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Tropical Storm Irwin Forecast Discussion Number 20
2017-07-27 10:55:16| National Hurricane Center (East Pacific)
Issued at 200 AM PDT Thu Jul 27 2017 000 WTPZ45 KNHC 270855 TCDEP5 Tropical Storm Irwin Discussion Number 20 NWS National Hurricane Center Miami FL EP102017 200 AM PDT Thu Jul 27 2017 Overall, Irwin's cloud pattern has change little during the past several hours. An earlier 0218 UTC SSMIS microwave pass shows a considerable structural vertical tilt toward the northwest due to the impinging southeasterly shear produced by Hilary to the east. Experimental GOES-16 channel 9 mid-level water vapor imagery indicates drier air intruding in the northeast quadrant of Irwin with subsequent convective erosion in that portion of the cloud canopy. The Dvorak satellite intensity estimates from SAB and TAFB support maintaining the initial intensity at 50 kt for this advisory. The dynamical intensity models indicate that the moderate southeasterly shear will back more toward the northeast during the next 12-24 hours as Hilary invades from the northeast. Beyond the 24 hour period, the intensity forecast becomes a bit problematic. The hurricane models, the HFIP Corrected Consensus, the Florida State Superensemble, and the GFS/ECMWF global models all show Irwin strengthening back into a hurricane under much improved diffluent flow aloft. The statistical-dynamical intensity models, on the other hand, show only slight strengthening. As a compromise for this forecast package, the official forecast shows modest strengthening, then gradual weakening after 48 hours. The initial motion is estimated to be west-southwestward, or 255/05 kt. As Hilary continues to moves closer and passes to the north of Irwin, a significant reduction in Irwin's forward speed, essentially drifting in a cyclonic fashion, is expected during the next 24-36 hours. After that time, the cyclone is forecast to accelerate northward and then northwestward up the eastern and northern portion of Hilary's peripheral circulation. The deterministic guidance remains in fair agreement with this synoptic pattern scenario, and the NHC forecast is similar to the previous one and is based on a blend of the TVCX and HCCA consensus models. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 14.7N 123.8W 50 KT 60 MPH 12H 27/1800Z 14.6N 124.4W 50 KT 60 MPH 24H 28/0600Z 14.5N 124.8W 55 KT 65 MPH 36H 28/1800Z 14.3N 124.7W 55 KT 65 MPH 48H 29/0600Z 14.7N 124.3W 55 KT 65 MPH 72H 30/0600Z 17.8N 124.4W 50 KT 60 MPH 96H 31/0600Z 22.0N 128.0W 40 KT 45 MPH 120H 01/0600Z 24.5N 133.5W 35 KT 40 MPH $$ Forecaster Roberts
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