Tropical Storm RAI Advisory lun, 13-12

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 28W (TWENTYEIGHT)
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 5.1N 141.7E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 340 NM SOUTHEAST OF YAP
   MOVEMENT PAST 6 HOURS: WESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY
EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC), DISPLACED JUST SOUTH
OF AN EXPANSIVE AREA OF DEEP, PERSISTENT CONVECTION. THE MOST
RECENT MICROWAVE IMAGE, A 122144Z 89GHZ SSMIS IMAGE, DEPICTED THE
DISORGANIZED STRUCTURE OF THE SYSTEM CONVECTIVE CORE, WITH THE LLCC
DISPLACED TO THE SOUTH. THE INITIAL POSITION IN ASSESSED WITH HIGH
CONFIDENCE BASED ON A SPIRAL BAND ANALYSIS OF THE EXPOSED LOW LEVEL
CLOUD LINES TO THE SOUTH OF THE CENTER. THE INITIAL INTENSITY OF 25
KNOTS IS ASSESSED WITH MEDIUM CONFIDENCE, BASED ON THE AVERAGE OF
MULTI-AGENCY DVORAK CURRENT INTENSITY ESTIMATES. THE WIND FIELD
REMAINS HIGHLY ASYMMETRIC, WITH EARLIER 121925Z SCATTEROMETER DATA
SHOWING 25-30 KNOTS TO THE NORTH, WITH EVEN HIGHER LIKELY OCCURRING
UNDER THE DEEPEST CONVECTION, WHILE WINDS WERE MEASURED AT 15-20
KNOTS ALONG THE SOUTHERN PORTIONS OF THE CIRCULATION. ENVIRONMENTAL
ANALYSIS REVEALS DIVERGENT EASTERLY FLOW OVER TOP OF THE SYSTEM,
AIDING THE CONVECTIVE ACTIVITY CURRENTLY BEING SEEN, THOUGH OVERALL
OUTFLOW REMAINS RELATIVELY WEAK AT PRESENT, BOTH POLEWARD AN
EQUATORWARD. SSTS ARE VERY WARM (29-30C) AND VWS IS LOW, LEADING TO
AN OVERALL FAVORABLE ENVIRONMENTAL ASSESSMENT.
INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).
CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE NORTH.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T1.5 - 25 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.5 - 35 KTS
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK POLEWARD AND EQUATORWARD
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.
FORECAST DISCUSSION: TROPICAL DEPRESSION 28W IS EXPECTED TO TRACK
WEST-NORTHWESTWARD THROUGH THE DURATION OF THE FORECAST PERIOD
ALONG THE SOUTHERN SIDE OF AN EXTENSIVE STR ENTRENCHED TO THE
NORTH. TRACK SPEEDS ARE FORECAST TO REMAIN RELATIVELY STEADY EXCEPT
FOR A BRIEF SLOWDOWN BETWEEN TAU 48 AND 72, IN RESPONSE TO A SLIGHT
WEAKENING OF THE STEERING GRADIENT DUE TO DEVELOPMENT OF A WEAKNESS
IN THE RIDGE TO THE NORTH. TD 28W IS EXPECTED TO CROSS THROUGH THE
SOUTHERN PHILIPPINE ISLANDS AFTER TAU 72, AND ULTIMATELY EMERGE INTO
THE SULU SEA AND TRACK TO NEAR PALAWAN BY TAU 120. OVER THE NEXT 12
TO 18 HOURS, THE LLCC IS EXPECTED TO FURTHER CONSOLIDATE WHILE
MOVING UNDER THE DEEP CONVECTION, ESTABLISHING A WELL DEFINED CORE.
THIS WILL MARK THE START OF A PERIOD OF STEADY INTENSIFICATION TO A
PEAK OF 85 KNOTS BY TAU 72. THE BULK OF THE INTENSIFICATION IS
EXPECTED IN THE FIRST 48 HOURS AS VWS REMAINS BELOW 15 KNOTS AND
OUTFLOW REMAINS RELATIVELY HIGH. BY TAU 72, SHEAR IS EXPECTED TO
INCREASE ABOVE 15 KNOTS AND THE OUTFLOW WILL DECREASE AS THE UPPER-
LEVEL FLOW SHIFTS TO A MORE EASTERLY DIRECTION, CUTTING OFF THE
POLEWARD OUTFLOW CHANNEL. THE SYSTEM WILL WEAKEN AS IT CROSSES
THROUGH THE PHILIPPINES DUE TO INFLOW AND CORE DISRUPTION CAUSED BY
INTERACTION WITH THE COMPLEX TERRAIN FEATURES. ONCE BACK OVER WATER,
THE SYSTEM WILL QUICKLY INTENSIFY ONCE MORE AS THE POLEWARD OUTFLOW
CHANNEL IS REESTABLISHED
MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH THE
DURATION OF THE FORECAST WITH A 90NM SPREAD AT TAU 48 INCREASING TO
210NM AT TAU 120, BUT ONLY 110NM IF THE NAVGEM TRACKER IS
DISCOUNTED AS THE SINGLE MAJOR OUTLIER. THE JTWC FORECAST LIES
AMONGST THE TIGHTEST PACKING OF CONSENSUS MEMBERS AND JUST SOUTH OF
THE CONSENSUS MEAN WITH HIGH CONFIDENCE THROUGH TAU 72, AND MEDIUM
CONFIDENCE THEREAFTER. INTENSITY GUIDANCE IS MIXED WITH TWO
DISTINCT PEAKS WITHIN THE FORECAST PERIOD. THE ENVELOPE OF PEAK
INTENSITIES RANGES FROM A LOW OF 65KNOTS TO A HIGH OF 110 KNOTS FOR
THE FIRST PEAK EAST OF THE PHILIPPINES, AND 65 KNOTS TO 135 KNOTS
FOR THE SECOND PEAK AT TAU 120. THE COAMPS-TC MODELS ARE THE MOST
AGGRESSIVE THROUGH THE FORECAST PERIOD, WITH THE SHIPS GUIDANCE
MARKING THE LOW END AND HWRF VERY AGGRESSIVE THROUGH TAU 24, BUT
THEREAFTER MORE MODEST, THOUGH THE HWRF DOES NOT WEAKEN THE SYSTEM
THROUGH THE PHILIPPINES. BASED ON ANALYSIS OF THE GLOBAL FIELDS,
THE COAMPS-TC AND HWRF TRACKERS ARE DETERMINED TO BE TOO AGGRESSIVE
WHILE THE GFS IS TOO BEARISH. THE JTWC FORECAST LIES AMONGST MOST
OF THE GUIDANCE THROUGH TAU 48, THEN DIVERGES LOWER THAN THE MOST
AGGRESSIVE MODELS THROUGH THE REMAINDER OF THE FORECAST. IN LIGHT
OF THE EXTREME UNCERTAINTY EXHIBITED BY THE MODEL GUIDANCE, AT THIS
TIME THERE IS LOW CONFIDENCE IN THE JTWC FORECAST INTENSITY.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: LOW
   INTENSITY 72-120 HR: LOW//
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