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Hurricane Hilary Forecast Discussion Number 24
2017-07-27 10:54:17| National Hurricane Center (East Pacific)
Issued at 200 AM PDT Thu Jul 27 2017 000 WTPZ44 KNHC 270854 TCDEP4 Hurricane Hilary Discussion Number 24 NWS National Hurricane Center Miami FL EP092017 200 AM PDT Thu Jul 27 2017 Hilary has lost organization due to the effects of moderate northerly vertical shear, with recent Windsat and GPM overpasses indicating that the center is now near the northwestern edge of the main convective area. Subjective satellite intensity estimates are 75-90 kt, while estimates based on microwave sounding data and the CIMSS ADT are 55-65 kt. Based on a blend of these, the initial intensity is reduced to a possibly generous 75 kt. The large-scale models forecast Hilary to move under a cyclonic shear axis between two upper-level high pressure areas during the next 24-48 h. This evolution may allow the current shear to decrease, but it is not an ideal environment for a tropical cyclone. Thus, the intensity forecast calls for little change in strength during this time, which is in good agreement with the intensity guidance. After 48 h, Hilary should move over cooler sea surface temperatures, and it is likely to significantly interact with Tropical Storm Irwin. The large-scale models differ on the details, but they agree that the combination of the interaction, dry air, and the presence of cool water should cause Hilary to quickly weaken as a result. Thus, the new intensity forecast now calls for the system to degenerate to a remnant low by the end of the forecast period. The initial motion is 290/10. For the first 48 h or so, Hilary should be steered generally west-northwestward by the subtropical ridge. After that, the track forecast becomes very uncertain due to the interaction with Irwin, where the latter cyclone is likely to move around the eastern and northern sides of Hilary before a possible merger. The Canadian model merges the two cyclones in about 72 h, while the GFS finishes the merger at about 120 h. On the other hand, the ECMWF delays the merger until after the forecast period, while the UKMET keeps the two systems separate. The new track forecast will not attempt to show the details of the interaction, but it will show more of a westward turn and changes in speed during the interaction than the previous forecast. Overall, the new track is close to the consensus models. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 17.3N 115.1W 75 KT 85 MPH 12H 27/1800Z 17.7N 116.5W 70 KT 80 MPH 24H 28/0600Z 18.4N 118.2W 70 KT 80 MPH 36H 28/1800Z 19.1N 119.9W 70 KT 80 MPH 48H 29/0600Z 19.8N 121.7W 70 KT 80 MPH 72H 30/0600Z 21.0N 126.0W 60 KT 70 MPH 96H 31/0600Z 21.5N 130.5W 45 KT 50 MPH 120H 01/0600Z 22.0N 133.0W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ Forecaster Beven
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