Tropical Cyclone Iris

05/04/2018

050300Z POSITION NEAR 19.5S 152.3E.

TROPICAL CYCLONE 17P (IRIS), LOCATED APPROXIMATELY 402 NM EAST-SOUTHEAST OF CAIRNS, AUSTRALIA, HAS TRACKED EAST-NORTHEASTWARD AT 06 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL (MSI) SATELLITE IMAGERY SHOWS A FULLY EXPOSED LLCC TO THE NORTH OF STRUGGLING, PERIODICALLY FLARING CONVECTION. THE INITIAL POSITION IS BASED ON THE LLCC EVIDENT IN THE MSI LOOP AND THE PGTW 00Z FIX POSITION. A 042325Z AMSU-B 89GHZ MICROWAVE IMAGE REVEALS DEEP CONVECTION ISOLATED TO THE SOUTH OF THE LLCC. THE INITIAL INTENSITY

IS BASED ON OBSERVATIONS OVER 100NM TO THE SOUTHWEST BETWEEN 34-37 KTS, AS WELL AS A 042127Z SATCON ESTIMATE OF 42 KTS. THIS ESTIMATE IS SLIGHTLY ABOVE THE PGTW AND KNES 00Z FIXES OF CURRENT INTENSITY (CI) T2.5 (35 KTS) BUT BELOW THE ABRF CI OF T3.0 (45 KTS). A

042326Z BULLSEYE ASCAT PASS REVEALS A PATCH OF 35-39 KT WINDS TO THE SOUTHWEST, AND A BROAD SWATH OF 30-34 KT WINDS SURROUNDING TC 17P WITH THE EXCEPTION OF THE NORTHWEST SECTOR. UPPER LEVEL ANALYSIS INDICATES THAT THE CYCLONE IS NOW IN AN AREA OF UNFAVORABLE (20-30 KNOTS) NORTHERLY VWS, WHICH IS PARTIALLY OFFSET BY EXCELLENT POLEWARD OUTFLOW. ALONG-TRACK SSTS REMAIN CONDUCIVE (27 TO 28 CELSIUS). THE SYSTEM IS TRACKING SLOWLY EAST-NORTHEASTWARD ALONG THE PERIPHERY OF A NEAR-EQUATORIAL RIDGE

EXTENSION TO THE NORTHEAST, WHICH IS RECEDING AS A MIDLEVEL TROUGH MOVES THROUGH THE AREA. TC 17P IS EXPECTED TO GRADUALLY WEAKEN DUE TO THE UNFAVORABLE VWS AND AN UPPER LEVEL LOW THAT WILL LIMIT CONVECTION, DROPPING BELOW 35 KTS AFTER 36 HOURS. AS TC 17P IS

SHEARED FROM THE TOP DOWN, IT IS EXPECTED TO TURN NORTHWESTWARD UNDER THE STEERING INFLUENCE OF A 700 MB STR TO THE SOUTHWEST, WHICH WILL BECOME THE PRIMARY STEERING MECHANISM. AVAILABLE NUMERICAL MODEL GUIDANCE HAS CONVERGED OVER RECENT RUNS TO AGREE UPON A GENERAL U-TURN AND THEN NORTH OR NORTHWESTWARD SOLUTION, BUT WITH REMAINING DIFFERENCES IN HOW TIGHTLY TC 17P TURNS NORTHWARD AND THE TRACK SPEED. THE JTWC FORECAST TRACK IS SLIGHTLY EASTWARD OF MODEL CONSENSUS AND FOR THIS REASON TGERE IS OVERALL LOW CONFIDENCE IN THE JTWC TRACK FORECAST. MAXIMUM SIGNIFICANT WAVE

HEIGHT AT 050000Z IS 15 FEET.

 

03/04/2018

030300Z POSITION NEAR 17.1S 149.1E.

TROPICAL CYCLONE 17P (IRIS), LOCATED APPROXIMATELY 184 NM EAST OF CAIRNS, AUSTRALIA, HAS TRACKED EASTWARD AT 01 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED MULTI-SPECTRAL (MSI) SATELLITE SHOWS A DEEP CONVECTION TO THE SOUTH OF A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE MSI LOOP AND WILLIS ISLAND RADAR LOOP. THE INITIAL INTENSITY OF 45 KNOTS IS BASED ON MULTI-AGENCY DVORAK CURRENT INTENSITY ESTIMATES OF T3.0 (45 KNOTS) AND SUPPORTED BY A SATCON ESTIMATE OF 47 KNOTS. UPPER LEVEL ANALYSIS REVEALS THE SYSTEM HAS AN EXCELLENT POLEWARD OUTFLOW CHANNEL DUE TO AN UPPER LEVEL ANTICYCLONE CENTERED TO THE NORTHEAST. ENVIRONMENTAL ANALYSIS SHOWS THE SYSTEM IS IN AN AREA OF LOW (5-10 KNOTS) VERTICAL WIND SHEAR, AND IS TRACKING THROUGH WARM (28 CELSIUS) SEA SURFACE TEMPERATURES. OVERALL FAVORABLE CONDITIONS WILL ALLOW FOR STEADY  INTENSIFICATION THROUGH TAU 36, REACHING A PEAK INTENSITY OF 65 KNOTS. TC 17P IS UNDER COMPETING STEERING INFLUENCES WITH A NEAR-EQUATORIAL RIDGE (NER) LOCATED TO THE NORTHEAST AND A SUBTROPICAL RIDGE (STR) ANCHORED OVER AUSTRALIA. TC IRIS IS FORECAST

TO  TRACK SOUTHEAST THROUGH TAU 48 ALONG THE NER. AROUND THIS TIME, THE STR WILL TRANSIT TO THE WEST AND A TROUGH MOVES IN OVER CENTRAL AUSTRALIA. AFTER TAU 48 THE SYSTEM WILL BECOME QUASI STATIONARY IN AN AREA OF INCREASINGLY UNFAVORABLE VERTICAL WIND SHEAR, THE RESULT OF THE TROUGH OVER AUSTRALIA. THE SYSTEM WILL STEADILY DISSIPATE BETWEEN TAU 72 AND TAU 120, WITH THE REMNANT SURFACE LOW LEVEL TRACKING BACK TO THE NORTHWEST DUE TO THE SOUTHEASTERLY SURFACE FLOW. HOWEVER, THERE IS A BIFURCATION IN THE AVAILABLE MODEL GUIDANCE. AN ALTERNATE SCENARIO, AND THE PREVIOUS FORECAST PHILOSOPHY, IS THAT THE SYSTEM COULD TRACK EASTWARD AROUND THE SOUTHERN EXTENT OF THE NER AND DISSIPATE UNDER INCREASING VWS. DUE TO THE BIFURCATION IN THE DYNAMIC MODEL GUIDANCE, THERE IS OVERALL LOW CONFIDENCE IN THE JTWC FORECAST TRACK. MAXIMUM SIGNIFICANT WAVE HEIGHT AT 030000Z IS 12 FEET. 

 

02/04/2018
020300Z POSITION NEAR 16.9S 148.7E.

TROPICAL CYCLONE 17P (IRIS), LOCATED APPROXIMATELY 167 NM EAST OF CAIRNS, AUSTRALIA, HAS TRACKED WEST-NORTHWESTWARD AT 08 KNOTS OVER THE PAST SIX HOURS. AVAILABLE METEOROLOGICAL DATA SUPPORTS THE REGENERATION OF TC 17P, THEREFORE, JTWC HAS RESUMED WARNINGS. THIS CANCELS AND SUPERSEDES REF A (WTPS21 011030). ANIMATED MULTISPECTRAL (MSI) IMAGERY DEPICTS AN EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC)

WITH CONVECTIVE BANDING WRAPPING IN FROM THE EAST AND NORTH. A 012036Z SSMIS 91 GHZ IMAGE REVEALS DEEP CONVECTION CONCENTRATED SOUTH AND EAST OF THE CENTER, AND IN THE TRAILING BAND TO THE NORTH. THE INITIAL POSITION IS BASED ON THE EXPOSED LLCC IN MSI IMAGERY AND RADAR IMAGERY FROM AUSTRALIA. THE INITIAL INTENSITY IS SET AT 35 KNOTS BASED ON A 012248Z PARTIAL ASCAT PASS SHOWING A SWATH OF 35 KT WIND BARBS OVER 50NM TO THE EAST OF THE CENTER, AS WELL AS OBSERVATIONS FROM FLINDERS REEF STATION RECORDING WINDS IN EXCESS OF 35 KTS FOR 4 HOURS. THIS INTENSITY ESTIMATE AGREES WITH THE KNES AND

PGTW DVORAK CURRENT INTENSITY ESTIMATES OF T2.5 (35 KTS). ENVIRONMENTAL ANALYSIS INDICATES THAT DIVERGENCE ALOFT IS SUPPORTING CONVECTION DESPITE MODERATE 15-20 KT VERTICAL WIND SHEAR (VWS). SEA SURFACE TEMPERATURES ARE A FAVORABLE 27-28 DEGREES CELSIUS. TC 17P MADE A TURN TO THE NORTHWEST THEN HAS REMAINED QUASISTATIONARY (QS)

IN THE PAST 9 HOURS. WHILE THE REMNANTS OF 17P HAD BEEN SUBTROPICAL ON 27 MAR, IT RE-ACQUIRED TROPICAL CHARACTERISTICS, GROWING INCREASINGLY MOIST AND WARM CORE, IN THE PAST SEVERAL DAYS AS IT MOVED NORTHWESTWARD. IT IS LOCATED IN A WEAK STEERING ENVIRONMENT BETWEEN A NEAR-EQUATORIAL RIDGE TO THE NORTHEAST AND ANOTHER RIDGE

OVER THE AUSTRALIAN CONTINENT. THE JTWC INTENSITY FORECAST SUGGESTS GRADUAL INTENSIFICATION TO A PEAK OF 55 KTS AT 36 HOURS. IN THE NEAR TERM, GLOBAL MODELS ALL INDICATE A SOUTHERLY COMPONENT TO THE STORM MOTION, WITH HWRF AND GFS BEING WESTERN OUTLIERS. ECMWF, NAVGEM, AND AFUM INDICATE A SOUTHEASTWARD TRACK AFTER A POSSIBLE LOOP. IN THE LONG TERM, NUMERICAL MODEL GUIDANCE IS TRIFURCATED, WITH HWRF AND GFS MOVING TC 17P CONTINUING WESTWARD TOWARD AUSTRALIA, COAMPS AND NAVGEM TRACKING TC 17P SOUTHEASTWARD THEN EASTWARD, AND ECMWF AND THE UKMET MODELS SHOWING EVENTUAL NORTHEASTWARD MOTION. AS A RESULT, MULTIPLE DISSIPATION SCENARIOS ARE POSSIBLE AT THE END OF THE FORECAST PERIOD. DUE TO HIGH SPREAD IN THE MULTI-MODEL CONSENSUS, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST TRACK. MAXIMUM SIGNIFICANT WAVE HEIGHT AT 020000Z IS 12 FEET.

 

26/03/2018

260900Z POSITION NEAR 18.0S 158.0E. TROPICAL CYCLONE 17P (IRIS), LOCATED APPROXIMATELY 531 NM WEST- NORTHWEST OF NOUMEA, NEW CALEDONIA, HAS TRACKED SOUTHWARD AT 10 KNOTS OVER THE PAST SIX HOURS. ANIMATED MSI SHOWS EXPANSIVE FLARING DEEP CONVECTION OVER A WEAK AND FULLY EXPOSED LLC. THE INITIAL POSITION IS BASED THE MSI WITH HIGH CONFIDENCE. THE INITIAL INTENSITY IS BASED ON THE HIGH END OF AGENCY DVORAK ESTIMATES RANGING FROM T2.0 TO T2.5. TC 17P WILL CONTINUE TO TRACK ALONG THE WESTERN PERIPHERY OF THE NER TO THE EAST UP TO TAU 36. AFTERWARD, THE STR TO THE SOUTH WILL ASSUME STEERING AND DEFLECT THE CYCLONE WESTWARD. THE SYSTEM WILL SLIGHTLY INTENSIFY UNDER MARGINALLY FAVORABLE CONDITIONS; HOWEVER, AFTER TAU 24, INCREASING VWS WILL MAINLY BE RESPONSIBLE FOR ITS RAPID DECAY AND EVENTUAL DISSIPATION BY TAU 48. THE AVAILABLE NUMERIC GUIDANCE IS IN OVERALL GOOD AGREEMENT BUT GIVEN THE WEAK ORGANIZATION AND TRACK UNCERTAINTY IN THE LONG TERM, THERE IS LOW CONFIDENCE IN THE JTWC TRACK FORECAST. MAXIMUM SIGNIFICANT WAVE HEIGHT AT 260600Z IS 14 FEET.

 

25/03/018

TROPICAL CYCLONE 17P (IRIS), LOCATED APPROXIMATELY 573 NM WEST-NORTHWEST OF PORT VILA, VANUATU, HAS TRACKED SOUTHWESTWARD AT 04 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED MULTI-SPECTRAL IMAGERY (MSI) DEPICTS AN EXPANSIVE AREA OF FLARING CONVECTION 

LOCATED TO THE NORTH OF A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE MSI LOOP AND A 242254Z METOP-A ASCAT PARTIAL IMAGE. THE INITIAL INTENSITY OF 35 KNOTS IS BASED ON THE UPPER END OF DVORAK CURRENT INTENSITY ESTIMATES RANGING FROM T2.0 TO T2.5 (30-35 KNOTS) FROM KNES AND PGTW. THE INITIAL INTENSITY IS ALSO SUPPORTED BY THE AFOREMENTIONED ASCAT IMAGE WHICH SHOWS 30 KNOT (30-34 KNOT) WIND BARBS IN THE EASTERN SEMI-CIRCLE. UPPER LEVEL ANALYSIS REVEALS THE SYSTEM HAS GOOD EQUATORWARD AND POLEWARD OUTFLOW WITH THE LATTER TAPPING INTO THE MID-LATITUDE WESTERLIES. ENVIRONMENTAL ANALYSIS SHOWS THAT TC IRIS IS CURRENTLY IN AN AREA OF LOW (5-10KTS) VERTICAL WIND SHEAR AND IS TRACKING THROUGH WARM (28-29 CELSIUS) SEA SURFACE TEMPERATURES. THE SYSTEM WILL CONTINUE TO TRACK GENERALLY SOUTHWARD ALONG THE WESTERN EXTENT OF A SUBTROPICAL RIDGE, WITH THE TRACK BECOMING SOUTHEASTWARD AFTER TAU 36. TC 17P WILL GRADUALLY INTENSIFY UNDER FAVORABLE CONDITIONS THROUGH TAU 48, REACHING A PEAK OF 55 KNOTS. AFTER TAU 48, INCREASING VWS AND COOLING SEA SURFACE TEMPERATURES WILL BEGIN TO WEAKEN THE SYSTEM. THE SYSTEM WILL CONTINUE TO TRACK SOUTHEASTWARD THROUGH TAU 72, AFTER WHICH THE CYCLONE WILL BECOME QUASI-STATIONARY BEFORE TRACKING BACK TO THE NORTHWEST AS AN EXTENSION OF A BUILDING SUBTROPICAL RIDGE OVER EASTERN AUSTRALIA TAKES OVER AS THE PRIMARY STEERING FEATURE. DYNAMIC MODEL GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 48 WHEN INDIVIDUAL CONSENSUS MEMBERS BEGIN TO VARY SIGNIFICANTLY ON THE TIMING AND SHARPNESS OF THE TURN TO THE NORTHWEST. THE JTWC TRACK FORECAST IS PLACED NEAR THE MULTI-MODEL CONSENSUS WITH OVERALL LOW CONFIDENCE DUE TO THE UNCERTAINTY IN THE MODEL GUIDANCE. MAXIMUM SIGNIFICANT WAVE HEIGHT AT 250000Z IS 14 FEET.

From JTWC.