WDXS31 PGTW 031500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 23S (HIDAYA) WARNING 
NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 8.2S 42.0E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 181 NM EAST-SOUTHEAST OF DAR ES SALAAM,
TANZANIA
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 25 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 23S (HIDAYA) HAS SET A NEW RECORD, AS THE
MOST INTENSE TC IN THIS REGION IN THE HISTORICAL DATABASE, PEAKING
AT AN ESTIMATED 80 KNOTS EARLIER IN THE DAY. RAPIDLY INCREASING
SHEAR AND AN INFLUX OF DRY AIR IN THE MID-LEVELS MEANS THAT THE
SYSTEM WILL WEAKEN FROM HERE FORWARD. ANIMATED MULTISPECTRAL
SATELLITE IMAGERY (MSI) DEPICTS A RAPIDLY DETERIORATING STRUCTURE,
WITH THE AREAL EXTENT OF THE INNER CORE QUICKLY DECREASING,
ESPECIALLY ON THE EASTERN SIDE, AS DRY AIR FLOWS IN FROM THE NORTH.
THE INNER CORE REMAINS WELL-DEFINED WITH INTENSE CONVECTION
OBSCURING THE LOW LEVEL CIRCULATION CENTER (LLCC), THOUGH THE
PREVIOUSLY VISIBLE EYE HAS NOW FILLED IN COMPLETELY. THE LAST
MICROWAVE IMAGE, A GMI PASS FROM 030748Z SHOWED A WELL-DEFINED
LOW-LEVEL MICROWAVE EYE FEATURE IN THE 36GHZ BAND, THOUGH THE
EYEWALL IS CLEARLY ERODING FROM THE NORTH. THE INITIAL POSITION IS
ASSESSED WITH HIGH CONFIDENCE BASED ON AN EXTRAPOLATION OF A WEAK
MICROWAVE EYE FEATURE IN A 031040 NOAA-20 ATMS IMAGE AND THE TIGHT
GROUPING OF AGENCY FIX POSITIONS. THE INITIAL INTENSITY IS ASSESSED
WITH MEDIUM CONFIDENCE AT 75 KNOTS, DOWN A NOTCH FROM THE PEAK,
HEDGED SLIGHTLY LOWER THAN THE ADT AND AIDT ESTIMATES AND IN LINE
WITH THE AGENCY DVORAK CURRENT INTENSITY ESTIMATES OF T4.5 (77
KNOTS). THE LATEST CIMSS SHEAR ANALYSIS INDICATES NORTHERLY
DEEP-LAYER SHEAR IS RAMPING UP, NOW ESTIMATED AT 15 KNOTS OR
HIGHER. GFS MODEL SOUNDINGS SUGGEST EVEN HIGHER MID-LEVEL SHEAR. AS
MENTIONED, DRY AIR IS ALSO FLOWING IN FROM THE NORTH ALONG THE
EASTERN SIDE OF THE SYSTEM, FURTHER ERODING THE INNER CORE AND
INHIBITING CONVECTION. OTHERWISE, ENVIRONMENTAL CONDITIONS ARE
GENERALLY GOOD, WITH MODERATE POLEWARD OUTFLOW AND WARM SSTS, BUT
THESE ARE JUST NOT ENOUGH TO OFFSET NEGATIVE INFLUENCE OF THE SHEAR
AND DRY AIR.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A LOW- TO
MID-LEVEL STR OVER SOUTH AFRICA. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   KNES: T4.5 - 77 KTS
   DEMS: T4.5 - 77 KTS
   FMEE: T4.5 - 77 KTS
   CIMSS SATCON: 71 KTS AT 030900Z
   CIMSS ADT: 84 KTS AT 031200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: DRY MID-LEVEL AIR ENTRAINING INTO THE SYSTEM FROM
THE NORTH. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE PREVIOUS FORECAST CALLED FOR
CONTINUED INTENSIFICATION, BUT THIS FORECAST CALLS FOR THE
OPPOSITE, RESULTING IN A LARGE CHANGE IN THE FORECAST INTENSITY,
PARTICULARLY IN THE FIRST 12 TO 24 HOURS. 

FORECAST DISCUSSION: TC 23S HAS CONTINUED TO TRACK WEST-NORTHWEST
OVER THE PAST DAY, ALONG THE NORTHEASTERN PERIPHERY OF THE
DEEP-LAYER STR CENTERED OVER SOUTH AFRICA. THE SYSTEM IS EXPECTED
TO MAINTAIN THIS GENERAL TRACK MOTION THROUGH THE NEXT 24 HOURS,
APPROACHING A LANDFALL ALONG THE COAST OF TANZANIA SOUTH OF DAR ES
SALAAM BY TAU 24. AS THE SYSTEM WEAKENS RAPIDLY, THE STEERING FLOW
BECOMES PROGRESSIVELY SHALLOWER, WHERE THE RIDGE IS ORIENTED MORE
NORTH-SOUTH. THE NET EFFECT WILL BE FOR THE SYSTEM TO TURN MORE
EQUATORWARD AS IT WEAKENS. THE CURRENT FORECAST CALLS FOR THE
SYSTEM TO TRACK VERY CLOSE TO DAR ES SALAAM AND THEN REMAIN INLAND
AS IT DISSIPATES. A FASTER RATE OF WEAKENING COULD RESULT IN THE
SYSTEM TURNING MORE SHARPLY NORTHWARD AND TRACKING BACK OUT OVER
WATER AS IT DISSIPATES, AS IT SUGGESTED BY SOME OF THE MODEL
GUIDANCE. IN TERMS OF INTENSITY, THE SYSTEM PEAKED AROUND 030000Z
AT 80 KNOTS BUT IS NOW STARTING TO COME DOWN, ALBEIT SLOWLY AS IT
IS JUST BEGINNING TO FEEL THE EFFECTS OF INCREASING SHEAR AND DRY
AIR ENTRAINMENT. SHEAR IS EXPECTED TO RAPIDLY INCREASE OVER THE
NEXT 12 TO 24 HOURS, WHICH QUICKLY ERODE THE VORTEX, AS WELL AS
USHER IN ADDITIONAL DRY AIR. ADD IN THE DISRUPTION OF THE LOW-LEVEL
INFLOW DUE TO FRICTIONAL EFFECTS AS THE SYSTEM APPROACHES LAND, AND
THE SYSTEM WILL RAPIDLY WEAKEN FROM HERE FORWARD. DISSIPATION OVER
OR JUST ALONG THE EAST COAST TANZANIA IS FORECAST BY TAU 48, IF NOT
A BIT EARLIER. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE CONTINUES TO SHOW A
VERY LARGE SPREAD BETWEEN SOLUTIONS. THE GFS, NAVGEM AND THE BULK
OF THE MESOSCALE MODELS SHOW A TRACK ALMOST DUE WEST THROUGH THE
FORECAST PERIOD, WHILE THE UKMET, GALWEM AND GFS ENSEMBLE TRACKERS
SHOW THE SYSTEM TURNING MORE EQUATORWARD AFTER TAU 12, AND KEEPING
THE VORTEX OVER WATER THROUGH ALL OR MOST OF THE FORECAST PERIOD.
THE ECMWF LIES CLOSE TO THE CONSENSUS MEAN, AS DOES THE JTWC
FORECAST, ON THE SOUTHWESTERN EDGE OF THE NORTHERN GROUP OF
TRACKERS. INTENSITY GUIDANCE IS IN GOOD AGREEMENT THAT THE SYSTEM
WILL RAPIDLY WEAKEN THROUGH THE FORECAST PERIOD. THE JTWC FORECAST
LIES ON THE HIGH SIDE OF THE GUIDANCE ENVELOPE WITH MEDIUM
CONFIDENCE. 

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