WTPZ45 KNHC 251435

Hurricane Irwin Discussion Number 13
NWS National Hurricane Center Miami FL EP102017
800 AM PDT Tue Jul 25 2017

Recent microwave imagery confirms that Irwin has a well-defined mid-level eye, which has also been apparent in shortwave infrared satellite imagery. On the whole, satellite intensity estimates have risen a little, so Irwin's initial intensity is now set at 70 kt. Various shear analyses indicate that about 15 kt of southerly shear has developed over Irwin, which isn't surprising since microwave data has hinted that the low-level center may be located just a bit south of the mid-level eye. This shear is expected to continue for another 24 hours or so, which will likely prevent Irwin from any additional significant strengthening. After 24 hours, Irwin's proximity to Hurricane Hilary is likely to induce some weakening, as well as the potential for upwelling of colder water due to Irwin's slow motion through 72 hours. More definitive weakening is likely by day 5 as Irwin reaches higher latitudes and much colder waters. The intensity models are fairly stable at the moment the latest runs did not suggest that any changes were needed to the previous forecast. It should be noted that while most of the global models maintain Irwin as a separate and distinct cyclone from Hilary through day 5, the ECMWF model indicates that Irwin could be absorbed by that time.

Irwin appears to be moving slowly westward, or 270/5 kt. The cyclone's future track will largely be dictated its the binary interaction with Hilary. First, as Hilary approaches from the east, Irwin is expected to dip west-southwestward and southwestward during the first 48 hours. It will likely stall by day 3, but then get pulled northward and northwestward around the eastern side of Hilary's circulation. Although there is still considerable spread in the track models after 72 hours, the regional HWRF and HMON models are now on board with the binary interaction. Therefore, the NHC track forecast is much closer to the TVCN multi-model consensus than it has been during the past few days.


INIT 25/1500Z 15.6N 119.4W 70 KT 80 MPH 12H 26/0000Z 15.2N 120.2W 70 KT 80 MPH 24H 26/1200Z 14.7N 121.4W 65 KT 75 MPH 36H 27/0000Z 14.2N 122.2W 60 KT 70 MPH 48H 27/1200Z 13.9N 122.8W 55 KT 65 MPH 72H 28/1200Z 14.4N 122.7W 55 KT 65 MPH 96H 29/1200Z 18.0N 122.0W 55 KT 65 MPH 120H 30/1200Z 21.5N 125.0W 50 KT 60 MPH

$$ Forecaster Berg