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Tropical Storm IDA Forecast Discussion Number 21
2015-09-23 16:46:32| National Hurricane Center (Atlantic)
Issued at 1100 AM AST WED SEP 23 2015 000 WTNT45 KNHC 231446 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 21 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST WED SEP 23 2015 Ida's cloud pattern has deteriorated since yesterday, and the upper-level outflow is very disrupted due to shear. The center of the cyclone has been placed in the middle of a couple of low cloud swirls, and to the west of a small area of deep convection. Based on continuity, the initial intensity is kept at 35 kt, however, a recent partial ASCAT suggests that the winds could be lower. Both ECMWF and GFS SHIPS intensity models indicate that the shear has peaked, and a gradual relaxation should soon begin. However, it will take 36 to 48 hours for the shear to be low enough to favor re-strengthening. On this basis, only a small increase in intensity is forecast beyond that time. This is consistent with the previous forecast and the intensity consensus model. The cyclone, as anticipated, has become well embedded in the base of an upper-level trough and is drifting eastward or 090 degrees at 4 kt. Most of the global models lift the trough out and keep the cyclone moving very slowly within weak steering currents for the next day or two. After that time, the subtropical ridge is forecast to build over the Atlantic, and this flow pattern will force the cyclone to move toward the north and then to the north-northwest. For the next 3 days or so, the guidance is in good agreement showing a northward turn. The model spread increases after four days, but the general trend is to keep the cyclone moving slowly while is trapped south of the subtropical ridge. The NHC forecast is in the middle of the track envelope and is very similar to the previous one. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 20.0N 47.0W 35 KT 40 MPH 12H 24/0000Z 20.0N 46.4W 35 KT 40 MPH 24H 24/1200Z 20.5N 46.0W 35 KT 40 MPH 36H 25/0000Z 21.0N 46.0W 35 KT 40 MPH 48H 25/1200Z 22.0N 46.5W 35 KT 40 MPH 72H 26/1200Z 23.0N 47.5W 40 KT 45 MPH 96H 27/1200Z 25.0N 47.5W 45 KT 50 MPH 120H 28/1200Z 25.5N 48.0W 50 KT 60 MPH $$ Forecaster Avila
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