AWIO20 FMEE 251219 TROPICAL CYCLONE CENTER / RSMC LA REUNION / METEO-FRANCE

BULLETIN FOR CYCLONIC ACTIVITY AND SIGNIFICANT TROPICAL WEATHER IN THE SOUTHWEST INDIAN OCEAN

DATE: 2020/12/25 AT 1200 UTC

PART 1: WARNING SUMMARY: Bulletins WTIO22 FMEE n°008/4 and WTIO30 FMEE n°8/4/20202021 issued at 06 UTC on Moderate Tropical Storm CHALANE. Next warnings will be issued at 12 UTC.

PART 2 : TROPICAL WEATHER DISCUSSION: The basin remains in a Monsoon Trough (MT) pattern, axed along 10°S. At the western edge, the tropical storm CHALANE is now evolving autonomously with respect to the MT. East of 70°E, in addition to the Area of Disturbed Weather which has been followed for several days but which is in the Indonesian zone, a new area of enhanced low levels vorticity is monitored within the MT southeast of the Chagos archipelago.

Moderate tropical storm CHALANE north of the Mascarene Islands: Position at 100 UTC: 16.1°S / 55.3°E Maximum wind over 10 minutes: 35 kt Estimated central pressure: 997 hPa Forward motion: West-Southwestwards at 8 kt

For more information, please refer to bulletins WTIO22 and WTIO30 issued at 06Z and followings.

Area of Disturbed Weather near the North-Eastern boarder of the basin : An area of vorticity is still present in the Indonesian area around 9S/94E according to the BOM's Tropical Cyclone Outlook for the Western Region (IDW10800). The latest ASCAT swaths show that the surface wind circulation is no longer closed. The minimum is under the influence of a strong easterly shear.

Over the next few days, this area is expected to move slowly towards the southwest and get closer to our area of responsibility. While the deep shear could decrease below the upper ridge thanks to the southward shift, the northerly mid level shear is expected to strengthen, limiting the benefit of the increased surface convergence on the polar side. Among the latest numerical models, only UKMO and some members of the American and European ensemble forecasts suggest a significant deepening in this unfavorable environment.

Over the next 5 days, the risk for the development or the entry of a moderate tropical storm in the north-eastern part of the basin becomes very low from Wednesday.

Southeast of the Chagos archipelago: The last scatterometric data show the presence of a wide and elongated circulation in the lower levels, associated with winds ranging up to 10 to 15kt. The apparent center of this ill-defined circulation is located near 9S/75E, in an also very sheared environment.

Over the next few days, the surface convergence on the equatorial and polar sides is expected to improve while at the same time the shear could decrease with a center below the upper ridge. This change could favor the development of a compact closed circulation or even a storm as suggested by some recent deterministic runs (UKMO and GFS0Z) and some members of the different ensemble forecasts. However, at the present time, this scenario still seems unlikely in view of the forecast range and dispersion of the forecast scenarios (no closed circulation in the latest European deterministic runs).

During the next 5 days, the risk for the development of a moderate tropical storm near the Chagos archipelago becomes very low from Wednesday.

NOTA BENE: The likelihood is an estimate of the chance of the genesis of a moderate tropical storm over the basin and 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.