AWIO20 FMEE 261354 TROPICAL CYCLONE CENTER / RSMC LA REUNION / METEO-FRANCE BULLETIN FOR CYCLONIC ACTIVITY AND SIGNIFICANT TROPICAL WEATHER IN THE SOUTHWEST INDIAN OCEAN DATE: 2025/12/26 A 1200 UTC PART 1: WARNING SUMMARY NIL. PART 2 : TROPICAL WEATHER DISCUSSION: The basin displays a Monsoon Trough (MT) configuration east of 50oE, with an MT axis between 5 and 10oS. Convective activity is moderate within convergences along the northern edge of the MT and is also present within severe tropical storm GRANT, still in the Australian region this Friday, at the southeastern edge of the MT. A short secondary MT branch has also formed in the Mozambique Channel between 17 and 22oS, fed by the Kashkazi flow along the African coast. Thunderstorm activity is locally strong there, particularly near the west coast of Madagascar. Two areas of vorticity are visible, associated with moderate convective activity : one near the southwestern coast of Madagascar, the other over Mozambique near the southeast of Malawi. The equatorial wave setup is becoming more favorable for cyclogenesis over the coming days as an active MJO phase moves in from the west. It is associated with a Kelvin wave at the beginning and middle of next week, thus enhancing convergence and cyclogenesis potential in the Mozambique Channel. We should also note the westward progression of an equatorial Rossby wave from the east of the basin, currently associated with tropical storm GRANT. Entry of tropical storm GRANT from the Australian area of responsibility : Tropical system GRANT, currently monitored by the BOM (see latest IDW27700 bulletin), was located at 06UTC near 11.8oS and 92.8oE. It was then at Severe Tropical Storm stage with 50 kt winds and a 990 hPa central pressure, moving west-north-west at 8 kt. GRANT is expected to continue westward and move into our basin this Saturday morning near 00 or 06UTC, with a slightly increasing intensity. RSMC La Reunion will take over monitoring from Saturday 06UTC onwards. There is a very high risk that tropical storm GRANT will enter the far east of our basin from the Australian area of responsibility from Saturday, December 27th. Suspect area near the coast of Mozambique : Synoptic convergence is strengthening over the center of the Mozambique Channel between the monsoon flow to the north and the trade winds to the south, around a low-pressure area stretching from Mozambique to Madagascar within a very moist environment (secondary branch of the Monsoon Trough). A small vortex is present off the southwestern coast of Madagascar, associated with winds nearing 20/25 kt in its northern part, but it should quickly be swept towards the Malagasy mainland on Friday evening, in a moderately sheared environment on the edge of an upper-level trough, which should prevent successful tropical cyclogenesis, even if some baroclinic deepening is likely. This small low-pressure system is expected to be carried away by the upper-level flow and cross southern Madagascar on Saturday, then re-emerge in the Indian Ocean by Sunday, remaining in an overly baroclinic environment. In addition, another area of vorticity is present over central-eastern Mozambique, currently over land, southeast of Malawi. In the short term, moderate vertical shear, proximity to land, and a lack of surface convergence are preventing cyclogenesis. However, wind shear is expected to decrease between Sunday and Tuesday, while low-level vorticity will shift further east over the warm waters of the Channel. This could make conditions more favorable for cyclogenesis, although uncertainty remains about more or less efficient surface wind convergence. Some deterministic model runs suggest a tropical storm developing from early next week, such as the 00Z run of IFS. Ensemble models probabilities have increased slightly since yesterday, suggesting a risk that could become moderate from Tuesday 30th or Wednesday December 31st. The risk of tropical storm development in the central Mozambique Channel is estimated to be very low from Sunday 28th, becoming low on Monday 29th, then moderate from Tuesday December 30th. 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.