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Subtropical Storm Oscar Forecast Discussion Number 4

2018-10-27 22:33:01| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Oct 27 2018 000 WTNT41 KNHC 272032 TCDAT1 Subtropical Storm Oscar Discussion Number 4 NWS National Hurricane Center Miami FL AL162018 500 PM AST Sat Oct 27 2018 Oscar is still located beneath an upper-level low, however it its convective structure is now more reminiscent of a tropical cyclone. Deep convection is primarily occuring in a small burst just south of the center of Oscar, however this convective activity appears to be displaced by light northerly shear associated with the upper-low. Earlier AMSU sounding data indicated that the cyclone has developed a weak but vertically deep warm core which also indicates that Oscar is nearly a tropical cyclone, if it isn't one already. Oscar is moving quickly westward away from the upper-low, and will likely become a tropical storm later tonight if its current structural trends continue. The initial intensity is held at 50 kt, based primarily on the latest Hebert-Poteat subtropical intensity estimate from TAFB. The intensity guidance has bounced back, and the most of the intensity guidance is higher than it was six hours ago. While little change was made to the NHC intensity forecast, it now lies very near the intensity consensus at all forecast hours. Gradual intensification is still expected and Oscar is forecast to approach hurricane strength by Monday. Slight additional intensification is possible through the middle of next week, until extratropical transition occurs by 120 h. Oscar moved very quickly westward this afternoon, but a more representative motion estimate is 265/15 kt. The latest runs of the typically reliable global models are in better agreement than they were this morning. Oscar is forecast to move west-southwestward or westward for the next day or two on the south side of a mid-level ridge to the north. The cyclone is then expected to recurve and accelerate northeastward ahead of a substantial mid-latitude trough moving across the western and central Atlantic by the middle of next week. The official track forecast has been adjusted westward to bring it closer to the latest track consensus, especially for the first 72 h of the forecast. By day 5, the official forecast is quite similar to the previous advisory, but with a slightly slower forward speed for Oscar. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 26.9N 50.7W 50 KT 60 MPH 12H 28/0600Z 26.1N 53.0W 50 KT 60 MPH...TROPICAL CYCLONE 24H 28/1800Z 25.7N 55.8W 55 KT 65 MPH 36H 29/0600Z 25.6N 57.9W 55 KT 65 MPH 48H 29/1800Z 26.4N 59.1W 60 KT 70 MPH 72H 30/1800Z 30.6N 57.5W 70 KT 80 MPH 96H 31/1800Z 38.5N 50.0W 70 KT 80 MPH 120H 01/1800Z 46.5N 38.0W 60 KT 70 MPH...POST-TROP/EXTRATROP $$ Forecaster Zelinsky

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Subtropical Storm Oscar Forecast Discussion Number 3

2018-10-27 16:45:00| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Oct 27 2018 000 WTNT41 KNHC 271444 TCDAT1 Subtropical Storm Oscar Discussion Number 3 NWS National Hurricane Center Miami FL AL162018 1100 AM AST Sat Oct 27 2018 Oscar's organization has improved this morning. Although the subtropical storm is still entangled with an upper-level low, convection has increased near the center of the cyclone since last night. The most recent Hebert-Poteat subtropical intensity estimate from TAFB has increased accordingly to 45-50 kt. Furthermore, Canadian drifting buoy 47546 recently reported a minimum pressure of just below 996 mb to the east of Oscar's center, suggesting the central pressure of the cyclone has decreased since the last advisory. The initial intensity is therefore increased to 50 kt for this advisory. Little change has been made to the intensity forecast. The GFS and many of its associated models (HWRF, DSHP, LGEM) have changed abruptly and forecast far less intensification than they did just 6 hours ago. However, the CTCI, HMON, and ECMWF-based statistical guidance still show Oscar reaching hurricane strength within a few days. Rather than chase a possible short-term trend in the intensity guidance, the official intensity forecast will stay the course for now and is a little above the intensity consensus, bringing Oscar to hurricane strength in around 48 h. After that time, some slight additional intensification is possible, but Oscar is ultimately expected to undergo extratropical transition by the end of the forecast period, which should cause the cyclone to steadily weaken. Oscar has turned toward the west and the initial motion estimate is now 270/11 kt. A west-southwestward motion is anticipated later today as Oscar moves around the back side of an mid- to upper-level trough over the central Atlantic, followed by a turn toward the west on the south side of a subtropical ridge to the north. As long as Oscar intensifies as forecast, it should turn sharply northeastward early next week ahead of a mid-latitude trough advancing across the western and central Atlantic, and then accelerate in that direction while undergoing extratropical transition. The GFS is an outlier, showing a much weaker and vertically shallow cyclone that does not fully recurve, but all of the other global models are in generally good agreement with the scenario listed above. Despite the inconsistency of the GFS, the model consensus has not changed significantly since the last advisory, so only minor adjustments were made to the NHC track forecast. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 27.3N 48.4W 50 KT 60 MPH 12H 28/0000Z 26.9N 50.6W 55 KT 65 MPH 24H 28/1200Z 26.2N 53.5W 60 KT 70 MPH...TROPICAL CYCLONE 36H 29/0000Z 25.9N 56.1W 60 KT 70 MPH 48H 29/1200Z 26.5N 57.4W 65 KT 75 MPH 72H 30/1200Z 30.2N 57.0W 70 KT 80 MPH 96H 31/1200Z 37.0N 50.0W 70 KT 80 MPH 120H 01/1200Z 47.0N 38.0W 60 KT 70 MPH...POST-TROP/EXTRATROP $$ Forecaster Zelinsky

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Subtropical Storm Oscar Forecast Discussion Number 2

2018-10-27 10:39:29| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sat Oct 27 2018 000 WTNT41 KNHC 270839 TCDAT1 Subtropical Storm Oscar Discussion Number 2 NWS National Hurricane Center Miami FL AL162018 500 AM AST Sat Oct 27 2018 Oscar's convective pattern has not changed much since the previous advisory. The surface center is displaced just to the southeast of a burst of deep convection, and other convective elements extend along a broken band over the eastern part of the circulation. Although satellite classifications have not increased, a drifting buoy (47546) near Oscar's center recently reported a pressure of 998.8 mb, implying a significant drop in the cyclone's central pressure from the previous estimate. Based on pressure-wind relationships, this low pressure would equate to maximum winds around 45 kt, and that is set as the initial intensity for this advisory. Oscar is careening around the northern side of a mid- to upper-level low, and its initial motion is now west-northwestward, or 300/10 kt. As the circulation becomes more vertically aligned in the coming days, the cyclone is forecast to be pushed westward to west-southwestward by the flow on the back side of a trough that is dropping southward over the eastern Atlantic. After 48 hours, Oscar is expected to recurve sharply and accelerate toward the north Atlantic ahead of an approaching mid-latitude trough. The global models have come into much better agreement on Oscar's eventual recurvature and acceleration, and confidence in the NHC track forecast has increased. In fact, the 00Z guidance suite required a significant increase in Oscar's forecast forward speed on days 4 and 5, and the new NHC track forecast is much faster than the previous one at the end of the forecast period, jumping northward by about 10 degrees of latitude. This new forecast is close to the TVCN multi-model consensus and HCCA model on day 5, but it's still not as fast as the 00Z GFS and ECMWF guidance. Gradually decreasing shear and warmer waters ahead of Oscar should allow for a steady increase in intensity during the next few days, and the cyclone is also expected to take on a more tropical convective pattern in about 36 hours. The intensity models are showing a little more intensification than before, and the new NHC intensity forecast has been nudged upward, showing Oscar becoming a hurricane in 3-4 days. Still, this forecast lies near the lower bound of the intensity guidance, and additional upward adjustments may be required in future advisories. Now that most of the models are showing more acceleration of Oscar toward the north Atlantic later in the forecast period, there is also more definitive consensus that Oscar will become an extratropical low by day 5, and that is now indicated in the official forecast. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 27.3N 47.0W 45 KT 50 MPH 12H 27/1800Z 27.1N 49.1W 45 KT 50 MPH 24H 28/0600Z 26.2N 52.0W 50 KT 60 MPH 36H 28/1800Z 25.7N 54.6W 55 KT 65 MPH...TROPICAL CYCLONE 48H 29/0600Z 25.9N 56.6W 60 KT 70 MPH 72H 30/0600Z 28.4N 57.4W 65 KT 75 MPH 96H 31/0600Z 35.0N 52.0W 70 KT 80 MPH 120H 01/0600Z 45.0N 39.0W 60 KT 70 MPH...POST-TROP/EXTRATROP $$ Forecaster Berg

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Subtropical Storm Oscar Forecast Discussion Number 1

2018-10-27 04:33:56| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Oct 26 2018 000 WTNT41 KNHC 270233 TCDAT1 Subtropical Storm Oscar Discussion Number 1 NWS National Hurricane Center Miami FL AL162018 1100 PM AST Fri Oct 26 2018 Satellite imagery and scatterometer data indicate the circulation of the broad low pressure area over the central Atlantic has become better defined, with a low-level center developing to the south of a convective burst. The scatterometer data indicate 35-40 kt winds about 70-90 n mi from the center, and satellite imagery shows that the cyclone is entangled with a developing upper-level low. Based on these factors, advisories are being initiated on the latest in this year's series of subtropical storms, Oscar. Due to the recent development of the center, the initial motion is an uncertain 335/8. The track guidance is in excellent agreement that Oscar should should turn sharply westward and west- southwestward during the next 24 h as it pivots around the north side of the upper-level low, with this general motion continuing through about 48 h. After that, the cyclone should turn more westward at a slower forward speed as it approaches a weakness in the subtropical ridge. A northeastward motion appears likely from 72-120 h due to a mid-latitude trough approaching Oscar from the west. However, there are significant differences in the forward speed between the GFS, ECMWF, and UKMET models that make this part of the forecast low confidence. Overall, the forecast track is close to the TVCN consensus model, and it splits the difference between the divergent global models late in the forecast period. Gradual strengthening is forecast as Oscar remains in the deep-layer cyclonic flow of the upper-level low during the first 36-48 h. After that time, the global models suggests the system will have a chance to develop a tropical cyclone outflow pattern, and based on this the forecast calls for Oscar to become a tropical cyclone near the 48 h point. Interaction with the aforementioned mid-latitude trough may allow some additional intensification before Oscar encounters stronger shear near the 120 h point. The FV3 model shows a much weaker Oscar than the other dynamical models, and it is possible that dry air entrainment may hamper development through the forecast period. Thus, the intensity forecast is below the intensity consensus and on the weaker side of the intensity guidance. FORECAST POSITIONS AND MAX WINDS INIT 27/0300Z 26.7N 45.7W 40 KT 45 MPH 12H 27/1200Z 27.2N 47.2W 45 KT 50 MPH 24H 28/0000Z 26.6N 50.3W 50 KT 60 MPH 36H 28/1200Z 25.6N 53.2W 50 KT 60 MPH 48H 29/0000Z 25.3N 55.3W 55 KT 65 MPH...TROPICAL CYCLONE 72H 30/0000Z 26.5N 57.5W 60 KT 70 MPH 96H 31/0000Z 30.0N 55.5W 60 KT 70 MPH 120H 01/0000Z 34.0N 50.0W 55 KT 65 MPH $$ Forecaster Beven

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Remnants of Willa Forecast Discussion Number 18

2018-10-24 16:33:36| National Hurricane Center (East Pacific)

Issued at 1000 AM CDT Wed Oct 24 2018 000 WTPZ44 KNHC 241433 TCDEP4 Remnants Of Willa Discussion Number 18 NWS National Hurricane Center Miami FL EP242018 1000 AM CDT Wed Oct 24 2018 Satellite images and surface observations indicate that Willa's surface circulation has dissipated over northeastern Mexico, so this is the final advisory. It should be noted that a non-tropical cyclone that is forecast to move from the Gulf of Mexico to the northeastern United States over the next few days is a separate system, and not directly associated with Willa's remnants. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 25.5N 101.5W 20 KT 25 MPH...REMNANTS OF WILLA 12H 25/0000Z...DISSIPATED $$ Forecaster Pasch

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