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Tropical Storm Philippe Forecast Discussion Number 5
2017-10-28 22:52:29| National Hurricane Center (Atlantic)
Issued at 500 PM EDT Sat Oct 28 2017 000 WTNT43 KNHC 282052 TCDAT3 Tropical Storm Philippe Discussion Number 5 NWS National Hurricane Center Miami FL AL182017 500 PM EDT Sat Oct 28 2017 Satellite images, radar data from Cuba and Key West, and surface observations indicate the depression has turned northward over the past few hours, and has also strengthened into a tropical storm. The initial intensity of 35 kt is based on a surface observation of 35 kt in a brief squall from Grand Cayman at 1444Z, a recent AMSU estimate of 37 kt, and average Key West Doppler radar velocities of 40 kt between 10,000-12,000 ft just north of the Cuban coast; the latter data equates to an approximate surface wind speed of 36 kt. The initial motion estimate is now 355/25 kt based on radar data and high-resolution GOES-16 visible satellite imagery over the past 2-3 hours. The low-level wind flow pattern for the next 6 hours or so is forecast to be complex, with a new non-tropical surface low having recently formed about 100 nmi west of Key West, Florida. This feature appears to have developed beneath an upper-level jet entrance region ahead of an approaching cold front. The global, regional, and even the convective-scale models are now showing the center of Philippe moving general northward across the Straits of Florida this evening and merging with the non-tropical low around 0000Z. After the merger over Florida Bay, the various models are in good agreement on Philippe moving northeastward to east-northeastward across extreme southern Florida or the Florida Keys overnight as the cyclone is accelerated ahead of the aforementioned frontal system and strong mid-/upper-level trough. After reaching the northwestern Bahamas by 1200Z Sunday morning, Philippe is forecast to accelerate further towards the northeast at forward speeds of 30-35 kt, by Sunday afternoon and evening, remaining well offshore of the Carolinas. By 36 hours, Philippe is expected to pass about midway between Bermuda and the southeastern United States, and gradually getting pulled north-northeastward up the east side of a powerful baroclinic low that is forecast to develop near Cape Hatteras and move northward near the U.S. east coast. The NHC track guidance is in reasonable agreement on this developing track scenario, and lies between the GFS model, which takes Philippe across the Florida Keys, and the ECMWF model, which moves the cyclone farther north over South Florida. The vertical wind shear affecting Philippe is expected to remain favorable for strengthening to occur for the next 24 hours, along with an additional baroclinic boost from the aforementioned upper-level jet maximum. By 36 hours, strong baroclinic forcing associated with the approaching frontal system are forecast to induce additional strengthening before the cyclone merges with the frontal system and becomes an extratropical low by 48 hours. Dissipation or absorption by a larger extratropical low is expected by 72 hours when the system is located over the cold waters of the North Atlantic. KEY MESSAGES: 1. Although the center of Philippe is now forecast to move across the Florida Keys or extreme south Florida, most of the strongest winds are expected to remain east and southeast of the center. However, tropical-storm-force winds, mainly in gusts, could occur in brief heavy squalls across the upper Florida Keys and southeast Florida overnight. For that reason, a tropical storm watch remains in effect for these areas. 2. Regardless of the exact track of Philippe, the primary threat from this system will be heavy rainfall that can cause localized flooding across portions of Cuba, the Florida Keys, and South Florida. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 23.0N 82.6W 35 KT 40 MPH 12H 29/0600Z 26.0N 80.1W 45 KT 50 MPH 24H 29/1800Z 31.5N 75.3W 50 KT 60 MPH 36H 30/0600Z 36.6N 70.5W 55 KT 65 MPH 48H 30/1800Z 42.0N 66.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 31/1800Z...DISSIPATED $$ Forecaster Stewart
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