TC 26S update// 3 Week Tropical Cyclone Formation Probability//1909utc

JTWC IS ISSUING 12 HOURLY WARNINGS AND 3 HOURLY SATELLITE BULLETINS ON TC 26S.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY EXPOSED, TIGHT LOW LEVEL CIRCULATION CENTER (LLCC) LURKING UNDER CONVECTIVE DEBRIS. FROM ABOUT 190300Z ONWARDS, A BURST OF CONVECTION DEVELOPED JUST WEST OF THE VORTEX CENTER AND THE LLCC BRIEFLY TUCKED UP UNDER THIS SMALL CONVECTIVE MASS. HOWEVER, BY 190700Z THE CONVECTION HAS DISSIPATED, LEAVING THE LLCC ONCE AGAIN EXPOSED AND HIGHLY TILTED WESTWARD WITH HEIGHT. CIMSS VWS ESTIMATES ARE CURRENTLY SHOWING EASTERLY SHEAR OF ABOUT 25-30 KNOTS OR EVEN SLIGHTLY HIGHER, WHICH IS CONSISTENT WITH THE SATELLITE PRESENTATION AND THE INABILITY OF THE SYSTEM TO MAINTAIN PERSISTENT CONVECTION IN AND AROUND THE INNER CORE OF THE VORTEX. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE PARTIALLY EXPOSED LLCC IN THE MSI. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON PERSISTENCE WITH SCATTEROMETER AND SMOS DATA FROM ABOUT 18 HOURS AGO, AND CONSISTENT WITH THE HIGHER T3.0 DVORAK CURRENT INTENSITY ESTIMATES NOTED ABOVE. THE ENVIRONMENT IS MARGINAL AT BEST, WITH WARM SSTS AND STRONG WESTWARD OUTFLOW OFFSET BY THE HIGH VWS AND RELATIVELY DRY AIR TO THE NORTH AND EAST OF THE SYSTEM.

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TC 26S REMAINS TRAPPED BETWEEN A BELT OF LOW- TO MID-LEVEL NORTHWESTERLIES TO THE NORTH AND A DEEP-LAYER STR TO THE SOUTH. OVER THE PAST 12 HOURS OR SO, THE STR HAS BEEN THE DOMINATE INFLUENCE, AND AFTER A BRIEF SOUTHEASTWARD EXCURSION, TC 26S HAS TRACKED FOR THE MOST PART TOWARDS THE SOUTHWEST SINCE THIS MORNING. OVER THE NEXT 12 HOURS OR SO, THE STR IS EXPECTED TO WEAKEN AND SHIFT EASTWARD, WHICH WILL ALLOW THE INFLUENCE OF THE WESTERLIES TO THE NORTH TO INCREASE, AND TURN TC 26S TOWARDS THE SOUTHEAST THROUGH TAU 72. A COMBINATION OF WEAKENING MONSOON WESTERLIES TO THE NORTH AND A RESTRENGTHENING STR TO THE SOUTH WILL GENERATE A WEAK STEERING PATTERN AND THE SYSTEM IS EXPECTED TO SLOW SIGNIFICANTLY, GENERALLY BECOMING QUASI-STATIONARY AFTER TAU 72. IN TERMS OF INTENSITY, THE RECENT BURSTS OF CONVECTIVE ACTIVITY HAVE PROVIDED THE IMPETUS TO CONTINUE SPINNING UP THE LOW-LEVEL WIND FIELD AND IT IS ASSUMED THE SYSTEM HAS MAINTAINED 45 KNOT INTENSITY, AS MEASURED BY AN EARLIER SMOS PASS. HOWEVER, THE SYSTEM REMAINS HIGHLY TILTED AND IT WILL BE DIFFICULT FOR THE SYSTEM TO OVERCOME THE PERSISTENT, STRONG, EASTERLY SHEAR AND ACHIEVE SYMMETRIZATION. THE WIND FIELD HOWEVER WILL NOT WEAKEN SIGNIFICANTLY IN THE SHORT-TERM, AND THE FORECAST CALLS FOR A STEADY STATE INTENSITY THROUGH TAU 24. THEREAFTER, WHILE SHEAR IS EXPECTED TO WEAKEN SLIGHTLY, LARGE-SCALE SUBSIDENCE AND SIGNIFICANT DRYING OF THE AIRMASS ALONG THE NORTHERN SIDE OF THE CIRCULATION WILL BEGIN TO IMPACT THE SYSTEM. THE DRY AIR WILL BE INGESTED INTO THE INNER CORE OF THE SYSTEM AFTER TAU 24, RESULTING IN A SLOW BUT STEADY WEAKENING TREND THROUGH TAU 48. SOME MODEST MOISTENING AND DECREASE IN SHEAR IS EXPECTED AFTER TAU 48, THOUGH BY THAT TIME, HAVING SPENT A SIGNIFICANT AMOUNT OF TIME OVER RELATIVELY THE SAME AREA, COOL WATER UPWELLING WILL OFFSET THE OTHERWISE MORE FAVORABLE ENVIRONMENTAL CONDITIONS. THE TUG OF WAR BETWEEN THESE COMPETING INFLUENCES MEANS THE SYSTEM WILL LIKELY MAINTAIN MINIMAL TROPICAL STORM STRENGTH THROUGH THE END OF THE FORECAST PERIOD, THOUGH IT IS POSSIBLE IT COULD WEAKEN BELOW WARNING THRESHOLD PRIOR TO TAU 120.

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE HAS COME INTO BETTER OVERALL AGREEMENT WITH THIS NEW RUN. ALL MEMBERS OF THE CONSENSUS EXCEPT NAVGEM NOW SHOW A TRACK THAT TURNS SHARPLY SOUTHWARD WITH THE NEXT SIX HOURS FOLLOWED BY A GENERAL TRACK TOWARDS THE SOUTHEAST THROUGH TAU 72. AFTER THIS POINT, THE GUIDANCE BEGINS TO FAN OUT WITH MOST MEMBERS INDICATING AN INFLECTION POINT AROUND TAU 96 WHERE THE TRACK TURNS BACK TOWARDS THE SOUTH AND WEST. AS MENTIONED, THE NAVGEM TRACKS THE SYSTEM STRAIGHT SOUTHWEST THROUGH THE FORECAST PERIOD, WHILE THE ECMWF ENSEMBLE MEAN TRACKS THE SYSTEM SOUTHWARD THEN WEST AFTER TAU 72 AND THE ECMWF AIFS LOOPS THE SYSTEM BACK NORTHWARD AFTER TAU 72. GEFS AND ECENS ENSEMBLE MEMBERS DEPICT A SQUASHED SPIDER TYPE SCENARIO, WITH MEMBERS ROUGHLY EQUALLY SPREAD OUT ACROSS A WIDE FAN OF POTENTIAL TRACKS FROM WEST TO SOUTHEAST TO EAST OR EVEN LOOPING. THE JTWC FORECAST ROUGHLY CONSISTENT WITH THE CONSENSUS MEAN THROUGH THE FORECAST PERIOD, WITH LOW CONFIDENCE. INTENSITY GUIDANCE IS SPLIT, WITH THE DECAY-SHIPS (BOTH GFS AND NAVGEM) SHOWING MODEST INTENSIFICATION TO 50-60 KNOTS BEFORE WEAKENING AFTER TAU 72. THE REMAINDER OF THE GUIDANCE PACKAGE SHOWS EITHER A FLAT TRACE OR SLIGHT INTENSIFICATION IN THE SHORT-TERM, FOLLOWED BY A SLOW WEAKENING THAT LEVELS OFF AROUND 35 KNOTS AFTER TAU 48. THE JTWC FORECAST IS CONSISTENT WITH THE CONSENSUS MEAN, THE HAFS AND COAMPS-TC, WITH MEDIUM CONFIDENCE.

Valid - 03/26/25 - 04/08/25 Since entering the Indian Ocean at a slowed phase speed earlier in month, RMM observations show a more canonical eastward propagation of the MJO signal, though its amplitude has gradually weakened during the past few days in phase 3. Objective filtering of upper-level velocity potential and OLR anomaly observations offer a slightly different perspective, which shows the core of the MJO enhanced envelope closer to 110E, where according to composites during Feb-Apr, is more reflective of a phase 4 MJO expression at present. During the next week, RMM forecasts favor a very fast phase speed over the Maritime Continent, while maintaining a low to near zero amplitude as it enters the Western Pacific. Such a phase speed is uncharacteristic of the MJO, and the thinking is that this behavior is related to the rapid development of suppressed divergence aloft favored in the western Indian Ocean tied to exiting equatorial Kelvin wave activity in the region. This would cause the RMM index to “catch-up” to the enhanced convective MJO envelope that actually lies closer to the Western Pacific, consistent with the latest OLR and velocity potential short-term forecast fields. Beyond week-1 however, dynamical models remain at odds with the strength and evolution of the MJO once in the Western Pacific. The ECMWF continues to devolve the upper-level velocity potential fields into a multiple wave pattern by weeks 2 and 3, suggesting other modes of variability, namely, an emerging low frequency signal in the eastern Pacific characteristic of El Nino conditions, are playing a growing role in the global tropics. The GEFS and CFS have been less supportive of this feature, likely owing to the shallowness of the anomalously warm water in eastern Pacific, and favor a more organized MJO over the western Pacific. However, these models also struggle to maintain its eastward propagation and its potential to break through the low-frequency enhanced trade wind regime along Date Line heading into April. Given the unusual structure of the base state, and its potential interactions with the evolving subseasonal activity, there is a higher degree of uncertainty in the updated outlook. Despite these uncertainties, there continues to be decent support in the model guidance for additional tropical cyclone (TC) development in the southern Indian Ocean and South Pacific later in March. Conversely, decreased chances for TC development are favored over the southeastern and south-central Indian Ocean following a pretty active period from late February into early March. No TCs formed during the past week, with no active TCs at present. The Joint Typhoon Warning Center (JTWC) is monitoring a disturbance (91S) in the southeastern Indian Ocean with increased chances of formation during week-1. With the GEFS and ECMWF both favoring the persistence of strong anomalous lower level westerlies over this part of the basin and extending eastward by week-2, conditions look to remain favorable for additional development off the Kimberley Coast eastward to the Coral Sea. Therefore, 40% chances are posted from approximately 120E to 145E to the north of Australia, with a broader area of 20% chances, consistent with the week-3 GTH outlook issued last week. Probabilistic genesis tools also feature increased signals north of the equator in the Philippine Sea; however, the environment may not be yet conducive for genesis, precluding any TC shapes at this time. TC development is also possible to the northeast of Madagascar based on these tools, but forecast confidence is too low especially if the suppressed phase of the MJO should prevail over this part of the basin. Given the aforementioned differences in the model guidance with respect to the MJO, the week-3 perspective is unclear, which is compounded by a quieter genesis climatology entering April. However, 20% chances are posted to the north of Australia and extending eastward into the South Pacific where models favor the persistence of anomalous lower-level westerlies with modest signals in the probabilistic tools.

Forecasts for enhanced and suppressed precipitation are based on a historical skill weighted blend of CFSv2, GEFS, and ECMWF ensemble systems, with some consideration of MJO and ENSO composites for Feb-Apr due the uncertainties associated with predicted subseasonal activity, as well as the non-canonical low frequency base state. The model blend does well reflect the suppressed precipitation near the Date Line tied to the enhanced trades, with warming SSTs supporting an expanded coverage of enhanced precipitation over the eastern and central equatorial Pacific, where the highest probabilities are issued. Above-normal temperatures are favored over much of the western CONUS and many parts of eastern South America, where drought related conditions are being registered. In addition, excessive heat conditions are possible in portions of western Africa and the Indian subcontinent where daytime temperatures could exceed 105 degrees F. Precipitation forecasts over Africa are made in coordination with the International Desk at CPC. For hazardous weather concerns in your area in the U.S. during the next two weeks, please refer to your local NWS office, the Medium Range Hazards Forecast produced by the Weather Prediction Center, and the CPC Week-2 Hazards Outlook.