXIS IN AN AREA OF MODERATE EASTERLY VERTICAL WIND SHEAR. 3. FORECAST REASONING. A. THIS IS THE INITIAL PROGNOSTIC REASONING AND SETS THE FORECAST PHILOSOPHY. B. TD 20W IS EXPECTED TO TRACK GENERALLY NORTHWESTWARD OVER THE NEXT 72 HOURS AS SUBTROPICAL RIDGING BECOMES FIRMLY ESTABLISHED TO THE NORTH. THE SYSTEM SHOULD INTENSIFY SLOWLY IN THE NEAR-TERM WHILE VERTICAL WIND SHEAR REMAINS UNFAVORABLE. AFTER TAU 48, THE SYSTEM WILL LIKELY INTENSIFY MORE RAPIDLY AS VERTICAL WIND SHEAR DECREASES AND UPPER-LEVEL OUTFLOW INCREASES. DYNAMIC MODEL GUIDANCE IS IN GENERALLY GOOD AGREEMENT REGARDING THE FORECAST TRACK THROUGH TAU 72. THERE IS HIGH CONFIDENCE IN THIS PORTION OF THE OFFICIAL FORECAST, WHICH LIES CLOSE TO THE MULTI-MODEL CONSENSUS. C. IN THE EXTENDED PERIOD, TD 20W WILL TURN POLEWARD AROUND THE STEERING RIDGE AXIS. STEADY TO RAPID INTENSIFICATION IS EXPECTED AS THE SYSTEM MAKES THE TURN, BUT INCREASING VERTICAL WIND SHEAR AND PASSAGE OVER COOLER WATER SHOULD PRODUCE GRADUAL WEAKENING AND INITIATE THE EARLY STAGE OF EXTRATROPICAL TRANSITION BY TAU 120. DYNAMIC MODEL GUIDANCE IS IN GOOD AGREEMENT REGARDING TIMING OF THE TURN AROUND OR JUST AFTER TAU 72, BUT ARE IN LESS AGREEMENT REGARDING THE ORIENTATION OF THE TRACK FOLLOWING THE TURN. ONE GROUPING OF MODELS, INCLUDING GFS AND HWRF, PREDICTS A TIGHTER TURN INSIDE THE MULTI-MODEL CONSENSUS AND ANOTHER GROUPING, INCLUDING THE UKMET AND ECMWF MODELS, PREDICTS A WIDER TURN TO THE WEST OF THE MULTI-MODEL CONSENSUS. THE CURRENT FORECAST LIES CLOSE TO THE CONSENSUS BUT SLIGHTLY FAVORS THE WESTERN GROUPING GIVEN A WESTWARD SHIFT IN MODEL FORECASTS OVER THE PAST SEVERAL MODEL RUNS. GIVEN MODEL FORECAST SPREAD, CONFIDENCE IN THE EXTENDED TRACK FORECAST IS LOW.// NNNN NNNN