000 WTNT42 KNHC 160833 TCDAT2 TROPICAL STORM KARL DISCUSSION NUMBER 8 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122016 500 AM AST FRI SEP 16 2016 Water vapor satellite imagery indicates that Karl is moving directly toward the base of a mid-/upper-level trough while being affected by at least 20 kt of westerly shear. A recent SSMIS microwave pass showed that the deep convection remains displaced to the northeast of the center. Since the structure of the cyclone has not changed since the last advisory, the initial intensity is held at 40 kt. The SSMIS data indicated that the center has moved a little north of previous position estimates, and the initial motion is now 280/11 kt. Karl may gain a little more latitude during the next 12-24 hours, but after that time a strong subtropical ridge is expected to force the cyclone to move south of due west or possibly west- southwestward between 36-48 hours. Once it reaches the western portion of the ridge and intensifies, Karl is likely to again gain some latitude, turning west-northwestward by days 4 and 5. The track models all agree on this scenario and keep Karl well to the east and northeast of the Leeward Islands during the five-day forecast period. Mainly because of the adjustment in the initial position, the new NHC track forecast is a little north of the previous forecast, and it lies closest to an average of the GFS and ECMWF solutions. Strong to moderate vertical shear is expected to persist for the next 24-36 hours, and little to no change in strength is anticipated during that time. Even after 36 hours, Karl will be moving through a dry environment over the central tropical Atlantic, but lower shear and warming sea surface temperatures could allow for at least gradual strengthening. The intensity models have trended a little bit higher at the end of the forecast period, and the new NHC forecast shows Karl potentially reaching hurricane intensity by day 5. This forecast is very close to the SHIPS and LGEM models. The initial 12-ft sea radii have been expanded considerably on this advisory based on recent altimeter wave height data. FORECAST POSITIONS AND MAX WINDS INIT 16/0900Z 18.3N 33.4W 40 KT 45 MPH 12H 16/1800Z 18.7N 35.3W 40 KT 45 MPH 24H 17/0600Z 18.7N 38.0W 40 KT 45 MPH 36H 17/1800Z 18.3N 40.7W 40 KT 45 MPH 48H 18/0600Z 17.7N 43.3W 45 KT 50 MPH 72H 19/0600Z 17.6N 48.1W 50 KT 60 MPH 96H 20/0600Z 19.0N 53.0W 55 KT 65 MPH 120H 21/0600Z 21.5N 57.5W 65 KT 75 MPH $$ Forecaster Berg