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Tropical Storm Irwin Forecast Discussion Number 10
2017-07-24 22:33:27| National Hurricane Center (East Pacific)
Issued at 200 PM PDT Mon Jul 24 2017 000 WTPZ45 KNHC 242033 TCDEP5 Tropical Storm Irwin Discussion Number 10 NWS National Hurricane Center Miami FL EP102017 200 PM PDT Mon Jul 24 2017 Irwin has been displaying a dimple in visible satellite imagery, which appears to coincide with a mid-level eye that has been apparent in recent microwave imagery. However, the microwave data also suggest that Irwin's circulation is tilted from south to north with height, with the low-level center located south of the feature noted in visible imagery. Subjective Dvorak estimates have risen to T3.5/55 kt from TAFB and T4.0/65 kt from SAB, while the objective ADT is still down around T3.1/47 kt. Given the cyclone's tilted structure, I'm comfortable going down the middle and initializing the intensity at 55 kt. Vertical shear is currently low over Irwin and should remain so for the next 24 hours or so. This will likely allow Irwin to continue strengthening, reaching hurricane status overnight. Increased shear, possibly enhanced by Hilary's outflow, could begin to impinge on Irwin after 24 hours, which would likely limit the intensification trend. The new intensity guidance has come down a bit after 24 hours, and only gradual weakening is anticipated through the end of the forecast period as Irwin and Hilary interact. Irwin will also be approaching the 26 deg C SST isotherm in about 5 days, which will also foster some weakening. The updated NHC forecast is close to the HCCA guidance, and it's just slightly below the previous forecast from 36 hours and beyond. Irwin's is drifting westward with an initial motion of 280/3 kt. As Irwin and Hilary's circulations get closer together over the next few days, Irwin will respond by first moving slowly west-southwestward on days 2 and 3, and then turning northwestward and accelerating behind or around the southeast side of Hilary's circulation. The regional HWRF and HMON models continue to show a persistent westward track that ignores Hilary's existence, while the global models show varying degrees of interaction between the two cyclones. Because of the unrealistic scenarios shown by the regional models, the NHC track forecast continues to lie east of the multi-model consensus on days 4 and 5, and it's very close to HCCA. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 15.0N 117.8W 55 KT 65 MPH 12H 25/0600Z 15.2N 118.3W 65 KT 75 MPH 24H 25/1800Z 15.2N 119.2W 70 KT 80 MPH 36H 26/0600Z 15.1N 120.3W 65 KT 75 MPH 48H 26/1800Z 14.8N 121.3W 65 KT 75 MPH 72H 27/1800Z 14.2N 122.7W 60 KT 70 MPH 96H 28/1800Z 15.0N 123.0W 60 KT 70 MPH 120H 29/1800Z 17.5N 124.5W 55 KT 65 MPH $$ Forecaster Berg
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