WTIO30 FMEE 171849
RSMC / TROPICAL CYCLONE CENTER / LA REUNION
TROPICAL CYCLONE FORECAST WARNING (SOUTH-WEST INDIAN OCEAN)

0.A WARNING NUMBER: 7/11/20202021
1.A MODERATE TROPICAL STORM 11 (GUAMBE)

2.A POSITION 2021/02/17 AT 1800 UTC:
WITHIN 20 NM RADIUS OF POINT 22.2 S / 37.5 E
(TWENTY TWO DECIMAL TWO DEGREES SOUTH AND
THIRTY SEVEN DECIMAL FIVE DEGREES EAST)
MOVEMENT: SOUTH 6 KT

3.A DVORAK ANALYSIS: 2.5/3.0/D 0.5/24 H

4.A CENTRAL PRESSURE: 988 HPA
5.A MAX AVERAGE WIND SPEED (10 MN): 45 KT
RADIUS OF MAXIMUM WINDS (RMW): NIL

6.A EXTENSION OF WIND BY QUADRANTS (KM):
28 KT NE: 240 SE: 205 SW: 110 NW: 260
34 KT NE: 205 SE: 0 SW: 0 NW: 240

7.A FIRST CLOSED ISOBAR (PRESSURE / AVERAGE DIAM): 1004 HPA / 800 KM
8.A VERTICAL EXTENSION OF CYCLONE CIRCULATION: DEEP

1.B FORECASTS (WINDS RADII IN KM):
12H: 2021/02/18 06 UTC: 22.8 S / 37.3 E, VENT MAX= 045 KT, MODERATE
TROPICAL STORM
28 KT NE: 280 SE: 250 SW: 100 NW: 150
34 KT NE: 195 SE: 150 SW: 85 NW: 140

24H: 2021/02/18 18 UTC: 23.4 S / 37.0 E, VENT MAX= 050 KT, SEVERE
TROPICAL STORM
28 KT NE: 295 SE: 295 SW: 110 NW: 120
34 KT NE: 205 SE: 175 SW: 85 NW: 110
48 KT NE: 95 SE: 55 SW: 65 NW: 35

36H: 2021/02/19 06 UTC: 24.3 S / 36.4 E, VENT MAX= 060 KT, SEVERE
TROPICAL STORM
28 KT NE: 335 SE: 335 SW: 305 NW: 120
34 KT NE: 220 SE: 205 SW: 185 NW: 110
48 KT NE: 100 SE: 55 SW: 75 NW: 65

48H: 2021/02/19 18 UTC: 24.8 S / 35.8 E, VENT MAX= 070 KT, TROPICAL
CYCLONE
28 KT NE: 295 SE: 325 SW: 250 NW: 130
34 KT NE: 185 SE: 205 SW: 185 NW: 120
48 KT NE: 100 SE: 85 SW: 95 NW: 85
64 KT NE: 65 SE: 45 SW: 35 NW: 55

60H: 2021/02/20 06 UTC: 25.4 S / 35.5 E, VENT MAX= 080 KT, TROPICAL
CYCLONE
28 KT NE: 305 SE: 350 SW: 215 NW: 130
34 KT NE: 195 SE: 215 SW: 150 NW: 120
48 KT NE: 110 SE: 55 SW: 65 NW: 95
64 KT NE: 55 SE: 35 SW: 30 NW: 65

72H: 2021/02/20 18 UTC: 26.9 S / 35.7 E, VENT MAX= 090 KT, INTENSE
TROPICAL CYCLONE
28 KT NE: 325 SE: 305 SW: 215 NW: 195
34 KT NE: 195 SE: 195 SW: 165 NW: 140
48 KT NE: 110 SE: 85 SW: 95 NW: 95
64 KT NE: 65 SE: 45 SW: 35 NW: 65

2.B LONGER-RANGE OUTLOOK:
96H: 2021/02/21 18 UTC: 30.6 S / 39.5 E, VENT MAX= 075 KT,
POST-TROPICAL DEPRESSION
28 KT NE: 435 SE: 445 SW: 360 NW: 350
34 KT NE: 230 SE: 195 SW: 270 NW: 230
48 KT NE: 120 SE: 95 SW: 95 NW: 110
64 KT NE: 95 SE: 75 SW: 65 NW: 75

120H: 2021/02/22 18 UTC: 33.5 S / 49.5 E, VENT MAX= 055 KT,
POST-TROPICAL DEPRESSION
28 KT NE: 490 SE: 545 SW: 630 NW: 445
34 KT NE: 260 SE: 260 SW: 350 NW: 280
48 KT NE: 120 SE: 80 SW: 90 NW: 120

2.C ADDITIONAL INFORMATION:
T=2.5 CI=3.0-.

THE VERY DEEP CONVECTION LOCATED IN THE NORTH OF THE CENTER HAS SHOWN
LARGE FLUCTUATIONS SINCE THE PREVIOUS ADVISORY. AFTER A PHASE OF
MARKED WEAKENING, A NEW IMPORTANT BURST IS TAKING PLACE. DESPITE
THESE FLUCTUATIONS, THE LATEST SMAP PASS (1535Z) CONTINUES TO SHOW
STRONG WINDS REACHING 50 KT IN THE NORTHERN SEMICIRCLE. SINCE THESE
WINDS CAN BE POTENTIALLY CONTAMINATED BY HEAVY RAINFALL, THE
ESTIMATED INITIAL INTENSITY IS A COMPROMISE BETWEEN THE DVORAK
ESTIMATES (40 KT) AND THE SMAP DATA.

THE SYSTEM REMAINS UNDER THE INFLUENCE OF OPPOSITE DIRECTIONAL FLOWS:
ON THE NORTHEAST FACE OF THE SUBTROPICAL GEOPOTENTIAL HIGHS, THE
SYSTEM IS EXPOSED TO A SOUTHEASTERLY FLOW, WHILE TO THE NORTHEAST, A
NEAR EQUATORIAL RIDGE IMPARTS A NORTHWESTERLY FLOW. THE SUBTROPICAL
RIDGE, EXPECTED TO RECONSTITUTE OVER MADAGASCAR, ORIENTING THE
TRAJECTORY MORE SOUTHWARD ON ITS WESTERN SIDE, SEEMS TO HAVE STARTED
TO TAKE OVER THE STEERING FLOW. ON THE LAST RUNS, THE SET OF
NUMERICAL GUIDANCES TEND TO REINFORCE THE SOUTHERN PART OF THIS
SUBTROPICAL RIDGE DURING THE NEXT 48 HOURS, WHICH RESULTED IN
SHIFTING QUITE SIGNIFICANTLY THE FORECAST TRACK TOWARDS THE WEST. THE
OFFICIAL FORECAST FOLLOWS THIS TREND. WITH THE PERSISTENCE OF A RIDGE
OVER SOUTHERN AFRICA, THIS SHIFT SHOULD REMAIN RATHER SLOW. AT THE
END OF THE WEEK, THE PRESENCE OF A DEEP MID-LATITUDE TROUGH SOUTH OF
THE CHANNEL SHOULD FAVOR AN ACCELERATION OF THE SOUTHEASTWARD SHIFT.
THIS TRAJECTORY IS A COMPROMISE BETWEEN THE MAIN AMERICAN AND FRENCH
NUMERICAL GUIDANCE IN AGREEMENT WITH SEVERAL MEMBERS OF THE EUROPEAN
ENSEMBLE FORECAST.

THE SYSTEM IS UNDER THE EFFECT OF A SOUTHEASTERLY SHEAR NORTH OF THE
UPPER LEVELS RIDGE. THIS SHEAR IS EXPECTED TO FADE IN THE NEXT 24
HOURS. THE ENVIRONMENT IS MOIST AT ALL LEVELS. THE CONVERGENCE OF LOW
LAYERS, ACCENTUATED ON THE POLAR COAST BY A SOUTHERLY BURST STILL
PRESENT TODAY, IS NOT OPTIMAL ON THE NORTHEAST SIDE OF THE SYSTEM,
THE FASTEST MONSOON FLOW REMAINING QUITE DISTANT FOR THE MOMENT,
ATTRACTED BY THE ZONE OF LOW PRESSURE STRETCHING SOUTHWARD FROM
MADAGASCAR, BUT IN THE PROCESS OF FILLING IN. GRADUALLY, A MORE
PRONOUNCED INTENSIFICATION OF THE SYSTEM IS LIKELY. THESE FAVORABLE
CONDITIONS, COMBINED WITH WARM WATERS, SHOULD ALLOW GUAMBE TO REACH
THE STAGE OF A TROPICAL CYCLONE OR EVEN AN INTENSE TROPICAL CYCLONE.
HOWEVER, AT THE END OF THE PERIOD, ON THE EDGE OF THE MID-LATITUDE
TROUGH, THE NORTHWESTERN SECTOR SHEAR COULD GRADUALLY STRENGTHEN AND
ADVECT DRY AIR INTO THE INNER CORE OF THE SYSTEM, LEADING TO ITS
WEAKENING, ALTHOUGH THIS IS MITIGATED BY THE HIGH SPEED OF MOVEMENT
OF THE SYSTEM IN THE SHEAR DIRECTION.

THE PRESENT FORECAST IS MUCH CLOSER OF THE MOZAMBICAN COAST THAN THE
PREVIOUS ONE. GUAMBE IS MORE AND MORE LIKELY TO AFFECT SIGNIFICANTLY
THE METEOROLOGICAL CONDITIONS IN SOUTHERN MOZAMBIQUE, ESPECIALLY ON
FRIDAY AND SATURDAY, ESPECIALLY BETWEEN VILANKULO IN THE NORTH AND
XAI-XAI IN THE SOUTH: HEAVY RAINS THAT CAN REACH AND LOCALLY EXCEED
100 MM IN 24 HOURS ARE POSSIBLE ON THESE SECTORS. DEPENDING ON THE
FINAL TRAJECTORY OF GUAMBE, THEY COULD EXTEND FURTHER WEST TO MAPUTO.
ALTHOUGH THE STRONGEST WINDS ASSOCIATED WITH GUAMBE SHOULD BE MAINLY
IN THE EASTERN SEMICIRCLE OF THE SYSTEM, STRONG WINDS OF UP TO 100
KM/H GUSTING ARE POSSIBLE FROM THURSDAY EVENING OR FRIDAY MORNING.
THE RISK OF STORM SURGE REMAINS TO BE SPECIFIED. IT COULD BE
SIGNIFICANT IF THE TRAJECTORY OF THE SYSTEM WOULD THREATEN MORE
DIRECTLY INHAMBANE.
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