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Post-Tropical Cyclone Oscar Forecast Discussion Number 20

2018-10-31 21:35:03| National Hurricane Center (Atlantic)

Issued at 500 PM AST Wed Oct 31 2018 000 WTNT41 KNHC 312034 TCDAT1 Post-Tropical Cyclone Oscar Discussion Number 20 NWS National Hurricane Center Miami FL AL162018 500 PM AST Wed Oct 31 2018 Satellite imagery and scatterometer data indicate that Oscar has become a hurricane-force extratropical low, as the central convection has all but dissipated and frontal-band-type cloud features have become better defined. The scatterometer data show hurricane-force winds about 70 n mi south of the center, and that the overall wind field has expanded considerably since the previous overpass. The cyclone is expected to maintain an intensity of 60-65 kt for the next 48 h, then gradually weaken as the baroclinic energy wanes, with dissipation occurring between 96-120 h over the far northeastern Atlantic. The initial motion is now 030/30 kt. Oscar is now well embedded in the mid-latitude westerlies, and for the next 3-4 days it should move generally northeastward with a gradual decrease in forward speed. Much of the current forecast, especially the intensity and the size, is based on input from the NOAA Ocean Prediction Center. This is the last advisory on Oscar from the National Hurricane Center. Additional information on this system can be found in High Seas Forecasts issued by the National Weather Service, under AWIPS header NFDHSFAT1, WMO header FZNT01 KWBC, and available on the Web at https://ocean.weather.gov/shtml/NFDHSFAT1.shtml. FORECAST POSITIONS AND MAX WINDS INIT 31/2100Z 39.3N 49.6W 65 KT 75 MPH...POST-TROP/EXTRATROP 12H 01/0600Z 42.6N 46.6W 65 KT 75 MPH...POST-TROP/EXTRATROP 24H 01/1800Z 46.8N 41.4W 65 KT 75 MPH...POST-TROP/EXTRATROP 36H 02/0600Z 50.7N 35.1W 60 KT 70 MPH...POST-TROP/EXTRATROP 48H 02/1800Z 54.2N 28.4W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 03/1800Z 59.5N 15.0W 55 KT 65 MPH...POST-TROP/EXTRATROP 96H 04/1800Z 67.0N 2.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 05/1800Z...DISSIPATED $$ Forecaster Beven

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Hurricane Oscar Forecast Discussion Number 19

2018-10-31 15:33:06| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Wed Oct 31 2018 000 WTNT41 KNHC 311432 TCDAT1 Hurricane Oscar Discussion Number 19 NWS National Hurricane Center Miami FL AL162018 1100 AM AST Wed Oct 31 2018 Oscar is quickly transforming into an extratropical low. Although there is still a little bit of deep convection just north of the center, a more prominent cloud shield extends northward from the western part of the circulation. In addition, GOES-16 derived products show Oscar's center nearly embedded within a frontal zone and cold air advection occurring on the back side of the system. The maximum winds are a bit uncertain, but for now they are held at 65 kt based on the Dvorak Current Intensity number from TAFB and the latest microwave estimates, which range from 60-70 kt. Oscar is expected to complete extratropical transition later today when it becomes fully attached to the frontal boundary. Baroclinic energy is likely to keep the cyclone's intensity relatively steady for the next 48 hours or so, although it should be noted that the GFS shows some intensification later today as a sting jet develops to the west of the center. After 48 hours, the post-tropical cyclone is expected to gradually lose strength, and the NHC intensity forecast continues to most closely follow the GFS model, which is at the high end of the guidance envelope. Despite this weakening, Oscar's wind field is expected to grow substantially, affecting a large portion of the north Atlantic Ocean over the next several days. A northeastward acceleration continues with an initial motion of 035/25 kt. Further acceleration toward the north Atlantic is expected during the next 48 hours while Oscar becomes more fully embedded within the mid-latitude flow. The new NHC track forecast was shifted slightly northward and westward from the previous forecast to trend closer to the latest consensus aids, but otherwise the track forecast reasoning remains unchanged. Large swells from Oscar are expected to continue to affect portions of the coast of Bermuda through today. Please consult products from your local weather office, as these conditions could cause life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 31/1500Z 36.6N 51.6W 65 KT 75 MPH 12H 01/0000Z 40.2N 48.7W 65 KT 75 MPH...POST-TROP/EXTRATROP 24H 01/1200Z 44.5N 44.3W 65 KT 75 MPH...POST-TROP/EXTRATROP 36H 02/0000Z 48.4N 38.7W 60 KT 70 MPH...POST-TROP/EXTRATROP 48H 02/1200Z 52.1N 32.4W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 03/1200Z 57.8N 18.6W 55 KT 65 MPH...POST-TROP/EXTRATROP 96H 04/1200Z 64.0N 5.5W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 05/1200Z...DISSIPATED $$ Forecaster Berg

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Hurricane Oscar Forecast Discussion Number 18

2018-10-31 09:43:01| National Hurricane Center (Atlantic)

Issued at 500 AM AST Wed Oct 31 2018 000 WTNT41 KNHC 310842 TCDAT1 Hurricane Oscar Discussion Number 18 NWS National Hurricane Center Miami FL AL162018 500 AM AST Wed Oct 31 2018 Oscar continues to become less tropical-looking in appearance. Central convection is now minimal, and the overall cloud pattern is quite asymmetric, with the main area of dense overcast over the western portion of the circulation, stretching northward over an approaching frontal boundary. The current intensity estimate is 65 kt in accord with the latest Dvorak Current Intensity numbers from TAFB and SAB. Oscar is expected to merge with the nearby front in 12-18 hours, by which time the global models depict considerable cold and warm advection around the center. This indicates that Oscar will become an extratropical cyclone tonight, but energy from baroclinic processes will likely maintain the system near hurricane strength for the next couple of days. Only gradual weakening is expected thereafter and post-tropical Oscar will likely be a strong cyclone for the next 4 days or so. Post-tropical Oscar is forecast to merge with another extratropical cyclone at high latitudes over the weekend. The official intensity forecast is close to the latest GFS prediction, which should be appropriate for a mid-latitude system. Oscar is moving quickly toward the northeast, or 035/19 kt. The hurricane is being steered by the flow on the eastern side of a trough that is passing through Atlantic Canada. In a day or so, post-tropical Oscar should become embedded within the trough and move rapidly northeastward in the mid-latitude westerlies over the north-central and northeastern Atlantic. The track guidance is in fairly good agreement, aside from some speed differences in the latter part of the forecast period. The official forecast is close to the latest dynamical model consensus, and is similar to the previous NHC track. Large swells from Oscar are expected to continue to affect portions of the coast of Bermuda through today. Please consult products from your local weather office, as these conditions could cause life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 31/0900Z 34.1N 53.6W 65 KT 75 MPH 12H 31/1800Z 37.6N 50.9W 65 KT 75 MPH 24H 01/0600Z 42.2N 47.1W 65 KT 75 MPH...POST-TROP/EXTRATROP 36H 01/1800Z 46.0N 42.2W 65 KT 75 MPH...POST-TROP/EXTRATROP 48H 02/0600Z 49.7N 36.4W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 03/0600Z 56.0N 23.0W 60 KT 70 MPH...POST-TROP/EXTRATROP 96H 04/0600Z 61.5N 10.5W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 05/0600Z...MERGED WITH LARGER EXTRATROPICAL LOW $$ Forecaster Pasch

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Hurricane Oscar Forecast Discussion Number 17

2018-10-31 03:32:59| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Tue Oct 30 2018 000 WTNT41 KNHC 310232 TCDAT1 Hurricane Oscar Discussion Number 17 NWS National Hurricane Center Miami FL AL162018 1100 PM AST Tue Oct 30 2018 While conventional satellite imagery continues to show an area of deep convection over the center of Oscar, recent microwave data suggest that the inner core has become fragmented and that the circulation is tilted from southwest to northeast. The overall cloud pattern has also expanded northward as Oscar begins to interact with a frontal zone that is approaching the system from the northwest. A blend of the latest Dvorak T- and Current Intensity (CI) numbers yields an initial wind speed of 70 kt for this advisory. Oscar will be moving over decreasing SSTs and into an area of higher vertical wind shear tonight and Wednesday which is likely to cause some additional weakening during that time. The hurricane should complete its extratropical transition in about 24 hours, and the dynamical models indicate that the post-tropical cyclone is likely to maintain 60-65 kt winds for at least another couple of days. Some weakening is expected by 96 hours before the system merges with another low pressure area over the far northeastern Atlantic. Oscar has turned northeastward and continues to accelerate. An additional increase in forward speed is anticipated during the next day or two, as the cyclone becomes embedded within deep-layer southwesterly flow ahead of a mid-latitude trough. Late in the period, the post-tropical low is forecast to slow down over the northeastern Atlantic. The track guidance remains in good agreement, except for some forward speed differences late in the period. The NHC track forecast is once again close to the various consensus aids and very similar to the previous advisory. Large swells from Oscar are expected to continue to affect portions of the coast of Bermuda through Wednesday. Please consult products from your local weather office as these conditions could cause life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 31/0300Z 32.8N 55.2W 70 KT 80 MPH 12H 31/1200Z 35.6N 52.6W 65 KT 75 MPH 24H 01/0000Z 40.2N 48.9W 65 KT 75 MPH...POST-TROP/EXTRATROP 36H 01/1200Z 44.3N 44.4W 65 KT 75 MPH...POST-TROP/EXTRATROP 48H 02/0000Z 48.2N 38.7W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 03/0000Z 55.2N 24.3W 60 KT 70 MPH...POST-TROP/EXTRATROP 96H 04/0000Z 61.0N 12.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 05/0000Z...MERGED WITH LARGER EXTRATROPICAL LOW $$ Forecaster Brown

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Hurricane Oscar Forecast Discussion Number 15

2018-10-30 15:35:56| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Tue Oct 30 2018 000 WTNT41 KNHC 301435 TCDAT1 Hurricane Oscar Discussion Number 15 NWS National Hurricane Center Miami FL AL162018 1100 AM AST Tue Oct 30 2018 Cloud-top temperatures have warmed a bit overall, and the convective pattern is becoming more asymmetric as dry air is infiltrating the southern and eastern part of Oscar's circulation. However, the hurricane is still producing plenty of inner-core convection and some lightning strikes. Dvorak Current Intensity numbers from TAFB and SAB still support maximum winds of 90-100 kt, but objective numbers are much lower (65-75 kt), so Oscar's initial intensity is lowered slightly to 85 kt. Increasing shear and stronger upper-level divergence will likely offset each other in the short term, causing Oscar to maintain its intensity or only slightly weaken during the next 24 hours. However, a cold front is quickly approaching Oscar from the northwest, and their interaction is expected to cause Oscar to complete extratropical transition and become fully embedded within the frontal zone in about 36 hours. Oscar's winds should gradually diminish after it becomes extratropical, but the NHC official forecast remains above the various consensus aids from 36 hours and beyond and lies closest to the GFS and ECMWF global models, which should have a good handle on the cyclone's structure during the post-tropical phase. Oscar continues to accelerate and has turned north-northeastward, or 020/12 kt, while entering the flow between a large high over the eastern/central Atlantic and a mid-latitude trough now moving over the western Atlantic. Oscar is expected to become embedded within the trough by 36 hours (the completion of extratropical transition), with the entire system becoming a cut-off low north of the jet stream by days 4 and 5. The track models are in fairly good agreement on Oscar's future path, but there are speed differences by the end of the forecast period. Of particular note, the ECMWF is much faster than the other models, showing a more progressive pattern on day 5. The NHC track forecast lies close to the previous official forecast to maintain continuity, but it is still faster than the GFS, HWRF, and the TVCN multi-model consensus at day 5. Large swells from Oscar will affect Bermuda through Wednesday. Please consult products from your local weather office as these conditions could cause life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 30/1500Z 29.7N 57.7W 85 KT 100 MPH 12H 31/0000Z 31.8N 56.1W 80 KT 90 MPH 24H 31/1200Z 35.6N 52.6W 80 KT 90 MPH 36H 01/0000Z 40.6N 48.7W 75 KT 85 MPH...POST-TROP/EXTRATROP 48H 01/1200Z 45.1N 44.4W 65 KT 75 MPH...POST-TROP/EXTRATROP 72H 02/1200Z 52.6N 31.8W 55 KT 65 MPH...POST-TROP/EXTRATROP 96H 03/1200Z 59.0N 18.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 04/1200Z 63.5N 6.5W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Berg

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