WTNT45 KNHC 140837 RRA

TCDAT5



TROPICAL STORM IAN DISCUSSION NUMBER   8

NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102016

500 AM AST WED SEP 14 2016



THE CLOUD PATTERN OF IAN HAS CHANGED LITTLE DURING THE PAST SEVERAL

HOURS.  THE LOW-LEVEL CENTER OF THE TROPICAL STORM IS COMPLETELY

EXPOSED AND LOCATED MORE THAN 100 N MI SOUTH OF THE MAIN AREA OF

DEEP CONVECTION.  THIS POOR STRUCTURE IS THE RESULT OF ABOUT 30 KT

OF SOUTH-SOUTHWESTERLY VERTICAL WIND SHEAR.  THE INITIAL INTENSITY

IS HELD AT 45 KT, IN AGREEMENT WITH THE EARLIER SCATTEROMETER DATA

AND A DVORAK CI-NUMBER FROM TAFB.



IAN IS MOVING NORTHWARD AT ABOUT 13 KT, AND IS BEING STEERED

BY THE SOUTHERLY FLOW BETWEEN A MID- TO UPPER-LEVEL LOW TO ITS WEST

AND A RIDGE TO ITS EAST.  A NORTHWARD TO NORTH-NORTHEASTWARD MOTION

AT ABOUT THE SAME FORWARD SPEED IS FORECAST DURING THE NEXT 24 HOURS

AS THE STEERING PATTERN MORE OR LESS PERSISTS.  AFTER THAT TIME, A

SHORTWAVE TROUGH IS EXPECTED TO MOVE EAST OF ATLANTIC CANADA, AND

THAT SHOULD CAUSE IAN TO TURN NORTHEASTWARD AND SHARPLY ACCELERATE

UNTIL THE CYCLONE BECOMES ABSORBED BY A LARGE EXTRATROPICAL LOW IN 3

TO 4 DAYS. THE MODELS ARE IN GOOD AGREEMENT, AND THE OFFICIAL

FORECAST LIES CLOSEST TO A CONSENSUS OF THE GFS AND ECMWF MODELS.



THE STRONG SHEAR THAT HAS BEEN AFFECTING IAN SINCE GENESIS IS

EXPECTED TO LET UP SOME DURING THE NEXT 24 TO 36 HOURS.  THE

PREDICTED LOWER SHEAR, FAIRLY WARM WATER, AND BAROCLINIC FORCING

SHOULD ALLOW IAN TO STRENGTHEN A LITTLE DURING THE NEXT DAY OR TWO.

BEYOND A COUPLE OF DAYS, IAN IS FORECAST TO MOVE OVER SHARPLY

COOLER SEA SURFACE TEMPERATURES AND INTO AN ENVIRONMENT OF STRONG=