WDXS31 PGTW 210300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 24S (IALY) WARNING NR 
010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 5.2S 43.0E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 241 NM EAST-NORTHEAST OF DAR ES SALAAM,
TANZANIA
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A RAGGED
INNER CORE CONSISTING OF PATCHES OF DEEP CONVECTION WITH CLOUD TOPS
AS COLD AS -70 TO -80 DEGREES CELSIUS, SURROUNDING AN INTERMITTENT
WARM SPOT WHICH WAS ASSOCIATED WITH AN EYE-LIKE FEATURE ABOUT SIX
HOURS AGO. A 201901Z ASCAT-B PASS INDICATED THAT THE LOW-LEVEL
CIRCULATION CENTER (LLCC) HAD BECOME SLIGHTLY DISPLACED NORTH OF
THE EMBEDDED CENTER IN INFRARED IMAGERY AND THE MICROWAVE EYE IN
EARLIER MICROWAVE OVERPASSES. A RECENT 202236Z ATMS 88.2 GHZ IMAGE
SUGGESTS THAT THIS SOUTH TO SOUTHWESTWARD TILT WITH HEIGHT HAS
PERSISTED, WITH AN EXPOSED LOW-LEVEL CURVED BAND EVIDENT IN THE
NORTHEAST QUADRANT OUTLINING THE NEAR-SURFACE CENTER AND A PARTIAL
DEEP CONVECTIVE BAND AROUND THE WESTERN SEMICIRCLE OUTLINING THE
MID-LEVEL CENTER. THE INITIAL POSITION IS ASSESSED WITH LOW
CONFIDENCE, HOWEVER, DUE TO UNCERTAINTY IN THIS TILT ANALYSIS
WITHOUT MORE RECENT SURFACE WIND DATA. THE INITIAL INTENSITY IS SET
AT 55 KT BASED ON THE ULTRA HIGH-RESOLUTION VERSION OF THE
AFOREMENTIONED ASCAT PASS, CONSISTENT WITH AGENCY DVORAK FIXES
RANGING FROM T3.5 TO T4.0, BUT ABOVE OTHER OBJECTIVE ESTIMATES SUCH
AS ADT, D-MINT, AND SATCON.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE OVER TANZANIA

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   KNES: T4.0 - 65 KTS
   DEMS: T3.5 - 55 KTS
   CIMSS SATCON: 42 KTS AT 202210Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 25-30 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE DUAL-CHANNEL
   OTHER FACTORS: DRY AIR MASS TO THE NORTHEAST

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST TRACK HAS BEEN SHIFTED
35 TO 50 NM WEST OF THE PREVIOUS FORECAST DUE TO A DELAY IN THE
ANTICIPATED NORTHWARD TURN

FORECAST DISCUSSION: TROPICAL CYCLONE 24S (IALY) HAS BEEN
STUBBORNLY HOLDING ONTO ITS INNER CORE STRUCTURE DURING THE PAST 12
HOURS IN THE FACE OF 25-30 KT MID-LEVEL SHEAR, WITH A MICROWAVE EYE
PERSISTING IN IMAGERY UNTIL ABOUT 202200Z. WITH THE VORTEX NOW
ASSESSED TO BE TILTING SOUTH OR SOUTHWESTWARD WITH HEIGHT, THE
DECAY PHASE OF THE CYCLONE HAS LIKELY BEGUN, AND GIVEN ITS COMPACT
SIZE, IT SHOULD SUCCUMB TO THE ENVIRONMENTAL SHEAR RATHER QUICKLY.
THE JTWC INTENSITY FORECAST SHOWS STEADY WEAKENING THROUGHOUT THE
FORECAST PERIOD, DECREASING BELOW WARNING THRESHOLD BY 36 HOURS.
INDEED, MOST DYNAMICAL MODEL FORECASTS OPEN THE CIRCULATION INTO A
WAVE AXIS BY THAT TIME DUE TO RAPID DECAY. HOWEVER, THERE IS SOME
UNCERTAINTY IN THE IMMEDIATE FUTURE AS TO WHEN THE RAPID COLLAPSE
OF THE INNER CORE AND FULL DECOUPLING OF THE VORTEX WILL OCCUR. THE
TRACK FORECAST IS STRAIGHT FORWARD, AS THE DECOUPLING VORTEX WILL
BEGIN TO RESPOND PRIMARILY TO THE SHALLOW SOUTHERLY STEERING FLOW
IN THE LOWER TROPOSPHERE, USHERING THE STORM NORTHWARD AND ROUGHLY
PARALLEL TO THE AFRICAN COASTLINE. MODELS ARE IN UNANIMOUS
AGREEMENT THAT THE VORTEX WILL DISINTEGRATE OFFSHORE OF KENYA OR
SOUTHERN SOMALIA, THOUGH A BAND OF ENHANCED OFFSHORE WINDS MAY
PERSIST ON THE WESTERN SIDE OF THE REMNANT CIRCULATION.

MODEL DISCUSSION: NUMERICAL GUIDANCE IS IN GENERALLY GOOD AGREEMENT
ON THE TRACK AND INTENSITY FORECASTS, WITH MOST TRACK SPREAD BEING
ARTIFICIALLY DRIVEN BY TRACKERS LOSING THE VORTEX AS IT RAPIDLY
DISSIPATES. THE JTWC TRACK FORECAST IS CLOSE TO THE MULTI-MODEL
CONSENSUS AND SHIFTED 35-50 NM WEST OF THE PREVIOUS FORECAST. THE
RECENT WESTWARD SHIFT WAS DUE TO A DELAY IN THE DECOUPLING OF THE
VORTEX AND THUS ITS NORTHWARD TURN, BUT IF DECOUPLING IS INDEED
BEGINNING, FURTHER SIGNIFICANT SHIFTS WESTWARD ARE UNLIKELY. THE
JTWC INTENSITY FORECAST IS CLOSE TO THE MULTI-MODEL CONSENSUS.

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