WTNT45 KNHC 140252 RRA

TCDAT5



TROPICAL STORM IAN DISCUSSION NUMBER   7

NWS NATIONAL HURRICANE CENTER MIAMI FL       AL102016

1100 PM AST TUE SEP 13 2016



IAN'S CLOUD PATTERN HAS CHANGED LITTLE IN ORGANIZATION SINCE THIS

AFTERNOON.  THE LOW-LEVEL CENTER REMAINS PATRIALLY EXPOSED TO THE

SOUTH OF A FAN-SHAPED MASS OF DEEP CONVECTION.  THIS STRUCTURE IS

CONSISTENT WITH A DEEP-LAYER SOUTHWESTERLY VERTICAL SHEAR OF NEARLY

40 KT ASSOCIATED WITH A MID- TO UPPER-TROPOSPHERIC CYCLONE TO THE

WEST OF THE CYCLONE. A 0018Z ASCAT PASS SHOWED RELIABLE SURFACE

WINDS NEAR 45 KT WELL NORTH OF THE CENTER, WHICH AGREES WITH A TAFB

SATELLITE CLASSIFCIATION OF T3.0.  ON THIS BASIS, THE INITIAL

INTENSITY IS INCREASED TO 45 KT.



THE INITIAL MOTION ESTIMATE HAS BEEN NUDGED A LITTLE MORE TO THE

RIGHT THAN EARLIER TODAY AND IS NOW 345/11 KT.  IAN SHOULD CONTINUE

TO MOVE GENERALLY NORTHWARD INTO A WEAKNESS IN THE SUBTROPICAL RIDGE

OVER THE CENTRAL ATLANTIC DURING THE NEXT 24 HOURS OR SO.  A

SIGNIFICANT ACCELERATION TOWARD THE NORTHEAST SHOULD OCCUR IN ABOUT

36 HOURS WHEN A SHORTWAVE TROUGH MOVING THROUGH ATLANTIC CANADA

APPROACHES IAN.  THE NHC TRACK FORECAST IS ADJUSTED TO THE RIGHT OF

THE PREVIOUS ONE, ESPECIALLY AFTER 24 HOURS, AND IS IN AGREEMENT

WITH THE THE LATEST TRACK CONSENSUS AIDS.



THE SHIPS OUTPUT INDICATES THAT THE SHEAR SHOULD DECREASE,

BEGINNING IN ABOUT 12 HOURS, ROUGHLY AROUND THE TIME THAT IAN

BECOMES CO-LOCATED WITH THE UPPER LOW TO ITS WEST.  THE SHEAR IS

FORECAST TO REACH A RELATIVE MINIMUM 24 HOURS LATER WHILE THE FLOW

ALOFT GRADUALLY BECOMES MORE DIFFLUENT.  THIS BAROCLINIC FORCING IS

EXPECTED TO RESULT IN SOME INTENSIFICATION WHILE IAN REMAINS OVER=