WDPN33 PGTW 271500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) 
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.8N 125.9E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 420 NM NORTHEAST OF MANILA, PHILIPPINES
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A SLOWLY
BE STEADILY CONSOLIDATING SYSTEM, WITH FORMATIVE SPIRAL RAIN BANDS
WRAPPING IN TO STILL BROAD LOW LEVEL CIRCULATION CENTER (LLCC). THE
STRONGEST RAIN BANDS LIE ON THE WEST SIDE OF THE CIRCULATION, WITH
THE EASTERN SIDE SEEING MUCH WEAKER CONVECTIVE BANDING FEATURES,
LIKELY DUE TO SOME MODEST NORTHEASTERLY SHEAR AND A BIT OF
UPPER-LEVEL CONVERGENCE INDUCED BY THE PROXIMITY OF A TUTT-CELL
ABOUT 450NM TO THE SOUTHEAST. A SERIES OF 89GHZ MICROWAVE IMAGES
BETWEEN 260930Z AND 261330Z PROVIDED MEDIUM CONFIDENCE TO THE
INITIAL POSITION, DUE TO THE BROAD AND STILL NASCENT NATURE OF THE
LLCC. THE INITIAL INTENSITY WAS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON THE HIGHER END OF THE AGENCY FIXES NOTED BELOW AND NEAR
THE CIMSS ADT ESTIMATE OF 28 KNOTS. ANALYSIS REVEALS A FAVORABLE
ENVIRONMENT, WITH LOW NORTHEASTERLY SHEAR, VERY WARM SSTS, HIGH OHC
AND GOOD EQUATORWARD OUTFLOW. THE SYSTEM IS CURRENTLY BEING STEERED
SOUTHWESTWARD ALONG THE EASTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED OVER THE SOUTH CHINA SEA.

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

CURRENT STEERING MECHANISM: EASTERN PERIPHERY OF A DEEP-LAYER STR
OVER THE SOUTH CHINA SEA. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS
   CIMSS ADT: 28 KTS AT 271150Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: ELEVATED MID-LEVEL, NORTHEASTERLY SHEAR AND
SLIGHTLY CONVERGENT FLOW ALOFT IN THE NORTHEAST QUADRANT

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: TD 20W WILL CONTINUE TO TRACK SOUTHWESTWARD
ALONG THE EAST SIDE OF THE STR OVER THE SOUTH CHINA SEA FOR THE
NEXT 12 TO 18 HOURS. THE RIDGE IS EXPECTED TO RAPIDLY RETREAT
WESTWARD AT THE SAME TIME, WHILE AN EXTENSION OF THE STR BUILDS IN
TO THE SOUTHEAST ALONG A MINDANAO TO GUAM AXIS. BY TAU 24, TD 20W
WILL REACH ITS SOUTHERN MOST POINT, SLOWING DOWN TO ABOUT 3 KNOTS
OF FORWARD SPEED AS IT MOVES INTO A WEAK STEERING PATTERN. THE
SYSTEM DRIFTS GENERALLY WEST-NORTHWESTWARD THROUGH TAU 36, AS THE
PATTERN SHIFTS, WITH A NEW ANTICYCLONE CENTER FORMING JUST WEST OF
GUAM, THE RIDGE OVER THE SOUTH CHINA DISAPPEARING AND ANOTHER RIDGE
DEVELOPING OVER EASTERN CHINA. BY TAU 48, THE RIDGE WEST OF GUAM
QUICKLY BUILDS AND BEGINS TO TRACK NORTHWARD, AND BECOMES THE
DOMINANT STEERING INFLUENCE, KICKING TD 20W OUT ONTO AN
ACCELERATING PATH NORTHWARDS AFTER TAU 48. AT THIS POINT THERE IS A
DIVERGENCE EMERGING BETWEEN THE ECMWF AND GFS SOLUTIONS IN HANDLING
THE RIDGES DRIVING THE TRACK MOTION. AFTER TAU 48, THE ECMWF BUILDS
THE RIDGE OVER CHINA MUCH FURTHER EAST, EXTENDING WELL INTO THE
EAST CHINA SEA, WHILE MOVING THE RIDGE IN THE PHILIPPINE SEA CLOSER
TO TD 20W. THE RESULT IS A STRONGER INFLUENCE OF THE RIDGE OVER
CHINA, WHICH PUSHES TD 20W ONTO A TRACK SOUTH OF TAIWAN BEFORE
RECURVING INTO SOUTHWESTERN TAIWAN. THE GFS ON THE OTHER HAND SHOWS
A WEAKER RIDGE OVER CHINA AND A STRONGER RIDGE OVER THE PHILIPPINE
SEA FURTHER TO THE EAST, RESULTING IN A TRACK THAT REMAINS EAST OF
TAIWAN FOR THE MOST PART, ONLY CLIPPING NORTHEASTERN TAIWAN FOR A
BRIEF PERIOD BEFORE RECURVING TO THE NORTHEAST. THE DYNAMICS OF THE
RIDGE PATTERN DEVELOPMENT INTRODUCE SIGNIFICANT UNCERTAINTY INTO
THE MID- TO LONG-RANGE FORECAST AS THE SYSTEM APPROACHES TAIWAN. IN
TERMS OF INTENSITY, THE SYSTEM WILL SLOWLY BUT STEADILY INTENSIFY
FOR THE FIRST 36 HOURS AS IT CONTINUES TO CONSOLIDATE. AFTER TAU
36, THE SYSTEM IS EXPECTED TO RAPIDLY INTENSIFY AS THE VORTEX
BECOMES FULLY CONSOLIDATED AND VERTICALLY ALIGNED, UPPER-LEVEL
OUTFLOW IS ENHANCED AND THE SYSTEM TRACKS SLOWLY OVER A POOL OF
HIGH OHC WATERS. THE PEAK INTENSITY IS EXPECTED TO OCCUR AROUND TAU
96 AT OR NEAR 110 KNOTS. INTERACTION WITH TAIWAN AND INCREASING
SHEAR WILL LEAD TO A GENERAL WEAKENING AFTER TAU 96. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN FAIR AGREEMENT
IN THE EARLY STAGES OF THE FORECAST, BUT EVEN AS EARLY AS TAU 48
STARTS TO SHOW A DECENT AMOUNT OF SPREAD. THE GEFS ENSEMBLE MEAN
TURNS THE SYSTEM SHARPLY NORTHWARD AFTER TAU 24, AND IS THE EASTERN
MOST OUTLIER, WHILE THE GALWEM AND THE ECMWF TAKE THE SYSTEM
WESTWARD INTO THE LUZON STRAIT BY TAU 72, GENERATING A SPREAD OF
ABOUT 220NM BY TAU 72. BY TAU 120, ALL OF THE GUIDANCE SHOWS THE
SYSTEM RECURVING TO THE NORTHEAST BUT WITH A LARGE SPREAD IN THEIR
STARTING AND ENDING POINTS. THE GALWEM STARTS THE FURTHEST WEST AND
ENDS UP OVER TAIWAN, THE GFS LIES OVER NORTHEASTERN TAIWAN AND THE
GEFS ENSEMBLE MEAN IS SITUATED WEST OF OKINAWA AT TAU 120,
RESULTING IN A SPREAD OF ABOUT 350NM. IT IS NOTED THAT THE 06Z
ECEPS RUN SHOWED A LARGE SHIFT WESTWARD OF THE MEAN, WHICH HAS
INTRODUCED MORE UNCERTAINTY INTO THE FORECAST SINCE THE PREVIOUS
RUN, WHICH WAS MORE ALIGNED WITH THE GEFS. THE JTWC FORECAST
REMAINS CONSISTENT WITH THE PREVIOUS FORECAST HEDGES CLOSE TO THE
CONSENSUS MEAN FOR THE DURATION OF THE FORECAST. INTENSITY GUIDANCE
IS SHOWING INCREASING SPREAD AS WELL, WITH THE COAMPS-TC NOW GIVING
UP ON THE SYSTEM AFTER PEAKING AT ABOUT 85 KNOTS, WHILE THE HAFS-A
PEAKS AT AN EXTREME 155 KNOTS. IT SHOULD BE NOTED HOWEVER, THAT
NEITHER OF THESE MODELS ARE IN THE TRACK CONSENSUS AND ARE EXTREME
OUTLIERS FROM THE TRACK GUIDANCE WITH THE COAMPS-TC TAKING THE
SYSTEM INTO THE TAIWAN STRAIT WHILE THE HAFS-A ENDS UP JUST WEST OF
OKINAWA. SO IN SUMMARY, BOTH ARE COMPLETELY UNREPRESENTATIVE OF THE
ACTUAL FORECAST AND SHOULD BE DISCOUNTED. DUE TO UPSTREAM OUTAGES,
THESE ARE THE ONLY INTENSITY MODELS AVAILABLE AND THUS THERE IS
EXTREMELY HIGH UNCERTAINTY IN THE FORECAST INTENSITY, WHICH IS
HEDGED SLIGHTLY HIGHER THAN THE CONSENSUS WITH LOW CONFIDENCE.  

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