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The western part of the basin remains in a winter configuration. However, east of 70E, a
Near-Equatorial Trough (NET) pattern has begun to form this Sunday August 11th, with a NET axis
between 1 and 4S. Convective activity in the vicinity of this NET has risen compared with previous
days.
This pattern change was triggered by the onset of a large-scale equatorial westerly wind burst
driven by the passage of a Kelvin wave over the basin in recent days. Vorticity and moisture
convergence within the NET should be further reinforced over the next few days by the additional
contribution of an Equatorial Rossby wave and by the installation of the active phase of the MJO
over the Indian Ocean this mid-August, thus providing an atmospheric background which is
unusually conducive for the heart of the south-hemispheric cool season. A new Kelvin wave is also
set to reinforce convergence next weekend. Moreover, ocean heat content north of 10S is
unseasonably high, with SSTs close to 29C around 5S/75E and 27C around 10S.
Numerical models thus suggest the formation of a closed low-pressure circulation early in the week
near 5S/75E, which could more or less develop over the following days as it moves close to the
Chagos Archipelago. This development could, however, be hampered by the presence of moderate
northeasterly shear or sub-optimal low-level convergence. Some deterministic models, in particular
GFS, have been simulating the formation of a tropical storm at the end of the week for several runs
now. The deterministic IFS remains less reactive, but the GEFS and EPS ensembles have an
increasingly high number of members forming a tropical storm by next weekend, illustrating
increasing cyclogenesis potential.
For the next 5 days, there is a low risk of tropical storm development near the Chagos
archipelago from Friday August 16th onwards.
Beyond the next 5 days, this risk could increase and become moderate during the weekend of
August 17-18th.
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