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Hurricane Hilary Forecast Discussion Number 20
2017-07-26 10:56:39| National Hurricane Center (East Pacific)
Issued at 300 AM MDT Wed Jul 26 2017 000 WTPZ44 KNHC 260856 TCDEP4 Hurricane Hilary Discussion Number 20 NWS National Hurricane Center Miami FL EP092017 300 AM MDT Wed Jul 26 2017 The structure of Hilary has changed little during the past several hours. Conventional satellite imagery shows a central dense overcast that is slightly elongated north-to-south, most likely due to the impact of some northerly shear. An eye is occasionally appearing in infrared imagery, and microwave data continues to indicate an eye present under the overcast. However, the eye has not yet become well-developed enough to justify an increase in the initial intensity, which remains 90 kt in agreement with a blend of the various satellite estimates. The initial motion is 285/11. The track forecast is essentially unchanged for the next 72 h, while Hilary is expected to be steered by the mid-level subtropical ridge to the north. After that time, the guidance has become increasingly divergent due to disagreements on how much a mid- to upper-level trough just west of California affects the ridge. The GFS uses the trough to break the ridge and allow Hilary to move almost northward by the end of the period, which is a major shift to the right since the previous advisory. The UKMET and Canadian models keep the strongest ridge and forecast a more westward motion. The ECMWF is between these extremes, but has shifted northward since the previous advisory. An additional complication is the possibility of interaction with Hurricane Irwin. The new GFS forecast calls for little interaction, while the UKMET and Canadian merge the two cyclones in 3-4 days and the ECMWF has interaction at days 4-5. The latter part of the new forecast track will be moved northward based on the northward shift of the GFS, ECMWF, and the various consensus models. However, it lies to the south of the consensus models and the center of the guidance envelope in deference to the UKMET and Canadian forecasts. This part of the forecast is low confidence and additional adjustments may be needed later. Some slight strengthening is possible today even though Hilary is in marginal shear conditions. After that, the cyclone should move over progressively cooler sea surface temperatures and gradually weaken. The new intensity forecast has changed little through 72 hours, and then shows a faster rate of decay based on a track over colder water than previously forecast. It should be noted that the uncertainty in the track forecast also affects the intensity forecast, as either a more northward motion or interaction with Irwin would probably weaken the cyclone faster than currently forecast. FORECAST POSITIONS AND MAX WINDS INIT 26/0900Z 16.3N 111.0W 90 KT 105 MPH 12H 26/1800Z 16.7N 112.7W 95 KT 110 MPH 24H 27/0600Z 17.1N 114.7W 90 KT 105 MPH 36H 27/1800Z 17.6N 116.5W 85 KT 100 MPH 48H 28/0600Z 18.1N 118.2W 80 KT 90 MPH 72H 29/0600Z 19.5N 121.5W 75 KT 85 MPH 96H 30/0600Z 21.0N 125.0W 55 KT 65 MPH 120H 31/0600Z 22.5N 128.5W 35 KT 40 MPH $$ Forecaster Beven
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