000 WTNT45 KNHC 222036 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 18 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 PM AST TUE SEP 22 2015 The low-level center of Ida has been placed in the middle of a gyre defined by several small vortices. The cyclone is still sheared with the strongest convection located within a cyclonically- curved band to the south of the center. Given the lack of scatterometer data, and little change in the satellite presentation, the initial intensity is kept at 40 kt. Ida is embedded within the southern end of a mid-level trough, which is forcing the cyclone to drift east-southeastward at about 4 kt, and is also causing shear over the cyclone. The evolution of this trough will be crucial for the future of Ida. Global models indicate that Ida will drift generally eastward embedded in the trough for the next 48 hours. After that time, most of the models lift the trough out, leaving Ida south of an amplifying ridge. This long range forecast pattern should result in the cyclone turning toward the north-northwest as indicated in the NHC forecast, and also a decrease of the shear with favorable conditions for strengthening. Since the environment is currently quite hostile, no important change in intensity is indicated during the next 3 days. By the end of the forecast period, when the cyclone becomes detached from the trough, some slight increase in strength is forecast. The NHC track forecast continues to be in the middle of the guidance envelope, and is very close to the ECMWF and GFS consensus. If the global models are correct, Ida will be meandering over the Atlantic for several more days. FORECAST POSITIONS AND MAX WINDS INIT 22/2100Z 20.5N 47.8W 40 KT 45 MPH 12H 23/0600Z 20.2N 47.2W 40 KT 45 MPH 24H 23/1800Z 20.0N 46.5W 40 KT 45 MPH 36H 24/0600Z 20.0N 46.0W 40 KT 45 MPH 48H 24/1800Z 20.5N 45.5W 40 KT 45 MPH 72H 25/1800Z 22.5N 45.7W 40 KT 45 MPH 96H 26/1800Z 24.5N 47.0W 50 KT 60 MPH 120H 27/1800Z 26.5N 46.5W 55 KT 65 MPH $$ Forecaster Avila