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Tropical Storm Irwin Forecast Discussion Number 11
2017-07-25 04:35:32| National Hurricane Center (East Pacific)
Issued at 800 PM PDT Mon Jul 24 2017 000 WTPZ45 KNHC 250235 TCDEP5 Tropical Storm Irwin Discussion Number 11 NWS National Hurricane Center Miami FL EP102017 800 PM PDT Mon Jul 24 2017 Irwin has been gradually strengthening throughout the day. Satellite images show that deep convection has been persisting over the center and in fragmented curved bands, especially over the south and west portions of the circulation. Microwave images indicate that the system is vertically titled from south to north, likely due to southerly shear. The latest Dvorak CI numbers are 3.5/55 kt from TAFB and 4.0/65 kt from SAB. These estimates and higher ADT values from CIMSS at the University of Wisconsin support increasing the initial intensity to 60 kt. Irwin is crawling west-northwestward, with the initial motion estimated to be 285/3 kt. A slow westward to west-northwestward motion is forecast through early Tuesday as a weak mid-level ridge remains in place to the north of the storm. A west-southwestward motion is expected between 24 and 72 h as Hurricane Hilary approaches from the east. Beyond that time, the track forecast is very challenging as the models remain quite divergent with the spread being more than 700 n mi at day 5. The UKMET, ECMWF, and GFS global models show Irwin rotating around the east side of the circulation of Hilary and ultimately merging with that hurricane. The CMC model shows some binary interaction between the tropical cyclones, but keeps the systems separate, and the regional hurricane models HWRF and HMON show no interaction between the systems and continue to move Irwin westward. The NHC track forecast favors the global models and shows a slow northward motion on days 4 and 5. Needless to say, the long range track forecast is of low confidence. The environmental conditions should support some additional strengthening during the next day or so, and Irwin is expected to become a hurricane tonight or on Tuesday. Thereafter, an increase in vertical wind shear, associated with the outflow of Hilary, and slightly cooler waters along the forecast track should cause a gradual weakening trend. The NHC intensity forecast is similar to the previous one and in fair agreement with the consensus models. FORECAST POSITIONS AND MAX WINDS INIT 25/0300Z 15.2N 118.2W 60 KT 70 MPH 12H 25/1200Z 15.3N 118.8W 65 KT 75 MPH 24H 26/0000Z 15.1N 119.9W 70 KT 80 MPH 36H 26/1200Z 14.8N 120.9W 65 KT 75 MPH 48H 27/0000Z 14.5N 121.7W 60 KT 70 MPH 72H 28/0000Z 14.2N 122.8W 55 KT 65 MPH 96H 29/0000Z 15.5N 123.0W 55 KT 65 MPH 120H 30/0000Z 18.0N 123.0W 50 KT 60 MPH $$ Forecaster Cangialosi
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