TROPICAL CYCLONE 21S (HELLEN), LOCATED APPROXIMATELY 240 NM NORTH-
NORTHWEST OF ANTANANARIVO, MADAGASCAR, HAS TRACKED SOUTHEASTWARD AT
05 KNOTS OVER THE PAST SIX HOURS. THE CURRENT POSITION IS BASED ON
RECENT SATELLITE FIXES FROM PGTW AND KNES. THE INTENSITY OF 125 KNOTS
IS CONSISTENT WITH DVORAK CURRENT INTENSITY ESTIMATES FROM THE SAME
REPORTING AGENCIES. TC 21S HAS BEGUN TO WEAKEN OVER THE PAST SIX
HOURS FOLLOWING A 24-HOUR PERIOD OF EXPLOSIVE DEEPENING. INTERACTION
WITH LAND AND ENTRAINMENT OF DRIER ENVIRONMENTAL AIR INTO THE
CIRCULATION CENTER APPEAR TO BE THE MAIN FACTORS DRIVING THE RECENT
WEAKENING TREND. TC 21S CONTINUES TO TRACK SOUTHEASTWARD ALONG THE
WESTERN PERIPHERY OF A SUBTROPICAL STEERING RIDGE TO THE EAST. SLOW
SOUTHEASTWARD MOTION IS EXPECTED TO PERSIST IN THE NEAR-TERM, WITH
THE CENTER OF TC 21S MAKING LANDFALL JUST WEST OF MAHAJANGA IN ABOUT
12 HOURS. THEREAFTER, THE SYSTEM IS EXPECTED TO REMAIN INLAND AND
TRACK OVER FAIRLY ROUGH TERRAIN FOR ABOUT 30 HOURS. CONSEQUENTLY,
POST-LANDFALL FORECAST INTENSITIES HAVE BEEN REDUCED SIGNIFICANTLY
FROM THE PREVIOUS WARNING. SHOULD THE CYCLONE TRACK EVEN FURTHER
INLAND THAN CURRENTLY FORECAST, IT MAY DISSIPATE BELOW WARNING
THRESHOLD CRITERIA (35 KNOTS) OVER NORTHWESTERN MADAGASCAR.
ASSUMING THE CYCLONE REMAINS INTACT, IT IS EXPECTED TO TURN WESTWARD
UNDER THE INFLUENCE OF A LOW T0 MID-LEVEL SUBTROPICAL RIDGE BUILDING
TO THE SOUTH AND TRACK WESTWARD OVER THE MOZAMBIQUE CHANNEL BY TAU
36. LITTLE INTENSITY CHANGE IS ANTICIPATED OVER THE CHANNEL AS
MARGINAL UPPER LEVEL CONDITIONS, PASSAGE OVER WARM WATER, AND DRY
AIR ENTRAINMENT IMPACT THE SYSTEM. NUMERICAL MODEL TRACK FORECAST
GUIDANCE SUPPORTS SLOW TRACK SPEEDS AND A GENERAL TURN TOWARD THE
WEST OVER THE NEXT 24 HOURS, FOLLOWED BY A WESTWARD TRACK ACROSS THE
MOZAMBIQUE CHANNEL IF THE CIRCULATION HOLDS TOGETHER. THE CURRENT
FORECAST TRACK LIES CLOSE TO THE MULTI-MODEL CONSENSUS, BUT
CONFIDENCE IS LOW DUE TO POTENTIAL DISSIPATION OVER LAND IN THE NEAR-
TERM. MAXIMUM SIGNIFICANT WAVE HEIGHT AT 310000Z IS 37 FEET.
- JTWC
