WTIO30 FMEE 291813
RSMC / TROPICAL CYCLONE CENTER / LA REUNION
TROPICAL CYCLONE FORECAST WARNING (SOUTH-WEST INDIAN OCEAN)

0.A WARNING NUMBER: 25/4/20202021
1.A SEVERE TROPICAL STORM 4 (CHALANE)

2.A POSITION 2020/12/29 AT 1800 UTC:
WITHIN 20 NM RADIUS OF POINT 19.7 S / 37.3 E
(NINETEEN DECIMAL SEVEN DEGREES SOUTH AND
THIRTY SEVEN DECIMAL THREE DEGREES EAST)
MOVEMENT: WEST 10 KT

3.A DVORAK ANALYSIS: 4.0/4.0/D 1.0/18 H

4.A CENTRAL PRESSURE: 986 HPA
5.A MAX AVERAGE WIND SPEED (10 MN): 55 KT
RADIUS OF MAXIMUM WINDS (RMW): 28 KM

6.A EXTENSION OF WIND BY QUADRANTS (KM):
28 KT NE: 165 SE: 165 SW: 130 NW: 110
34 KT NE: 85 SE: 120 SW: 95 NW: 75
48 KT NE: 55 SE: 55 SW: 60 NW: 35

7.A FIRST CLOSED ISOBAR (PRESSURE / AVERAGE DIAM): 1006 HPA / 400 KM
8.A VERTICAL EXTENSION OF CYCLONE CIRCULATION: DEEP

1.B FORECASTS (WINDS RADII IN KM):
12H: 2020/12/30 06 UTC: 19.7 S / 34.4 E, VENT MAX= 050 KT, SEVERE
TROPICAL STORM
28 KT NE: 215 SE: 205 SW: 240 NW: 150
34 KT NE: 120 SE: 120 SW: 110 NW: 120
48 KT NE: 65 SE: 55 SW: 75 NW: 55

24H: 2020/12/30 18 UTC: 19.9 S / 31.5 E, VENT MAX= 025 KT, OVERLAND
DEPRESSION

36H: 2020/12/31 06 UTC: 20.7 S / 28.3 E, VENT MAX= 025 KT, OVERLAND
DEPRESSION

48H: 2020/12/31 18 UTC: 21.7 S / 25.1 E, VENT MAX= 025 KT, OVERLAND
DEPRESSION

60H: 2021/01/01 06 UTC: 22.5 S / 22.8 E, VENT MAX= 025 KT, OVERLAND
DEPRESSION

72H: 2021/01/01 18 UTC: 22.9 S / 20.1 E, VENT MAX= 020 KT, OVERLAND
DEPRESSION

2.B LONGER-RANGE OUTLOOK:
NIL


2.C ADDITIONAL INFORMATION:

THE CLOUD PATTERN OF CHALANE HAS EVOLVED OVER THE LAST 6 HOURS, THE
DEEP CONVECTION PRESENTS A STRONG ASYMMETRY, ESPECIALLY PRESENT IN
THE NORTHERN SEMICIRCLE NOW. MOREOVER, A TILT ALSO EXISTS BETWEEN THE
LOW AND HIGH ALTITUDE CIRCULATION CENTERS, REFLECTING THE EFFECT OF A SOUTHEASTERLYCONSTRAINTE ON THE SYSTEM.NEVERTHELESS, THE SYSTEM
CONTINUES TO BENEFIT FROM A GOOD DIVERGENCE ON THE EQUATORIAL SIDE,
AND GOES ON A SLOW INTENSIFICATION.

CHALANE CONTINUES ITS RAPID WESTWARD MOVEMENT, ON THE NORTHERN FACE
OF THE SUBTROPICAL RIDGE. THE LANDING IS FORECAST LATER TONIGHT OR
EARLY TOMORROW MORNING NEAR THE NORTH SIDE OF THE CITY OF BEIRA.

LITTLE CHANGE IN TERMS OF INTENSITY FORECAST:
THE ENVIRONMENTAL CONDITIONS ARE RATHER FAVORABLE FOR THE DEVELOPMENT
OF THE SYSTEM: MODERATE SHEAR, FAVORABLE OCEANIC POTENTIAL, GOOD
FEEDING OF THE MONSOON FLOW WHICH GOES DOWN TO THE MIDDLE OF THE
CHANNEL. HOWEVER, WE NOTE THE PRESENCE OF A LOT OF DRY AIR IN THE
IMMEDIATE ENVIRONMENT OF THE SYSTEM WHICH COULD SLOW DOWN THE RATE OF INTENSIFICATION BY THE TIME OF THE ARRIVAL ON LAND, ESPECIALLY AS A
SMALL SOUTHEASTERN CONSTRAINT COULD ALSO BE IN PLACE, HOWEVER LIMITED
BY THE SPEED OF THE SYSTEM.

WITH THIS SCENARIO, THE THREAT REPRESENTED BY CHALANE IS VERY REAL
FOR THE BEIRA REGION:
- IN TERMS OF WINDS, DESTRUCTIVE GUSTS (EXCEEDING 150 KM/H) ARE
POSSIBLE LESS THAN 60 KM FROM THE CENTER. (CHALANE IS A RATHER
COMPACT SYSTEM).
- IN TERMS OF PRECIPITATION, ACCUMULATIONS OF MORE THAN 100 MM IN 24
HOURS ARE POSSIBLE. DUE TO THE SHEAR, THE RAINS WILL MAINLY CONCERN
THE REGIONS LOCATED ON THE RIGHT OF THE TRAJECTORY (NORTH OF THE
SYSTEM). THESE RAINS CAN CAUSE FLASH FLOODS AND FLOODS THAT WILL
SPREAD FAR INLAND OVER ZIMBABWE ON DECEMBER 31, BOTSWANA ON JANUARY
01, AND EVEN NAMIBIA ON JANUARY 02.
- A STORM SURGE OF ABOUT 1M TO 2.5M IS POSSIBLE AT THE LANDING SITE
AND SOUTH OF IT. COUPLED WITH WAVES THAT CAN EXCEED 6M, AND AT HIGH
TIDE (IN PHASE WITH THE TIMING OF THE LANDING ON THE CURRENT
FORECAST), THIS STORM SURGE WILL BE ASSOCIATED WITH COASTAL FLOODING
AND SUBMERSION.
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