MEDIUM - 迪戈加西亚东南热带扰动第13号(99S) - 11.6S 74.8E

大水台6

sh992026 INVEST 20260329 1200 -8.9 71.4 S DB 15 0

Ck.

99S

pocketbox

ABIO10 PGTW 291230
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/SIGNIFICANT TROPICAL WEATHER ADVISORY FOR THE INDIAN OCEAN
REISSUED/291230Z-291800ZMAR2026//
RMKS/
1. NORTH INDIAN OCEAN AREA (MALAY PENINSULA WEST TO COAST OF AFRICA):
   A. TROPICAL CYCLONE SUMMARY: NONE.
   B. TROPICAL DISTURBANCE SUMMARY: NONE.
   C. SUBTROPICAL SYSTEM SUMMARY: NONE.
2. SOUTH INDIAN OCEAN AREA (135E WEST TO COAST OF AFRICA):
   A. TROPICAL CYCLONE SUMMARY: NONE.
   B. TROPICAL DISTURBANCE SUMMARY:
      (1) AN AREA OF CONVECTION (INVEST 99S) HAS PERSISTED NEAR 9.2S
76.0E, APPROXIMATELY 237 NM SOUTHEAST OF DIEGO GARCIA. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A DISORGANIZED LLCC WITH
FLARING CONVECTION AND ELEVATED WINDS BEGINNING TO WRAP ALONG THE
NORTHEASTERN PERIPHERY INTO THE SYSTEM. 99S IS CURRENTLY IN A MARGINALLY
FAVORABLE ENVIRONMENT FOR FURTHER DEVELOPMENT WITH VERY WARM (29-30 C)
SEA SURFACE TEMPERATURES, MODERATE POLEWARD OUTFLOW ALOFT, WHILE ALSO
BEING PLACED WITHIN A POCKET OF LIGHTER VERTICAL WIND SHEAR (10-15
KNOTS) OPPOSED TO THE SURROUNDING AREA (25-30 KNOTS). A FACTOR THAT CAN
NEGATIVELY AFFECT THE SYSTEM IS A LARGE DRY AIR MASS TO THE WEST THAT
WILL BE IN THE DIRECT PATH OF 99S AS IT CONTINUES TO DEVELOP. GLOBAL
DETERMINISTIC AND ENSEMBLE MODELS ARE IN GOOD AGREEMENT ON A
SOUTHWESTWARD TRANSIT AND SLIGHT INTENSIFICATION OVER THE NEXT 24-48
HOURS.  MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 18 TO 23 KNOTS.
MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1008 MB. THE
POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN
THE NEXT 24 HOURS IS LOW.
      (2) NO OTHER SUSPECT AREAS.
   C. SUBTROPICAL SYSTEM SUMMARY: NONE.
3. JUSTIFICATION FOR REISSUE: ADDED LOW AREA IN PARA 2.B.(1).//
NNNN

Ragasa

AWIO20 FMEE 291148
TROPICAL CYCLONE CENTER / RSMC LA REUNION / METEO-FRANCE

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

DATE: 2026/03/29 A 1200 UTC

PART 1: WARNING SUMMARY

NIL.

PART 2 : TROPICAL WEATHER DISCUSSION:

The basin features a monsoon trough (MT) that extends from 65oE to 88oE along 09oS. Convective activity is moderate north of the MT and weak in the northern part of the Mozambique Channel.

Despite a low-frequency environment causing easterly winds anomalies (positive IOD) over the central basin, an equatorial Rossby wave brings a surge of westerly winds that enhances equatorial convergence in the MT and, consequently, vorticity.

Over the central part of the basin :

Early next week, along the MT near 75E, the low-pressure circulation visible in the MSG satellite imagery could deepen in a favorable environment characterized by strong low-level convergence on both the equatorial and polar sides, as well as relatively weak upper-level wind shear. The ASCAT swath, which is outside the area, does not allow us to discern the hint of low-pressure circulation.
Ensemble models and AI scenarios do suggest the formation of a moderate tropical storm in most of their members by next Friday. The earliest deterministic models, such as the European model, show a tropical storm forming as early as Tuesday, while others reach this stage on Thursday the 2nd.

The likelihood of the formation of a moderate tropical storm becomes low from Tuesday, March the 31st, then moderate from Wednesday the 1st of April, south of Diego-Garcia.



10-day outlook:
10-day outlook : The Rossby wave moving westward across the basin will be supported by a moist MJO wave surging from the west of the basin by the end of next week.

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.

Ragasa

AWIO20 FMEE 301208
TROPICAL CYCLONE CENTER / RSMC LA REUNION / METEO-FRANCE

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

DATE: 2026/03/30 A 1200 UTC

PART 1: WARNING SUMMARY

NIL.

PART 2 : TROPICAL WEATHER DISCUSSION:

The basin exhibits a Near Equatorial Trough pattern, gradually transitioning to a Monsoon Trough (MT) structure in its central region. Convective activity is moderate to severe around this low-pressure area, particularly near a Zone of Disturbed Weather east of Diego Garcia.

Despite a low-frequency background that produces easterly anomalies (positive IOD) over the central basin, the TM is notably strengthened by an Equatorial Rossby wave that boosts equatorial westerlies and vorticity. The larger-scale context is also becoming more favorable.

Are of Disturbed Weather, over the central part of the basin :

The latest satellite imagery and scatterometric observationsparticularly this morning's ASCAT passessuggest the presence of a large low-pressure system near 8.5S 76E this Monday. Maximum winds do not exceed 25 knots.

Over the next few days, benefiting from good surface convergence on both the equatorial and polar sides, as well as low shear aloft, this area of disturbed weather is expected to gradually deepen. Most forecasts suggest the formation of a tropical storm by Friday. The EPS and some IA ensemble models indicate a significant risk for Thursday or even Wednesday. The system is expected to take a generally southerly track, which should keep it away from inhabited lands.

The likelihood of the formation of a moderate tropical storm becomes moderate on Wednesday the 1st of April, and high from Thursday 2nd of April onwards, south of Diego-Garcia.



10-day outlook:
Nothing to report

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.

Ragasa

AWIO20 FMEE 311249
TROPICAL CYCLONE CENTER / RSMC LA REUNION / METEO-FRANCE

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

DATE: 2026/03/31 A 1200 UTC

PART 1: WARNING SUMMARY

NIL.

PART 2 : TROPICAL WEATHER DISCUSSION:

The basin exhibits a  Monsoon Trough (MT) structure near 8oS and between 65oE and 85oE. Convective activity is moderate within the MT, particularly near a Zone of Disturbed Weather southeast of Diego Garcia.

Despite a low-frequency background that produces easterly anomalies (positive Indian Ocean Dipole) over the central basin, the TM is notably strengthened by an Equatorial Rossby wave that boosts equatorial westerlies and vorticity. The larger-scale context is also becoming more favorable.

Are of Disturbed Weather, over the central part of the basin :

The latest satellite imagery shows a large area of disturbed weather approximately 400 km southeast of Diego Garcia, with a low-level circulation center located near 08S/76E. The low-level circulation appears to have become more organized over the past 24 hours, although convection has become slightly less organized. However, in the absence of nearby ASCAT data, it is difficult to estimate maximum wind speeds.

Over the next few days, benefiting from good surface convergence on both the equatorial and polar sides, as well as low shear aloft, this area of disturbed weather is expected to gradually deepen. Most forecasts suggest the formation of a tropical storm by Thursday or Friday at the latest. The system is expected to take a generally southerly track, which should keep it away from inhabited lands.

The likelihood of the formation of a moderate tropical storm becomes high from Thursday 2nd of April onwards, south of Diego-Garcia.



10-day outlook:
Nothing to report

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.

Ragasa

ABIO10 PGTW 311800
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/SIGNIFICANT TROPICAL WEATHER ADVISORY FOR THE INDIAN
OCEAN/311800ZMAR2026-011800ZAPR2026//
RMKS/
1. NORTH INDIAN OCEAN AREA (MALAY PENINSULA WEST TO COAST OF AFRICA):
   A. TROPICAL CYCLONE SUMMARY: NONE.
   B. TROPICAL DISTURBANCE SUMMARY: NONE.
   C. SUBTROPICAL SYSTEM SUMMARY: NONE.
2. SOUTH INDIAN OCEAN AREA (135E WEST TO COAST OF AFRICA):
   A. TROPICAL CYCLONE SUMMARY: NONE.
   B. TROPICAL DISTURBANCE SUMMARY:
      (1) THE AREA OF CONVECTION (INVEST 99S) PREVIOUSLY LOCATED NEAR
9.6S 76.1E IS NOW LOCATED NEAR 8.6S 75.9E, APPROXIMATELY 213 NM EAST-
SOUTHEAST OF DIEGO GARCIA. ANIMATED ENHANCED INFRARED IMAGERY DEPICTS A
DEVELOPING LOW LEVEL CIRCULATION CENTER (LLCC) PARTIALLY OBSCURED BY
FLARING CONVECTION. A 311629Z METOP-B ASCAT PASS REVEALED 15-20 KNOT
WINDS WITHIN THE EASTERN SEMICIRCLE OF THE SYSTEM. ENVIRONMENTAL
ANALYSIS REVEALS FAVORABLE CONDITIONS FOR DEVELOPMENT WITH LOW VERTICAL
WIND SHEAR (10-15 KTS), WARM SEA SURFACE TEMPERATURES (29 TO 30 C), AND
MODERATE POLEWARD OUTFLOW. DETERMINISTIC AND ENSEMBLE MODELS ARE IN GOOD
AGREEMENT OF 99S FURTHER DEVELOPING AND TAKING A SOUTH-SOUTHWESTWARD
TRACK OVER THE NEXT 48 HOURS. MAXIMUM SUSTAINED SURFACE WINDS ARE
ESTIMATED AT 18 TO 23 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO
BE NEAR 1005 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT
TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS IS UPGRADED TO MEDIUM.
      (2) NO OTHER SUSPECT AREAS.
   C. SUBTROPICAL SYSTEM SUMMARY: NONE.//
NNNN

pocketbox

WTIO30 FMEE 011349
RSMC / TROPICAL CYCLONE CENTER / LA REUNION
TROPICAL CYCLONE FORECAST WARNING (SOUTH-WEST INDIAN OCEAN)

0.A WARNING NUMBER: 1/13/20252026
1.A TROPICAL DISTURBANCE 13

2.A POSITION 2026/04/01 AT 1200 UTC:
WITHIN 30 NM RADIUS OF POINT 11.0 S / 74.1 E
(ELEVEN    DECIMAL ZERO   DEGREES SOUTH AND
SEVENTY FOUR    DECIMAL ONE   DEGREES EAST)
MOVEMENT: SOUTH-WEST 6 KT

3.A DVORAK ANALYSIS: 2.0/2.0/D 0.5/12 H

4.A CENTRAL PRESSURE: 1003 HPA
5.A MAX AVERAGE WIND SPEED (10 MN): 25 KT
RADIUS OF MAXIMUM WINDS (RMW): NIL

6.A EXTENSION OF WIND BY QUADRANTS (KM):
NIL

7.A FIRST CLOSED ISOBAR (PRESSURE / AVERAGE DIAM): 1009 HPA / 600 KM
8.A VERTICAL EXTENSION OF CYCLONE CIRCULATION: MEDIUM

1.B FORECASTS (WINDS RADII IN KM):
12H: 2026/04/02 00 UTC: 11.5 S / 73.5 E, VENT MAX= 035 KT, MODERATE
TROPICAL STORM
28 KT NE: 140 SE: 165 SW: 220 NW: 95
34 KT NE: 65 SE: 110 SW: 140 NW: 65

24H: 2026/04/02 12 UTC: 12.6 S / 73.0 E, VENT MAX= 050 KT, SEVERE
TROPICAL STORM
28 KT NE: 140 SE: 195 SW: 220 NW: 100
34 KT NE: 85 SE: 120 SW: 140 NW: 65
48 KT NE: 65 SE: 45 SW: 35 NW: 35

36H: 2026/04/03 00 UTC: 14.4 S / 72.1 E, VENT MAX= 060 KT, SEVERE
TROPICAL STORM
28 KT NE: 150 SE: 215 SW: 220 NW: 110
34 KT NE: 85 SE: 130 SW: 140 NW: 75
48 KT NE: 65 SE: 45 SW: 45 NW: 45

48H: 2026/04/03 12 UTC: 16.5 S / 70.8 E, VENT MAX= 075 KT, TROPICAL
CYCLONE
28 KT NE: 155 SE: 230 SW: 220 NW: 120
34 KT NE: 95 SE: 130 SW: 140 NW: 75
48 KT NE: 65 SE: 55 SW: 45 NW: 45
64 KT NE: 35 SE: 35 SW: 35 NW: 35

60H: 2026/04/04 00 UTC: 19.1 S / 69.7 E, VENT MAX= 085 KT, TROPICAL
CYCLONE
28 KT NE: 165 SE: 260 SW: 220 NW: 130
34 KT NE: 95 SE: 140 SW: 140 NW: 75
48 KT NE: 65 SE: 55 SW: 55 NW: 45
64 KT NE: 35 SE: 35 SW: 35 NW: 35

72H: 2026/04/04 12 UTC: 22.1 S / 69.3 E, VENT MAX= 085 KT, TROPICAL
CYCLONE
28 KT NE: 175 SE: 280 SW: 220 NW: 140
34 KT NE: 100 SE: 150 SW: 140 NW: 85
48 KT NE: 65 SE: 55 SW: 55 NW: 55
64 KT NE: 45 SE: 45 SW: 45 NW: 45

2.B LONGER-RANGE OUTLOOK:
96H: 2026/04/05 12 UTC: 29.4 S / 72.4 E, VENT MAX= 060 KT, SEVERE
TROPICAL STORM
28 KT NE: 195 SE: 325 SW: 220 NW: 150
34 KT NE: 110 SE: 155 SW: 140 NW: 95
48 KT NE: 65 SE: 65 SW: 65 NW: 55

2.C ADDITIONAL INFORMATION:
T=2.0 CI=2.0

A LOW-PRESSURE AREA FORMED EARLIER THIS WEEK SOUTHEAT OF THE CHAGOS
AND IS CURRENTLY LOCATED APPROXIMATIVELY 430 KM SOUTH-SOUTHEAST OF
DIEGO GARCIA. OVER THE PAST 24 HOURS, CONVECTION HAS BECOME MORE
ORGANIZED AND PERSISTENT NEAR THE CENTER OF THE LOW-PRESSURE SYSTEM.
AN INITIAL DVORAK CLASSIFICATION OF 1.5 CAN THUS BE ESTABLISHED FROM
THIS WEDNESDAY APRIL 1 AT 00UTC.

THE ASCAT SWATHS FROM 1701Z YESTERDAY AND 0333Z THIS MORNING ALLOWED
US TO TRACK THE SYSTEM AND INDICATE THAT THE LOW-LEVEL CIRCULATION IS
BECOMING MORE SYMMETRICAL, WITH MAXIMUM WINDS OF 25 KT. HOWEVER,
CONVECTION SUBSIDED DURING THE DAY DUE TO THE DIURNAL CYCLE.
INTENSITY IS THEREFORE ESTIMATED AT 25 KT BASED ON THE LATEST
AVAILABLE ASCAT SWATH, WITH A SUBJECTIVE DVORAK ESTIMATE OF 2.0.

IN TERMS OF TRACK, THE SYSTEM IS STEERED BY A LOW-TO-MID-LEVEL RIDGE
LOCATED TO THE SOUTHEAST OF THE SYSTEM, FORCING IT ONTO A
SOUTH-TO-SOUTHWEST TRACK OVER THE NEXT 48 TO 72 HOURS. IN THE LONGER
TERM, THE SYSTEM WILL GRADUALLY ACCELERATE SOUTHWARD THEN
SOUTHEASTWARD UNDER THE GROWING INFLUENCE OF A DEEP UPPER-LEVEL
TROUGH TO THE WEST OF THE SYSTEM. UNCERTAINTY REGARDING THE TRACK IS
LOW, EVEN THOUGH THE U.S. GFS MODEL SUGGESTS A SLOWER MOVEMENT IN THE
EARLY STAGES DUE TO A RIDGE LOCATED FURTHER EAST THAN IN OTHER
AVAILABLE FORECASTS.

IN TERMS OF INTENSITY, THE SYSTEM IS CURRENTLY BENEFITING FROM GOOD
LOW-LEVEL CONVERGENCE AND IS MOVING OVER WATERS WITH HIGH OCEANIC
POTENTIAL (SEA SURFACE TEMPERATURE NEAR 29AOC). DEEP-LEVEL SHEAR IS
EXPECTED TO REMAIN WEAK OVER THE NEXT 3 DAYS, WHILE ALTITUDE
DIVERGENCE IS EXPECTED TO INCREASE. ENVIRONMENTAL CONDITIONS ARE
THEREFORE FAVORABLE FOR THE SYSTEM TO INTENSIFY, WITH A PEAK
INTENSITY EXPECTED ON SATURDAY, APRIL 4. IN THE LONGER TERM, STRONG
NORTHWESTERLY SHEAR IS EXPECTED TO SET IN AND RAPIDLY WEAKEN THE
SYSTEM.

NO SIGNIFICANT IMPACT IS EXPECTED ON INHABITED LAND OVER THE NEXT 72
HOURS.