AWIO20 FMEE 111236
TROPICAL CYCLONE CENTER / RSMC LA REUNION / METEO-FRANCE
BULLETIN FOR CYCLONIC ACTIVITY AND SIGNIFICANT TROPICAL WEATHER
IN THE SOUTHWEST INDIAN OCEAN
DATE: 2025/12/11 A 1200 UTC
PART 1: WARNING SUMMARY
NIL.
PART 2 : TROPICAL WEATHER DISCUSSION:
The basin displays a Near Equatorial Trough (NET) pattern east of 60E. Convective activity is weak to moderate west of 80E but locally strong over the eastern basin, especially near a low pressure area which has formed over the last few days.
The eastern part of the NET (east of 80E and in the Indonesian region) is actually becoming more convergent and moist due to more favorable wave activity : strengthening of equatorial westerlies associated with a powerful Kelvin wave until mid-December adding to a trans-equatorial wind component favored by Rossby waves (ER and MRG).
Over the eastern part of the basin :
The low-pressure system present since Tuesday between 80 and 85E and around 6 to 7S (designated as invest 92S by the JTWC) has remained fairly well defined according to yesterday's ASCAT passes. Convection temporarily weakened on Wednesday afternoon before strengthening again quite strongly overnight. Conventional satellite imagery and recent microwave passes showed a slight increase in convective organization and curvature on Thursday morning, but this remains insufficient and not long-lasting enough for the moment, as a new deterioration has appeared on the latest images. An initial Dvorak T1.0 classification is therefore not yet possible. At 12UTC on Thursday, the system can be estimated to be a Zone of Disturbed Weather with maximum winds of 25 kt and an estimated center near 6.3S/84E.
Over the next few days, environmental conditions are moderately favorable for its intensification (low vertical shear, good convergence on its equatorial side but very poor on its southern side). Some deterministic models forecast it could intensify into a tropical storm by Saturday morning, while it will still be in our area of responsibility. Over the weekend, the system is expected to leave our basin and enter the Indonesian area of responsibility, carried along by the equatorial westerlies. Its future development could then be more or less hampered by a Fujiwhara-type interaction with another circulation (91S) approaching its southeastern periphery, making the forecast complex. A new entry of the system into our basin cannot be completely ruled out in the longer term (after rotation around 91S and/or merger with it) but is not forecast between now and D+5 at this stage.
There is a low risk of tropical storm development over the extreme east of the basin between Friday and Saturday morning, before the system moves away east of 90E over the weekend.
Tropical low in the Indonesian then Australian regions :
Another low-pressure system is present near 06S/100E, designated as 91S by JTWC and BMKG and 05U by the Australian BOM. It has not yet reached tropical storm stage but most models suggest it could intensify and reach tropical storm stage in the coming days while moving west-south-westwards, and so getting closer to the 90E meridian. A Fujiwhara-like interaction with the aforementionned system (92S) is suggested by models, which makes the forecast complicated, as this could influence both its track and intensity (which could make it weaken or even be absorbed by 92S). Some models though suggest it could reach our area of responsibility from Sunday 14th or Monday 15th, probably after shortly moving across the Australian region. Chances of it moving into our region while still being at tropical storm stage in the next 5 days are estimated very low.
There is also a very low risk of entry of a tropical storm entering from the Australian or Indonesian areas of responsibility from Sunday December 14th onwards.
10-day outlook : After D+5, chances of one of these low-pressure systems located east of 90E entering our region (after possibly merging with one another) should slightly increase, especially after December 20th.
NOTA BENE: The likelihood is an estimate of the chance of genesis of a moderate tropical storm over the basin within the next five days:
Very low: less than 10% Moderate: 30% to 60% Very high: over 90%
Low: 10% to 30% High: 60% to 90%
The Southwestern Indian ocean basin extends from the Equator to 40S and from the african coastlines to 90E.
发表于 2025-12-10 07:36
