SIO: INVEST 99S

[cycloneye]

99S INVEST 210510 0000 4.2S 66.1E SHEM 20 1007

[MHC Tracking]

As for me, I think 99S won't form. But, if it does, in my opinion it will only peak as a TD or TS equivalent system.

[TorSkk]

A near-equatorial trough ( NET) on 3/4S is in place west of 70E in relation with a surge of
equatorial westerlies over the western part of the basin driven by an active phase of the MJO.
Related to this strong flow anomaly north-west of the basin, convective activity is strong near this
NET, especially on the equator north of the Seychelles.
Within this area, no clear circulation seems to be present for the moment according to the latest
scatterometer or satellite data. In the next few days, the equatorial westerly flow and the upper level
divergence associated with the MJO will also be strengthened by the passage of a Kelvin wave,
overlapped by a Equatorial Rossby wave, which should increase the vorticity and convective
activity on both sides of the equator. Moreover, surface convergence on the polar side could also
improve in the next few days with the arrival of the new high-pressure cell. In this favorable
large-scale context, a twinned cyclogenesis on either side of the equator is possible, but several
factors could limit the risk for the southern hemisphere. The strengthening of the westerly flow is
expected to occur mid-week on the northern side of the equator between 0 and 5N. The core of the
low-pressure circulation in the southern hemisphere should therefore be located near the equator
around 2/3S during the second part of the week. At this latitude, the weakness of the Coriolis force
could significantly limit the risk of cyclogenesis. In addition, a strong east-north-easterly shear
could also reduce the potential for an organized circulation.
The models are still in disagreement about the impact of this strong wave activity and the potential
for cyclogenesis: while the American models (GFS, GEFS) are still very reactive at the end of the
week, showing a deepening east of the Seychelles to the north of 5S, most of the other models,
including the European model and its ensemble, are maintaining a much more elongated circulation
on the southern hemisphere. As the American models are still rather isolated among the other
reliable models and suggest an atypical cyclogenesis (latitude very close to the equator), the latter
scenario remains for the moment discarded. It will nevertheless be worth monitoring over the next
few days.

The development of a tropical storm over the basin is not expected for the next 5 days.

[TorSkk]

Convective activity remains strong near the equator over the western half of the basin, under the
influence of an active MJO phase. A Kelvin wave has also developed over this area and is expected
to propagate Eastward over the next days, strengthening the equatorial westerlies.
Within this large-scale context, a near-equatorial trough is still located between 50E and 70E but the
strong northwesterly upper shear and the lack of equatorward low-level convergence strongly
impedes the development of a tropical low.
By the end of the week, an Equatorial-Rossby wave train is expected to cross the path of the
MJO/Kelvin waves. Some models thus suggest a cyclogenesis risk. This signal is mainly suggested
by the American ensemble and deterministic models but note that the last euro ensemble prediction
suggest a slight increase in its cyclogenesis probabilities early next week.

Development of a moderate tropical storm is not expected for the next 5 days.