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Tropical Storm IDA Forecast Discussion Number 8
2015-09-20 10:46:11| National Hurricane Center (Atlantic)
Issued at 500 AM AST SUN SEP 20 2015 000 WTNT45 KNHC 200846 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 8 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 AM AST SUN SEP 20 2015 Ida remains a sheared tropical cyclone, but there are some indications that the mid-level and deep-layer shear is beginning to abate somewhat. A burst of deep convection has developed during the past few hours near the center and in the eastern quadrant, which so.me cloud tops near -80C. The initial intensity is being held at 35 kt based on consensus Dvorak current-intensity estimates of T2.5/35 kt from TAFB and SAB, but this could be a little conservative based on earlier partial ASCAT overpass that contained slightly higher wind speeds in the eastern quadrant. The initial motion estimate is 295/13 kt. Ida is expected to move west-northwestward and gradually slow down over the next several days, possibly even stalling by days 4 and 5. Steering currents are forecast to weaken considerably after 72 hours as a strong mid- to upper-level trough digs southeastward across the central subtropical Atlantic and significantly erodes the ridge to the north of the cyclone. The global models are in good agreement on the developing synoptic flow scenario through day 3, but then diverge significantly after that with the ECMWF, UKMET, and GFDL models taking a deeper and stronger tropical cyclone more northward, whereas the GFS and HWRF models take a weaker and more shallow system westward. Given the uncertainty in the expected strength and vertical depth of Ida on days 3-5, the official forecast track lies down the middle of the guidance envelope, and is close to a blend of the consensus models TVCA and GFEX. The moderate to strong westerly vertical wind shear that Ida has been experiencing the past day or so is forecast by the GFS and ECMWF models to gradually subside and become less than 10 kt by 24 hours through 72 hours. With such a robust low-level circulation and the mid-level humidity values expected to be near 70 percent, convection should steadily increase and become better organized, producing at least some slow intensification through the next 48 hours. The leveling off of the intensity on days 3-5 is due to the uncertainty on whether or Ida moves beneath a small upper-level col region with weak winds as indicated by the ECMWF model, which would support more strengthening than what the official forecast is indicating. The NHC intensity forecast lies essentially midway between the two intensity extremes of the weaker GFS-HRWF and the stronger ECMWF-GFDL model solutions, and is supported by the intensity consensus model IVCN. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 16.7N 43.6W 35 KT 40 MPH 12H 20/1800Z 17.5N 45.2W 35 KT 40 MPH 24H 21/0600Z 18.8N 47.0W 40 KT 45 MPH 36H 21/1800Z 19.8N 48.1W 45 KT 50 MPH 48H 22/0600Z 20.5N 48.6W 50 KT 60 MPH 72H 23/0600Z 20.6N 48.7W 50 KT 60 MPH 96H 24/0600Z 20.7N 48.7W 50 KT 60 MPH 120H 25/0600Z 21.0N 48.7W 50 KT 60 MPH $$ Forecaster Stewart
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