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Hurricane Ophelia Forecast Discussion Number 21
2017-10-14 10:58:25| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sat Oct 14 2017 000 WTNT42 KNHC 140858 TCDAT2 Hurricane Ophelia Discussion Number 21 NWS National Hurricane Center Miami FL AL172017 500 AM AST Sat Oct 14 2017 Ophelia's 20-nmi-diameter eye has continued to become more distinct and cloud-free, with the eye temperature now reaching 15 deg C. Satellite intensity estimates range from T4.5/77 kt from TAFB to T5.0/90 kt from SAB and T5.5/102 kt from UW-CIMSS ADT. For now, the initial intensity will remain at 85 kt, which is an average of the available intensity estimates. The initial motion estimate is 060/21 kt. Ophelia is embedded within deep-layer southwesterly flow on the east side of a broad mid-latitude trough. The global and regional models remain in excellent agreement on the trough amplifying over the next 3-4 days, which will cause the hurricane to accelerate toward the northeast at forward speeds near 30 kt by 48 hours. The tight clustering of the NHC model guidance, which shows very little cross-track or along-track spread, increases the confidence in the official track. As a result, no significant changes were made to the previous advisory, and the new forecast track remains near the middle of the guidance envelope, close to the HCCA and TVCX consensus models. Ophelia is expected to remain in relatively low vertical wind shear environment for the next 12 hours or so, which should help the hurricane retain much of its current intensity during that time, even though SSTs are only going to be 24-25C. However, upper-level temperatures that are still about 2 deg C cooler than normal, which will help to create sufficient instability to continue to drive the development of inner-core convection. By 36 hours or so, the shear is forecast to increase to 30-40 kt and the troposphere is expected to become stable as sea-surface temperatures decrease to less than 20 deg C. However, even those SST values are about 2 deg C warmer than normal for this time of the year. Those above-average ocean temperatures are forecast to combine with strong baroclinic energy associated with a potent, negatively tilted, upper-level trough, causing Ophelia to transition into a powerful extratropical low pressure system. By 48 hours, the post-tropical cyclone is forecast to maintain sustained hurricane-force winds as it approaches Ireland, with stronger winds expected over higher terrain. Given that Ophelia is forecast to become extratropical, the wind field should expand, resulting in impacts over portions of the British Isles regardless of its exact location or strength. By 96 hours, Ophelia should have weakened due to the interaction with land, causing the surface circulation to become ill-defined, and dissipation is expected shortly thereafter. Although the center of Ophelia is not forecast to reach Ireland or the UK for another 48-60 hours, wind and rain effects will arrive well in advance of the cyclone center. Individuals in those locations should consult products from their local meteorological service for more information on local impacts. Tropical-storm-force winds are possible throughout the Azores beginning tonight, primarily due to an approaching cold front. However, any track deviation to the west could bring stronger winds associated with Ophelia's circulation to those islands. Interests in the Azores should refer to products issued by the Azores Weather Forecast and Watch Center. KEY MESSAGES: 1. Ophelia is expected to become a hurricane-force post-tropical cyclone by Monday before it moves near Ireland and the United Kingdom. Direct impacts from wind and heavy rain in portions of these areas are likely, along with dangerous marine conditions. For more details on the magnitude, timing, and location of impacts from post-tropical Ophelia, residents in Ireland should refer to products issued by Met Eireann, and residents in the United Kingdom should refer to products issued by the Met Office. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 33.9N 28.6W 85 KT 100 MPH 12H 14/1800Z 35.3N 25.4W 80 KT 90 MPH 24H 15/0600Z 38.3N 20.4W 80 KT 90 MPH 36H 15/1800Z 43.0N 16.0W 75 KT 85 MPH...POST-TROP/EXTRATROP 48H 16/0600Z 48.4N 12.5W 75 KT 85 MPH...POST-TROP/EXTRATROP 72H 17/0600Z 57.4N 6.3W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 18/0600Z 63.1N .5W 40 KT 45 MPH...POST-TROP/EXTRATROP 120H 19/0600Z...DISSIPATED $$ Forecaster Stewart
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