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Tropical Storm IAN Forecast Discussion Number 4
2016-09-13 10:42:43| National Hurricane Center (Atlantic)
Issued at 500 AM AST TUE SEP 13 2016 000 WTNT45 KNHC 130842 TCDAT5 TROPICAL STORM IAN DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102016 500 AM AST TUE SEP 13 2016 Ian is a strongly sheared tropical storm. The low-level center of the cyclone is completely exposed with the main area of deep convection located about 120 n mi northeast of the center. This asymmetric cloud pattern is the result of about 30 kt of southwesterly vertical wind shear. The initial intensity is held at 40 kt for this advisory, in agreement with the earlier ASCAT data. This estimate is a little higher than the latest Dvorak classifications. Satellite fixes indicate that Ian has slowed down during the last several hours. The initial motion estimate, using a 12-h average, is 335/8 kt. The tropical cyclone is situated between a mid- to upper-level low to its west and a mid-level high to its east. This pattern is expected to persist for the next couple of days, and should cause Ian to move north-northwestward to northward with a slight increase in forward speed. After that time, a shortwave trough is expected to approach the tropical storm, and it will likely cause Ian to turn northeastward and accelerate as it becomes embedded in the mid-latitude flow. The models are in fair agreement in this overall scenario, and only small changes were made to the previous NHC track forecast. This prediction is in best agreement with the various consensus aids. The strong shear currently affecting Ian is expected to persist for another 24 hours, so little change in strength is predicted during that time. The shear is forecast to lessen some in a couple of days while Ian is still over warm water. These environmental conditions, combined with some baroclinic forcing, could allow for slight strengthening in the 36- to 72-h time period. After that time, Ian is expected to move back into a strong shear environment and over progressively colder water, which should lead to extratropical transition in about 4 days. The NHC intensity forecast is largely the same as the previous one, and is in best agreement with the SHIPS guidance during its tropical phase and the global models during the extratropical portion. FORECAST POSITIONS AND MAX WINDS INIT 13/0900Z 24.0N 51.7W 40 KT 45 MPH 12H 13/1800Z 25.4N 52.5W 40 KT 45 MPH 24H 14/0600Z 27.8N 53.4W 40 KT 45 MPH 36H 14/1800Z 30.4N 53.8W 45 KT 50 MPH 48H 15/0600Z 33.2N 53.1W 50 KT 60 MPH 72H 16/0600Z 40.7N 46.1W 50 KT 60 MPH 96H 17/0600Z 50.7N 32.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 18/0600Z 58.5N 18.5W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Cangialosi
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