Powerful TC 13S(EMNATI): forecast to reach CAT 4 by 24hours, slowly approaching East Central coast of Madagascar, 20/09utc

JTWC IS ISSUING 12HOURLY WARNINGS AND 3HOURLY SATELLITE BULLETINS ON TC 13S(EMNATI).

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: OVER THE PAST FEW HOURS, TC 13S HAS TRACKED SLIGHTLY MORE WESTWARD THAN PREVIOUSLY EXPECTED WHILE ALSO SLOWING A FAIR AMOUNT, LIKELY IN RESPONSE TO A SECONDARY SUBTROPICAL RIDGE (STR) CENTER CURRENTLY SOUTH OF MADAGASCAR WHICH WILL QUICKLY SLIDE BY TO THE SOUTH OF THE SYSTEM AND TAKE UP RESIDENCE TO THE SOUTHEAST, MERGING WITH THE LARGER STR ALREADY IN PLACE. SO THE CURRENT WESTWARD TRACK WILL BE RELATIVELY SHORT-LIVED AND TC 13S IS EXPECTED TO RESUME A GRADUAL TURN TO A SOUTHWESTWARD TRACK OVER THE REMAINDER OF THE FORECAST AS IT GRACEFULLY ROUNDS THE RIDGE. SOMEWHAT UNEXPECTEDLY, TC 13S HAS PROVEN EXCEPTIONALLY RESILIENT AND ABLE TO INTENSIFY IN THE FACE OF UNFAVORABLE UPWELLING AND LOW OCEAN HEAT CONTENT (OHC), LIKELY IN RESPONSE TO THE ROBUST OUTFLOW ALOFT AND LOW SHEAR. NO SIGNIFICANT CHANGE TO THE ENVIRONMENT IS ANTICIPATED PRIOR TO LANDFALL, SO ADDITIONAL INTENSIFICATION IS EXPECTED, TO A PEAK OF 115 KNOTS/CAT 4 BY 36H. THE POTENTIAL FOR AT LEAST ONE EYE-WALL REPLACEMENT CYCLE (ERC) AND THE POSSIBILITY OF THE LACK OF OHC CATCHING UP WITH THE SYSTEM, ARE THE ONLY POTENTIAL LIMITING FACTORS. THE SYSTEM IS EXPECTED TO MAKE LANDFALL ALONG THE EAST-CENTRAL COAST OF MADAGASCAR AROUND 60H, FOLLOWED BY RAPID WEAKENING OVER THE MOUNTAINOUS TERRAIN. THE SYSTEM WILL TURN MORE SHARPLY POLEWARD AS IT MOVES BACK OVER WATER TOWARD A WEAKNESS IN THE RIDGE INDUCED BY AN APPROACHING MID-LATITUDE TROUGH. ONCE BACK OVER WATER, SOME INTENSIFICATION IS ANTICIPATED, THOUGH THE 26C ISOTHERM LIES ROUGHLY ALONG THE 25S LATITUDE LINE, AND THE LACK OF OHC WILL INHIBIT SIGNIFICANT REDEVELOPMENT. THE SYSTEM WILL BEGIN SUBTROPICAL TRANSITION BY 120H AS IT MOVES OVER 23-25C WATERS AND BEGINS TO INTERACT WITH A SUBTROPICAL 500MB LOW AND TROUGH.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TC 13S RAPIDLY INTENSIFIED FROM 65 KNOTS/CAT 1 US TO 105 KNOTS/CAT 3 THROUGH 200000Z TODAY BUT HAS SINCE PLATEAUED. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS THAT SINCE THE 0000Z HOUR, TC 13S HAS MAINTAINED A ROUGHLY 22KM EYE, BUT THE CONVECTIVE SIGNATURE HAS BEEN CONTINUALLY CYCLING, WITH THE CLOUD TOPS ALTERNATELY WARMING, THEN COOLING AGAIN, AND ERODING FIRST ON THE EAST, THEN THE WEST SIDE OF THE EYE. UNFORTUNATELY THERE HAS BEEN NO BULLSEYE MICROWAVE PASS SINCE 200050Z BUT TWO PARTIAL PASSES FROM 200125Z AND 200200Z INDICATE THE PRESENCE OF A VERY SMALL CENTRAL CORE OF CONVECTION SURROUNDING THE MICROWAVE EYE, AND A MUCH LARGER OUTER BAND OF CONVECTION WRAPPING INTO THE CENTER, INDICATIVE OF A POSSIBLE EYEWALL REPLACEMENT CYCLE (ERC). THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON A BLEND OF THE PGTW FIX ON THE VISIBLE EYE FEATURE AND THE CENTER INDICATED IN A 200551Z ASCAT-B NORMALIZED RADAR CROSS SECTION (NRCS) IMAGE. BASED ON THE THE NRCS DATA, THE LOW LEVEL CIRCULATION CENTER (LLCC) IS DISPLACED ABOUT 11KM NORTH OF THE VISIBLE EYE, WHICH ALIGNS WELL WITH THE SOUTHERLY SHEAR VECTOR INDICATED BY CIMSS UPPER-LEVEL ANALYSIS. DVORAK CURRENT INTENSITY ESTIMATES SHOW A LARGE SPREAD, BETWEEN T5.0 FROM FIMP, TO T6.0 FROM PGTW, WITH THE ADT AND SATCON LYING ROUGHLY AT THE AVERAGE, HOWEVER DATA-T NUMBERS ARE ROUGHLY ONE HALF T-NUMBER LOWER ACROSS THE BOARD. THE INITIAL INTENSITY ESTIMATE REMAINS HIGHLY CHALLENGING DUE TO THE RAPIDLY FLUCTUATING STRUCTURE CHARACTERISTICS. THE INITIAL INTENSITY IS THUS HELD AT 105 KNOTS/CAT 3 WITH MEDIUM CONFIDENCE.

MODEL DISCUSSION: TRACK GUIDANCE CONTINUES TO CONSOLIDATE ONTO A TIGHTER ENVELOPE, WITH THE GFS AND ECMWF OUTLIERS MOVING IN TOWARDS ONE ANOTHER. THE GUIDANCE DIVERGES AFTER 24H, WITH A 335KM SPREAD AT 72H, INCREASING TO 500KM AT 120H. THE JTWC FORECAST REMAINS CONSISTENT WITH THE PREVIOUS FORECAST, WITH ONLY A SLIGHT ADJUSTMENT IN THE FIRST 24 HOURS DUE TO THE RECENT WESTWARD TRACK MOTION, AND A SLIGHTLY SHARPER POLEWARD TURN AFTER 96H. AS DISCUSSED, THERE IS CONSIDERABLE UNCERTAINTY IN THE INTENSITY FORECAST. THE JTWC FORECAST LIES ON THE EXTREME HIGH EDGE OF THE DETERMINISTIC GUIDANCE PACKAGE, 15-20 KNOTS ABOVE THE INTENSITY CONSENSUS MEAN (ICNW), CLOSELY TRACKING THE HWRF SOLUTION, WHICH PEAKS AT 115 KNOTS/CAT 4. ALL OTHER GUIDANCE SUGGESTS NO ADDITIONAL INTENSIFICATION, FOLLOWED BY RAPID WEAKENING AFTER 24H. ENSEMBLE GUIDANCE INCLUDING COAMPS, ECMWF AND GEFS INDICATE A LOW TO MODERATE PROBABILITY OF MODERATE INTENSIFICATION, WITH LITTLE TO NO PROBABILITY OF RAPID INTENSIFICATION AND CONSISTENT WITH THE PREVIOUS RUN, NONE OF THE RAPID INTENSIFICATION AIDS HAVE BEEN TRIGGERED.